Late-Collisional Granites in the Variscan

JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 11
PAGES 1613–1645
1999
Late-Collisional Granites in the Variscan
Erzgebirge, Germany
H.-J. FÖRSTER∗, G. TISCHENDORF, R. B. TRUMBULL AND
B. GOTTESMANN
SECTION 4.2, GEOFORSCHUNGSZENTRUM POTSDAM, TELEGRAFENBERG, D-14473 POTSDAM, GERMANY
RECEIVED OCTOBER 7, 1998; REVISED TYPESCRIPT ACCEPTED MAY 4, 1999
The late-collisional Erzgebirge granites (~325–318 Ma) were
emplaced at shallow crustal levels in the Variscan metamorphic
basement shortly after large-scale extension caused by orogenic
collapse. These granites comprise mildly peraluminous transitional
I–S-types and strongly peraluminous S-type rocks, which can be
subdivided into three major groups: low-F biotite granites; low-F
two-mica granites; and high-F, high-P2O5 Li-mica granites. The
highest degree of differentiation is reached in the Li-mica granites,
which exhibit strongly elevated concentrations of P, F, Li, Rb, Cs,
Ta, Sn, W and U; but very low Ti, Mg, Co, Ni, Sr, Ba, Y, Zr,
Hf, Th and rare earth elements. Crystal–melt fractionation is the
dominant process controlling the bulk composition of all groups of
granites. However, metasomatic processes involving late-stage residual
melts and high-T orthomagmatic fluids became increasingly more
important in highly evolved units and have modified the abundances
of mobile elements (P, F, Li, Rb, Cs, Ba, Sr) in the Li-mica
granites particularly. Isotopic and geochemical characteristics suggest
that the three granite groups cannot be derived from a common
precursor magma. Their discrete compositions are source related,
and are attributed to melting of quartzo-feldspathic and pelitic
crustal lithologies in different proportions. Granites are common in
the central European Variscides, but the Erzgebirge is unusual for
the predominance of evolved Li-mica granites associated with
economically important Sn, W and U deposits. The abundance of
Li-mica granites is attributed to a combination of favourable factors:
(1) low degrees of anatectic melting of crustal protoliths; (2) wide
distribution of fertile lithologies rich in large-ion lithophile elements
and ore elements; (3) extended magmatic differentiation by crystal–
melt fractionation and subsequent autometasomatism.
INTRODUCTION
accessory minerals
The Erzgebirge is a NE–SW trending antiformal structure
exposing Variscan crystalline basement rocks at the northern margin of the Bohemian Massif. The region is famous
among economic geologists for its extensive and highly
variable metallic ore deposits, which have been mined
since the Middle Ages (e.g. Štemprok & Seltmann, 1994;
Tischendorf & Förster, 1994), and for the classic studies
in the fields of economic geology, mining and metallurgy,
chemistry and mineralogy (e.g. Werner, 1791; Breithaupt,
1849; Agricola, 1974).
As in many of the other Variscan provinces in central
Europe, granitic rocks make up a large proportion of the
exposed rocks in the Erzgebirge, and a great variety of
compositional and textural types occur (Fig. 1). Knowledge of the processes involved in the generation and
evolution of these silicic magmas is essential for understanding heat and mass transport during the Variscan
orogeny, and also for the formation of metallic ore
deposits. Several lines of evidence indicate that early,
high-temperature Sn–W deposits are associated with the
emplacement of highly evolved, volatile-rich granitic
melts (Tischendorf, 1986; Förster & Tischendorf, 1992;
Štemprok, 1993). The role of the granites in formation of
later Permian (U, Pb–Zn) and younger ore mineralization
(F–Ba) is controversial, but good evidence exists that the
vein-type U deposits owe their uranium to leaching from
the uraninite-rich granites (e.g. Tischendorf & Förster,
1994).
The Erzgebirge granites were extensively studied between 1945 and the 1980s during the search for mineral
resources. Political changes in Europe in the early 1990s,
however, have greatly accelerated research progress because of access to modern analytical techniques, improved
∗Corresponding author. e-mail: [email protected]
 Oxford University Press 1999
KEY WORDS: Erzgebirge; collision-zone magmatism; granite; geochemistry;
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 11
NOVEMBER 1999
Fig. 1. Generalized geological map of the Erzgebirge, showing the distribution of the different groups of Variscan granites and rhyolites. EHD,
Ehrenfriedersdorf; POD, Podlesı́; TLH, Tellerhäuser; ASGZ, Aue–Schwarzenberg granite zone.
exchange of ideas and information with the international
community, and access to previously restricted data, drill
cores and outcrops. The Erzgebirge contains almost the
complete suite of silicic igneous rocks that characterize
collisional settings; these rocks were produced within a
few million years only, and were emplaced within a small
volume of the upper crust. Composite plutons with up
to four cogenetic sub-intrusions, which are exposed in
three dimensions by extensive underground mine workings and drill cores, allow a detailed study of their
magmatic evolution. Equally important, the tectonic and
metamorphic history of the country rocks and their
chemical composition are well known.
In the light of new data on the mode of occurrence,
petrography, mineralogy, age and composition, this paper
discusses the petrogenesis and evolution of the latecollisional (Namurian) suite of Variscan granites in the
German Erzgebirge. These granites are classed into three
major groups (low-F biotite granites, low-F two-mica
granites and high-F, high-P2O5 Li-mica granites). Their
principal features are illustrated using data from the
largest well-exposed plutons of each group that also show
the greatest range of internal differentiation, namely,
Kirchberg (biotite granite), Bergen (two-mica granite) and
Eibenstock (Li-mica granites) (Fig. 1). The compositional
diversity of the low-F biotite granites requires inclusion
of data from the Niederbobritzsch massif and an assemblage of small granite bodies from the Aue–
Schwarzenberg granite zone (ASGZ). Special attention
is given to the Li-mica granites because of their highly
evolved character, close association with ore deposits,
and their relative abundance in the Erzgebirge compared
with other Variscan regions of Europe. Furthermore,
these granites have been intensively investigated for the
composition of late-stage melts deduced from silicate melt
inclusions (Breiter et al., 1997a; Thomas & Klemm, 1997;
Webster et al., 1997) and pegmatites (stockscheiders;
Seltmann et al., 1995).
Variscan magmatism in the Erzgebirge ended with the
emplacement of a suite of post-collisional granites and
rhyolites of probably post-Westphalian age. These small
but geochemically evolved biotite and Li-mica granites
have chemical characteristics trending toward aluminous
A-type granites. They are rich in F and poor in P2O5,
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FÖRSTER et al.
LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
and contain elevated concentrations of Sc, Zn, Ga, Rb,
Y, Zr, Nb, Hf, Ta, W, Th, U and the heavy rare earth
elements (HREE) (e.g. Breiter et al., 1991; Förster et al.,
1995). A detailed description of these rocks is the subject
of a separate publication.
GEOLOGICAL SETTING
The Variscan fold belt of central Europe formed during
the early Palaeozoic by collision of Laurentia, Baltica,
Africa and a number of Gondwana-derived microplates
(Tait et al., 1997). The Erzgebirge antiform is located in
the Saxothuringian zone of the Variscan belt, at the
northwestern margin of the Bohemian massif. It forms a
discontinuous belt extending ~160 km along the Czech–
German border and comprises an area of ~9000 km2
(Fig. 1).
The tectonic and metamorphic history of the Erzgebirge basement has been the subject of renewed interest
in the 1990s. The prevailing view until this time was that
the metamorphic basement consisted of a succession
of lithostratigraphic units with a regular increase in
metamorphic grade inward from chlorite-zone phyllites
to high-grade, partially anatectic gneisses in the core of
the antiform (e.g. Lorenz & Hoth, 1990). The recent
work has shown that the basement rocks form a subducted
part of a rifted passive margin of Gondwana (Mingram,
1998), represent a tectonic stack of units of different
metamorphic grade, and received a major metamorphic
and deformational imprint in the early Carboniferous
(Willner et al., 1997; Rötzler et al., 1998). The lowest unit
and core of the Erzgebirge antiform is composed of
granodioritic orthogneisses whose protolith age is ~550
Ma (Kröner et al., 1995). Overlying the orthogneiss core
are several metamorphic units, each consisting of similar
Proterozoic and Palaeozoic protoliths (pelites, greywackes, marbles, rhyolites, metabasites and conglomerates) but with different metamorphic histories.
Relics of eclogite and other high-pressure assemblages
in these supracrustal units (P > 20 kbar) indicate that
during the Variscan collision lower-crustal rocks were
thrust over the medium-pressure orthogneiss massif. Mingram (1998) recognized a repetition of distinct sequences
of bulk chemical composition in the different metamorphic units, which underscores the concept of stacking.
Radiometric dating of key units has established the following stages in the assemblage of the Erzgebirge Variscan crust (Fig. 2): peak HP–HT metamorphism
(eclogite facies) of Palaeozoic metasedimentary and metavolcanic rocks at 350–340 Ma, followed by rapid uplift
and cooling (Ar–Ar and K–Ar mica ages 340–326 Ma),
erosional unconformity with deposition of sediments at
326 Ma, and intrusion of oldest lamprophyres (e.g.
Werner & Lippolt, 1998b) and granites at 325 Ma.
Shortly after thermal relaxation and extensional collapse of the orogen in the late Carboniferous and early
Permian, the Erzgebirge crust was intruded by postkinematic granitic plutons of various sizes and compositions, and minor lamprophyre dykes. Permian erosion
and Tertiary block faulting produced differential uplift
such that abundant rhyolitic and rhyodacitic lavas and
subvolcanic dykes are exposed in the eastern Erzgebirge,
whereas far more plutons are exposed in the western
Erzgebirge.
Important general features of the granites are: (1) none
show penetrative deformation, and even cataclasis is only
a local phenomenon; (2) the level of emplacement was
shallow (3–6 km) and the temperature interval of crystallization was large (from about 720 to 580°C; Thomas
& Klemm, 1997); (3) the granites were not accompanied
by important mafic and intermediate magmatism except
for the volumetrically insignificant lamprophyre dykes
mentioned. Seismic profiles through the Erzgebirge and
gravity data show no evidence for large mafic intrusions
beneath the granitic upper crust (Bankwitz & Bankwitz,
1994).
THE LATE-COLLISIONAL GRANITES
The Erzgebirge granites have been grouped in a number
of ways in the past (see Lange et al., 1972; Štemprok,
1986; Tischendorf et al., 1987; Breiter et al., 1991; Förster
& Tischendorf, 1994). The classification still widely used
subdivides the granites into an older and younger intrusive
complex (OIC and YIC, respectively). The ‘older’ OIC
series includes weakly to moderately evolved biotite and
two-mica granites, whereas highly evolved and mineralized Li-mica granites of both S- and A-type affinity
make up the ‘younger’ series. This subdivision is generally
accepted but it is untenable in practice because it imposes
age significance on a classification based on compositional
characteristics. Field relations and radiometric dating in
the German Erzgebirge give no justification for the
assumption that all plutons of the OIC group are older
than those assigned to the YIC group. Förster et al. (1998,
1999) introduced a more workable subdivision based on
objective criteria of mineralogical and chemical composition and their structural setting with respect to the
Variscan collision (i.e. late-collisional and post-collisional).
According to this, the late-collisional granites are divided into three groups: (1) low-F biotite granites; (2)
low-F two-mica granites; (3) high-F, high-P2O5 Li-mica
granites. In this paper we refer to the granites of groups
1–3 simply as biotite granites, two-mica granites and Limica granites. The presentation focuses on the largest
and best-studied composite plutons of each group
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VOLUME 40
NUMBER 11
NOVEMBER 1999
Fig. 2. Geochronology of Variscan metamorphic and magmatic events in the Erzgebirge. (See Fig. 1 and Table 1 for abbreviations of the
names of the granite plutons.) bio, biotite; mus, muscovite; Kfs, K-feldspar; mon, monazite; xen, xenotime; ur, uraninite. ∗Age calculated from
electron-microprobe data by the method of Rhede et al. (1996). Bold vertical lines mark 2d uncertainties in age. References (superscript numbers):
1, Gerstenberger (1989); 2, Seifert (1994); 3, Velichkin et al. (1994); 4, Gerstenberger et al. (1995); 5, Schmädicke et al. (1995); 6, Willner et al.
(1996); 7, Werner et al. (1997); 8, Tichomirowa (1997); 9, Kröner & Willner (1998); 10, Werner & Lippolt (1998a); 11, Werner & Lippolt (1998b);
12, authors’ unpublished data.
(Table 1). The Bergen two-mica granite and the Eibenstock Li-mica granite can be taken as ‘type’ plutons for
their respective groups in the entire Erzgebirge. The
Kirchberg granite does not have the same status as a
‘type’ pluton for the biotite granite group because the
biotite granites are too variable in composition. In particular, the abundant biotite granites in the Czech part
of the region differ from the Kirchberg granite in composition and differentiation trends, and closely resemble
an assemblage of small and poorly exposed granites in
the Aue–Schwarzenberg zone (ASGZ; see Fig. 1).
Figure 2 compiles all available age information on the
Erzgebirge granites. Data sources are listed in the caption.
Conventional U–Pb monazite ages, microprobe U–total
Pb monazite + xenotime + uraninite ages [calculated
by the method of Rhede et al. (1996)], K–Ar biotite ages
and Ar–Ar muscovite ages are generally concordant and
older than the Rb–Sr whole-rock isochron and mineral
ages. The same discrepancy between Rb–Sr isochron
ages and U–Pb zircon ages has been recognized in studies
of Variscan granites in the Oberpfalz and Fichtelgebirge
of Bavaria (Siebel et al., 1997) and elsewhere. The Rb–Sr
method is not suitable for dating the emplacement of
these granites, particularly the Li-mica group with their
very high Rb/Sr ratios and late-magmatic and hydrothermal mobility of both Rb and Sr (Gerstenberger,
1989; Irber et al., 1997). The bulk of reliable age dates
imply that the three groups of late-collisional granites
were emplaced within a short time span between 325
and 318 Ma. Westphalian vs Namurian K–Ar and Ar–Ar
cooling ages and slightly younger U–total Pb mineral
ages suggest, however, that the Li-mica granites postdate the intrusion of the biotite and two-mica granites.
GRANITE CLASSIFICATION
The Li-mica granites exhibit most of the characteristic
mineralogical and geochemical features of S-type granites
as described by Chappell & White (1974), i.e. they are
dominantly Si rich, strongly peraluminous and reduced,
have crustal isotopic signatures (eNd(t) < –5; Sri > 0·71;
see Table 14, below) and are associated with Sn–W
mineralization. As will be shown below, the biotite and
two-mica granites have a transitional I- and S-type character, indicated partly by their low initial Sr isotopic
ratios (0·705–0·707) and high eNd(t) values (–2·9 to –4·9)
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FÖRSTER et al.
LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
Table 1: Subdivision of late-collisional multi-phase granite plutons in the Erzgebirge
Sub-unit
Texture
Low-F biotite granites
Low-F two-
High-F, high-P2O5 Li-mica granites
mica granites
Kirchberg
Niederbobritzsch
Bergen
Eibenstock
Pobershau (POB)
(KIB)
(NBZ)
(BRG)
(EIB)
and Satzung (SZU)
Aplitic
very fine-grained
A-KIB
A-NBZ
A-BRG
A-EIB
Most
fine- (to medium-) grained,
KIB3
NBZ3
BRG3p
EIB3
POB3, SZU3
evolved (3)
occasionally porphyritic
More
medium-grained, slightly
evolved (2)
porphyritic
Evolved (1)
BRG3
KIB2
NBZ2
BRG2
EIB2
POB2, SZU2
coarse- to medium-grained,
KIB1a
NBZ1
BRG1
EIB1
POB1
porphyritic
KIB1m
EIB0
POB0, SZU0
Least
fine- to medium-grained,
evolved (0)
porphyritic
Magmatic Es
variably textured
E-KIB
E-NBZ
E-BRG
A, aplite; E, enclave. BRG3p, P2O5-rich facies of granite BRG3. (See text for further explanations.)
and partly by the presence of mafic microgranular enclaves. The I-type affinity is strongest for the least silicic
of the biotite granite plutons, Niederbobritzsch, which
is the only amphibole-bearing Variscan granite in the
German Erzgebirge.
