JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 5
RW3ES 1025-1029
1996
J. M. STUSSI1* AND M. CUNEY1
'CENTRE DE RECHERCHES PETROGRAPHIQUES ET GEOCHIMIQUES, B.P. 20, 54501 VANDOEUVRE-CEDEX, FRANCE
'CREGU AND GDR CNRS-CREOU, B.P. 23, 54501 VANDOEUVRE-CEDEX, FRANCE
Nature of Biotites from Alkaline,
Caloalkaline and Peraluminous Magmas
by Abdel-Fattah M. Abdel-Rahman:
A Comment
KEY WORDS biotite, gnmites, magma natun
STATE OF THE
ART
quartz—feldspar component is poorly discriminant.
The paper of Abdel-Rahman (1994) represents, after
the proposal of Nachit et al. (1985), another attempt
using biotite chemistry to discriminate between
alkaline (A), calc-alkaline (C) and peraluminous (P)
granite suites in MgO—AI2O3 and FeO—AI2O3 diagrams and to specify their geotcctonic environment.
Granites are usually classified in a number of magmatic suites of alkaline (A), calc-alkaline (C), highK calc-alkaline (HKC), shoshonitic (SH) and peraluminous (P) nature (Peccerillo & Taylor, 1976; La
Roche et al., 1980; Miller, 1985; Lameyre, 1987;
Debon & Le Fort, 1988; Pitcher, 1993). In most
granite suites, the decrease of the differentiation
index DI=Fe+Mg+Ti is generally correlated with an
increase of the Fe/(Fe+Mg) ratio and of the aluminous index AI=Al-(K+Na+2Ca) or AI=A12O3/
(CaO+Na2O+K.2O). Weakly peraluminous compositions commonly characterize the felsic members
of the C, HKC, SH and also of some A-type suites.
In P suites, characterized by the same range of DI
values as C, HKC and SH granite suites, AI generally increases with decreasing DI but can decrease
in some suites (e.g. Kosciusko S-type suite; Millevaches M1-M2 suites, Fig. 1; Stussi & Cuney, 1993).
In all of these suites, the variation of biotite composition defines a negative Al-Mg correlation [see
point (5) below]. In many granites, biotite is the
single dark mineral; the DI and Fe/(Fe+Mg) of
bulk-rock compositions mainly represent the abundance and the composition of the biotite whereas AI
reflects the excess of alumina in biotite with respect
to the one-to-one balance of Na+K+2Ca vs Al in
feldspars. Therefore, the composition of biotite can
be a discriminating tool in the identification of the
nature of granites and granite suites in which the
Fig. 1. Bulk-rock chemical variation trend of granite mites in the
AI=Al-(K+Na+2Ca) vj DI=Fe+Mg+Ti diagram of Debon & Le
Fort (1988). BN: Ben Nevia (Hailam, 1968); SN: Sierra Nevada
(Bateman it al., 1963); Po: Portugal (De Albuquerque, 1971;
Neiva, 1976); M l , M2: Millevachei, France (Stussi & Cuney,
1993); PI: Ploumanac'h (Barriere, 1977): G: Gueret, France
(Debon & Le Fort, 1988); K: Kosciuiko S-type (Hine et al.,
1978); A: Corsica peralkaline (Bonin, 1980); S, J: Strathbogie,
Jindabyne (Burkhard, 1991). Hb: hornblende; bi: biotite; mu:
muscovite; cd: cordierite; grt: garnet; ail: sillimanitc. C—P: transposition of the biotite C—P boundary in the bulk composition
diagram.
•Corresponding author.
THephonc (33) 83 59 42 4 a Fix: (33) 83 51 17 9a
e-mail: [email protected]
© Oxford University Press 1996
100
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JOURNAL OF PETROLOGY
NUMBER 5
VOLUME 37
DISCUSSION
(1) In Abdel-Rahman's paper, a first discrimination is obtained between the composition of
biotite from (i) A suites and (ii) G and P suites. The
boundary drawn between these two principal compositional fields defines a negative AljOs-MgO correlation trend which agrees rather well with the
boundary proposed by Nachit et al. (1985) using
similar techniques and a data base including mainly
Variscan granites from Western Europe.
