JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 5
PAGES 1013–1043
2004
DOI: 10.1093/petrology/egh002
The Relationship between Tectonometamorphic Evolution and Argon Isotope
Records in White Mica: Constraints from
in situ 40Ar---39Ar Laser Analysis of the
Variscan Basement of Sardinia
GIANFRANCO DI VINCENZO1*, RODOLFO CAROSI2 AND
ROSARIA PALMERI3
1
ISTITUTO DI GEOSCIENZE E GEORISORSE---CNR, VIA MORUZZI 1, I-56124 PISA, ITALY
2
DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI PISA, VIA S. MARIA 53, I-56126 PISA, ITALY
3
MUSEO NAZIONALE DELL’ANTARTIDE, UNIVERSITÀ DI SIENA, VIA LATERINA 8, I-53100 SIENA, ITALY
RECEIVED JUNE 7, 2003; ACCEPTED OCTOBER 30, 2003
The basement of Sardinia represents a nearly complete section of a
segment of the Variscan belt that experienced a polyphase tectonometamorphic evolution and Barrovian metamorphism. This
basement is well suited to investigate the relationship between
tectono-metamorphic evolution and argon isotope records in white
mica. Micaschists from the garnet zone (maximum T of up to
520---560C) contain two texturally and chemically resolvable
generations of white mica: (1) deformed celadonite-rich flakes,
defining a relict S1 foliation preserved within the main S2 foliation
or enclosed in rotated albite porphyroblasts; (2) celadonite-poor
white micas aligned along the main S2 foliation. The S1 foliation
developed earlier and at a deeper crustal level with respect to that
at which the thermal peak was reached. From the staurolite zone
( T of up to 590---625C) to the sillimanite þ K-feldspar zone, white
mica is nearly uniform in composition (muscovite) and is predominantly aligned along the S2 foliation or is of later crystallization.
In situ 40Ar---39Ar laser analyses of white mica yielded ages of
340---315 Ma in the garnet zone, and 320---300 Ma in the
staurolite and sillimanite þ K-feldspar zones. Results highlight a
close link between argon isotope records in white mica and both
textures and structure-forming major elements. The oldest ages were
detected in samples where the earlier syn-D1 white mica generation
did not texturally and chemically re-equilibrate at upper-crustal
levels. This study suggests that the white micas that escaped recrystallization retain argon isotope records of an earlier metamorphic
In the last decade, K---Ar mineral ages have widely been
interpreted to record cooling below a critical temperature
defined as the closure temperature (Tc; Dodson, 1973).
Minerals that developed and/or re-equilibrated at temperatures above their Tc yield K---Ar ages that represent
cooling ages. The use of the closure temperature concept,
however, implies that temperature was the only parameter controlling the rate of isotope transport during
tectono-metamorphic events and that argon diffusion
parameters are known, so that for a given grain size and
cooling history the appropriate Tc can be calculated.
Nevertheless, metamorphic rocks commonly exhibit disequilibrium textures, and each mineral often comprises
different generations developed under different P---T conditions and/or different deformational regimes and fluid
activities. Regarding white mica, numerous studies have
*Corresponding author. Telephone: þ39 050 3152270. Fax: þ39 050
3152360. E-mail: [email protected]
Journal of Petrology 45(5) # Oxford University Press 2004; all rights
reserved
stage that survived a later event characterized by temperatures higher
than 500 C.
K---Ar systematics; 40Ar---39Ar laserprobe dating; white
mica; Barrovian metamorphism; Variscan belt
KEY WORDS:
INTRODUCTION
JOURNAL OF PETROLOGY
VOLUME 45
shown that variations in argon isotope age records can be
closely linked to microtextural and microchemical variations (e.g. Chopin & Maluski, 1980; Hammerschimdt &
Frank, 1991; Hames & Cheney, 1997; Di Vincenzo et al.,
2001), thus assigning a major role in the control of argon
transport to chemical reactivity, strain and circulation of
fluids. These studies have also shown that, in certain
circumstances, the argon diffusion rate in white mica
may be significantly slower than previously believed.
However, most of those studies dealt with rock samples
that had experienced very extreme physical conditions
and/or were based on bulk 40Ar---39Ar analyses. Other
studies highlighted age gradients in micas independent of
crystal-chemical variations and compatible with volume
diffusion processes (e.g. Hodges et al., 1994). It is therefore
important to study the relationship between tectonometamorphic evolution and argon isotope records in
white mica from carefully selected, natural samples with
well-defined P---T---deformation paths.
The basement of Sardinia represents a nearly complete
section of a segment of the Variscan belt comprising lowto high-grade metamorphic rocks. This is a suitable setting
in which to study the relationship between tectonometamorphic evolution and argon isotope records in
white micas. This paper reports the results of a detailed
microtextural and microchemical study and in situ
40
Ar---39Ar laser experiments on metamorphic samples
from the garnet zone to the sillimanite þ K-feldspar
zone of northern Sardinia. The research aimed to examine the relationship between argon age records and both
superimposed fabric-forming episodes and chemical
variations in white mica.
GEOLOGICAL BACKGROUND
Sardinia and Corsica represent a segment of the Southern European Variscan belt extending from the Axial
Zone in Corsica and northern Sardinia to the External
Zone in southwestern Sardinia (Carmignani et al., 1994).
The crystalline basement of the Sardinia---Corsica block
formed in the Early Carboniferous as a result of the
collision between the northern Armorica and southern
Gondwana continents, which produced intense deformation and synkinematic regional metamorphism accompanied by widespread igneous activity.
In the metamorphic basement the metamorphic grade
increases from very low grade in SW Sardinia to medium
and high grade in the northern sector of the island.
Polyphase deformation and metamorphism were
recorded during the collisional and post-collisional stages
(Carmignani et al., 1994, and references therein). The
study area (Fig. 1) represents a portion of the mediumto high-grade basement belonging to the Internal
Nappes and High-Grade Metamorphic Complex. The
southern part of the study area consists of micaschists,
NUMBER 5
MAY 2004
porphyroblastic paragneisses, and Ordovician granodioritic orthogneisses and granitic augen gneisses. The
Posada---Asinara suture zone (Fig. 1, Posada---Asinara
Line, Cappelli et al., 1992) separates the High Grade
Metamorphic Complex from the Internal Nappes
(Fig. 1). Migmatites and migmatitic gneisses with lenses
and bodies of amphibolites, granulites and retrogressed
eclogites crop out north of the Posada---Asinara Line
(Franceschelli et al., 1982). The progression from lowgrade to high-grade metamorphism is considered a prograde evolution caused by collision-related Barrovian
metamorphism (Franceschelli et al., 1989).