All the granites have low values of magnetic susceptibility [(2·7 to 0·01) × 10–3 SI; Förster & Tischendorf,
1994] which permits their classification in the ilmenite
series of Ishihara (1977). The granites are further characterized by high radioactive heat production, which
increases with differentiation within each granite group
as follows: biotite granites (from 2 to 10 lW/m3), twomica granites (from 3 to 9 lW/m3) and Li-mica granites
(from 6 to 12 lW/m3).
The most highly fractionated granites of the Li-mica
group correspond to the ‘high-P’ subtype of topaz granites
proposed by Taylor (1992). They are similar to other Prich, rare metal granites in the Variscan chain of Europe
including St Austell, SW England (Manning & Hill,
1990), Beauvoir, French Massif Central (Raimbault et al.,
1995), and Argemela, Portugal (Charoy & Noronha,
1996). Associated pegmatites are of the LCT family
(Li–Cs–Ta as indicative elements) according to the classification of Černy (1991).
FIELD OCCURRENCE AND
PETROGRAPHIC DESCRIPTION
The late-collisional granites form composite plutons built
up by a succession of texturally and geochemically dis-
tinct, comagmatic intrusions. Locally, sharp contacts can
be recognized, which allow a sequence of intrusion to
be established (e.g. Schust, 1965). Most plutons studied
show the same characteristic textural types and these are
indicated by arabic numbers in order of increasing degree
of fractionation (Table 1). The ultimate phase in each
pluton constitutes aplitic dykes or complex aplite–
pegmatite dykes and schlieren.
Enclaves are generally rare in the Erzgebirge granites.
Rounded or ellipsoidal enclaves of centimetre to decimetre size are common only in the least-evolved Niederbobritzsch biotite and Bergen two-mica granites. Two
types can be distinguished: country-rock xenoliths (gneiss,
schist) near the pluton roof and dark, cognate and microgranitoid enclaves that occasionally contain centimetre-sized newly grown feldspar phenocrysts. The
enclaves have reacted strongly with the host granite,
resulting in substantial recrystallization and changes in
bulk composition (Rösler & Budzinski, 1994). Thus, they
are of limited use for petrogenetic or geochemical modelling.
Table 2 provides a petrographic summary of the three
main granite groups. The descriptions are based on
detailed petrographic studies of a representative pluton
from each group, and they apply, unless otherwise stated,
to the group as a whole. To avoid repetition from Table 2,
the following section discusses only those petrographic
features that distinguish the groups from one another.
Biotite is the principal mafic mineral in the biotite
granites (monzogranites), and its abundance falls markedly from the coarse-grained facies to the late aplites.
1617
f.g., equigranular, light
grey to reddish colour
m.g., equigranular,
pink–grey, some
rounded Qz
phenocrysts, local Kfs
phenocrysts
porphyritic, m.g. brown
matrix; c.g. Qz, pink Kfs
phenocrysts, rare biotite
schlieren
f.g., equigranular; grey
or pink
Kirchberg
KIB 3
Kirchberg
KIB 2
1618
40:22:30
4:2
40:23:29
5:1
anh., seldom in sheafs;
rims Bi; replaces Top
and Kfs; late
older generation
subh.–tabular, cloudy;
young albites clear,
blocky
very pale, zoned with
anh., interstitial or rims subh., most cloudy,
darker cores, anh., late on Bi, local sheafs; late some Qz embayments;
also clear late Ab
present
pale brown, subh. to
subh. to anh., interstitial subh., some sericitized
anh.; commonly zoned, or rims on Bi, replaces
several generations, late Top and Kfs; late
pale cream colour,
homogeneous, anh.,
late
dk brown, red tint,
same as above
subh. at Qz, Kfs, anh. at
Pl; kink bands
subh. to anh., some
older cores, corroded
by Qz
phenocr. subh., blocky,
zoned, older cores;
matrix anh.; myrmekite
phenocr. subh., zoned,
sericitic cores;
myrmekite; anh. matrix
grains clear
subh. to anh., corroded
by Kfs, Qz, Mu
weakly zoned, subh.to
anh., corroded byQz,
myrmekite
subh., commonly
zoned, sericite in cores,
mymekite
Plagioclase
Top, Ap, Zir, Mon, Xen,
Op, Crd, rutile, fluorite,
uraninite, cassiterite
Ap, Zir, Mon, Crd, Xen,
Op tourmaline, uraninite,
rutile
Ap, Zir, Mon, Op, Xen,
Crd, tourmaline,
uraninite, rutile
Ap, Zir, Mon, Op, Xen,
Crd, tourmaline, rutile,
uraninite
Ap, All, Zir, Op, thorite,
Crd, Xen, uraninite
Ap, Zir, Mon, Op, Xen,
Crd, uraninite, thorite
Ap, Zir, Mon, Op, Xen,
thorite, uraninite, Top
Accessories (dominant
minerals in italics)
Top, Ap, Zir, Mon, Op,
Xen, Crd, fluorite,
tourmaline, uraninite,
cassiterite, rutile
phenocr. subh.,
Top, Ap, Zir, Mon, Op,
Carlsbad twins, no
Xen, tourmaline, rutile,
microcline; coarse
cassiterite, uraninite,
perthite, Ab-rims;matrix fluorite
anh.
most anh., Carlsbad or
microcline twins or
untwinned, no perthite
anh., Carlsbad and/or
microcline twins, film
and vein perthite
phenocr. tabular; matrix
anh., Carlsbad and/or
microcline twins, film
and vein perthite
mostly anh., no
twinning, no perthite;
rare phenocrysts are
tabular, Carlsbad twins
subh. to anh., Carlsbad
twins, no microcline;
abundant perthite
same as above
phenocr. euh., Carlsbad
twins, coarse perthite;
matrix anh., microcline
twins, rare perthite
anh., some microcline
twinning, rare perthite
K-feldspar
∗ granite sub-units 3–1 are listed in order from youngest, most fractionated (3) to oldest, least fractionated (1). Qz–Pl–Kfs–Bi–Mu proportions from modal analysis
from Herrmann (1967) and Lange et al. (1972).
Mineral abbreviations: Ab, albite; All, allanite; Ap, apatite; Bi, biotite (including Li-mica); Crd, cordierite; Kfs, K-feldspar; Mon, monazite; Mu, muscovite (including
phengite); Op, opaques; Pl, plagioclase; Qz, quartz; Top, topaz; Zir, zircon. Other abbreviations: anh., anhedral; c.g., coarse grained; dk, dark; euh., euhedral; f.g.,
fine grained; m.g., medium grained; phenocr., phenocryst; subh., subhedral.
Eibenstock
EIB 1
Eibenstock
EIB 2
36:33:23
3:2
31:31:30
5:2
same as above
subh., included in Pl
and Kfs; late
included in Bi and Pl,
locally interstitial, late
Muscovite
dk brown, red tint; anh., large, anh. poikiloreplaced by Mu
blastic flakes embay Bi,
Pl, Kfs; also interstitial
dk brown, subh.,
same as above
replaced by Mu
same as above
dk brown, subh. to anh.;
corroded by Qz and Pl;
early
dk brown, euh. to subh.
flakes; early
Biotite
NUMBER 11
m.g., mostly
equigranular, local Kfs
and Qz phenocrysts;
pink
mostly porphyritic, m.g.
to c.g. matrix; coarse
Kfs, Pl, Qz phenocrysts;
pink
porphyritic, m.g. matrix;
c.g. Kfs, Pl, Qz
phenocrysts; grey to
light reddish
f.g., equigranular; very
light colour, locally pink
Bergen
BRG 1
35:32:24
3:5
37:33:24
1:5
28:36:27
8:0·3
33:32:28
6:0·6
36:32:29
2:0·3
Qz:Pl:Kfs
Bi:Mu
VOLUME 40
Eibenstock
EIB 3
m.g. equigranular; grey
to light brownish
Bergen
BRG 2
Bergen
BRG 3
Kirchberg
KIB 1a
Macroscopic features
Granite
Sub-unit∗
Table 2: Petrographic features of the major sub-intrusions forming the Kirchberg (biotite granite), Bergen (two-mica granite) and Eibenstock
(Li-mica granite) plutons
JOURNAL OF PETROLOGY
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FÖRSTER et al.
LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
Feldspars include zoned plagioclase associated with myrmekite and perthitic alkali-feldspar with prominent
microcline twinning. Cordierite (pinitized) is rare. Allanite-(Ce) and thorite occur only in this granite group.
Apatite with dark cores caused by pyrrhotite inclusions
is frequently observed and is tentatively interpreted as
representing assimilated crustal material (Gottesmann &
Wirth, 1997). Magnetite, ilmenite and minor rutile are
the most common Fe–Ti oxides. Ore minerals include
wolframite, molybdenite and scheelite. Apatite-free
aplites in the Kirchberg massif locally contain abundant
sulphides (arsenopyrite, galena, chalcopyrite, molybdenite, sphalerite), scheelite and columbite.
The two-mica monzo- to syenogranites generally show
less perthite texture and microcline twinning than the
biotite granites, and cordierite pseudomorphs are more
widespread. Dioctahedral micas (muscovite and phengite)
occur in several settings. They overgrow siderophyllite
and both feldspars, and also occur as rosettes and subhedral flakes in the grain boundary network. The granites
contain locally abundant tourmaline and rarely carry
molybdenite and wolframite.
The Li-mica granite group comprises a range of compositions from Li siderophyllite-bearing syenogranites to
protolithionite–zinnwaldite-bearing alkali-feldspar granites. The trioctahedral Li–Fe micas are accompanied
by minor but ubiquitous near end-member fluortopaz
(19–21 wt % F). Dioctahedral micas (muscovite and
phengite), which are typically rich in lithium, overgrow
the trioctahedral micas and feldspars, and also occur as
rosettes and subhedral flakes in the grain boundary
network. There is a greater compositional variety of
accessory minerals in the Li-mica granites than in the
other granite groups. For example, the Eibenstock pluton
is the only granite world-wide in which the rare monazitegroup mineral brabantite has been discovered to date
(Förster, 1998a). Rutile may contain several wt % Nb,
Ta, Sn and W. More differentiated intrusions may also
carry cassiterite, wolframite and scheelite, and minor
molybdenite and columbite.
of pre-existing minerals and growth of new minerals in
the intergranular space and cracks (excluding weathering). The bulk chemical influence of late and secondary
mineral growth varies from negligible to essential, as
discussed in a later section.
Secondary effects in the biotite granites are confined
to overgrowths and partial replacement of pre-existing
minerals (chlorite ± titanite ± epidote ± rutile in
biotite, white mica in cordierite, white mica ± clinozoisite
± fluorite ± carbonate in plagioclase). Local hydrothermal mobilization of the rare earth elements (REE)
led to the formation of rare synchysite-(Ce), bastnaesite(Ce) and secondary allanite-(Ce). The two-mica granites
show mineral replacements similar to those of the biotite
granite group, but the effects are more intense, and
cataclastic deformation with recrystallization of feldspars
and quartz is more common. Both late and secondary
muscovite occurs, and apatite also crystallizes early
(euhedral) and late (interstitial). U mobility is manifest in
various secondary uraniferous micas (bergenite, autunite,
torbernite, zeunerite, uranocircite).
Replacement, mineral overgrowths and late interstitial
phases are essential features of the Li-mica granites and
can be attributed to the elevated contents of volatile and
incompatible elements in the magmas. Chlorite is lacking,
but growth of late and secondary dioctahedral micas is
common. White mica replaces plagioclase and topaz.
Late albite occurs interstitially and as rims on K-feldspar.
Late quartz is abundant in the grain boundary network,
from which it partially replaces feldspars. Topaz, which
often is sericitized or even kaolinitized, is a common
interstitial mineral and it locally forms in clusters with
quartz and greenish protolithionite. Other common volatile-bearing late phases include fluorite, tourmaline
(some as tourmaline–quartz nodules) and a second generation of interstitial apatite. Li-mica granites from the
locality Pobershau (Fig. 1) occasionally carry andalusite
instead of topaz.
ANALYTICAL METHODS
Mineral chemistry
LATE-MAGMATIC AND SECONDARY
EFFECTS
The different groups of Erzgebirge granites show variably
strong petrographic evidence for recrystallization and
mineral replacement. None of the granites studied is free
of such effects, and in the case of the Li-mica granite
group, they are so prominent as to be a distinctive
feature of these rocks. It can be difficult or impossible
to distinguish late-magmatic and post-magmatic effects
petrographically (i.e. was a melt present or not?), and
this paper uses the term ‘secondary effects’ in a general
sense for phenomena of recrystallization or replacement
Mineral analyses were performed using CAMEBAX SX50 and SX-100 electron microprobes at the GFZ Potsdam
operating in wavelength-dispersive mode. The operating
conditions during analysis of accessory minerals were as
follows: accelerating voltage 20 kV, beam current 40–60
nA and beam diameter 1–2 lm. Counting times, data
reduction, analysing crystals, standards, analytical precision and detection limits have been described in detail
by Förster (1998a, 1998b). Analysis of major silicates and
apatite was performed at 15 kV, 10–20 nA and 5–15
lm beam diameter, using natural minerals and oxides
as standards.
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JOURNAL OF PETROLOGY
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Whole-rock geochemistry
For each granite pluton, a representative number of
samples (n) were trimmed of weathering material and
between 2 and 10 kg were crushed for analysis (n was
27 for Kirchberg, 15 for Niederbobritzsch, 15 for the
Aue–Schwarzenberg zone, 33 for Bergen, 48 for Eibenstock, 29 for Pobershau–Satzung).
A variety of analytical techniques at the GFZ Potsdam
were used to obtain whole-rock geochemical data on
homogenized rock powders. Several trace elements were
analysed by various methods, which allows checks on
dissolution procedures and inter-technique calibrations
for a given element in a certain concentration range.
The major elements, and some trace elements (Zn, Ga,
Rb, Sr, Y, Zr and Ba) were determined by wavelengthdispersion X-ray fluorescence spectrometry (XRF) using
fused lithium tetraborate discs. Pressed powder pellets
were used for Zr, Nb, Sn, Pb, Th and U measurements
by XRF. All XRF analyses were made with an automated
Siemens SRS303AS spectrometer using a Rh tube operated at 50 kV and 45 mA. Analysis for fluorine was
performed using ion-selective electrodes. Total water and
CO2 were determined by combustion–IR detection. The
REE plus Rb, Sr, Y, Zr, Cs, Ba, Hf, Pb, Th and U were
analysed by inductively coupled plasma-mass spectrometry (ICP-MS; Perkin-Elmer–Sciex Elan Model 500)
according to the method and with the precision and
accuracy outlined by Dulski (1994). Analysis for Li, Sc,
Co, Ni, Zn, Ga, Rb, Sr, Nb, Mo, Sn, Sb, Cs, Ta,
W, Tl, Pb, Bi, Th and U was performed by ICP-MS
(Fisons–VG Plasma Quad PQ 2+) as described by
Plessen et al. [in Govindaraju et al. (1994)]. Concentrations
of Ti, Mg, Li, Be, Sc and Sr were determined by
inductively coupled plasma-atomic emission spectrometry
on a Varian Liberty 200 ICP-emission spectrometer.
Supplementary data on Sc, Co, Cs, Hf, Ta, Th and U,
as well as Sc, Rb, Mo, Sb, Cs, Ba, Ta, W, Th and
U, were produced by instrumental neutron activation
analysis (INAA) at the Technical University of Munich
and at Bondar-Clegg & Company Ltd, Ottawa, respectively.
Isotopes
Isotopic compositions of Sr and Nd were determined on
whole-rock sample powders (grain size <63 lm) that
were dissolved in pressurized Teflon vessels for 3 days
in 5:1 HF–HNO3, then dried and taken up in HCl
for chemical separation. Cation exchange resin (Biorad
AG50W × 12) in quartz glass columns was used to
separate the Sr and the REE fraction. Neodymium was
separated from the other REE in quartz glass columns
filled with Teflon (PFTE) powder coated with HDEHP
NUMBER 11
NOVEMBER 1999
(bis-2ethyl-hexyl-phosphoric acid). Sr isotopic compositions were measured on a VG Sector 54-30 mass
spectrometer operated in dynamic mode. The average
87
Sr/86Sr value of NBS 987 Sr standard obtained during
the measuring campaign was 0·710246 ± 5. Nd analysis
was carried out on a Finnigan TIMS MAT 262 mass
spectrometer operated in static mode with a doublefilament procedure (Ta evaporation filament, Re ionization filament). The 143Nd/144Nd ratios were corrected
for mass fractionation using the 146Nd/144 Nd ratio 0·7219.