(2) Contrary to the Nachit et al. (1985) classification, the boundary between the C and P fields of
Abdel-Rahman is surprisingly transverse to the
negative Al2O3-MgO correlation trend defined by
the variation of biotite composition from the most
mafic to the most felsic granite phase in the same
suite (Fig. 2). This boundary also does not fit with
the boundary expected from interrelated mineral
and bulk composition variation trends of C and P
suites (Fig. 1). As a whole, the classification of
Abdel-Rahman approximately separates biotite of
peraluminous magmas from biotite of metaluminous
magmas whatever the magma nature of the granite
suites. This point will be discussed more thoroughly
below.
(3) The boundary proposed by Abdel-Rahman
would mean that all Al-rich biotites with high Fe/
Mg ratios belong to peraluminous magmas. As a
OCTOBER 1996
consequence, such a relationship would also imply
that peraluminous magmas result from the differentiation of calc-alkaline magmas. The most
fractionated members of calc-alkaline suites may
become slightly peraluminous, but the origin of most
peraluminous granites is certainly different (Miller,
1985;Turpin^a/., 1990).
(4) In fact, Al content of biotite crystallizing in
equilibrium with a silicate melt reflects the peraluminosity of the melt. As exemplified by the Variscan granites, under comparable P, T, fo
conditions, the composition of biotites becomes
increasingly aluminous from peralkaline to peraluminous suites (Fig. 2). For example, biotite from
the mafic members (cordierite ± garnet bearing
granodiorites) of peraluminous suites is richer in Al
than biotite of hornblende granodiorites from calcalkaline suites with similar DI. Biotite of the cordierite-biotite granodiorites (i.e. Gueret in Fig. 2)
plots in the P field of the classification of Nachit et
al., in accordance with the chemistry of the magma,
instead of the C field of Abdel-Rahman's classification. In the same way, the felsic members of peraluminous suites contain biotite characterized by a
higher Al content than that of the felsic members
from calc-alkaline suites, even if biotite is not associated with a more aluminous mineral. Biotite from
these felsic calc-alkaline granites plots in the C field
instead of the P field, in accordance with the bulk
4.00
3.50 -
3.00 3.5 -
2.50 •
2.00
0.00 0.50 1.00 1.50
2.5
1.5
2.50 3.00 3.50
Fig. 2. (a) Biotite composition typology according to Nachit it al. (1985). Al, Mg in number of atoms per formula unit. ( 9 , Hepburn
batholith; A , Zaer; O, Sierra Nevada batholith; • , Ploumanac'h—Armorican massif, France; A . Ballonj massif, France (Pagel, 1980);
+, (Ml) and X (M2), Millevaches, France, unpublished data of the authors; X, Gueret, Sabourdy (1988) and Vauchelle (1988). S:
Strathbogie; J: Jindabyne; P: peraJuminoui; C: calc-alkaline; SA: subalkah'ne; A—PA: alkaline-peralkaline. C—P: transposition in the
Nachit et al. (1985) diagram of the boundary between C and P biotite fieldi of Abdel-Rahman (1994). Sierra Nevada, Hepburn, Zaer,
Ploumanac'h—Armorican massif, Strathbogie, Jindabyne data: see references in Abdel-Rahman (1994). (b) Biotite end-members in the
Al-Mg diagram. Sid: siderophytlite; East: eastonite; Phlog: phJogopite; Ann: annite; MTS: tetrasilicic mica (Mg=25; Al=00); Zw:
zinnwaldite; (a), (b), (c), (d), (e), substitutions cited in text; MDT, mean differentiation trend.
1026
STUSSI AND CUNEY
COMMENT
composition (i.e. Sierra Nevada; Ploumanac'h,
Armorican massif). Other apparent overlaps
between the C and P fields (i.e. Morocco Zaer
granites) correspond to the presence of two distinct
calc-alkaline and peraluminous granite suites (Giuliani et al., 1989). The composition overlaps which
may still exist between P and C fields as defined by
Nachit et al. result either from differences in the
physical conditions prevailing during granite genesis
from one orogen to another, or from late magmatic
to subsolidus alteration, modifying pristine compositions as discussed in the following points.