Metamorphic basement
Structural data indicate the presence of four main phases
of ductile deformation. South of the study area, the D1
deformation phase is characterized by overturned, SWfacing folds and syn- to late-D1 ductile to brittle shear
zones with a top-to-SW sense of shear (Franceschelli et al.,
1982; Carmignani et al., 1994). An S1 penetrative axial
plane foliation developed during D1 deformation. A
heterogeneous mylonitic D2 deformation, related to a
transpressional phase (Carosi & Palmeri, 2002), overprinted the D1 structures on a regional scale. The D2
strain increases northward until the main pervasive foliation becomes the S2 foliation. The S2 foliation strikes
WNW---ESE and strongly dips SSW. The D2 deformation
accompanied much of the exhumation of the low- to
medium-grade metamorphic rocks (Carosi & Palmeri,
2002). The main foliation and stretching lineation in the
migmatitic complex has the same attitude as that of the
mylonitic foliation in the micaschists and gneisses. The D2
deformation is overprinted by two main systems of open
to tight folds [F3 and F4 according to Carosi & Palmeri
(2002)]. Franceschelli et al. (1982) mapped seven metamorphic zones (from south to north): (1) chlorite;
(2) biotite; (3) garnet; (4) staurolite þ biotite; (5) kyanite þ
biotite; (6) sillimanite þ muscovite; (7) sillimanite þ
K-feldspar. Franceschelli et al. (1989) highlighted the
diachroneity of mineral growth. The index minerals in
the low-grade rocks are aligned along S1. Starting from
the garnet zone, the index minerals are predominantly
post-D1 and pre- to syn-D2 porphyroblasts. The index
minerals (white mica, sillimanite and K-feldspar) in the
high-grade zones are aligned along the S2 foliation.
Variscan plutonic rocks
The Variscan Sardinia---Corsica Batholith is one of the
largest batholiths in SW Europe. Widespread plutonic
activity consisting of two orogenic associations took
place during and after the Variscan tectono-metamorphic events (Rossi & Cocherie, 1991): (1) an earlier
syntectonic Mg---K calc-alkaline association, cropping
1014
DI VINCENZO et al.
METAMORPHIC EVOLUTION AND ARGON AGES
quartzites
Fig. 1. Geological map of NE Sardinia (modified after Carmignani & Rossi, 1999) and sample location. Mineral symbols according to Kretz (1983).
Olig, oligoclase.
out only in northern Corsica and emplaced during peak
amphibolite-facies metamorphism; (2) a late- to posttectonic, high-K calc-alkaline association that postdates
the D2 structures. In Sardinia, the high-K calc-alkaline
association also includes strongly peraluminous granitoids, which were emplaced at the same time as the
high-K calc-alkaline sensu stricto granitoids (Di Vincenzo
et al., 1994a, and references therein).
Previous geochronology of the Variscan
cycle in the Sardinia---Corsica basement
The cumulative probability distribution diagrams in
Fig. 2 display previous geochronological data for the
metamorphic basement, for high-K calc-alkaline granitoids from both Sardinia and Corsica, and for Mg---K
calc-alkaline granitoids from Corsica. Published data
indicate that the emplacement of late- to post-tectonic,
high-K, calc-alkaline granitoids in both Corsica and
Sardinia occurred from 310 to 280 Myr ago (Fig. 2a).
The youngest ages (280---290 Ma) refer to the posttectonic leucogranites. The earliest syntectonic Mg---K
association, instead, was emplaced at 330---345 Ma
(Fig. 2a). Rb---Sr data on biotite and muscovite from
Sardinian calc-alkaline granitoids yielded ages of
275---295 Ma and 290---305 Ma for biotite and muscovite,
respectively (Fig. 2b). The emplacement of the peraluminous Sos Canales pluton, cropping out in the study area
1015
JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 5
MAY 2004
Figure 2c shows that available muscovite and biotite
Rb---Sr and K---Ar ages from the metamorphic basement
largely overlap at 300---320 Ma. Most of the biotite ages
are younger (280---290 Ma) and overlap with the youngest
biotite ages of the high-K calc-alkaline granitoids. Older
ages were reported by Del Moro et al. (1991) for muscovite--whole-rock pairs (ages of 336 8 and 350 16 Ma) and
for actinolite 40Ar---39Ar data (age of 345 4 Ma), from
micaschists and a metagabbro sampled in the Internal
Nappes of NW Sardinia. Figure 2c also reports the Rb---Sr
age of 344 7 Ma obtained using six bands from a
stromatic migmatite with trondhjemitic leucosomes
(Ferrara et al., 1978), which was considered to approximate the Variscan metamorphic climax.
(a)
EXPERIMENTAL PROCEDURES
(b)
(c)
Fig. 2. Cumulative probability distribution of geochronological data
for the Variscan basement of Sardinia and Corsica. Data sources:
Ferrara et al. (1978); Beccaluva et al. (1985); Macera et al. (1989);
Del Moro et al. (1991); Di Vincenzo et al. (1994a, 1994b); Carmignani
& Rossi (1999). The cumulative probability distribution was calculated
using Isoplot/Ex, v. 2.3 (Ludwig, 2000). Mineral symbols according to
Kretz (1983), and: WR, whole rock. (1) refers to the age of 344 7 Ma
from a banded migmatite and (2) to the actinolite 40Ar---39Ar age of
345 4 Ma from a metagabbro.
(Fig. 1b), was dated to 298 4 Ma by whole-rock Rb---Sr
data; biotite and muscovite yielded Rb---Sr ages (mineral--whole-rock pairs) of 285 4 to 299 4 Ma (Di Vincenzo
et al., 1994b).
Petrographical observations on polished thin sections
(derived from the opposite face of the rock chip used for
40
Ar---39Ar dating) were carried out using a light microscope and by scanning electron microscopy (SEM) using
a Philips XL30 instrument operating at 20 kV and
equipped with an X-ray energy dispersive system (EDS).
Electron microprobe (EMP) analyses were carried out
using a JEOL JX 8600 electron microprobe fitted with
four wavelength-dispersive spectrometers, using an accelerating voltage of 15 kV and sample current of 10 nA.
Natural standards were used for calibration.
Rock chips 85 mm in diameter were drilled from
polished sections ( 300 mm thick) for in situ 40Ar---39Ar
laserprobe analysis. They were irradiated for 30 h in the
TRIGA reactor at ENEA-Casaccia (Rome). The neutron
flux was monitored using the standard MMhb-1 (age
5204 Ma; Samson & Alexander, 1987). In situ laser analyses were carried out along the length of the mica grains
(i.e. parallel to the basal cleavage) using a CW diodepumped Nd---YAG infrared (IR) laser (mainly for biotite)
connected to an external computer-controlled shutter
and a pulsed Nd---YAG ultraviolet (UV) laser (frequency
quadrupled and Q-switched). Although this orientation
differs from that commonly used in diffusion studies (i.e.
perpendicular to the cleavage plane), it allows the direct
comparison between argon data and microstructural
information. Single short pulses (5---7 ms) of the IR laser
(operating at 12 W ) were focused to produce circular melt
pits 100 mm in diameter. The UV laser, operating at
20 Hz and 05---1 mJ per pulse, was focused to 10 mm
and repeatedly rastered by a computer-controlled x---y
stage over areas of 00036---0010 mm2, to produce pits
50---100 mm deep. The sample was observed by a CCD
camera coaxial with the laser beam. After cleanup
(10---15 min for IR and UV analyses, respectively),
extracted gases were equilibrated via automated valves
with an MAP215-50 mass spectrometer fitted with a
Balzers SEV217 secondary electron multiplier. Blanks
1016
DI VINCENZO et al.
METAMORPHIC EVOLUTION AND ARGON AGES
were analysed every two to four analyses. The interference factors, measured on phases of Ca and K, were:
(36Ar/37Ar)Ca ¼ 000029; (39Ar/37Ar)Ca ¼ 000069;
(40Ar/39Ar)K ¼ 00076; (38Ar/39Ar)K ¼ 0013. The Ca/
K and Cl/K ratios were calculated following the procedure given by Di Vincenzo & Palmeri (2001). After
40
Ar---39Ar analyses, the location of the analysed pits in
the irradiated rock chips of the finer-grained samples
(micaschists) was checked by SEM.