Repeated analysis of the La Jolla Nd standard gave an
average value of 0·511855 ± 4.
MINERAL COMPOSITIONS
Mineral assemblages and the composition of minerals,
determined by electron microprobe, reflect the similarities
and differences among the three main granite groups.
Mineral compositions provide a monitor of magmatic
differentiation and an indicator of late-stage and secondary processes. The accessory minerals play a particularly important role, as they host many of the
economically and petrogenetically important trace elements in silicic magmas (U, Th, REE, Zr, Y, Nb, Sn)
and therefore control whole-rock element abundances.
Alkali feldspar
The albite (Ab) content of anorthite-poor (An < 1 mol
%) alkali feldspar in the biotite granites varies between
22 and 4 mol % and generally decreases with increasing
differentiation. The P2O5 content is constantly low (<0·07
wt %) and Rb2O is at or below the detection limit. Kfeldspar in the two-mica granites varies from Ab12 to Ab2
(An<1 in all cases) with progressive differentiation. The
Rb and P contents generally increase with degree of
fractionation (up to 0·2 wt % for Rb2O and 0·7 wt %
for P2O5) but there is considerable overlap among the
sub-intrusions. Magmatic K-feldspar from the Li-mica
granites is variable in composition (19–3 mol % Ab and
0·2–1·2 wt % P2O5). The content of Rb2O systematically
increases from <0·1 wt % in less evolved granite facies
to 0·2–0·3 wt % in the most evolved granites. Postmagmatic, near end-member K-feldspar (Ab<3) is usually
lower in P2O5 (<0·05–0·6 wt %) than late-magmatic Kfeldspar.
Plagioclase
The composition of plagioclase correlates with the degree
of fractionation within each granite group. Plagioclase
in the biotite granites varies from An30 to An5. Nearly
pure end-member, metasomatic albite (An1) forms the
1620
FÖRSTER et al.
LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
outermost rims of plagioclase grains. Plagioclase is poor
in P2O5, with contents typically <0·05 wt %, and exceptionally (in aplite) up to 0·3 wt %. The most calcic
plagioclase in less-fractionated two-mica granites has a
composition of An18. Magmatic albite (An>2) in highly
evolved granites of this group differs from that of metasomatic origin (An<2) and plagioclase in the less-differentiated rocks by having higher contents of P2O5
(0·25–0·5 wt % vs <0·2 wt %, respectively). The content
of Rb2O generally does not exceed 0·1 wt %. The Limica granites contain albitic plagioclase in most samples
(An5 to An0·5); more calcic plagioclase with anorthite
contents ranging from 15 to 5 mol % is restricted to
the least-fractionated parts of composite plutons. P2O5
contents in plagioclase cover a wide range (<0·05–0·8
wt %), which correlates roughly with degree of fractionation. As in the two-mica granites, sub-solidus albite
tends to be P poor. The contents of Rb2O are uniformly
low (<0·08 wt %) and show no correlation with progressive differentiation.
Trioctahedral micas
The composition of trioctahedral micas differs systematically with differentiation in all granite groups
(Fig. 3, Table 3). Mica compositions in the biotite granites
range from Mg biotite in least-evolved intrusions through
Fe biotite to siderophyllite (in late aplite) that contains
up to 0·5 wt % Rb2O and 0·12 wt % Cs2O. The most
primitive trioctahedral mica in the two-mica granites is
Fe biotite, and the mica evolves with fractionation to
siderophyllite rich in F (maximum 2·7 wt %), Rb2O (0·5
wt %) and Cs2O (0·3 wt %) (Fig. 4a). Mica compositions
in the Li-mica granites extend from siderophyllite through
protolithionite to zinnwaldite, with a steady increase in
F from about 1·5 to 8 wt % (Fig. 4b). The zinnwaldite
in these rocks is rich in Rb2O and Cs2O, with maximum
values of 1·25 and 0·15 wt %, respectively. Maximum
measured and calculated (see Table 3) Li2O concentrations amount to 4·3 wt %. Secondary zinnwaldite
from metasomatic zones in the granites contains the
highest measured values of F, Rb2O and Cs2O (9·1, 1·3
and 0·17 wt %, respectively; see Fig. 4b).
Dioctahedral micas
Late- (to post?)-magmatic white micas show an evolution
in composition with rock fractionation similar to that for
the trioctahedral micas (Figs 3 and 4). F-poor phengite
and muscovite with low calculated Li concentrations
(Table 4) crystallized in early formed two-mica granites,
whereas Li- and Mn-rich phengite containing up to 1·7
wt % F and 0·3 wt % Rb2O is typical of the late subintrusions. Mn-poor muscovite and phengite characterize
the least fractionated Li-mica granites, and the more
evolved sub-intrusions contain FeO-rich (up to 11 wt %)
and Rb2O-rich (0·2–0·8 wt %) but relatively Cs2O-poor
lithian phengite with up to 4·6 wt % F.
Apatite
Cl- and OH-poor fluorapatite differs notably in minor
element composition between the granite groups
(Table 5). Early-magmatic apatite in the biotite granites
is silica rich and typically displays a systematic increase
in the REE, Y, Mn, Fe and Na with differentiation.
Apatite from the two-mica granites is silica poor. Early
crystallized apatite is abundant in the least-evolved granites, whereas late-crystallized interstitial, anhedral grains
predominate in more evolved sub-intrusions. The composition of the late apatite evolves in a systematic way,
i.e. the contents of Na, Y and the light REE (LREE)
decrease, and the content of Mn dramatically increases
(up to 8·8 wt % MnO) with differentiation. In the aplites
apatite grains frequently occur, which contain variable
amounts of MnO (0·8–8·5 wt %) but are enriched in SrO
(0·4–1·6 wt %), and which are interpreted as metasomatic.
Early REE-, Y- and Fe-apatite is restricted in occurrence
to the Li siderophyllite-bearing sub-intrusions of the Limica granite group. Late apatite became enriched in Mn
and depleted in Y, Na and the LREE with continuing
granite differentiation. Metasomatic apatite is represented
either by a Sr-poor near end-member F-apatite or a Fapatite that is strongly enriched in SrO (0·3–1·7 wt %).
Allanite-(Ce)
Primary (magmatic) allanite is restricted to the leastevolved sub-intrusions of the Kirchberg and Niederbobritzsch biotite granites, and it does not coexist with
monazite. This is illustrated by two facies of the Kirchberg
massif (e.g. Table 1): one contains magmatic allanite
(KIB1a), and the other bears either secondary (hydrothermal) allanite associated with monazite or monazite
alone (KIB1m). Microprobe analyses indicate strong enrichment in the LREE, relatively high concentrations of
Th, and low contents of U, Y and the HREE in primary,
metamict and strongly hydrated allanite (Table 6). Anhedral secondary allanite usually occurs along cleavages
in Mg–Fe biotites and, compared with primary allanite,
contains higher concentrations of Ca, Al, Y and the
HREE, lower concentrations of U, Th and Mg, and is
less intensively hydrated.
Thorite
Thorite occurs throughout the entire fractionation range
in the Kirchberg and Niederboboritzsch biotite granite
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JOURNAL OF PETROLOGY
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NUMBER 11
NOVEMBER 1999
Table 3: Representative electron-microprobe analyses of trioctahedral micas (in wt %)
Group:
biotite granites
two-mica granites
Li-mica granites
Sub-unit: KIB1a
KIB2
KIB3
A-KIB2
BRG1
BRG2
BRG3
BRG3p
EIB0
EIB1
EIB2
EIB3
A-EIB1
Sample: 587
785
1072
784
524
884
523
521
821
820
812
819
800
SiO2
35·8
TiO2
4·30
36·6
3·66
37·1
2·72
33·6
3·58
38·2
3·58
34·3
36·6
3·34
1·71
36·5
34·0
1·47
3·04
36·7
1·51
40·5
0·53
43·5
0·37
48·2
0·25
Al2O3
13·5
16·2
17·8
18·0
16·4
19·2
21·7
22·7
19·8
21·9
23·8
22·3
19·5
FeO∗
21·9
20·6
22·5
26·5
21·1
21·8
20·3
21·3
26·7
22·3
17·0
13·9
11·8
MnO
0·52
MgO
10·3
0·76
1·63
0·85
0·27
0·51
2·61
1·70
0·20
0·44
0·32
0·33
0·14
7·65
3·61
3·25
6·99
6·30
2·58
1·53
1·88
2·01
0·59
0·44
0·13
CaO
0·02
0·01
0·02
0·02
0·02
0·01
Li2O
0·11†
0·17†
0·44†
0·49†
0·20†
0·23†
n.d.
0·63†
n.d.
1·02†
n.d.
0·85†
0·03
0·94‡
n.d.
2·06‡
n.d.
2·92‡
n.d.
Na2O
0·07
0·06
0·07
0·07
0·17
0·19
0·07
0·11
0·18
0·28
0·22
0·28
0·12
K 2O
9·19
9·84
9·20
9·10
9·08
9·18
9·36
9·34
9·26
9·31
9·61
9·63
9·73
1·26
4·28‡
Rb2O
0·12
0·14
0·15
0·33
0·16
0·16
0·43
0·19
0·47
0·62
0·91
Cs2O
0·03
0·04
0·04
0·06
0·07
0·07
0·08
0·05
0·10
0·13
0·14
0·11
H2O§
3·48
3·48
3·61
3·35
3·60
3·18
3·04
2·99
2·64
2·52
1·47
1·05
0·57
F
0·72
0·88
0·54
0·83
0·73
1·46
1·89
2·04
2·38
2·90
5·51
6·53
7·78
Cl
0·21
0·05
0·04
0·20
0·09
0·05
0·01
0·02
0·25
0·19
0·03
0·01
Sum
100·2
O=
0·35
100·1
0·38
99·6
0·24
100·2
0·39
99·5
0·33
100·0
100·5
0·62
0·80
101·2
101·4
0·86
1·06
101·5
1·26
102·4
2·33
102·4
2·75
0·07
103·9
3·29
(F+Cl)
Total
Variety
99·9
99·8
99·3
99·8
99·1
99·4
Mg bi
Fe bi
sid
sid
Fe bi
Fe bi
99·7
100·4
100·4
100·3
100·1
sid
sid
sid
sid
prot
99·6
znw
100·6
znw
Blank, not analysed; n.d., analysed but not detected. Mg bi, Mg biotite; Fe bi, Fe biotite; sid, siderophyllite; prot, protolithionite;
znw, zinnwaldite.
∗Total iron as FeO. †Calculated according to equation (1) of Tischendorf et al. (1999). ‡Calculated according to equation
(tri1) of Tischendorf et al. (1997). §Calculated assuming the (F,Cl,OH) site is filled.
massifs but is rare in the ASGZ biotite granites, where
it is confined to the least evolved intrusions. Like primary
allanite, it is always metamict and hydrated, and may
contain appreciable amounts of F (Table 7). Thorite
exhibits strong variation in the contents of the lanthanide
and actinide elements. The compositional data suggest
extensive solid solution of thorite (tetragonal ThSiO4)
with isostructural coffinite (USiO4), zircon (ZrSiO4) and
xenotime [(Y,HREE)PO4].
Th-, U- and Hf-poor euhedral, non-metamict crystals
close to the ZrSiO4 end-member to subhedral–anhedral,
corroded and pitted grains that exhibit substantial contents of Hf, P, Y, HREE, Th and U, as a result of
the isomorphic components hafnon (HfSiO4), xenotime,
thorite and coffinite. These metamict and strongly hydrated zircon grains are also depleted in Zr and Si, and
enriched in Sc, Al, Ca and Fe. Evolution in zircon
compositions is accompanied by a systematic decrease
in the Zr/Hf ratio from the least-evolved to the highly
evolved members of cogenetic fractionation series.
Zircon
Zircon can be highly variable in composition at all scales
of observation, i.e. between granite groups, within a
pluton or even at a thin section or individual grain
scale. On average, however, systematic shifts in zircon
composition during magmatic differentiation are common to all groups (Table 8). With increasing degree of
granite fractionation, zircon characteristics change from
Monazite-(Ce)
Like zircon, monazite exhibits considerable compositional
variations at different scales (Förster, 1998a), but the
common monazite compositions reflect the differences
in bulk composition between the groups of granites and
record the compositional changes attending magmatic
1622
FÖRSTER et al.
LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
Fig. 3. Position of trioctahedral and dioctahedral micas in the mica
classification diagram of Tischendorf et al. (1997). Al phl, aluminophlogopite; Al Mg bi, Al–Mg biotite; Fe bi, Fe biotite; Li mus, Li
muscovite; Li phe, Li phengite; lpm, lepidomelane; lpl, lepidolite; Mg
bi, Mg biotite; mus, muscovite; phe, phengite; phl, phlogopite; prot,
protolithionite; sid, siderophyllite; znw, zinnwaldite.
differentiation within a given pluton (Table 9). The mole
fractions of huttonite (monoclinic ThSiO4) and brabantite
[CaTh(PO4)2] and their U equivalents, as well as of
xenotime, increase with fractionation; hence, monazite
in more differentiated rocks typically is enriched in Th,
U, Y and the HREE, and depleted in the LREE. Furthermore, late monazite in the fractionated granites typically displays discontinuous chondrite-normalized LREE
patterns (e.g. downward kinks at La or Nd, or both). The
LaN/CeN and NdN/SmN ratios in monazite systematically
decrease and the chondrite-normalized REE patterns
become flatter with granite differentiation.
Xenotime
In contrast to monazite and zircon, xenotime does not
vary significantly in composition during granite differentiation (Table 10). A feature of xenotime distinctive of
the three granite groups is the shape of its chondritenormalized HREE pattern (Förster, 1998b). Whereas
xenotime from the biotite and two-mica granites typically
shows flat patterns, xenotime from the Li-mica granites
shows a substantial preference for the elements of the
Gd–Ho group over the elements of the Er–Lu group
(Table 10). Another feature of interest is that the proportion of xenotime grains with discontinuous HREE
Fig. 4. Relations between F and FeOtot in primary and secondary
micas from the Bergen two-mica granites (a) and Eibenstock Limica granites (b). Arrows indicate evolution of mica composition with
magmatic differentiation.
patterns (e.g. downward kinks at Gd, Lu or more commonly Ho, and non-chondritic Y/Ho ratios) is greatest
in highly fractionated two-mica and Li-mica granites.
Uraninite
Magmatic uraninite is common in granites from all three
groups. Mass balance calculations show that it is the
dominant U host in the rocks (Förster, 1999). Uraninite
from the two-mica and Li-mica granites is consistently
poor in ThO2 (0·8–6·5 wt %) and Y2O3 (0–0·8 wt %),
and contains only between 0·1 and 0·6 wt % REE2O3.
In contrast, uraninite in the biotite granites is enriched
in these components (ThO2 = 5·6–11·0 wt %, Y2O3 =
0·6–5·5 wt %, REE2O3 = 0·7–5·5 wt %).
WHOLE-ROCK GEOCHEMISTRY
The biotite granites, two-mica granites and Li-mica granites from the Erzgebirge are all peraluminous in com-
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JOURNAL OF PETROLOGY
VOLUME 40
Table 4: Representative electron-microprobe
analyses of dioctahedral micas (in wt %)
Group:
two-mica granites
Li-mica granites
Sub-unit:
BRG1
BRG3
BRG3p
EIB0
EIB1
EIB3
A-EIB1
Sample:
524
523
521
821
820
819
800
SiO2
51·0
TiO2
Al2O3
0·40
30·1
45·6
0·11
32·0
45·9
0·18
32·2
44·7
0·20
34·0
44·7
0·50
29·8
45·5
0·16
28·8
44·0
0·22
28·1
FeO∗
2·37
4·33
4·48
4·03
6·12
7·68
10·56
MnO
0·03
0·33
0·46
0·05
0·14
0·16
0·10
MgO
0·64
1·24
0·94
1·03
1·30
0·37
0·06
Li2O†
0·03
0·56
0·61
0·83
1·91
1·77
1·56
Na2O
K2 O
0·57
10·1
0·65
10·0
Rb2O
0·59
10·1
0·82
10·1
0·50
10·2
0·15
10·4
0·60
0·45
H2O‡
4·46
3·80
3·78
3·63
2·81
2·88
2·91
F
0·13
1·30
1·39
1·76
3·29
3·11
2·83
Cl
0·03
0·01
0·01
0·01
0·07
0·01
0·14
Sum
O=(F+Cl)
0·25
0·16
10·4
99·9
0·06
Total
99·8
Variety
mus
100·0
0·55
99·4
phe
100·9
0·59
101·2
0·74
101·4
1·40
101·6
1·31
100·3
101·5
1·22
100·3
100·4
100·0
100·3
phe
phe
Li phe Li phe Li phe
Blank, not analysed; mus, muscovite; phe, phengite; Li phe,
Li phengite.