(5) The negative correlation between Al and Mg
observed among biotite compositions is usually
accounted for by a number of substitutions operating
between four end-members (Fig. 2): (a)
(phlogopite—annite substitution) and (b) M 22 + V I
S i I V ^ A l V I , A1 I V (siderophyllitic substitution)
(Speer, 1984); (c) M 2 + V l , 2 A l l V ^ D V I , 2Si IV along
the annite/phlogopite—MTS (tetrasilicic mica) join
and (d) 3 M 2 + 1 " ^ 2A1 VI , D V I along the annite/
phlogopite—muscovite join (Robert, 1981; Monier &
Robert, 1986a); (e) T i + + ^ 2 F e 2 + (Patino Douce,
VI
VI
in Li-rich
1993) and (f) 2M 2+VI. -A1 , Li
systems (Monier & Robert, 19864). Substitution (d)
is specific of biotites from peraluminous granites;
coupled with substitution (a), it generates a negative
Al—Mg correlation. Calc-alkaline biotite compositions are mainly controlled by substitutions (a), (c)
and (d) which also result in a negative Al—Mg correlation. The correlation trend will occur at lower
average Al content in calc-alkaline granites than in
peraluminous granites because of the lower activity
of Al in calc-alkaline magmas. Substitution (c) is
specific of biotite from alkaline and pcralkaline
granites. The increase of octahedral vacancies produced by substitution (d) lowers the increase of Fe
with regard to Mg and thus decreases the discriminating efficiency of the Al-Fe diagram with
respect to the Al-Mg diagram in biotite typology.
(6) The composition of biotites depends on a
number of interdependent variables—P, T,fOt,fHtO,
element activities—one of which must be defined
either empirically or indirectly so as to determine the
others. In addition, the Fe +/Fe + ratio of biotite
cannot be determined from microprobe analyses, so
the redox buffer and / o , cannot be correctly estimated. Wet chemical analyses of biotite separates
fail to yield accurate compositions and Fe + /Fe 2+
ratios because of the usual presence of unseparated
chlorite and/or oxide phases. Experimental data
show that the stability field of Al-rich biotite is
enlarged towards lower temperatures, whereas the
solubility of Ti and Mg increases with an increase in
temperature (Rutherford, 1973; Robert, 1976, 1981;
1027
Monier & Robert, 1986a; Patino Douce, 1993).
Transposed into the Al-Mg biotite classification,
these data indicate that the crystallization temperature of biotite of a given granite suite would
decrease from Ti-Mg-rich and Al-poor towards Mgpoor and Al-rich compositions. Some discrepancies,
however, are observed with respect to this general
trend, e.g. for similar Mg values, the temperature of
crystallization of the biotite from the Strathbogie
peraluminous granite determined by Burkhard
(1991) is higher (860-1026°C) than that determined
on the biotite of the European Variscan peraluminous granites [600-760°C at 5 kbar according
to the Ti-substitution geothermometry of Patino
Douce (1993)]. The high temperature of crystallization of the Strathbogie biotite is also evidenced
by the presence of orthopyroxene and magmatic
cordierite (Phillips et al., 1981). The low Al content
of this biotite results from the lower solubility of Al
in the biotite with increasing temperature (Robert,
1976; Puziewicz & Johannes, 1988; Patiflo Douce,
1993). However, the biotites of high-temperature
peraluminous granites such as the Strathbogie one
can be discriminated from C-type biotite by their
higher Ti content.