RESULTS
Studied samples
Samples (location in Fig. 1b) are metasedimentary rocks
and orthogneisses from the garnet zone (C3, C15,
GFS280), the staurolite þ biotite zone (C6), and the
sillimanite þ K-feldspar zone (RS2BA, RS5A, GFS294).
Sample C15 is a porphyroblastic micaschist collected at
the beginning of the garnet zone. It consists of millimetresized albite porphyroblasts showing evidence of post-D1
and pre-D2 growth and including an internal foliation (S1)
defined by inclusion trails of white mica, quartz, garnet
and minor biotite. The external foliation envelops porphyroblasts and is an advanced, discrete, S2 crenulation
cleavage dominated by white mica and minor chlorite
and biotite. Garnet, oligoclase and opaque minerals are
also found along S2. Microlithon relics of the S1 foliation
and F1 fold hinges are preserved within S2. Sample C3
is a micaschist collected 5 km north of sample C15.
Although its petrographic and structural features are
comparable with those of sample C15, it is characterized
by a better-developed D2 deformation. GFS280 is an
augen gneiss collected 2 km from the contact with the
Sos Canales Intrusion (Fig. 1b). The orthogneiss consists
of K-feldspar, quartz and plagioclase with a crenulated
foliation (S2) defined by biotite and white mica; zircon,
apatite and opaque minerals are accessory phases. Chlorite is a common secondary phase after biotite.
Sample C6 is a micaschist with pre-D2 millimetre-sized
garnet and staurolite porphyroblasts. It was collected at
the start of the staurolite zone (Fig. 1b), where D2 strain
was high. It is characterized by a well-developed D2
shear-band cleavage, a significant coarsening of grain
size in the matrix and by the disappearance of primary
chlorite. White mica is more abundant than biotite. Biotite locally shows partial retrogression to chlorite, staurolite to white mica, and garnet to a fine-grained
association of micas.
Samples RS5A and GFS294 are orthogneisses consisting of K-feldspar, quartz and plagioclase, with biotite and
white mica aligned along the main foliation (S2); apatite,
zircon and opaque minerals are accessory phases. Biotite
is locally chloritized in RS5A, whereas it seems pristine in
GFS294. Sample RS2BA is a metasedimentary stromatic
migmatite; mesosomes consist of biotite, quartz, plagioclase, K-feldspar and white mica, with accessory zircon,
apatite and opaque minerals, whereas leucosomes consist
of K-feldspar, plagioclase and quartz, minor biotite, small
garnet grains and apatite. Mineral phases in the mesosomes grew along the main foliation (S2). Biotite is
partially replaced by chlorite.
Microtextural and microchemical data
Samples C3 and C15 contain different texturally resolvable generations of potassic white mica: (1) relics of the
S1 foliation; (2) slightly crenulated, fine-grained flakes
along the S2 foliation and enveloping porphyroblasts
(Fig. 3a---c). Rare, undeformed, later flakes cross-cut the
main foliation or occur with biotite as degradation products of garnet. Syn-D1 white micas occur: (1a) within
albite porphyroblasts (Fig. 3a); (1b) in microlithons where
white micas along with biotite---chlorite constitute
the hinges of microfolds enveloped by syn-D2 micas
(Fig. 3b); (1c) ‘coarse-grained’ relics enveloped by mica
sheaves aligned along S2 and consisting of finer-grained
white micas with biotite---chlorite interlayers of variable
thickness (Figs 3b and c, and 4a). Type-1 white micas are
invariably celadonite-rich and paragonite-poor, and
show no significant chemical zoning (Table 1, Fig. 4).
Low-celadonite and high-paragonite compositions were
detected along with subordinate high-celadonite and lowparagonite contents in white micas on the S2 foliation
(Table 1). X-ray maps of the microtextural type-2 white
micas (Fig. 4c) show that the celadonite content is locally
heterogeneously distributed at the microscopic scale.
White micas in both samples exhibit nearly continuous
variations, from earlier high-celadonite and low-paragonite to later low-celadonite and high-paragonite compositions (Fig. 5). White micas along S2 in sample C3, when
compared with those of C15, are characterized by lower
celadonite and higher paragonite contents (Fig. 5b).
Microtextural and mineral features in these samples suggest that white mica was involved in different continuous
metamorphic reactions. The AKFM reaction
( Thompson, 1976) chlorite þ K-white mica þ quartz ¼
garnet þ biotite þ H2O (R1) can be regarded as responsible for the observed paragenesis in both syn-D1 and synD2 assemblage. In addition, type-1 and type-2 white
micas are thought to be the result of a reaction in which
celadonite-rich white mica is a reactant and celadonitepoor white mica a product (Thompson, 1976): cel-rich
K-white mica þ Fe-rich chlorite ¼ cel-poor K-white
mica þ Mg-rich chlorite þ biotite (R2). The presence of
oligoclase in the syn-D2 assemblage after the albite
porphyroblasts and the decrease of the grossular content
from core to rim of syn-D2 garnets (Carosi & Palmeri,
2002) suggest the involvement of the reaction (Crawford,
1966) Ca-rich garnet þ albite þ K-white mica þ chlorite þ
quartz ¼ Ca-poor garnet þ oligoclase þ biotite (R3).
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JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 5
MAY 2004
(b)
(a)
200 µm
(c)
(d)
(e)
(f)
200 µm
Fig. 3. (a) Photomicrograph of an albite porphyroblast including syn-D1 white mica and enveloped by syn-D2 white mica (garnet zone-----micaschist
C3). (b) Back-scattered electron photomicrograph of microlithon constituting the hinges of microfolds enveloped by syn-D2 micas (garnet
zone-----micaschist C15). (c) Back-scattered electron photomicrograph of a ‘coarse-grained’ relict celadonite-rich white mica enclosed within synD2 celadonite-poor white mica sheaves (garnet zone-----micaschist C3). (d) Micaschist from the staurolite þ biotite zone (micaschist C6) in which synD2 white mica envelops porphyroblasts and late white mica replaces staurolite. (e) and (f), samples from the sillimanite þ K-feldspar zone in which
white mica flakes lie along or cross-cut the main foliation [(e) sample RS5A; (f) sample GFS294]. Mineral symbols according to Kretz (1983), and:
WM, white mica; Ab, albite; Olig, oligoclase.
Although microtextural variations in biotite are similar to
those in white mica, biotite shows no significant chemical
variations (Fig. 6). A few data points for C3 biotite show
higher AlIV and XFe, and lower K, and are attributed to
the possible occurrence of interlayered chlorite. White
mica in sample GFS280 occurs as: (1) rare, deformed,
small flakes enveloped by the main foliation, or within
biotitic glomeroblastic associations suggesting a previous
1018
DI VINCENZO et al.
METAMORPHIC EVOLUTION AND ARGON AGES
(a)
Rt
Ilm
Grt
Grt
(b)
Bt-Chl
100µm
WM-1
WM-2
(b)
Bt-Chl
Rt
WM-1
Bt-Chl
WM-2
25µm
(c)
Si Kα
Al Kα
Fe Kα
Mg Kα
Fig. 4. (a) Close-up of Fig. 3c showing the ‘coarse-grained’ relict white
mica enclosed within syn-D2 white mica sheaves. Both syn-D1 and synD2 white micas contain biotite---chlorite interlayers tens of microns thick.