∗Total iron as FeO. †Calculated according to equation (di1)
of Tischendorf et al. (1997). ‡Calculated assuming the (F, Cl,
OH) site is filled.
position (A/CNK = 1·0–1·3; Fig. 5a) and cover a
considerable range of SiO2 contents (67–77 wt %, 71–76
wt % and 73–76 wt %, respectively; Fig. 5b). Representative whole-rock chemical analyses of the three
groups are given in Tables 11–13. The analyses were
selected to show typical compositions of each sub-intrusion from the multi-phase plutons and thus document
the compositional range present. Particular emphasis is
placed on differentiation trends within the three granite
groups because these potentially reveal information about
the processes and/or physical conditions of magma evolution. Thus, Fig. 5 presents a series of variation diagrams
for petrogenetically and metallogenetically important elements with 1/TiO2 as an index of differentiation. This
index is superior to SiO2 for these rocks because Ti is
less easily remobilized by secondary effects and because
TiO2 contents in whole rock decrease smoothly and
continuously with progressive differentiation, whereas
silica first increases then falls at high levels of fractionation
(see Fig. 5b).
NUMBER 11
NOVEMBER 1999
To cover the compositional heterogeneity within the
biotite granite group we have plotted analyses from
Kirchberg, Niederbobritzsch and the ASGZ separately
in Fig. 5. Compared with Kirchberg, the ASGZ granites
have higher P2O5 and F, but distinctly lower Li, Rb, Cs,
Y, Zr, Nb, Hf, Ta, Pb, Th and HREE contents at
the same level of differentiation. The Niederbobritzsch
granite shares most of the compositional signatures of
the ASGZ granites but is similar to the Kirchberg granites
with respect to P2O5, F and Th. The two-mica granites
are represented by the ‘type’ pluton Bergen. The Bergen
samples (surface outcrops) are, however, variably affected
by secondary processes as apparent in the scattered
variation of mobile elements F, Rb, Cs, Sn, Pb and U
with TiO2. In the case of the Li-mica granites, where
alteration processes are more widespread, care was taken
to select only data from the freshest available samples:
drill core and quarry material from the Eibenstock and
Pobershau–Satzung plutons.
The differentiation trends in all three granite groups
are indistinguishable for components that are most compatible in feldspars and biotite (MgO, CaO, Co, Sr, Ba).
The groups show distinctly different trends for P2O5, F
and Li, and for trace elements whose abundance is
controlled by accessory minerals such as Y, Zr, Sn, the
HREE, Hf, Pb, Th and, less markedly, Nb and the LREE.
Whereas P2O5, F and Li increase with differentiation in
the two-mica and Li-mica granites, the opposite is true in
the biotite granite group. Differentiation in the Kirchberg
biotite granite increases or does not affect the concentration of Y, Th, Pb and the HREE whereas these
same elements decrease steadily with differentiation in
the other two groups. The separate character of differentiation in the granite groups is also reflected in the
behaviour of Na2O and K2O, although the variation of
both of these components is irregular and may not be
entirely magmatic.
Characteristic chondrite-normalized REE patterns are
shown in Fig. 6. The granites of all three groups have
the common feature of a steady depletion of LREE
and increasing negative Eu anomaly with magmatic
differentiation. In the Kirchberg biotite granites, this
LREE depletion is accompanied by increasing HREE
contents whereas in the other granites HREE contents
also fall with progressive fractionation. In contrast to the
two-mica and biotite granites, which have relatively flat
HREE abundance patterns (TbN/YbN ≈ 1), the HREE
patterns from the Li-mica granites are inclined. The most
highly evolved granites and aplites from the two-mica
and Li-mica groups display characteristically curved REE
patterns with concave-upward segments between Gd and
Ho, which Bau (1996) described as the lanthanide tetrad
effect.
The end stage of magmatic differentiation in each
granite group (residual melt composition) is represented
1624
FÖRSTER et al.
LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
Table 5: Representative electron-microprobe analyses of apatite (in wt %)
Group:
biotite granites
Sub-unit:
KIB1a
KIB2
KIB3
BRG1
BRG2
BRG3p
A-BRG
EIB0
EIB1
EIB3
A-EIB1
POB3
Sample:
587
785
1072
524
478
521
779
504
509
796
800
911
Origin:
early
early
early
early
late
late
metasom.
early
late
late
metasom. metasom.
41·9
41·5
41·6
42·2
P2O5
two-mica granites
40·9
SiO2
0·23
0·25
0·22
0·04
0·07
Y 2 O3
0·16
0·63
0·80
0·28
0·36
Li-mica granites
40·3
41·1
0·03
n.d.
0·05
n.d.
42·1
0·10
0·54
41·0
n.d.
0·37
41·9
n.d.
n.d.
41·6
0·05
n.d.
41·8
0·02
n.d.
LREE2O3∗
0·68
0·77
1·11
0·21
0·38
0·30
0·07
0·76
0·24
0·10
0·15
0·10
HREE2O3†
0·10
0·25
0·33
0·16
0·01
0·19
0·06
0·32
0·30
0·26
0·16
0·02
CaO
54·4
53·2
50·5
52·9
49·9
48·4
46·5
51·7
50·6
50·0
52·3
54·6
MnO
0·12
0·57
1·65
1·21
3·71
6·93
8·46
0·96
2·49
3·15
1·55
0·48
FeO‡
0·05
0·20
0·49
0·32
0·98
0·47
0·19
1·29
1·51
1·51
0·80
0·03
SrO
1·38
n.d.
Na2O
0·09
0·20
0·28
0·14
0·21
0·04
0·17
0·23
0·16
0·03
0·04
n.d.
F
3·43
4·44
4·09
3·52
3·52
4·21
3·81
3·47
3·95
3·62
4·19
Cl
0·05
0·01
0·01
0·09
0·10
0·03
0·06
0·10
0·05
0·10
H2O§
Sum
O=(F+Cl)
Total
n.d.
n.d.
n.d.
0·14
101·4
1·46
99·9
n.d.
0·08
102·0
1·87
100·2
100·5
1·72
99·3
101·2
1·50
99·7
n.d.
n.d.
0·47
n.d.
0·04
100·2
n.d.
0·11
100·9
1·51
98·7
n.d.
100·9
1·78
99·1
1·60
99·3
101·6
1·47
100·1
n.d.
4·77
n.d.
0·02
100·8
1·68
99·1
100·6
1·53
99·0
102·3
1·79
100·6
101·9
2·01
99·9
n.d., analysed but not detected.
∗LREE2O3 = La2O3 + Ce2O3 + Nd2O3 + Sm2O3. †HREE2O3 = Gd2O3 + Dy2O3 + Yb2O3. ‡Total iron as FeO. §Calculated
assuming the (F,OH,Cl) site is filled.
by aplite dykes. Aplites from all three groups plot at the
extension of the differentiation trend defined by the
granite samples. The overall degree of differentiation
represented by the aplites increases from the biotite
granites to the two-mica and Li-mica granites (1/TiO2 =
18, 29 and 34, respectively). Clearly, whole-rock analyses
can only imperfectly document the composition of residual melts, particularly with reference to volatile elements. Melt inclusions potentially contain this
information as shown by several recent studies of the Limica granites (Breiter et al., 1997a; Thomas & Klemm,
1997; Webster et al., 1997). These studies indicate that
the residual melts from Li-mica granites are strongly
peraluminous (A/CNK = 1·3–2·0) and volatile rich,
with contents of F and P2O5 from 2 to 9 wt % and 2 to
6 wt %, respectively. The residual melts contain much
less Si, Ca, Sr and Y, and more Al, P, F, Li, B, Be, Rb,
Nb, Sn, Cs and Ta than the most highly evolved aplite
samples. As shown in Fig. 7, many of the melt inclusion
compositions plot along a continuation of the differentiation trend established by whole-rock data.
Sr AND Nd ISOTOPES
Most previous radiogenic isotopic studies of the Erzgebirge granites were concerned with age dating by
the Rb–Sr isochron method. Gerstenberger (1989) and
Gerstenberger et al. (1995) obtained statistically valid
isochrons and well-defined 87Sr/86Sr intial ratios from the
Niederbobritzch and Kirchberg biotite granites and the
Bergen two-mica granite (note the Rb–Sr ages of Kirchberg and Bergen may be slightly too young; see Fig. 2).
The initial Sr ratios for these granites overlap (0·706–
0·708). However, the extremely high Rb/Sr ratios of the
Li-mica granites, accentuated in some cases by secondary
Rb enrichment and/or Sr loss (Gerstenberger, 1989;
Irber et al., 1997), lead to imprecise or geologically
meaningless values of 87Sr/86Sr initial ratios. Neodymium
and samarium are much less mobile than Rb and Sr,
and the Nd-isotopic ratios are therefore more reliable
source indicators. Gerstenberger et al. (1995) reported
Nd ratios of the Bergen and Kirchberg granites. Table 14
combines these data with new Nd-isotopic analysis from
Li-mica granites of the Pobershau and Satzung massifs.
1625
JOURNAL OF PETROLOGY
VOLUME 40
Table 6: Selected electron-microprobe analyses of
allanite from biotite granites (in wt %)
Origin: primary
Sub-
KIB1a
KIB1a
KIB1a
NBZ1
KIB1m KIB1m
235
235
235
334
792
(core)
(mid)
(rim)
unit:
P2O5
n.d.
SiO2
31·1
TiO2
ThO2
Sub-unit:
KIB1a
KIB1m
KIB3
KIB3
A-KIB2
Sample:
235
441
305
1072
784
P2O5
Sample: 307
1·33
1·71
UO2
n.d.
Al2O3
11·2
0·04
29·0
1·65
0·03
32·1
1·93
0·06
33·8
2·67
0·04
1·20
2·61
3·59
1·08
0·03
0·06
0·05
11·9
13·5
0·04
32·5
0·02
32·8
3·35
0·05
13·2
SiO2
792
TiO2
30·9
11·4
NOVEMBER 1999
Table 7: Selected electron-microprobe analyses of
thorite (in wt %)
secondary
KIB1a
NUMBER 11
1·20
18·6
0·03
ZrO2
n.d.
ThO2
54·9
0·19
18·6
0·04
66·1
3·27
14·1
0·67
60·5
10·3
11·4
0·13
1·88
16·6
n.d.
10·2
n.d.
39·0
53·0
UO2
3·74
0·00
0·00
3·22
13·5
Al2O3
1·06
0·04
0·10
0·36
n.d.
n.d.
Y 2 O3
3·55
1·24
1·42
6·44
3·53
n.d.
n.d.
La2O3
0·27
0·09
0·12
0·07
0·03
20·5
18·3
Ce2O3
2·20
0·69
0·31
0·45
0·25
0·15
Y 2 O3
0·09
0·16
0·12
0·14
0·32
0·50
0·29
Pr2O3
0·54
0·18
0·05
0·05
0·09
La2O3
5·08
5·75
6·21
4·36
5·71
5·57
4·76
Nd2O3
2·26
1·66
0·25
0·54
0·43
9·02
Sm2O3
0·79
0·51
0·12
0·34
0·25
0·64
0·35
1·21
0·69
Ce2O3
10·9
11·6
12·5
9·37
12·3
10·6
Pr2O3
0·88
1·13
0·99
0·95
1·16
1·13
0·90
Gd2O3
0·90
Nd2O3
2·50
3·49
3·06
3·00
3·74
3·67
3·09
Tb2O3
0·07
Sm2O3
0·25
0·26
0·31
0·26
0·47
0·56
0·40
Dy2O3
0·65
Gd2O3
0·05
0·14
0·14
0·15
0·19
0·35
0·34
Ho2O3
0·15
0·07
Dy2O3
FeO∗
0·06
15·4
0·02
12·1
0·04
n.d.
8·84
8·86
n.d.
13·6
0·46
0·79
0·47
0·17
0·06
0·26
0·76
0·23
11·7
13·6
CaO
1·35
2·35
2·49
3·01
1·94
1·89
5·31
0·05
0·10
0·19
0·30
8·57
9·13
MgO
0·68
0·96
0·60
0·65
1·15
0·17
0·26
Fe2O3∗
1·74
MnO
0·35
0·49
0·74
0·79
0·60
0·56
0·16
PbO
0·27
F
0·11
0·40
0·06
O=F
0·05
0·17
0·02
90·1
90·8
0·08
n.d.
0·03
95·1
0·64
Yb2O3
7·81
90·9
1·29
Er2O3
9·66
91·5
0·42
12·6
0·04
9·92
Total
0·21
10·1
CaO
n.d.
0·28
98·2
96·6
F
0·04
1·20
0·80
O=F
0·02
0·51
0·34
Total
Blank, not analysed; n.d., analysed but not detected.
∗Total iron as FeO.
94·9
94·6
89·7
90·8
94·3
Blank, not analysed; n.d., analysed but not detected.
∗Total iron as Fe2O3.
Figure 8 shows a synthesis of Sr and Nd isotopic data
from the Erzgebirge granites and compares these with
compositional fields of coeval granites from NE Bavaria
(Fichtelgebirge and Oberpfalz) and of potential source
rocks from the Bohemian massif (data sources on figure
caption). For the Kirchberg, Bergen and Pobershau–
Satzung granites, the range of Nd values from Table 14
is shown by vertical arrows plotted at the values of initial
87
Sr/86Sr determined by isochron analysis (Seifert, 1994;
Gerstenberger et al., 1995). All isotopic ratios including
the source-rock fields have been calculated for an age of
320 Ma. The important points to note from the figure
are that the biotite and two-mica granites of Kirchberg
and Bergen overlap in terms of both Sr and Nd isotopic
ratios and they plot at the isotopically less-evolved end
of the range defined by the NE Bavarian granites. In
terms of their eNd(t) values (–4·5 to –7·3), the Li-mica
granites have slightly more evolved compositions than
the other Erzgebirge granites. The initial Sr-isotope ratios
of our Pobershau and Satzung samples (Table 14) are
meaningless (<0·7), and this illustrates the problem of
interpreting Sr isotopic data from these evolved granites.
The petrogenetic implications of the granite data compared with potential source fields in Fig. 8 are discussed
in a later section.
DISCUSSION
Element behaviour during magmatic
evolution
Smooth trends in variation diagrams of compatible and
incompatible elements (Fig. 5) suggest that crystal–melt
1626
FÖRSTER et al.
LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
Table 8: Selected electron-microprobe analyses of zircon (in wt %)
Group:
biotite granites
two-mica granites
Sub-unit:
KIB1a
KIB2
KIB3
A-KIB2
BRG1
BRG2
BRG3
BRG3p
EIB0
EIB1
EIB2
EIB3
Sample:
235
785
305
784
823
478
945
521
504
320
814
1197
P2O5
n.d.
n.d.
0·13
1·50
0·11
0·20
Li-mica granites
0·65
2·12
0·17
0·22
1·89
1·94
SiO2
32·6
32·4
28·9
23·9
32·4
31·6
29·6
29·1
32·3
31·9
28·2
22·7
ZrO2
66·0
65·6
59·4
49·3
65·2
63·9
60·4
57·4
65·8
66·0
60·5
49·4
HfO2
1·69
ThO2
0·04
UO2
n.d.
Al2O3
n.d.
Sc2O3
1·75
n.d.
0·16
n.d.
n.d.
La2O3
n.d.
n.d.
0·09
n.d.
Nd2O3
0·07
n.d.
n.d.
Gd2O3
1·75
2·87
3·81
0·27
1·38
0·04
0·05
0·08
0·04
n.d.
0·63
5·41
0·35
1·66
0·79
n.d.
n.d.
0·62
0·58
0·70
0·36
0·55
n.d.
n.d.
0·61
1·46
0·28
0·65
0·40
2·00
0·82
0·51
0·49
0·03
n.d.
0·09
n.d.
Yb2O3
Lu2O3
1·84
0·04
0·22
n.d.
0·14
0·03
2·78
0·09
n.d.
0·23
0·67
n.d.
Ce2O3
Dy2O3
1·85
0·03
Y 2 O3
Sm2O3
1·88
0·02
0·14
0·04
n.d.