(7) Biotite composition can be re-equilibrated
when fluid-oversaturation occurs in highly fractionated granites, during the late or subsolidus stages of
crystallization, and thus cannot be representative of
the mineral-melt equilibrium. The strong partitioning of the alkalis in chloride-rich magmatic fluids
leads to an increase of the aluminium-alkalis
balance in the residual melt. Thus, biotite compositions are shifted towards Al-richer compositions and
muscovite may crystallize in initially metaluminous
magmas (Harrison, 1990). As a consequence, for
typological purposes, it is preferable to select the
composition of biotite crystals included in early
crystallizing phases, which will not be re-equiclibrated by late magmatic fluids when vapour
oversaturation is reached in the magma. Incipient
chloritization, which is frequently undetectable by
microscopic observation, is the most common
phenomenon increasing Al and Mg content of biotite
during subsolidus alteration.
(8) The C field of Abdel-Rahman corresponds to
the C and SA (subalkaline) fields of Nachit et al.
(1985). This subdivision reflects the granite typology
used for the Variscan granites (Barriere, 1977; La
Roche et al., 1980; Lameyre, 1987; Debon & Le
Fort, 1988; Stussi, 1989). The SA granites correspond nearly to the SH granite suites of Peccerillo &
Taylor (1976) and most of the felsic phases are
comparable with the highly fractionated calcalkaline granites as defined by Sylvester (1989). For
JOURNAL OF PETROLOGY
VOLUME 37
similar Mg content, biotites of SA suites are characterized by lower Al content than C suites, i.e. discrimination between the calc-alkaline Sierra Nevada
felsic granites and the SA granites from Ploumanac'h, Armorican massif. Comparatively, the biotite
composition of peralkaline granites is characterized
by a lower Al content.
(9) As pointed out by Abdel-Rahman, the link
between biotite composition (as well as the magma
nature determined by bulk-rock compositions) and
tectonic environment in pre-Mesozoic orogenies is
not straightforward (Sylvester, 1989). This is best
exemplified by the granite suites of the West European Variscan belt. The SA, C and P gTanites of this
orogen are all emplaced during the post-collisional
stage. They do not present any obvious preferential
space—time pattern of distribution and can be even
observed in the same plutonic complex in which they
occur as coeval phases (Stussi, 1989; Lagarde et al.,
1992). Their major element composition is comparable with that of granite suites from either converging plate (for SA and C granites) or collisional
(for P granites) tectonic environments, except for
their isotopic (Sheppard, 1986) and to a certain
extent for their trace element signature (Harris et al.,
1986; Thieblemont & Cabanis, 1990). As the biotite
composition reflects the bulk-rock composition, this
example clearly shows that the chemistry of this
mineral cannot be readily used for geotectonic
implications.
CONCLUSION
The Al-Mg diagram appears to be one of the most
discriminating tools for the appraisal of the granite
magma nature based on mineral criteria. However,
the discrimination between the C and P biotite
compositions in the AI2O3—MgO and AI2O3—FeO
diagrams of Abdel-Rahman (1994) does not fit with
interrelated mineral-whole-rock chemical variations.
The discrepancies between Abdel-Rahman's study
and previous studies using similar techniques and
data sets do not result from the approach used. They
obviously result from an ambiguous use, in granite
magma typology, of the meaning of the term peraluminous for either bulk-rock composition whatever
the origin of the peraluminous character, or for the
definition of specific granite suites derived from peraluminous sources.
Anyhow, such diagrams have to be used with
great care, taking into account (i) the shift of the
biotite composition related to the variation of
intensive parameters such as 7" and To,) (») the possible alteration of biotite by late- or post-magmatic
fluids, which may modify the original composition
NUMBER 5
OCTOBER 1996
inherited from mineral-melt equilibrium, (iii) the
fact that the Al-Mg plane represents only a part of
the biotite compositional space. As discussed by
Patifio Douce (1993), a more general experimental
calibration of biotite substitutions and stability field
is necessary to obtain quantitative constraints on the
intensive variables operating in the crystallization of
biotite in the various granite types. Finally, owing to
the large number of independent parameters controlling the biotite chemistry, the use of the biotite
composition of granitoids from well-characterized
Cenozoic—Mesozoic orogenic environments for the
geotectonic reconstruction of ancient orogens can be
misleading.
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REVISED TYPESCRIPT ACCEPTED 1 JANUARY 1996
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