(b) Close-up of (a). (c) X-ray maps of Si, Al, Fe and Mg of the area shown
in (b) corresponding to 0024 mm2 (256 200 pixels). The maps were
acquired with an SEM-EDS system. The lightness is proportional to the
element concentration. The absence of significant chemical zoning in
the ‘coarse-grained’ white mica relic (left) and the heterogeneous distribution of Si, Al and Mg in the smaller white micas aligned with S2
(right) should be noted.
crystallization with respect to the micas along the main
foliation; (2) millimetre-sized flakes along the main foliation; (3) rare, later flakes showing no preferred orientation. Celadonite-rich compositions were rarely detected
within the glomeroblastic associations, whereas lower
celadonite contents characterize the other microtextural
occurrences. White mica data for sample GFS280 define
a nearly flat trend in Fig. 5b, with paragonitic contents
significantly lower than those of micaschists C3 and C15.
Ti contents (Fig. 5c) increase with decreasing celadonite
contents and are, at intermediate celadonite values,
higher than those of white micas from micaschists C3
and C15. Again, the biotite composition is uniform
(Fig. 6).
Micaschist C6 (staurolite þ biotite zone) also shows
white mica characterized by different microtextural features: (1) rare and tiny earlier flakes within porphyroblasts; (2) sheaves aligned along the main foliation;
(3) later flakes cross-cutting the main foliation and/or
replacing staurolite and (with biotite) garnet. In spite of
the different microtextural occurrences, EMP analyses
reveal only the presence of celadonite-poor white micas
in this metamorphic zone (Fig. 5). Celadonite contents
are comparable with those of white micas growing along
the main S2 foliation of the lower-grade zone. As shown
in Fig. 5b, however, the paragonite content of white
micas in sample C6 (XNa up to 030) is higher than that
of white micas in micaschists C3 and C15 (XNa up to
025). In this metamorphic zone, the growth of staurolite
coexisting with biotite and idioblastic garnet and the
absence of primary chlorite suggest that white mica was
involved in the discontinuous reaction of the KFMASH
system (Hoschek, 1969): chlorite þ K-white mica ¼
staurolite þ biotite þ quartz þ H2O (R4). Moreover,
the decrease of the grossular content from core to rim of
porphyroblastic garnets and the occurrence of only
celadonite-poor white micas suggest that reactions
(R3) and (R2) were still active. Biotite microtextural
variations are comparable with those of white mica.
Biotite locally occurs as very tiny flakes within white
mica sheaves (Fig. 3d). Lower XFe compositions at
nearly constant AlIV characterize some biotite flakes
enclosed in staurolite.
White mica and biotite in samples from the sillimanite þ K-feldspar zone (RS2, RS5, GFS294) mostly grow
along the S2 foliation (Fig. 3e and f ). Minor flakes crosscut the main foliation or replace feldspars and sillimanite
(white mica) or garnet (white mica þ biotite). EMP analyses
reveal that white micas are celadonite- and paragonitepoor (510% paragonite) but contain more Ti (40025 up
to 007 a.p.f.u.; Fig. 5c) than lower-grade samples. It
should be noted that chemical data are much more
homogeneous for sample GFS294 than for RS2BA or
RS5B (Fig. 5). Slight but systematic variations in Ti
contents were observed within single grains of RS5A
white mica, with TiO2 ranging from 08 to 11 wt %
from core to rim (Table 1). The sillimanite þ K-feldspar
zone is marked by the disappearance of prograde white
mica and by the generation of stromatic migmatites.
White mica was therefore involved in the dehydration
1019
1020
------0.251
0.001
0.07
3.062
0.013
2.867
0.06
------0.042
0.001
0.163
3.048
0.018
2.871
0.051
------0.053
------0.125
Si
Al
Na
------6.928
------6.938
F
Cl
Total
0.72
-------
0.772
-------
K
0.757
------6.877
------------6.876
-------
0.767
------0.060
------0.232
2.375
0.095
3.331
0.016
n.d.
94.65
0.745
------6.884
-------
------6.838
-------
0.703
0.000
0.056
------0.298
0.001
0.294
0.001
0.082
2.256
0.116
3.394
0.015
n.d.
95.10
n.d.
8.39
b.d.
0.44
b.d.
3.05
29.14
2.12
51.65
0.31
2.195
0.143
3.408
0.015
n.d.
95.55
n.d.
8.87
0.02
0.65
0.02
3.00
28.29
2.61
51.78
0.31
WM5
in albite
WM6
------6.851
-------
0.713
0.001
0.070
0.001
0.270
2.280
0.118
3.380
0.018
n.d.
94.69
n.d.
8.45
0.02
0.55
0.03
2.75
29.26
2.14
51.12
0.37
in albite
Bt1
------15.439
-------
1.587
0.012
0.055
------2.431
3.385
2.289
5.483
0.197
n.d.
95.66
n.d.
8.34
0.08
0.19
b.d.
10.93
19.25
18.35
36.76
1.76
in albite
Bt2
------15.531
-------
1.721
0.013
0.068
0.010
2.331
3.500
2.288
5.511
0.089
n.d.
96.61
n.d.
0.24
9.11
10.55
0.08
0.11
20.05
18.47
37.20
0.80
along S2
Bt3
------15.538
-------
1.686
------0.042
0.016
2.220
3.432
2.547
5.427
0.168
n.d.
94.29
n.d.
8.62
b.d.
0.14
0.12
9.71
18.99
19.86
35.39
1.46
along S2
Bt4
------15.381
1.506
0.330
0.020
0.084
0.014
2.379
3.505
2.214
5.501
0.158
b.d.
98.46
8.14
0.72
0.13
0.30
0.11
10.99
20.49
18.24
37.90
1.44
along S2
Grt1
Grt2
33.27
0.82
34.01
0.92
7.999
-------
-------
-------
-------
0.393
0.062
0.298
2.022
2.264
-------
2.960
n.d.
100.78
n.d.
b.d.
b.d.
7.996
-------
-------
-------
-------
0.443
0.055
0.271
2.012
2.218
-------
2.997
n.d.
100.56
n.d.
b.d.
b.d.
2.28
5.19
b.d.
21.41
b.d.
21.55
2.51
4.60
37.59
rim
37.19
rim
Pl1
4.986
-------
-------
0.160
0.817
0.005
-------
-------
-------
1.192
-------
100.37
2.812
n.d.
n.d.
9.61
0.08
b.d.
3.41
b.d.
b.d.
b.d.
23.08
64.19
rim
NUMBER 5
Ca
Mg
Mn
Fe
2.341
0.095
3.346
0.016
n.d.
94.97
n.d.
9.08
b.d.
0.47
WM4
along S2
VOLUME 45
Ti
Total
n.d.
95.53
n.d.
94.83
n.d.
Cl
n.d.
n.d.
F
8.99
0.01
0.55
b.d.
2.35
0.01
2.56
50.29
0.33
30.42
1.71
K2O
8.62
0.02
1.28
0.06
0.98
WM3 rim
relic in S2
30.11
1.72
9.14
Na2O
CaO
MgO
MnO
FeO(total)
b.d.
0.44
36.79
0.93
Al2O3
b.d.
0.54
37.1
1.10
46.02
0.37
SiO2
TiO2
46.70
0.27
along S2
50.70
0.32
WM3 core
relic in S2
WM2
along S2
WM1
C3 (garnet zone)
Table 1: Representative electron microprobe analyses of white mica (WM), biotite (Bt), garnet (Grt) and plagioclase (Pl)
JOURNAL OF PETROLOGY
MAY 2004
b.d.