0·02
n.d.
n.d.
0·07
0·09
n.d.
n.d.
0·04
0·02
n.d.
0·35
n.d.
0·18
n.d.
0·71
0·03
n.d.
CaO
0·03
0·07
n.d.
0·07
n.d.
PbO
n.d.
n.d.
0·03
1·35
n.d.
0·02
0·05
n.d.
0·02
n.d.
n.d.
n.d.
0·02
0·20
n.d.
n.d.
0·03
0·02
n.d.
n.d.
0·04
n.d.
2·98
n.d.
3·02
0·43
3·17
n.d.
0·04
n.d.
0·06
0·08
0·05
n.d.
0·09
0·06
0·05
0·04
0·04
0·02
0·10
0·11
0·15
0·35
0·24
0·06
0·04
0·10
0·08
0·07
0·04
0·13
0·36
n.d.
0·12
0·28
n.d.
1·68
0·04
1·34
FeO∗
0·31
1·53
n.d.
n.d.
0·20
0·08
F
n.d.
n.d.
0·03
0·62
0·33
n.d.
0·02
0·07
n.d.
0·07
0·76
n.d.
n.d.
1·95
4·17
0·10
0·28
O=F
0·12
Total
100·5
100·2
Zr/Hf
34
33
92·6
91·1
27
23
100·0
31
98·5
97·5
95·9
32
18
13
100·1
100·1
38
34
97·8
91·4
18
14
Blank, not analysed; n.d., analysed but not detected.
∗Total iron as Fe2O3.
fractionation is the dominant process controlling the bulk
composition of the granites. Secondary effects disturb or
overprint these trends and can be readily recognized.
These are discussed in the subsequent section.
Phosphorus
The contrasting behaviour of phosphorus among the
three groups of granites can be explained in terms of the
strong control of melt peraluminosity (A/CNK) on apatite
solubility (Pichavant et al., 1992; Wolf & London, 1994).
Thus, depletion in P during differentiation of the biotite
granites is due to the low apatite solubility in weakly
peraluminous melts. In contrast, apatite solubility is high
in the strongly peraluminous Li-mica granites and differentiation produces an increase in P2O5. The Bergen twomica granites show an intermediate behaviour. The P
contents fall during early differentiation of the granites
(BRG1 to BRG3; see Table 1 for abbreviations), after
which a threshold value of peraluminosity is reached,
apatite solubility changes from low to high (A/CNK
~1·15; e.g. Wolf & London, 1995) and P contents build
up again in the late-stage units (BRG3p and A-BRG).
The formation of F-, P- and Al-bearing complexes in
such late residual melts suppressed the partitioning of Al
and P into feldspars and prevented the early crystallization of topaz, fluorite and aluminium phosphates
(Webster et al., 1997). In the Ehrenfriedersdorf and Podlesı́
Li-mica granites, extreme enrichment of phosphorus is
expressed mineralogically by the presence of late-magmatic, very P-rich K-feldspar (up to 2·5 wt % P2O5) and
by a complex assemblage of Li, Al, Fe and Mn phosphates
including triplite, berlinite, triphylite, childrenite and
zwieselite–eosphorite (Breiter et al., 1997a; Webster et al.,
1997).
1627
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 11
NOVEMBER 1999
Table 9: Selected electron-microprobe analyses of monazite (in wt %)
Group:
biotite granites
Sub-unit:
KIB1m
KIB2
KIB3
A-KIB2
BRG1
BRG2
BRG3
A-BRG
EIB0
EIB1
EIB2
EIB3
Sample:
792
785
1072
784
823
478
521
1069
821
820
812
819
P2O5
29·5
27·6
two-mica granites
25·5
SiO2
0·64
1·96
ThO2
7·69
8·88
UO2
0·05
0·10
0·50
Y 2 O3
1·42
2·14
2·17
3·08
12·4
La2O3
15·4
13·9
10·1
Ce2O3
28·6
29·0
24·9
25·4
3·58
29·2
29·8
0·81
0·70
6·06
8·26
0·45
0·13
0·38
2·46
0·98
2·58
16·7
7·44
21·1
15·9
12·3
30·5
27·4
Li-mica granites
30·3
0·61
30·4
1·29
6·03
1·11
0·77
1·31
2·32
3·12
3·18
3·43
4·04
2·43
6·85
6·97
9·71
7·83
4·78
22·7
22·5
Nd2O3
9·52
Sm2O3
1·33
1·58
2·62
2·93
1·26
1·80
3·68
3·47
Gd2O3
0·93
1·08
1·79
1·80
0·90
1·23
1·94
Tb2O3
0·10
0·10
0·21
0·20
0·08
0·09
0·24
0·25
0·49
0·39
0·59
0·25
0·43
0·57
0·07
0·06
0·09
0·07
0·05
0·06
0·22
0·14
0·17
0·07
0·02
0·06
1·10
0·80
1·58
Dy2O3
Ho2O3
Er2O3
Yb2O3
CaO
PbO
Total
n.d.
0·25
n.d.
1·28
0·09
99·9
10·0
0·14
n.d.
0·35
0·14
100·4
12·5
n.d.
0·14
0·20
99·6
2·91
12·9
0·26
100·2
2·75
10·2
2·77
3·07
10·3
20·7
17·0
9·30
8·31
2·20
2·19
2·50
3·20
2·58
1·95
2·11
2·28
2·15
0·21
0·26
0·19
0·29
0·24
0·83
0·84
0·80
0·85
0·73
0·09
0·12
0·05
0·25
0·28
0·22
0·25
0·29
0·03
0·07
0·04
0·04
0·05
0·08
2·71
2·45
2·00
2·32
2·93
4·82
n.d.
0·35
99·9
23·0
1·88
9·97
11·2
100·0
99·6
2·58
0·43
21·0
2·51
0·10
100·2
24·7
14·2
2·70
12·2
2·91
12·1
0·83
30·1
9·88
10·3
0·66
29·5
5·82
11·3
2·80
3·09
29·8
0·52
Pr2O3
2·84
29·8
0·32
10·1
0·17
100·0
0·22
100·3
2·38
n.d.
0·42
99·8
0·18
0·44
100·1
LaN/CeN
1·39
1·23
1·04
0·91
1·34
1·16
0·78
0·79
1·08
1·08
0·97
0·72
NdN/SmN
2·31
2·06
1·54
1·42
2·62
1·85
1·15
1·04
1·49
1·47
1·20
0·93
Blank, not analysed; n.d., analysed, but not detected.
LILE (Rb, Cs, Sr, Ba)
The mineral–melt partitioning of the large-ion lithophile
elements (LILE) is controlled largely by the compositions
of the major silicates, particularly the micas and feldspars
(Icenhower & London, 1995, 1996). The enrichment of
rubidium and caesium with fractionation indicates that
the bulk distribution coefficient for these elements was
less than unity despite the crystallization of micas. In
contrast, strong partitioning of strontium into potassium
feldspar and albite-rich plagioclase and of barium into
micas and K-rich alkali feldspar (Blundy & Wood, 1991;
Icenhower & London, 1996) accounts for the depletion
of these elements. The enhanced levels of Sr (see Fig. 5j)
and Ba in the most highly fractionated samples of the
two-mica and Li-mica granites cannot be explained by
crystal fractionation and are attributed instead to fluid
interaction.
High field strength elements (Zr, Hf, Th, U) and the REE
Zirconium and hafnium contents decrease steadily during
differentiation, indicating that zircon was present
throughout crystallization. This is in accord with the
expected low solubility of zircon in low-T peraluminous
melt (Watson & Harrison, 1983). The continuous decrease of the Zr/Hf bulk-rock ratio during differentiation
(Kirchberg: from 39 to 12; Bergen: from 38 to 11;
Eibenstock: from 34 to 12) is also reflected in the Zr/Hf
ratios in magmatic zircon (see Table 8). The decrease in
the Zr/Hf ratio in both zircon and residual melt can be
explained in terms of the higher solubility of hafnon
relative to zircon in granitic melts (Linnen, 1998).
Like Zr, thorium also decreases continuously with
evolution of the ASGZ biotite, two-mica and Li-mica
granites as a result of fractionation of monazite, which
is an abundant accessory phase and rich in Th. The
Kirchberg and Niederbobritzsch biotite granites display
only a slight decrease in Th with fractionation because
low-Th allanite takes the place of monazite in the early
stages of crystallization (Table 6). The Th level in these
biotite granite magmas remained high enough for the
crystallization of thorite, which occurs together with a
Th-rich uraninite. Magmatic uraninite is present in the
1628
FÖRSTER et al.
LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
Table 10: Selected electron-microprobe analyses of xenotime (in wt %)
Group:
biotite granites
two-mica granites
Li-mica granites
Sub-unit:
KIB2
KIB3
A-KIB2
BRG1
BRG2
BRG3
BRG3p
EIB0
EIB1
EIB2
EIB3
Sample:
785
305
784
823
478
945
521
821
509
507
1081
P2O5
34·0
33·9
34·4
33·5
33·5
33·6
33·9
34·0
34·1
34·4
33·7
SiO2
0·69
0·72
0·38
0·93
0·74
0·94
0·49
0·92
0·57
0·52
0·44
ThO2
1·26
0·84
0·66
0·34
0·33
0·56
0·23
0·67
0·70
0·62
0·21
UO2
1·70
1·50
0·78
4·81
3·91
3·53
2·11
3·79
4·06
2·70
Y 2 O3
41·7
43·0
39·7
39·0
40·0
40·0
40·8
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
42·7
4·02
39·2
41·6
40·0
n.d.
n.d.
n.d.
La2O3
0·02
Ce2O3
0·09
0·02
0·02
0·04
0·13
0·12
0·17
0·16
0·07
0·11
0·10
Pr2O3
0·04
0·07
0·02
0·06
0·07
0·04
0·04
0·10
0·03
0·06
0·08
Nd2O3
0·38
0·44
0·21
0·46
0·62
0·40
0·41
0·42
0·44
0·43
0·57
Sm2O3
0·32
0·42
0·42
0·43
0·38
0·42
0·53
0·39
0·74
0·47
0·81
Gd2O3
2·85
3·28
4·65
2·97
3·68
3·23
4·09
3·30
4·69
3·96
4·03
Tb2O3
0·53
0·56
0·90
0·61
0·72
0·68
0·76
0·76
1·18
0·89
1·05
Dy2O3
4·14
4·41
5·96
4·55
4·90
4·45
5·40
5·46
6·85
6·16
6·48
Ho2O3
1·21
1·31
1·28
0·97
1·15
1·05
1·05
1·17
1·08
1·08
0·96
Er2O3
4·15
3·69
4·59
4·06
3·68
3·87
3·40
3·28
2·34
2·97
2·69
Tm2O3∗
0·66
0·58
0·67
0·67
0·59
0·64
0·59
0·44
0·32
0·41
0·37
Yb2O3
5·08
3·99
4·31
4·85
4·23
4·70
4·28
2·32
1·87
2·45
2·28
Lu2O3
0·61
0·48
0·58
0·64
0·55
0·73
0·52
0·32
0·28
0·31
0·34
CaO
0·09
0·06
0·13
0·32
0·35
0·55
0·19
0·29
0·69
0·31
0·60
PbO
0·10
0·08
0·22
0·14
0·02
0·19
Total
99·6
99·2
99·8
99·4
99·5
99·7
99·0
Y/Ho
31
30
28
36
31
34
35
100·3
33
0·16
99·1
99·2
99·6
33
35
37
Blank, not analysed; n.d., analysed, but not detected.
∗Interpolated (straight line between nearest adjoining REEs).
biotite and two-mica granites, consistent with experimentally constrained low uraninite solubility (10–30
ppm) in F-poor weakly peraluminous melts (Peiffert et
al., 1996). Few experimental data exist for F-rich, strongly
peraluminous melts like the Li-mica granites, but the fact
that magmatic uraninite is also present in these rocks
suggests that uraninite solubility is similar. Nevertheless,
U remained incompatible during fractionation of all three
groups of Erzgebirge granites, resulting in a systematic
decrease of the Th/U bulk-rock ratio (Kirchberg: from
4·4 to 1·1; Bergen: from 4·7 to 0·1; Pobershau-Satzung:
from 1·0 to 0·1).
The LREE are steadily depleted during differentiation
in all three groups, and this can be attributed to early
fractionation of allanite and then monazite (Montel,
1993; Wolf & London, 1995). In the ASGZ biotite,
two-mica and Li-mica granites, xenotime was present
throughout the entire differentiation history, which accounts for the continuous decrease of Y and the HREE
in these granites. Lower degrees of peraluminosity can
explain the relative paucity of xenotime in the Kirchberg
and Niederbobritzsch biotite granites, which allows the
contents of Y + HREE in these granites to increase
with differentiation.
Ore-forming elements (Sn, Nb, Ta, Mo, W)
Tin concentrations generally increase steadily with
differentiation, reaching values of tens of ppm in aplites
from two-mica and Li-mica granites. However, Sn contents in aplites from the Kirchberg and Niederbobritzsch
biotite granites show that tin is not enriched in the
residual melts. This could be due to higher oxygen
fugacity during crystallization of the biotite granites than
in the other granite groups, which favours its incorporation in biotite and Fe–Ti oxides (Förster & Tischendorf, 1992). Melt inclusions from the Li-mica granite
of Ehrenfriedersdorf have Sn concentrations of 500–1000
1629
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 11
NOVEMBER 1999
Table 11: Representative analyses of biotite granites from the Kirchberg (KIB) and Niederbobritzsch (NBZ)
plutons
Sub-unit:
Sample:
wt %
SiO2
TiO2
Al2O3
Fe2O3a
MnO
MgO
CaO
Na2O
K 2O
P 2 O5
H 2 O+
CO2
F
O=F
Total
ppm
Li
Be
Sc
Co
Ni
Zn
Ga
Rb
Sr
Y
Zr
Nb
Mo
Sn
Sb
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
W
Tl
Pb
Bi
Th
U
A/CNK
LaN/LuN
Eu/Eu∗
KIB1a
587
KIB1m
782
KIB2
785
KIB2
788
69·2
0·52
14·7
2·98
0·054
1·27
1·74
3·45
4·59
0·20
0·90
0·19
0·091
0·038
99·8
70·8
0·48
14·1
2·77
0·075
0·88
1·62
3·31
4·66
0·21
0·76
0·12
0·096
0·040
99·8
73·6
0·31
13·3
1·89
0·064
0·59
1·10
3·53
4·57
0·14
0·53
0·14
0·138
0·058
99·8
75·4
0·20
12·8
1·40
0·036
0·36
0·67
3·38
4·82
0·075
0·54
0·08
0·138
0·058
99·9
93
6·4
6·5
5·0
5·0
65
20
243
225
25·3
272
22
0·60
8·1
0·10
18·1
620
56·5
108
11·9
42·6
6·95
1·27
5·45
0·810
4·23
0·840
2·49
0·360
2·56
0·370
6·93
2·3
5·7
1·2
24·3
0·10
33·5
13·0
1·07
15·6
0·605
135
8·7
6·5
3·8
4·2
92
22
285
186
40·0
250
31
0·17
20
0·14
26·8
481
56·2
116
12·8
56·2
7·37
0·84
6·39
1·100
6·70
1·34
3·91
0·610
3·85
0·560
6·77
3·9
1·2
1·9
33·6
0·10
33·0
7·9
1·05
10·2
0·361
190
10·0
5·8
2·3
14
54
20
354
105
39·2
176
27
1·4
14
0·20
33·1
212
32·3
68·4
8·23
30·3
6·32
0·660
6·17
1·01
6·25
1·19
3·68
0·590
4·13
0·570
5·45
4·0
2·3
2·0
35·0
0·28
32·0
9·6
1·05
5·79
0·317
138
8·5
4·1
2·1
2·1
39
18
346
66
32·0
132
23
0·81
13
0·14
20·4
148
26·5
57·8
7·08
27·2
5·89
0·490
5·34
0·900
5·16
1·02
3·38
0·570
3·85
0·540
4·39
4·0
2·0
2·0
40·0
0·85
36·9
27·4
1·07
5·02
0·260
KIB3
305
KIB3
1072
A-KIB1
439
A-KIB2
784
76·7
0·13
12·6
1·08
0·030
0·29
0·65
3·28
4·80
0·033
0·56
0·02
0·075
0·032
100·2
77·1
0·092
12·8
0·65
0·019
0·15
0·42
3·65
4·60
0·019
0·52
0·05
0·028
0·012
100·0
76·8
0·055
13·0
0·55
0·014
0·040
0·36
3·46
5·31
0·016
0·21
0·12
0·025
0·011
99·9
77·1
0·054
12·9
0·43
0·008
0·043
0·37
3·63
4·85
0·060
0·29
0·07
0·017
0·007
99·8
76
9·5
3·3
1·0
1·0
20
18
371
30
37·2
96
25
47
9·0
3·8
0·9
3·0
65
17
405
16
37·0
97
28
0·20
9·3
0·10
23·4
18
14·9
36·9
4·94
19·3
5·11
0·233
4·82
0·905
5·91
1·22
3·88
0·617
4·54
0·667
4·33
5·5
7·4
2·2
68·3
1·7
36·1
29·7
1·09
2·28
0·140
18
19
5·2
0·2
0·4
19
22
476
3·5
40·8
86
42
0·07
6·2
0·32
49·7
13
16·7
42·5
5·83
20·2
5·45
0·058
5·13
1·03
7·04
1·49
5·00
0·945
7·55
1·22
7·37
16
6·6
2·2
66·8
4·5
15·1
13·5
1·08
1·40
0·033
27
6·0
3·1
2·3
1·3
92
19
447
7·5
33·7
109
36
26
2·7
0·74
16·6
11
7·50
17·2
2·34
8·86
3·48
0·070
4·03
0·870
5·99
1·25
4·25
0·660
4·54
0·680
5·35
6·4
46
2·5
184
3·0
22·5
56·3
1·08
1·13
0·057
6·6
0·13
21·1
52
17·5
40·0
5·34
20·4
4·99
0·360
4·77
0·880
5·53
1·10
3·49
0·570
4·12
0·620
3·95
4·5
1·8
45·9
34·7
15·0
1·07
2·89
0·221
a
Total iron as Fe2O3.
A/CNK is molar Al2O3/(CaO + Na2O + K2O).
1630
NBZ1
334
NBZ2
335
66·8
0·59
15·6
3·54
0·059
1·40
2·46
3·80
4·39
0·23
0·93
0·09
0·079
0·033
99·9
71·2
0·34
14·2
2·33
0·045
0·67
1·29
3·57
5·01
0·18
0·79
0·08
0·079
0·033
99·8
58
2·4
6·7
6·4
57
3·9
4·6
2·2
69
21
167
390
16·2
203
13
0·29
2·9
0·08
6·26
878
46·9
89·1
10·5
35·0
5·83
1·36
4·48
0·607
3·23
0·595
1·61
0·239
1·47
0·241
6·14
1·0
0·33
0·95
21·3
0·03
14·8
3·4
1·01
19·9
0·778
43
21
268
171
19·8
162
13
1·2
13
0·04
15·9
633
37·3
73·6
8·82
29·5
5·60
0·807
4·71
0·719
3·90
0·719
1·98
0·304
1·97
0·306
5·28
2·1
7·1
2·0
39·4
1·2
20·7
14·8
1·04
12·5
0·465
NBZ3
336
A-NBZ
343
74·2
0·25
13·1
1·64
0·026
0·45
0·65
3·22
5·07
0·11
0·99
0·13
0·043
0·018
99·8
76·2
0·12
12·7
0·61
0·011
0·15
0·51
3·28
5·17
0·030
0·93
0·13
0·025
0·011
99·9
60
4·1
3·7
1·4
5·6
38
18
257
87
19·5
115
13
0·48
11
0·16
13·5
455
26·4
53·4
6·46
22·2
4·53
0·566
3·93
0·631
3·56
0·699
2·02
0·329
2·12
0·325
4·21
2·6
3·8
1·6
37·6
1·2
25·2
16·1
1·10
8·31
0·398
18
5·5
2·4
0·4
2·3
13
15
258
45
20·0
92
16
0·27
2·2
0·08
13·4
152
15·8
33·7
4·41
15·9
3·69
0·258
3·40
0·578
3·43
0·663
2·08
0·350
2·39
0·375
3·94
3·1
7·0
1·4
41·0
1·2
31·0
12·3
1·07
4·31
0·217
FÖRSTER et al.
LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
Table 12: Representative analyses of two-mica granites from the Bergen pluton
Sub-unit:
Sample:
wt %
SiO2
TiO2
Al2O3
Fe2O3a
MnO
MgO
CaO
Na2O
K 2O
P 2 O5
H 2 O+
CO2
F
O=F
Total
ppm
Li
Be
Sc
Co
Ni
Zn
Ga
Rb
Sr
Y
Zr
Nb
Mo
Sn
Sb
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
W
Tl
Pb
Bi
Th
U
A/CNK
LaN/LuN
Eu/Eu∗
BRG1
524
BRG1
824
70·6
0·37
14·6
2·25
0·046
0·82
1·11
3·58
4·93
0·25
1·15
0·21
0·092
0·039
100·0
71·5
0·33
14·5
2·04
0·045
0·68
1·10
3·01
5·47
0·24
0·91
0·32
0·085
0·036
100·2
111
11
5·2
2·8
2·2
53
21
276
159
22·3
169
16
0·13
18
0·10
19·5
462
42·4
80·9
9·01
32·1
5·99
0·770
4·49
0·690
3·93
0·640
1·88
0·280
1·93
0·290
4·50
2·2
1·4
1·5
39·5
0·13
23·9
5·1
1·11
15·0
0·433
94
6·2
4·1
2·9
53
21
283
137
17·4
163
16
0·16
17
0·27
25·4
498
36·9
72·7
8·27
28·8
5·47
0·728
4·32
0·627
3·40
0·601
1·65
0·231
1·55
0·240
4·64
1·9
2·1
1·8
32·9
0·18
20·7
6·3
1·13
15·7
0·439
BRG2
780
74·4
0·18
14·0
1·29
0·058
0·34
0·57
3·50
4·39
0·16
0·82
0·16
0·089
0·037
99·9
204
16
3·2
1·1
32
18
344
49
14·9
89
20
0·31
14
0·67
36·7
156
17·8
36·1
4·30
14·2
2·74
0·310
2·39
0·415
2·45
0·506
1·52
0·265
1·83
0·280
2·85
4·2
5·0
2·1
27·3
0·88
10·2
18·2
1·21
6·50
0·360
BRG2
1085
74·8
0·15
14·0
0·93
0·030
0·28
0·49
3·91
4·49
0·18
0·69
0·07
0·078
0·033
100·1
86
18
3·8
0·4
1·8
22
17
343
43
13·6
72
22
0·17
8·1
0·18
18·9
117
13·2
26·7
3·03
10·6
2·16
0·234
1·88
0·320
2·03
0·399
1·23
0·205
1·60
0·238
2·38
6·4
9·6
1·8
28·5
2·4
9·25
22·1
1·15
5·67
0·345
BRG3
523
75·8
0·089
13·5
0·80
0·057
0·20
0·37
3·41
4·44
0·15
0·84
0·11
0·12
0·051
99·8
160
0·4
0·7
24
23
503
18
10·9
42
20
0·26
13
0·28
63·0
57
7·12
12·6
1·51
5·54
1·32
0·110
1·14
0·270
1·73
0·340
1·17
0·210
1·76
0·250
1·83
6·3
8·3
2·9
15·3
0·89
5·70
11·5
1·22
2·91
0·266
a
Total iron as Fe2O3.
A/CNK is molar Al2O3/(CaO + Na2O + K2O).
1631
BRG3
1089
75·3
0·084
13·7
0·86
0·058
0·15
0·41
3·88
4·33
0·16
0·76
0·05
0·132
0·056
99·9
184
19
2·5
0·6
1·1
31
18
461
15
10·1
37
23
2·4
19
0·13
28·3
38
6·26
13·1
1·56
5·53
1·32
0·093
1·21
0·250
1·63
0·320
1·05
0·187
1·51
0·218
1·67
5·7
10
2·4
25·1
0·25
4·95
33·5
1·16
2·94
0·220
BRG3p
776
75·5
0·065
14·0
0·71
0·059
0·13
0·37
3·79
4·11
0·21
0·82
0·17
0·172
0·072
100·0
384
13
2·9
0·3
27
20
554
7·1
5·9
27
27
0·26
19
0·88
97·8
17
2·63
5·68
0·746
2·55
0·740
0·038
0·729
0·165
1·00
0·179
0·569
0·111
0·888
0·134
1·61
6·3
12
3·2
13·8
5·0
2·32
3·2
1·23
2·01
0·155
BRG3p
521
A-BRG1
779
A-BRG1
1069
75·3
0·058
14·2
0·70
0·054
0·13
0·34
3·65
4·22
0·20
0·81
0·19
0·162
0·068
99·9
74·9
0·045
14·6
0·91
0·032
0·025
0·39
3·97
3·86
0·33
0·76
0·14
0·124
0·052
100·0
74·3
0·035
14·6
0·73
0·047
0·045
0·50
4·25
3·80
0·53
0·73
0·15
0·15
0·063
99·8
294
14
2·0
0·5
0·9
24
25
595
6·3
6·4
27
25
0·13
21
0·90
90·5
27
2·96
6·56
0·860
3·17
0·900
0·040
0·950
0·180
1·16
0·180
0·590
0·110
0·920
0·140
1·60
7·6
13
3·3
12·2
3·65
2·62
12·1
1·27
2·16
0·130
143
<3
<2
0·3
0·6
32
28
654
22
2·1
25
43
0·12
25
0·54
30·8
6·2
1·02
1·94
0·290
1·09
0·360
0·021
0·310
0·070
0·380
0·060
0·200
0·040
0·320
0·057
1·83
16
22
2·9
7·2
17
0·90
4·5
1·28
1·83
0·186
138
4·9
2·1
0·3
2
49
27
800
28
2·9
21
54
0·10
46
0·60
144
15
1·52
2·85
0·400
1·54
0·602
0·060
0·565
0·119
0·659
0·096
0·263
0·047
0·407
0·055
1·83
20
18
3·7
6·0
12
0·85
3·8
1·21
2·83
0·308
1632
0·142
2·4
Ni
Sn
Mo
38
0·35
19
Y
Nb
26·0
Sr
149
38
Rb
Zr
27
704
Ga
77
1·5
Co
26
0·60
18
113
22·7
28
504
25
48
2·1
1·0
3·0
11
163
35
0·35
17
85
16·0
22
683
28
46
0·8
0·7
2·8
11
326
99·9
0·148
0·352
0·03
0·56
0·23
4·99
34
0·40
20
75
14·4
13
814
25
50
1·2
0·7
300
99·7
0·198
0·471
0·09
0·73
0·27
4·91
3·13
0·44
0·16
0·028
1·45
13·6
0·11
74·6
509
EIB1
50
0·30
26
53
10·7
14
1082
33
41
0·8
0·4
2·5
17
578
99·8
0·288
0·685
0·09
0·50
0·40
4·47
3·54
0·38
0·09
0·016
1·23
14·0
0·072
74·6
814
EIB2
52
0·18
25
31
7·1
7·2
1028
31
33
0·3
1·9
22
1001
99·7
0·349
0·829
0·11
0·28
0·39
4·30
3·76
0·37
0·08
0·033
1·00
14·8
0·057
74·0
811
EIB3
29
0·06
19
23
6·5
4·9
795
28
36
1·2
0·2
2·1
11
329
99·7
0·289
0·687
0·05
0·49
0·35
4·36
3·49
0·35
0·041
0·022
1·06
14·2
0·053
74·8
1081
EIB3
97
0·18
38
34
6·6
15
1603
37
48
0·8
0·1
4·7
5·7
1226
99·5
0·496
1·18
0·03
0·72
0·51
4·34
3·87
0·43
0·06
0·028
0·95
15·1
0·047
72·8
1197
EIB3
28
0·16
33
31
5·5
14
1498
36
34
0·8
0·1
2·0
950
99·5
0·561
1·33
0·10
0·40
0·50
4·00
4·15
0·35
0·020
0·022
0·95
15·2
0·035
73·0
510
A-EIB1
73
0·30
41
27
3·0
45
1325
44
68
0·7
0·5
<2
4·0
800
99·7
0·516
1·23
0·18
0·75
0·40
3·77
3·69
0·67
0·031
0·024
1·21
15·4
0·032
72·9
800
A-EIB1
18
1·2
15
86
16·0
21
522
25
52
0·7
0·5
211
99·8
0·105
0·249
0·11
0·67
0·25
4·81
3·13
0·58
0·14
0·027
1·52
13·5
0·12
74·8
896
POB0
22
1·0
16
75
17·6
13
533
27
58
0·5
0·4
2·0
8·2
212
99·9
0·121
0·288
0·08
0·58
0·23
4·81
3·12
0·47
0·11
0·025
1·70
13·3
0·11
75·1
899
POB1
20
1·5
18
78
18·8
11
617
28
48
0·6
2·5
7·0
287
99·8
0·138
0·328
0·11
0·71
0·22
4·46
3·04
0·48
0·09
0·025
1·62
13·0
0·092
75·8
915
POB2
25
0·80
25
51
10·7
7·8
935
31
48
0·2
<0·1
<2
8·9
465
99·7
0·235
0·559
0·07
0·46
0·37
4·46
3·57
0·40
0·035
0·031
1·34
14·2
0·063
74·4
908
POB2
29
0·30
22
27
7·8
7·0
915
30
38
0·4
<0·1
1·8
9·3
392
99·8
0·280
0·665
0·07
0·58
0·39
4·26
3·55
0·42
0·039
0·022
1·19
14·4
0·042
74·4
911
POB3
21
0·70
15
137
19·9
55
457
23
54
1·8
1·5
164
99·7
0·107
0·254
0·11
0·76
0·27
4·85
3·03
0·83
0·33
0·029
1·72
13·7
0·22
73·7
926
SZU0
27
1·7
18
71
13·3
16
666
25
51
1·0
0·6
2·4
256
99·8
0·124
0·294
0·11
0·70
0·27
4·43
3·21
0·55
0·14
0·034
1·52
13·3
0·11
75·3
934
SZU2
26
0·40
19
61
13·5
10
693
26
43
1·0
0·2
2·0
14
296
99·9
0·163
0·388
0·06
0·68
0·28
4·39
3·21
0·47
0·08
0·029
1·43
13·3
0·073
75·7
935
SZU2
60
0·90
32
46
11·8
20
1232
34
54
0·6
0·1
2·7
4·3
626
99·6
0·442
1·05
0·08
0·54
0·56
4·53
3·26
0·66
0·09
0·036
0·80
15·2
0·051
73·2
923
SZU3
NUMBER 11
Zn
4·2
405
Sc
Be
Li
99·7
0·088
0·210
0·22
1·01
0·25
3·27
0·48
0·17
0·028
1·50
13·8
0·13
74·6
820
EIB1
VOLUME 40
ppm
99·9
O=F
Total
0·338
1·02
H2O+
F
0·25
0·14
4·78
4·84
K2 O
P2 O 5
CO2
3·06
3·32
Na2O
0·44
0·24
0·28
0·69
CaO
0·024
1·76
13·3
0·17
74·3
821
EIB0
MgO
2·46
0·044
MnO
14·0
0·24
Fe2O3a
Al2O3
TiO2
SiO2
72·4
504
Sample:
wt %
EIB0
Sub-unit:
Table 13: Representative analyses of Li-mica granites from the Eibenstock (EIB), Pobershau (POB), and Satzung (SZU) plutons
JOURNAL OF PETROLOGY
NOVEMBER 1999
1633
5·04
0·930
5·32
0·940
2·60
0·35
2·20
0·350
4·35
2·7
7·3
3·9
19·0
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
W
Tl
Pb
7·7
1·17
7·60
0·253
U
A/CNK
LaN/LuN
Eu/Eu∗
0·30
0·198
10·0
1·21
13·2
17·0
1·3
18·7
2·5
13
3·8
3·40
0·180
1·28
0·23
1·71
0·750
4·71
0·860
4·74
0·300
4·40
18·1
4·94
39·2
17·6
140
38·7
821
EIB0
0·29
0·231
10·1
1·19
23·3
12·6
0·65
16·4
3·7
12
4·1
2·55
0·130
1·04
0·15
1·33
0·510
3·28
0·620
2·98
0·230
2·99
12·3
3·54
29·7
12·9
113
84·6
820
EIB1
0·50
0·164
8·65
1·21
22·9
11·6
0·57
12·0
4·4
12
4·6
2·74
0·110
0·920
0·14
1·12
0·450
2·80
0·520
2·48
0·140
2·62
9·74
2·62
20·9
9·30
52
93·9
509
EIB1
1·7
0·098
4·74
1·24
8·5
7·4
1·3
6·9
5·3
11
7·7
2·39
0·094
0·660
0·11
0·80
0·320
2·04
0·340
1·51
0·048
1·45
4·49
1·26
10·4
4·36
20
156
814
EIB2
a
Total iron as Fe2O3.