2.72
b.d.
0.41
0.02
2.83
0.01
0.03
0.92
9.70
0.26
9.08
8.58
1021
------0.271
------0.053
2.199
0.18
0.001
0.284
0.001
0.066
2.732
0.082
0.001
0.092
------0.143
2.797
0.065
------0.070
------0.142
Na
K
------7.022
------6.941
Cl
Total
0.853
0.054
0.759
-------
F
Ca
Mg
Mn
Fe
0.818
------6.943
------------6.862
-------
0.731
2.227
0.171
3.395
0.014
Al
3.385
0.009
3.100
0.019
3.089
0.019
Ti
Total
Si
n.d.
94.17
n.d.
b.d.
94.84
b.d.
94.28
b.d.
0.51
9.52
28.29
3.07
50.82
0.28
n.d.
95.81
Cl
n.d.
K2O
F
b.d.
1.10
b.d.
1.12
Na2O
CaO
MgO
MnO
FeO(total)
b.d.
0.72
27.72
3.19
34.64
1.47
36.20
1.18
Al2O3
50.27
0.19
46.33
0.39
47.12
0.39
TiO2
SiO2
WM5
Bt1
------7.094
0.854
0.038
0.001
0.055
0.001
0.176
2.65
0.119
3.221
0.017
Bt2
------15.497
-------
-------
1.711
0.006
0.018
0.016
2.208
3.478
2.433
5.447
0.18
n.d.
94.97
n.d.
0.04
0.06
8.84
0.12
9.76
19.46
19.19
35.92
1.58
along S2
15.533
-------
1.647
0.007
0.056
0.007
2.235
3.344
2.602
5.445
0.190
n.d.
95.85
n.d.
0.17
b.d.
94.22
0.19
8.55
0.05
0.05
9.92
18.78
20.60
36.04
1.67
in albite
0.42
9.95
0.02
1.75
0.02
31.72
2.12
47.88
0.34
along S2
------7.162
0.959
0.138
------0.038
0.003
0.11
2.506
0.195
3.177
0.036
b.d.
95.42
11.08
0.64
b.d.
0.29
0.06
1.09
31.32
3.43
46.79
0.72
along S2
7.103
0.036
0.001
0.038
0.991
0.122
0.003
0.206
0.004
0.033
2.518
93.33
3.151
0.16
0.01
0.29
11.20
0.04
0.07
1.18
30.78
3.55
45.41
0.64
along S2
WM2
WM1
WM4
along S2
in albite
WM3
along S2
WM2
along S2
WM1
GFS280 (garnet zone)
C15 (garnet zone)
WM3
------6.961
-------
0.900
0.002
0.022
0.005
0.218
2.112
0.263
3.428
0.011
n.d.
95.48
n.d.
0.03
0.17
10.49
0.10
2.18
26.64
4.68
50.96
0.22
relic in S2
WM4
------6.996
-------
0.934
------0.024
0.004
0.151
2.33
0.237
3.295
0.021
n.d.
94.8
n.d.
10.79
b.d.
0.18
0.08
1.5
29.12
4.18
48.53
0.41
relic in S2
WM4
------7.107
0.953
0.088
------0.03
0.002
0.119
2.497
0.196
3.189
0.033
b.d.
95.13
11.01
0.41
b.d.
0.23
0.04
1.17
31.2
3.46
46.96
0.65
relic in S2
Bt1
15.377
0.223
0.005
0.053
1.613
1.052
0.019
3.420
0.035
0.254
3.459
95.26
5.472
0.45
0.02
0.11
0.18
8.05
0.26
4.49
18.68
26.04
34.83
2.15
along S2
Bt2
------15.398
-------
1.669
0.012
0.029
0.049
1.049
3.318
3.491
5.464
0.317
n.d.
96.04
n.d.
0.07
0.10
8.39
0.37
4.51
18.05
26.77
35.03
2.70
across S2
DI VINCENZO et al.
METAMORPHIC EVOLUTION AND ARGON AGES
b.d.
0.42
b.d.
1.86
0.02
0.33
0.02
1.86
7.50
1022
3.078
0.019
2.858
0.049
------0.040
------0.233
3.063
0.017
2.878
0.045
0.001
0.033
0.001
0.241
Si
Al
Na
0.061
-------
------6.338
0.639
-------
------6.918
K
F
Cl
Total
Bt1
------6.926
-------
0.658
0.001
0.213
0.001
0.040
2.909
0.052
3.036
0.016
n.d.
96.31
n.d.
------6.925
-------
0.659
0.004
0.203
------0.079
2.833
0.072
3.057
0.018
n.d.
95.15
n.d.
0.07
1.59
7.86
0.02
0.04
0.81
36.57
1.32
1.70
7.96
0.02
0.42
38.09
0.97
46.51
0.38
Bt2
------15.358
-------
-------
1.486
0.013
0.047
0.004
2.085
3.831
2.397
5.383
0.112
n.d.
96.25
n.d.
15.604
-------
1.782
0.025
0.030
0.010
1.954
3.551
2.735
5.376
0.141
n.d.
96.52
n.d.
0.17
7.87
9.45
0.08
8.66
0.15
0.10
9.23
21.96
19.34
0.03
19.91
36.37
1.01
near Grt
21.62
0.08
35.52
1.24
along S2
Bt3
15.643
-------
0.093
0.118
1.617
0.003
2.501
3.518
2.240
5.387
0.166
b.d.
97.34
8.62
0.20
b.d.
0.41
11.41
18.22
0.03
20.30
36.65
1.50
in St
Bt4
15.711
-------
1.624
0.25
0.006
0.029
0.009
2.049
3.548
2.607
5.436
0.153
b.d.
96.07
0.52
0.10
8.42
9.09
0.04
20.62
0.07
19.91
35.95
1.35
along S2
Grt1
8.000
-------
-------
-------
-------
0.202
0.055
0.335
2.044
2.425
2.938
0.005
n.d.
100.99
n.d.
b.d.
b.d.
0.81
2.37
36.38
2.82
21.75
36.86
0.09
inner rim
Pl1
5.033
-------
-------
0.039
0.118
0.870
-------
-------
-------
1.154
-------
2.852
n.d.
101.03
n.d.
10.30
0.07
b.d.
2.52
b.d.
b.d.
b.d.
22.46
65.42
along S2
------7.008
0.932
0.011
------0.052
0.002
0.072
2.751
0.076
3.079
0.033
b.d.
94.42
10.85
0.06
b.d.
0.40
0.04
0.72
34.64
1.36
45.69
0.66
along S2
WM2 rim
------6.954
-------
0.874
0.001
0.044
0.002
0.072
2.763
0.078
3.069
0.051
n.d.
95.45
n.d.
0.34
10.32
0.02
0.04
0.73
35.32
1.41
46.23
1.04
across S2
WM2 core
------6.964
-------
0.86
0.003
0.060
------0.075
2.799
0.074
3.051
0.042
n.d.
94.56
n.d.
0.46
10.07
0.05
b.d.
0.75
35.49
1.33
45.58
0.83
across S2
WM2 rim
------6.934
-------
0.832
0.001
0.043
------0.090
0.083
0.058
2.778
97.3
3.049
n.d.
n.d.
0.02
0.34
10.05
b.d.