A/CNK is molar Al2O3/(CaO + Na2O + K2O).
20·3
Th
0·16
0·460
Eu
Bi
5·79
25·9
Sm
Nd
6·85
56·3
Ce
Pr
26·0
243
Ba
La
107
Cs
Sb
0·40
504
Sample:
ppm
EIB0
Sub-unit:
Table 13: continued
2·2
0·081
3·22
1·29
6·2
4·8
4·6
7·2
6·2
28
9·4
1·57
0·068
0·535
0·08
0·521
0·208
1·33
0·218
0·892
0·023
0·822
2·33
0·663
5·22
2·14
12
213
811
EIB3
0·062
3·26
1·28
4·4
3·5
5·1
9·5
4·3
7·0
5·0
1·08
0·060
0·477
0·066
0·463
0·175
1·08
0·175
0·850
0·014
0·550
1·91
0·540
4·21
1·91
6·2
86·3
0·42
1081
EIB3
0·008
1·74
1·28
8·7
5·9
6·2
3·4
7·9
24
13
2·29
0·060
0·489
0·072
0·437
0·176
1·15
0·184
0·762
0·002
0·685
1·39
0·403
3·06
1·02
5·9
174
1·3
1197
EIB3
1·5
0·052
2·42
1·29
7·3
2·8
10
4·9
6·2
35
11
1·98
0·070
0·450
0·060
0·370
0·160
1·11
0·220
0·830
0·014
0·780
1·93
0·500
3·69
1·66
5·0
118
510
A-EIB1
0·47
0·059
2·02
1·36
6·7
2·8
7·9
5·6
5·9
34
19·3
1·48
0·035
0·250
0·040
0·200
0·080
0·590
0·120
0·410
0·009
0·490
0·910
0·240
1·75
0·69
16
66·2
800
A-EIB1
0·130
10·1
1·18
25·3
14·0
<0·1
18·4
3·0
11·5
3·9
2·69
0·120
0·930
0·160
1·27
0·550
3·52
0·620
3·22
0·140
3·27
12·3
3·39
27·0
11·8
72
33·9
<0·1
896
POB0
0·10
0·089
9·53
1·19
33·6
13·7
1·5
18·5
3·1
8·9
2·8
2·80
0·099
0·805
0·155
1·18
0·551
3·74
0·671
3·22
0·090
2·93
10·1
2·90
22·2
9·23
39
36·0
899
POB1
0·12
0·079
8·21
1·21
31·5
13·6
0·38
13·6
3·8
7·3
2·6
3·33
0·117
0·944
0·163
1·36
0·600
4·10
0·718
3·32
0·086
3·26
10·5
2·89
22·9
9·39
36
38·2
915
POB2
0·046
5·25
1·24
34·0
8·4
1·7
11·0
5·1
23
6·7
2·30
0·066
0·528
0·094
0·71
0·315
2·19
0·386
1·61
0·023
1·41
3·96
1·13
8·52
3·39
23
95·0
<0·1
908
POB2
0·20
0·026
4·63
1·29
30·0
5·5
6·5
7·1
4·7
30
7·0
1·47
0·042
0·364
0·060
0·469
0·209
1·66
0·280
1·11
0·009
0·983
2·17
0·626
4·91
1·90
30
79·4
911
POB3
0·232
11·5
1·17
18·7
18·5
0·20
21·4
2·7
13
4·3
4·18
0·200
1·42
0·220
1·70
0·660
3·90
0·660
4·04
0·340
4·66
20·8
5·78
47·2
22·4
204
36·5
<0·1
926
SZU0
0·52
0·126
8·32
1·21
30·8
12·6
0·76
13
3·8
17
4·7
2·51
0·110
0·87
0·140
1·14
0·450
2·82
0·480
2·41
0·100
2·37
8·84
2·44
20·0
8·95
54
62·1
934
SZU2
0·10
0·087
5·37
1·22
38·6
9·4
5·3
13·6
4·2
20
4·4
2·31
0·100
0·783
0·129
0·97
0·411
2·58
0·435
1·91
0·053
1·77
5·58
1·62
12·6
5·25
26
66
935
SZU2
1·6
0·053
2·68
1·33
33·2
4·9
39
7·2
6·7
52
12
2·45
0·106
0·875
0·124
0·833
0·320
2·17
0·353
1·37
0·023
1·26
3·17
0·887
6·92
2·78
34
140
923
SZU3
FÖRSTER et al.
LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 11
NOVEMBER 1999
Fig. 5. Element variation diagrams using 1/TiO2 as a differentiation index, for selected major and trace element components of the various
groups of Erzgebirge granites. (See text for discussion.)
1634
FÖRSTER et al.
LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
Fig. 6. Chondrite-normalized whole-rock REE patterns for biotite granites (a–c), two-mica granites (d) and Li-mica granites (e, f ) from the
German Erzgebirge. Arrows indicate the evolution of REE concentrations during progressive magma differentiation. Chondrite abundance data
are from Anders & Grevesse (1989).
ppm (Webster et al., 1997). These magmatic values are
in agreement with experimentally determined Sn solubilities in granitic liquids by Linnen et al. (1997).
Both tantalum and niobium show a systematic increase
during melt evolution (Fig. 7a), and the Nb/Ta ratios
decrease with differentiation from the biotite granites
(from 13 to 2) to the two-mica granites (from 7 to 3) and
to the Li-mica granites (from 7 to <1). The shift in melt
composition towards lower Nb/Ta ratios reflects the
higher solubility of manganotantalite over manganocolumbite in granitic melts (Keppler, 1993; Linnen &
Keppler, 1997; Linnen, 1998) and the preferential substitution of Nb over Ta in rutile and ilmenite (Wolf et
al., 1994).
The constant low molybdenum contents in all Erzgebirge granites, even in the stage of extreme differentiation represented by melt inclusions in the Li-mica
granites (Webster et al., 1997), are consistent with the
observation that Al-rich magmas typically contain less
Mo than peralkaline magmas (e.g. Lowenstern et al.,
1993). In contrast to Mo, the tungsten contents systematically increase with differentiation in all granite
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limit of wolframite in such melts. The different behaviour
of W and Mo can be explained by f O2-dependent
differences in partitioning of Mo and W between melt
and ferromagnesian minerals (e.g. Candela & Bouton,
1990) and argues for low oxygen fugacities [ΖQFM
(quartz–fayalite–magnetite)] during differentiation, particularly for the Li-mica and two-mica granite groups.
Secondary element remobilization
Fig. 7. Variation of Nb vs Ta (a), Rb vs F (b) and Sn vs F (c) in wholerock samples and melt inclusions from the Li-mica granite group.
Arrows indicate trends of differentiation by fractional crystallization
(qualitative). Data sources: Podlesı́, Breiter et al. (1997a, 1997b); melt
inclusions, Webster et al. (1997).
groups (W/Mo in Kirchberg: from 10 to 95; Bergen:
from 10 to 180; Eibenstock: from 20 to 225). The highest
W values, 40–60 ppm, are found in differentiated Limica granites that contain accessory wolframite. The
same range of W values was established in residual melt
inclusions from Ehrenfriedersdorf (Webster et al., 1997)
and in the wolframite-bearing Podlesı́ pegmatite (Breiter
et al., 1997a), suggesting that 60 ppm is near the solubility
The complexity of secondary phenomena, particularly
in the Li-mica granites, was discussed above. In this
section we focus on the geochemical effects that are
probably related to high-T late- to post-magmatic alteration. Bau (1996) discussed geochemical indicators for
the action of such processes. One is deviation of Y/Ho
ratios in bulk rock from the common magmatic values
of 24–34 and another is the characteristic curvature of
REE distribution patterns (lanthanide tetrad effect).
Fluid interaction in the biotite granites is shown by
chloritization and sericitization, and the occurrence of
hydrated, metamict allanite, thorite and zircon. However,
the uniform element differentiation trends (Fig. 5), normal
REE distribution patterns (Fig. 6), and bulk-rock Y/Ho
ratios from 27 to 33 suggest that remobilization of elements by high-T fluid–magma interaction was minimal
and on a local scale only.
The Bergen two-mica granites show abundant petrographic evidence of secondary alteration (see above), and
several samples display erratic distributions of the more
mobile elements (e.g. Ca, Na, Li, Rb, Sr, Sn, Cs, Ba, W,
Pb, U) when plotted against the fractionation index 1/
TiO2 (Fig. 5, Table 12). Furthermore, evolved intrusions
are distinguished by high Y/Ho ratios in bulk rock (up
to 36) and xenotime (see Table 10), and display the
curved, chondrite-normalized REE patterns in bulk rock,
monazite and xenotime typical for the lanthanide tetrad
effect. As samples richest in post-magmatic white mica
have low concentrations of the mobile elements such as
F, Li, Rb, Cs, Pb and U, it appears reasonable to conclude
that the magmatic composition of this group of granites
is best represented by the highest measured concentrations of these elements.
The role of secondary effects is most prominent, and
the extent of their influence is best quantified, in the case
of the Li-mica granites. Petrographic features (discussed
above), occurrence of F-rich, hydrated zircon, the composition of whole rocks and the data from melt inclusion
studies (e.g. Webster et al., 1997) suggest that the evolved
Li-mica granites formed from water-rich melts containing
elevated concentrations of the fluxing and ligand-forming
components F, P, B and Li. The fractionated granites
show high Y/Ho ratios (up to 42) and prominent lanthanide tetrad effect in bulk-rock REE patterns (controlled by monazite and xenotime REE distribution;
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LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
Table 14: Sr and Nd isotopic data from late-collisional granites of the Erzgebirge
143
eNd (t)
Rb
0·512280
0·512003
−4·3
364
51·2
0·79828
0·70460
0·512339
0·512076
−2·9
365
108
9·751
0·75052
0·70611
0·09820
0·512282
0·512076
−2·9
243
242
2·801
0·71985
0·70710
0·10741
0·512200
0·511975
−4·9
309
193
4·635
0·72752
0·70641
0·11640
0·512260
0·512016
−4·1
323
130
7·201
0·73884
0·70605
0·13291
0·512350
0·512072
−3·0
411
26·8
0·90291
0·70052
0·11015
0·512274
0·512043
−3·6
337
99·6
9·782
0·75039
0·70584
478
380
54·6
20·160
0·79561
0·70379
481
445
45·5
28·320
0·83035
0·70137
Sm
Nd
147
036
5·20
23·7
0·13207
785
6·32
30·3
0·12555
587
6·95
42·6
782
7·37
41·3
789
6·42
33·2
Sample
Sm/144Nd
143
Nd/144Nd
Nd/144Nd(t)
Sr
87
Rb/86Sr
87
Sr/86Sr
87
Sr/86Sr(t)
Kirchberg
20·57
Bergen
479a
1·91
781
3·55
8·65
19·4
44·44
Pobershau
891
1·64
896
3·27
902
2·98
910
1·47
913
1·34
0·20826
0·512284
0·511848
−7·4
714
7·2
0·16002
0·512257
0·511922
−5·9
524
16·6
9·62
0·18646
0·512331
0·511940
−5·6
512
14·4
4·27
0·20723
0·512416
0·511982
−4·8
819
4·4
3·68
0·21918
0·512378
0·511919
−6·0
838
7·6
4·74
12·3
320·07
1·88900
0·43129
1·03066
0·60080
107·74
1·19190
0·70120
696·51
3·70618
0·53404
361·17
2·05840
0·41350
94·385
Satzung
926
4·66
0·13485
0·512280
0·511997
−4·5
453
53·5
0·81942
0·70663
929
1·23
3·67
0·20174
0·512363
0·511940
−5·6
699
13·3
161·71
1·35624
0·61977
934
2·37
8·84
0·16138
0·512296
0·511958
−5·2
649
12·1
162·75
1·20619
0·46497
20·8
24·766
Sr and Rb concentrations determined by isotope dilution; Nd and Sm concentrations by ICP-MS. Initial ratios were calculated
for t = 320 Ma with the following values for CHUR: 147Sm/144Nd = 0·1967; 143Nd/144Nd = 0·512638.
Förster 1998a, 1998b). The magnitude of these effects
can be seen by comparing the chemical composition of
Li-mica granites from unweathered surface outcrops and
drill core samples. Figure 9 shows the comparison of
surface samples of the Eibenstock massif and the few
available drill cores from the same intrusion (locality
Tellerhäuser). The figure also includes data from drill
core of the same textural facies from the subsurface
Pobershau–Satzung pluton. The drill core samples from
both granites show an identical covariance of Rb and F
with fractionation index 1/TiO2, whereas most of the
Eibenstock surface samples deviate from the magmatic
trend toward higher Rb and F contents. The same is
true for Cs and Li, which, in extreme cases, reach
concentrations in metasomatized samples that are twice
as high as the magmatic values. Secondary addition of
F, Li, Rb and Cs is manifested in the rocks by the
formation of autometasomatic zinnwaldite (Fig. 4b), after
crystallization
of late-magmatic
protolithionite–
zinnwaldite.
Element redistribution within the Li-mica granites is
probably related to multiple events of metasomatic re-
working. A number of elements that are enriched in the
altered granites also occur in elevated concentrations in
melt inclusions and pegmatites (Thomas & Klemm, 1997;
Webster et al., 1997), and are thus attributed to interaction
with more-evolved, volatile-rich late-stage melts or aqueous fluids exsolved from them. This holds for Al, P, F,
Li, B, Be, Rb, Cs and Sn. Elevated contents of Ca, Sr
and Ba, however, require another explanation because
they are depleted in late-stage melts. We suggest that these
elements originate from the breakdown of plagioclase (e.g.
Schwartz, 1992). The Ca, Sr and Ba released by this
process can be transported by high-temperature aqueous
fluids from less evolved parts of the plutons to the site of
reprecipitation where the elements are fixed in newly
formed minerals such as fluorite, albite and apatite (e.g.
Van Gaans et al., 1995).
The most prominent evidence for low-T element remobilization is the U depletion in surface samples of the
Eibenstock massif relative to the drill core samples. Because
U and Sn–W cannot be transported together in the same
fluid because of their opposite solubility dependence
on f O2 (e.g. Dubessy et al., 1987), granite-derived
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Fig. 8. Sr and Nd isotopic compositions of Erzgebirge granites compared with coeval granites from NE Bavaria and with possible source
lithologies in the Variscan basement of the Bohemian massif. All isotope ratios are recalculated to a common age of 320 Ma. The Erzgebirge
data are shown as a range of measured eNd values (vertical arrows) plotted at the value of 87Sr/86Sri derived from isochron analysis of the
respective plutons. Data sources: Rb–Sr isochrons for Bergen and Kirchberg granites from Gerstenberger et al. (1995); Rb–Sr isochron for
Pobershau granite from Seifert (1994); NE Bavarian granites from Siebel et al. (1997); mafic crust and Erbendorf–Vohenstrauss Zone (ZEV)
from a compilation by Gerdes (1997); orthogneisses from Kröner et al. (1995) and Siebel et al. (1997); Erzgebirge schists from Mingram (1996);
Fichtelgebirge enclaves from Schödlbauer et al. (1997).
hydrothermal fluids that produced orthomagmatic Sn–W
mineralization cannot also have leached U. Given its high
mobility as U6+, uranium is suggested to have been remobilized from the crystallized granites by oxidizing aqueous solutions (note the reddish colour of the granites) during
low-T hydrothermal alteration.
Constraints on the granite sources
The Erzgebirge late-collisional granites share many of
the mineralogical and compositional features of granites
from elsewhere in the German and European Variscides
but there is one important difference to which the rich
Erzgebirge mining history owes its origin; namely, highly
evolved Li-mica and topaz-bearing granites are comparatively abundant in the Erzgebirge. An important
question is, therefore, what causes the compositional
uniqueness of the granites in this particular part of the
Variscides? Some workers (e.g. Štemprok, 1993) have
suggested that the biotite, two-mica and Li-mica granites
are related to a common magma type via continuous
fractional crystallization. However, the bulk of the geochemical and mineralogical data from the Erzgebirge
granites presented here are incompatible with a common
origin for all three granite groups, and such an origin is
clearly contradicted by the Sr and Nd isotopic data
(Fig. 8). An explanation for the compositional diversity
of the granites in the Erzgebirge must lie, at least in part,
in specific characteristics of the source rocks.
Several aspects of the Erzgebirge granites make the
identification of their source materials very difficult. First,
many of the granites, and especially those of the Li-mica
group, are highly evolved geochemically and the magmas
have clearly undergone extensive differentation since
their formation. Related to this is the fact that latemagmatic, volatile-rich residual melts and/or exsolved
fluids have caused locally extensive element remobilization and, again, this is particularly true for the
Li-mica group. Second, the late-collisional granites in
the Erzgebirge are allochthonous and were emplaced at
very shallow levels (<5 km). The root zones of the granites
are not exposed and the granites are exceedingly poor
in enclaves or restite minerals that might shed light on
the source lithologies. In view of these difficulties, no
attempt is made here to propose a detailed petrogenetic
model for the different granite groups. Our aim in this
discussion is to present broad constraints on magma
sources, which can be inferred from the compositional
data presented in this paper and from a comparison with
coeval granites and potential source lithologies in the
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FÖRSTER et al.
LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
Fig. 9. Whole-rock variation diagrams of 1/TiO2 vs Rb (a) and F (b)
demonstrating the potential effects of autometasomatic processes on
samples of the Li-mica granite group. Arrows mark qualitative trends of
magmatic differentiation by fractional crystallization and metasomatic
addition.
Variscan basement in the Erzgebirge and neighbouring
regions of the Bohemian massif. Emphasis is placed on
radiogenic isotope ratios because these have the best
potential to reveal magma source characteristics despite
the extensive differentiation history of the granites.
Both the Kirchberg biotite granite and the Bergen
two-mica granite from the Erzgebirge overlap with the
‘older granites’ of NE Bavaria in Fig. 8. These ‘older
granites’ [340–325 Ma, phase Ib of Siebel et al. (1997)] are
transitional I–S type granitoids with relatively primitive
isotopic compositions (eNd(320 Ma) > –4 and 87Sr/86Sr(320 Ma)
< 0·708). Siebel et al. were unable to distinguish between a
purely crustal origin for the older granites (i.e. unevolved,
more mafic lower crust) or a mixed origin involving
mantle magmas and more mature crustal sources (paragneisses). Tischendorf et al. (1992) argued from geochemical modelling that the Kirchberg granite could be
derived from partial melting of Cadomian orthogneisses
in the basement but the new Sr–Nd isotopic data contradict this view (Fig. 8). However, as in the NE Bavarian
examples, the isotopic data do not allow a distinction
between an origin for these granites from non-evolved
crust or from a mixed crust–mantle source. The latter
possibility is attractive for NE Bavaria because quartz
diorites (redwitzites) and minor gabbros with mantle
isotopic compositions occur with the older granites (Siebel
et al., 1997). Equivalent rocks are lacking in the Erzgebirge
and the only evidence for coeval mantle magmas are
small-volume lamprophyre dykes. The isotopic compositions of the Bergen and Kirchberg granites are indistinguishable, which suggests a common source for
them. However, only two samples from Bergen have
been analysed for both Sr and Nd isotopes (Table 14),
so this conclusion may be preliminary. It should be noted
that the chemically similar two-mica granites from NE
Bavaria (younger series) have very different isotopic compositions from the Bergen granites.
The Li-mica granites from the Erzgebirge (Pobershau
and Satzung drill core samples) have well-constrained
Nd isotopic compositions but their Sr isotopic values
cannot be meaningfully interpreted because of the extremely high Rb/Sr ratios of the samples and the prevalence of secondary remobilization of Sr. The eNd(320 Ma)
values of the Li-mica granites (–4·5 to –7·4), correspond
to those of the younger group of two-mica granites from
NE Bavaria [315–295 Ma, phases II and III of Siebel et
al. (1997)] and the metamorphic enclaves in the S-type
Fichtelgebirge Kösseine granite (Fig. 8). Therefore, the
isotopic data show no apparent difference between the
source of the Li-mica granites in the Erzgebirge and the
typical two-mica S-type granites of neighbouring regions
of the Bohemian massif. In terms of their Nd isotope
composition, the orthogneisses could be a potential source
for the Li-mica granites but the metasediments (schists)
exposed in the Erzgebirge have such low eNd(320 Ma) values
(–9·5 to –11·5) that they can be ruled out except as a
minor component in the source.
From a geochemical point of view, metapelites, metagreywackes and felsic orthogneisses all constitute possible sources for production of peraluminous granite
melts. Comparison of the Erzgebirge granite bulk composition with experimental melting experiments of a wide
range of crustal rocks reviewed by Montel & Vielzeuf
(1997) shows that the experimental melts have lower
Fe, Mg and Ca contents than the least differentiated
Erzgebirge biotite granites. Consequently, these granites
must involve other components, either cumulate mineral
concentrations, mafic restite material or admixed mafic
melts. This conclusion agrees with their relatively primitive isotopic composition and with the broad compositional spectrum of mafic magmatic enclaves observed
in the Niederbobritzsch biotite granite.
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Pelitic sediments are rich in the suite of lithophile trace
elements (e.g. P, Li, Rb, Cs, Be, B) that are anomalously
concentrated in the Erzgebirge Li-mica granites, and
mica-rich, quartz-poor and feldspar-poor schists with
high concentrations of these elements as well as Sn (12–44
ppm) and W (5–19 ppm) are present in the Erzgebirge
basement (Mingram, 1998). However, melt experiments
on such rocks show that they tend to produce only small
quantities of strongly peraluminous melts with A/CNK
> 1·3, and they cannot be suggested as the dominant
source for the large volumes of S-type melts with lower
A/CNK values. Furthermore, Montel & Vielzeuf (1997)
showed that the K2O/Na2O ratio of melts from a pelitic
source is significantly higher (average 4–24) than the
ratio in the least evolved samples of Li-mica granite (<2).
The more feldspar- and quartz-rich metagreywackes and
orthogneisses are potentially fertile sources and could
give rise to comparatively large volumes of moderately
peraluminous melt. Moreover, chemical analyses of these
rocks in the Erzgebirge basement (Mingram, 1996)
showed concentrations of Sn (on average 4–8 ppm), W
(2–3), Li (60–120), Rb (150–250) and, importantly, Cs
(12–20) high enough to account for the evolved composition of the Li-mica granites. The Erzgebirge basement
is heterogeneous and highly layered (e.g. Bankwitz &
Bankwitz, 1994), and in a crustal section of ‘fertile’
quartz–feldspar rocks interlayered with ‘infertile’
metapelites,
melting
of
the
quartz–feldspar
lithologies can enhance melting in the less fertile interlayers, thus increasing the overall quantity of melt generated (Skjerlie et al., 1993). For the Erzgebirge, this
process may contribute significantly to the lithophile
element enrichment in the evolved granites.
The high Cs contents in the granites rule out derivation
of the magmas from a source that had experienced a
previous episode of melting, because the highly incompatible Cs would have been lost (e.g. London, 1995).
Moreover, data on mineral–melt partitioning between
micas, feldspar and peraluminous granite melt from
Icenhower & London (1995, 1996) suggest that the simultaneous enrichment of Li, Rb, Cs and F in the Erzgebirge granites requires nearly complete melting of
biotite or muscovite in the source.
The isotopic data indicate that fundamental differences
in the type of source lithologies cannot be invoked to
explain the predominance of evolved granites in the
Erzgebirge relative to other Variscan regions. The special
character of the Erzgebirge granites is probably caused
by a combination of several factors at the source: (1)
low degrees of melting; (2) wide distribution of fertile
lithologies in the source region (metagreywackes and
orthogneisses), which were not depleted by earlier melting
events and have relatively high primary LILE concentrations; (3) interlayering of less fertile but very LILE-
NUMBER 11
NOVEMBER 1999
and Sn–W-rich metapelites among the fertile lithologies,
which increases their capacity to be melted.
Tectono-thermal processes of granite
formation
One of the key features of the Variscan Erzgebirge is
the coeval emplacement of diverse granite types in a
closely confined area shortly after the end of collisional
orogeny. Can this diversity be explained by a single
tectono-thermal process? A commonly favoured hypothesis for the origin of the Variscan S-type granites
involves crustal melting in response to radioactive heating
within a thickened crust (>60 km; Kröner & Willner,
1998). Meta-igneous and metasedimentary quartzo-feldspathic rocks, which form the major part of the Erzgebirge
basement, are fertile sources for felsic magmas at midcrustal pressures between 10 and 15 kbar, with or without
externally derived fluids (Patiño Douce & Beard, 1996;
Skjerlie & Johnston, 1996). High average radioactive heat
productivity (2–4 lW/m3) in the basement lithologies
could facilitate fluid-absent melting without additional
heat supplied by mantle-derived magmas. The span of
time between the culmination of thickening [peak of the
HP metamorphism; see Fig. 2 and Kröner & Willner
(1998)] and emplacement of the late-collisional granites
is at least 20 my, which is sufficient to allow build-up of
radioactive heat to values high enough for anatexis (Zen,
1992).
An alternative or additional source of heat for crustal
fusion and the formation of granites is intrusion or
underplating of the crust by hot mafic magma derived
from the mantle. This mechanism has been advocated
for compositionally similar granitic rocks from other
tectonometamorphic units within the European Variscides, in particular the Moldanubian zone, where highT, low-P regional metamorphism is widespread (e.g.
Williamson et al., 1996; Finger et al., 1997; Siebel et
al., 1997). However, Gerdes (1997) presented a twodimensional thermal model for the Variscan crust in the
Moldanubian zone (southern Bohemian massif ), which
showed that intracrustal heat production is sufficient to
explain the generation of granites in that region. He
concluded that mantle-derived heat sources are neither
required nor are they compatible with the chemical and
isotopic composition of the granites.
Evidence against a significant mantle heat input for
the Erzgebirge granites is the lack of surface expression
for equivalent magmas and the fact that seismic and
gravity surveys of the Erzgebirge show no evidence for
abundant mafic intrusions in the lower crust (Bankwitz
& Bankwitz, 1994). The only definite mantle-derived
igneous rocks in the Erzgebirge are volumetrically insignificant lamprophyre dykes. The first generation of
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FÖRSTER et al.
LATE-COLLISIONAL GRANITES OF THE ERZGEBIRGE
Variscan lamprophyres in the study area (325–320 Ma,
Werner & Lippolt, 1998b), pre-dates emplacement of the
Li-mica granites, and this led Seifert (1997) to postulate
a substantial contribution of heat (and material) from
fluids derived from enriched, lithospheric mantle in the
formation of the Li-mica granites. However, as discussed
by Gerdes (1997) for the southern Bohemian massif, such
lamprophyres are products of low-degree partial melting
of metasomatized lithospheric mantle and are unlikely
to make a significant material or thermal contribution to
generation of the large volumes of crustal magmas present. Delamination of the lithospheric mantle after crustal
thickening is a possible mechanism to increase the heat
input to the base of the crust and may have occurred in
the Variscides (Zulauf, 1997). This mechanism, if mantle
magmas are not intruded in quantity, would enhance
the process of intracrustal melting in the thickened orogen
only as a result of a higher heat flux from the base.
We suggest, therefore, that the weight of evidence
favours an intracrustal origin for the Erzgebirge granites,
with heat provided by radioactive decay in a thickened
continental crust. Additional heat from mafic magmas
cannot be ruled out but does not seem to be required to
explain the granites and is not supported by any tangible
observation.
CONCLUSIONS
The late-collisional Variscan granites in the Erzgebirge
can be classed in terms of composition and mineralogy
into three major groups: low-F biotite granites, low-F
two-mica granites, and high-F, high-P2O5 Li-mica granites. The still-widespread classification into an older (OIC)
and younger intrusive complex (YIC) is not consistent
with existing age determinations. Emplacement ages of
all three groups of granites in the German Erzgebirge
(concordant K–Ar mica and U–Pb ages) overlap between
about 325 and 318 Ma. Emplacement thus post-dates
the peak of collisional orogeny (high-P metamorphism)
by about 20 my and was associated with extension during
post-orogenic collapse.
All granite groups are peraluminous and their chemical
and isotopic compositions suggest a crustal origin. Systematic and regular differentiation trends indicate that
crystal–melt fractionation is the principal process that
controlled the behaviour of elements during evolution of
the granitic magmas. However, petrographic evidence of
mineral overgrowths and replacements, together with
distinctive features of trace-element fractionation patterns
(e.g. non-chondritic Y/Ho ratios, segmented REE distribution patterns), confirm that metasomatic processes
became increasingly more important in late-stage melts.
High-T alteration was caused by volatile-rich, highly
reactive residual silicate melts and/or aqueous fluids
exsolved from them. In the Li-mica granites such metasomatic effects (with enrichment of P, F, Li, Rb, Cs, Sn)
are so prominent as to be a distinctive feature of these
rocks. The quantitative contribution of magmatic and
metasomatic processes to the bulk-rock composition is
impossible to determine precisely because both can have
the same effect for many elements. Near-complete leaching of U and enrichments in Ca, Mg, Sr or Ba in
late intrusive phases, however, provide unambiguous
evidence for medium- to low-T alteration.
The Sr and Nd isotopic composition of the biotite
granites and two-mica granites are indistinguishable
(eNd(320 Ma) = –2·9 to –4·9 and 87Sr/86Sr(320 Ma) 0·706–
0·708) and permit a common origin. Lithologies in the
local Erzgebirge basement exposed at present, however,
are isotopically too evolved to be the source for the
biotite and two-mica granite magmas. Significant mixing
of mantle material is not ruled out by the compositional
data but is unsupported by field evidence. Crustal rocks
with appropriate isotopic compositions occur in the Moldanubian zone of the Bohemian massif west and south
of the study area, and such rocks are thought to be the
likely source.
The highly differentiated Li-mica granites do not yield
meaningful initial Sr-isotopic compositions because of
their high Rb/Sr ratios and late-stage Rb and Sr remobilization. However, their eNd(320 Ma) values, from –4·5
to –7·4, are considerably lower than those of the biotite
and two-mica granites, and rule out an origin of the Limica granites by extended differentiation of a common
precursor magma. Like the Li-mica granites, metapelites
in the Erzgebirge basement are rich in LILE and Sn–W
but they are isotopically too evolved to be a major
component in the source (eNd(320 Ma) < –10) and, furthermore, the metapelites are too infertile to generate
the large volumes of magma represented by the Li-mica
granites. The most important crustal source rocks are
thought to be orthogneisses and metagreywackes, which
are common in the Erzgebirge basement and have fertile
bulk compositions as well as the appropriate isotopic
ratios. Interlayering of these rocks with the less fertile
but geochemically specialized metapelites may have enhanced melting in the latter and thus help explain the
high LILE and ore element concentrations in the granites.
The high Cs and Li contents in the granites indicate that
the melts must have originated from crustal sources
not depleted in these highly incompatible, mica-hosted
elements by a previous melting event.
The Erzgebirge is distinguished from other regions of
the central European Variscides by its relative abundance
of Li-mica granites and granite-related ore mineralization. The reason for this is not entirely clear but
this study suggests some contributing factors. Isotopic
compositions of the Li-mica granites are not significantly
different from those of the non-specialized peraluminous
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JOURNAL OF PETROLOGY
VOLUME 40
granites from other regions but the Erzgebirge basement
gneisses and metasediments contain higher than average
contents of the suite of incompatible elements typically
enriched in the granites. Even slight differences in the
incompatible element composition of the starting magma
can be significant because of the extensive crystal–melt
fractionation and final late- to post-magmatic element
remobilization that the granites have undergone.
ACKNOWLEDGEMENTS
The work embodied in this paper was mainly carried
out at the GeoForschungsZentrum Potsdam (GFZ). The
authors are particularly indebted to R. Naumann,
P. Dulski and M. Zimmer for performing whole-rock
chemical analyses. D. Rhede and O. Appelt provided
valuable assistance during the microprobe work. U–Pb
isotopic dating of monazite separates was carried out at
IGDL Göttingen by F. Warkus under the supervision of
B. Hansen. The work benefited from numerous discussions with many associates, including in particular
R. Thomas, J. D. Webster and R. Seltmann. Constructive
reviews by K. Breiter, W. Siebel and A. Willner helped
to improve the manuscript.
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