0.93
36.29
1.54
46.94
1.19
across S2
NUMBER 5
Ca
Mg
Mn
WM4
in St
VOLUME 45
Fe
Ti
Total
n.d.
96.27
b.d.
95.42
Cl
n.d.
b.d.
F
K2O
Na2O
CaO
MgO
MnO
FeO(total)
7.44
37.56
0.91
36.57
0.81
Al2O3
46.84
0.34
47.68
0.40
45.86
0.34
SiO2
TiO2
along S2
WM1
WM3
along S2
WM2
along S2
WM1
RS5A (sillimanite þ K-feldspar zone)
C6 staurolite þ biotite zone)
Table 1: continued
JOURNAL OF PETROLOGY
MAY 2004
b.d.
0.38
7.86
0.06
0.10
9.61
0.36
8.22
n.d.
n.d.
95.66
5.273
0.408
3.337
2.792
0.049
1.808
0.010
0.029
1.892
------------15.598
0.10
96.63
5.335
0.294
3.369
2.794
0.046
1.879
------0.007
1.929
0.140
0.026
15.819
Cl
1023
Si
Al
K
Cl
15.719
0.128
0.019
------1.888
-------
1.806
2.748
0.046
0.32
3.439
96.88
5.325
0.27
0.07
b.d.
9.72
19.16
21.58
------7.057
0.895
0.081
------0.06
------0.093
2.712
0.075
3.097
0.044
b.d.
95.19
10.49
0.39
b.d.
0.47
b.d.
0.94
34.39
1.35
46.29
0.87
------7.007
-------
0.93
0.001
0.055
------0.114
2.646
0.104
3.112
0.045
n.d.
94.79
n.d.
15.684
0.144
0.005
------1.897
1.902
0.005
2.615
0.032
0.312
3.343
94.83
5.429
0.30
0.02
0.03
b.d.
9.62
0.43
10.84
0.25
8.25
18.34
20.22
35.11
2.69
1.13
0.02
0.01
33.36
1.86
46.24
0.90
along S2
------15.540
-------
1.789
0.005
0.052
0.036
1.990
3.382
2.603
5.400
0.283
n.d.
94.76
n.d.
9.12
0.03
0.18
0.28
8.68
18.66
20.24
35.12
2.45
across S2
Bt2
------7.001
-------
0.935
------0.053
------0.077
2.717
0.091
3.082
0.046
b.d.
95.45
b.d.
10.98
b.d.
0.41
b.d.
0.78
34.54
1.64
46.18
0.92
along S2
Ms1
------7.093
0.937
0.077
0.003
0.065
0.003
0.082
2.706
0.093
3.077
0.05
b.d.
94.29
0.36
0.50
10.81
0.06
0.81
0.04
33.8
1.64
45.29
0.98
across S2
Ms2
15.871
0.325
0.007
0.073
1.790
1.598
0.009
2.962
0.049
0.349
3.340
96.6
5.369
0.67
0.03
0.25
9.1
0.05
0.37
6.95
18.38
22.97
34.82
3.01
along S2
Bt1
GFS294 (sillimanite þ K-feldspar zone)
------15.526
-------
1.874
0.002
0.032
0.048
1.508
3.337
2.973
5.375
0.377
n.d.
97.96
n.d.
0.11
9.71
0.01
0.38
6.69
18.72
23.50
35.53
3.31
across S2
Bt2
Cations per formula unit are based on 11 oxygens for white mica, 22 for biotite, 12 for garnet and eight for plagioclase. b.d., below detection limit; n.d., not determined.
Total
F
Na
Ca
Mg
Mn
Fe
Ti
Total
F
9.87
0.29
b.d.
0.02
0.36
7.95
18.34
21.63
18.64
21.79
34.97
2.80
34.16
3.52
34.79
2.55
K2O
Na2O
CaO
MgO
MnO
FeO(total)
Al2O3
TiO2
SiO2
along S2
Bt1
across S2
across S2
along S2
Ms2
Ms1
Bt3
Bt2
Bt1
along S2
RS2BA (sillimanite þ K-feldspar zone)
RS5A (sillimanite þ K-feldspar zone)
DI VINCENZO et al.
METAMORPHIC EVOLUTION AND ARGON AGES
JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 5
MAY 2004
AlIV (a.p.f.u.)
3.0
(a)
2.8
2.6
2.4
2.2
(a)
0.5
C15
RS2A
C3
RS5B
GFS280
GFS294
C6
0.6
0.7
0.8
0.6
0.7
0.8
Ti (a.p.f.u.)
0.5
0.4
0.3
0.2
0.1
(b)
0
(b)
0.5
Fe/(Fe+Mg)
Fig. 6. Compositional variations in biotite. Symbols in (b) are the same
as in (a).
(c)
Fig. 5. Compositional variations in white mica. Symbols in (a) and (c)
are the same as in (b).
reaction of the KFMASH system (Le Breton & Thompson,
1988): K-white mica þ albite þ quartz ¼ K-feldspar þ
sillimanite þ melt. This reaction accounts for both the
disappearance of white mica during the prograde evolution and the growth of muscovite during cooling. Biotite
compositions in all samples are nearly homogeneous
(Fig. 6).
Pressure and temperature estimates and P---T path
Different calibrations of the garnet---biotite (Fe---Mg
exchange) thermometer were used to constrain the synD1 and the syn-D2 thermal conditions of the garnet zone
(Table 2). Temperatures were evaluated using different
garnet---biotite pairs enclosed in albite porphyroblasts,
with the assumption of equilibrium between garnet and
biotite inclusions, or growing along the main S2 foliation.
The calibrations based on the experimental dataset (i.e.
Hodges & Spear, 1982; Perchuck & Lavrent’eva, 1983)
gave comparable results, whereas calibrations (i.e.
Ganguly & Saxena, 1984; Kleeman & Reinhardt, 1994)
based on a thermodynamic dataset yielded slightly lower
and higher values than those obtained using the former
(Table 2). Results yielded temperatures of 497---544 C for
the syn-D1 couples and of 521---560 C for the syn-D2
phases, and suggest a slight increment of temperature
from the D1 to D2 phase. Temperature estimations are
remarkably consistent with previous determinations on
the same metamorphic zone by Franceschelli et al. (1989).
A minimum pressure of 10 GPa can be deduced for
the D1 stage based on the celadonite content in white
mica relics (Massonne & Schreyer, 1987). The GFS280
orthogneiss, which belongs to the same metamorphic
zone and contains a limiting assemblage consisting of
phengite, K-feldspar, biotite and quartz, yielded a pressure of 11 GPa for the rare syn-D1 relics. Pressures of
07---09 GPa were calculated for the syn-D2 peak using
different calibrations of the garnet---biotite---muscovite--oligoclase barometer (Table 2). The staurolite þ biotite
zone (Table 2) is characterized by peak temperatures of
588---624 C (garnet rim, matrix biotite) and pressures of
06---09 GPa (garnet rim, matrix biotite, muscovite,
oligoclase).
1024
DI VINCENZO et al.
METAMORPHIC EVOLUTION AND ARGON AGES
Table 2: P and T estimates for samples from the garnet and staurolite þ biotite zones
Sample
Temperature ( C)
Growth stages
Garnet---biotite
n
P*
HS
GS
PL
KR
Syn-D1 within albite
4
497 16
524 18
544 12
4
1.0
0.6
513 20
Syn-D2 along S2
545 16
521 13
540 9
560 9
C6
Syn-D2 along S2
5
0.7
624 15
588 18
604 3
597 17
Sample
Growth stages
C3
Pressure (GPa)
GMBP
n
Syn-D1 within albite
C3
Syn-D2 along S2
GFS280
Syn-D1 relic
C6
Syn-D2 along S2
Ph
T*
GSt Fe
GSt Mg
HC
H Fe
H Mg
MS
0.74 0.06
0.71 0.06
0.84 0.07
0.90 0.08
0.91 0.08
1.0
0.3---0.5
0.64 0.09
0.63 0.10
0.75 0.09
0.84 0.11
0.86 0.12
0.4
500
3
550
1.1
500
4
600
Reported values represent the average (standard deviation) of values by using the same calibration but different couples (T )
or assemblages (P). n, number of couples or assemblages used for calculation. Pressure estimates based on the phengite
content are given as minimum values. Temperatures ( C) were calculated using the Fe---Mg exchange between garnet and
biotite. Calibrations: HS, Hodges & Spear (1982); GS, Ganguly & Saxena (1984); PL, Perchuck & Lavrent’eva (1983); KR,
Kleeman & Reinhardt (1984). Pressures were calculated by the garnet, muscovite, biotite and plagioclase barometer (GMBP)
or on the basis of phengite content of white mica (Ph, Massonne & Schreyer, 1987). Calibrations: GSt Fe---GSt Mg, Ghent &
Stout (1981), Fe and Mg end-members; HC, Hodges & Crowley (1985); H Fe---H Mg, Hoisch (1990), Fe and Mg endmembers; MS, Massonne & Schreyer (1987). P* and T* are the nominal P and T used for calculations. The precision of
thermometers varies from 15 C (PL and KR) to 50 C (HS).
40
40
4+
Si
3.5
D1
Sil+Kfs
zone
3.4
0.8
4+
Si
3.3
Grt
zone
4+
Si
Chl+
Ms
St+B
t+Q
tz+H
2O
D2
St+Bt
zone
Ky+Bt
zone
D2
Ky
Sil
D2
D2
4+ 3.1
Si
3.2
0.4
Ky
And
Ar---39Ar dating
0.0
39
Ar--- Ar investigations concentrated on white mica
and, when resolvable, on biotite. The IR laser was mainly
used to analyse biotite, whereas the UV laser was used to
obtain most of the data on white mica because of the
possible poor absorption of IR radiation in white mica.
Unfortunately, biotite was not identified within all the
analysed rock chips of samples from the lower-grade
zones. Biotite in these samples is a minor phase commonly interlayered with chlorite. In situ IR spot-fusion
data and UV laser-ablation analyses are listed in Table 3
Si
Ms
+A
b+Q
Kfs
tz
+A
ls+
me
lt
4+
1.2
P(GPa)
Pressure and temperature estimates, along with petrographical constraints, were used to reconstruct the P---T
paths of the studied rock samples from the garnet and
staurolite þ biotite zones (Fig. 7). The reconstructed
trajectories in Fig. 7 are also compared with those derived
for the higher-grade zones by Franceschelli et al. (1989).
After the D1 deformation, the path of the lower-grade
samples describes a stage with rising temperatures and
decreasing pressures, followed by a stage with falling
temperatures and pressures. Samples from the staurolite
zone to sillimanite zone record only the retrogressive path
after the thermal climax.
400
2
1
500
Sil
And
600
700
800
T(°C)
Fig. 7. P---T paths for some metamorphic zones of the Variscan basement of NE Sardinia. P---T paths for the garnet zone and staurolite þ
biotite zone were reconstructed using the P---T estimates reported in
Table 2; P---T paths for the remaining metamorphic zones are from
Franceschelli et al. (1989). Reaction 1 is from Hoschek (1969); reaction 2
is from Le Breton & Thompson (1988). Isopleths of Si4þ for white mica
Si contents (atoms per formula unit) are from Massone & Schreyer
(1987). The shaded areas indicate the results of thermobarometric
calculations for the garnet zone (horizontal lines) and staurolite þ biotite
zone (vertical lines).
1025
Laser
UV
5
1026
UV
UV
UV
UV
30
31
32
33
27
UV
UV
26
UV
UV
25
28
UV
24
WM in Pl core
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM(Bt---Chl)
WM in Pl rim
WM in Pl rim
WM in Pl rim
WM in Pl rim
WM
WM
------0.00010
-------
0.00866
------0.00186
0.00712
0.00039
0.00020
0.00229
0.00525
0.00088
0.00041
------0.00217
------0.00006
0.00635
0.00354
-------
0.00028
0.00032
0.00098
------0.00020
-------
-------
------0.00292
0.00004
0.00019
0.00010
0.00022
0.00073
------0.00044
-------
------0.00009
-------
------0.00066
0.00031
0.00004
0.00046
0.00026
------0.00210
0.00334
-------
0.00288
0.00050
0.00050
0.00009
0.00073
-------
0.00471
0.00145
0.00054
0.00009
-------
0.00416
0.00548
0.00079
0.00045
------0.00003
-------
------0.00007
------0.00035
------------0.00224
0.00026
0.00054
-------
-------
0.00040
0.00060
0.00056
0.00041
-------
0.00609
0.00245
0.00016
0.00005
0.00021
0.00007
Ar(Cl)
38
Ar(K)
Ar(Tot)
0.2221
0.2756
0.4156
5.836
6.649
10.106
5.548
11.528
6.770
0.2321
6.806
7.269
7.533
8.846
1.842
2.636
7.682
5.670
10.122
2.222
6.423
6.848
6.917
8.762
8.952
13.738
10.524
8.394
8.981
8.340
4.887
5.880
4.182
5.033
40
0.4889
0.2847
0.2826
0.2964
0.3148
0.3698
0.07374
0.1128
0.3076
0.2253
0.4137
0.08932
0.2661
0.2784
0.2674
0.3673
0.3628
0.5653
0.4360
0.3421
0.3758
0.3516
0.1972
0.2436
0.1682
0.2022
39
336.3
322.1
325.3
318.3
318.7
320.2
321.2
330.3
3 .0
4 .4
3 .0
2 .2
4 .2
2 .6
1 .9
2 .4
2 .6
7 .9
2 .3
292.5
317.9
320.7
11
3 .3
4 .4
13
3 .6
2 .6
3 .9
4 .5
3 .8
6 .0
2 .8
4 .6
5 .1
5 .0
5 .7
5 .0
6 .9
4 .9
5 .9
2s
332
330.4
330.5
332
325.2
315.1
325.7
335.3
318.3
330.0
324.5
322.6
324.5
317.5
316.6
326.7
322.9
331.4
334.3
Age
95.0
98.7
99.0
99.7
99.0
99.0
98.6
99.8
98.1
99.1
98.5
91.8
98.1
97.4
98.4
98.8
96.3
98.0
96.2
98.6
99.1
98.8
98.8
97.9
98.1
98.5
97.6
99.0
98.8
99.7
Ar* (%)
40
-------
0.04
0.016
------0.02
------0.005
0.017
0.005
------0.012
------0.05
0.02
0.021
0.03
0.029
0.04
0.016
0.027
------0.007
-------
-------
0.038
------0.009
0.08
0.07
0.02
Ca/K
0.06
0.05
0.049
0.045
0.024
0.028
0.022
0.09
0.00048
0.06
0.10
0.043
0.21
-------
0.034
0.046
------0.00058
-------
------0.00014
-------
0.00008
------0.00098
-------
------0.00027
-------
-------
-------
-------
------0.00006
------0.00017
------0.00061
-------
-------
------0.00016
-------
0.00078
0.00023
Cl/K
0.022
0.043
0.041
0.043
0.06
0.07
0.06
2s
0.00011
0.00013
0.00015
0.00007
0.00012
0.00014
0.00017
0.00010
0.00018
0.00019
2s
NUMBER 5
29
UV
UV
23
UV
UV
19
UV
UV
18
20
UV
17
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
Ar(Ca)
37
VOLUME 45
21
UV
16
UV
UV
12
13
UV
UV
10
11
UV
UV
8
9
UV
UV
4
UV
UV
3
6
UV
2
7
UV
1
Ar(atm)
36
Ar---39Ar in situ analyses (argon data 1015 moles)
Description
40
Sample C3, J ¼ 0.008204 0.000025
No.
Table 3:
JOURNAL OF PETROLOGY
MAY 2004
Laser
IR
UV
UV
UV
UV
UV
15
22
34
35
36
37
Chl---Bt
Chl---Bt
Chl---Bt
Bt---Chl
Bt---Chl(WM)
Bt(Chl)
Bt(Chl)
Description
1027
UV
49
UV
UV
IR
IR
54
55
56
57
UV
UV
48
53
UV
47
UV
UV
46
52
UV
45
UV
UV
44
UV
UV
43
50
UV
42
51
UV
UV
41
UV
39
40
UV
38
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM in Pl
WM in Pl
WM in Pl
WM
WM
WM
WM
WM
WM
WM
Sample C15, J ¼ 0.008191 0.000025
IR
14
Total gas age
No.
-------
-------------
-------
-------
------0.00931
0.00015
0.00014
0.00015
0.00108
0.00114
------0.00392
0.00040
0.00017
------0.01245
-------
------0.00267
0.00029
0.00014
0.00052
------0.00007
0.00080
0.00025
0.00018
0.00004
------0.00006
-------
-------
-------
------0.00972
0.00012
0.00022
------0.00001
-------
-------
-------
0.00006
0.00005
------0.00009
Ar(Cl)
38
-------
0.00462
0.00018
0.00031
0.00033
0.00032
0.00030
0.00001
0.00026
0.00039
0.00033
0.00010
0.00024
0.00038
-------
0.01209
-------
0.00001
0.00042
0.00023
------0.00472
0.00216
0.00005
0.00043
------0.00375
-------
Ar(Ca)
37
0.00067
0.00002
0.00023
0.00013
Ar(atm)
36
5.415
5.936
7.806
42.088
0.3248
1.760
4.775
3.854
8.750
6.612
6.992
7.390
0.5973
1.178
10.578
7.099
5.215
7.066
8.344
11.085
7.663
9.827
0.1479
0.2915
0.6849
1.665
0.6788
3.641
2.356
Ar(Tot)
40
0.2231
0.2385
0.1848
0.1585
0.3614
0.2723
0.2896
0.2994
0.02016
0.04700
0.4327
0.2778
0.2113
0.2910
0.3470
0.4505
0.3148
0.4115
0.00722
0.01454
0.03066
0.07546
0.03150
0.1513
0.1054
Ar(K)
39
320.4
332.5
322.5
323.9
324.6
323.1
338.6
328.3
324.6
321.4
336
339
337.8
326.0
327.1
328.2
326.4
321.9
323.2
320.2
273
252
263
291
267.2
299
323.6
319.4
Age
2.1
1.1
3.3
3.3
6.1
4.5
3.0
2.2
3.6
3.8
22
49
1.6
2.3
2.4
2.4
2.6
2.5
1.4
2.4
256
33
108
30
9.2
11
2.2
9.2
2s
Ar* (%)
99.2
99.3
99.4
98.7
99.1
98.6
97.5
------0.01
------0.01
------0.05
0.1
-------
-------
------0.02
99.6
98.6
98.7
99.3
-------
-------
------0.04
-------
------0.02
-------
0.1
-------
0.13
------0.6
------0.09
-------
Ca/K
98.3
85.1
99.5
99.2
98.9
98.1
99.1
98.4
99.3
97.1
95.2
81.6
88.2
98.9
98.1
98.3
40
0.11
0.80
1.4
0.9
0.78
0.43
0.46
0.6
1.4
0.59
0.14
2s
0.00010
0.00036
0.00101
------0.00020
------0.00271
0.00008
0.00044
0.0008
0.00039
-------
-------
-------
-------
-------
0.00022
0.00030
------0.00002
-------
-------
0.00015
0.00001
0.00007
0.00008
0.00040
0.00011
0.00010
0.0007
0.00011
0.00007
0.00006
0.00009
0.00109
0.00093
-------
0.00033
0.00030
2s
0.00054
0.00053
-------
Cl/K
DI VINCENZO et al.
METAMORPHIC EVOLUTION AND ARGON AGES
Laser
Description
1028
UV
UV
80
81
UV
UV
UV
UV
UV
85
86
87
88
89
UV
UV
79
84
UV
78
UV
UV
77
UV
UV
76
WM---Bt(Chl)
WM
WM
WM
WM
WM
WM
WM
WM---Bt(Chl)
WM
WM
WM
WM
WM
WM
------0.00038
0.00022
0.00003
------0.03451
------0.01107
-------
-------
0.00036
0.00016
0.00053
------0.00676
------0.01613
0.01514
-------
0.00011
0.00023
0.00022
0.00008
0.00046
0.00059
0.00012
0.00015
-------
------0.00001
------0.00003
0.00027
0.00017
------0.00006
0.00005
------------0.10196
0.00012
0.00041
------0.00013
-------
-------
0.00002
------0.00021
0.00053
-------
0.00015
0.00017
0.00005
0.00010
-------
------0.00033
-------
0.00033
0.00002
0.01533
0.00037
0.00011
-------------
------------0.01892
0.00036
0.00012
0.00019
0.1549
0.09271
0.2885
0.1646
0.08936
0.3254
0.1966
0.1472
0.1767
0.1961
0.2665
0.1501
0.2521
0.3048
0.5581
0.1488
0.3222
0.4810
0.5360
0.4349
0.5253
0.3803
0.4823
2.362
3.848
7.084
3.995
4.552
2.203
3.531
7.930
4.317
4.702
6.520
3.742
6.181
7.353
13.430
3.579
7.894
11.729
12.894
10.697
12.731
9.135
11.664
9.462
6.857
0.3869
0.2821
-------
0.00594
0.00386
6.031
8.556
0.2451
0.3481
------0.00009
0.00052
0.00027
8.778
5.138
------0.00593
0.3530
0.2079
-------
0.00774
0.00010
0.00049
7.334
4.914
8.735
0.3003
Ar(Tot)
40
0.2013
0.3524
Ar(K)
39
-------
-------
0.00028
0.00009
0.00000
0.00002
Ar(Cl)
38
------0.01032
0.00971
0.00021
0.00045
Ar(Ca)
37
324.0
320.3
329.0
323.8
309.2
328.8
320.8
325.8
326.5
315.8
327.5
331.1
329.5
324.2
322.4
324.1
323.9
325.8
323.4
328.0
325.9
322.9
324.6
326.9
322.9
324.0
325.5
333.5
330.8
320.6
330.6
326.4
Age
6.7
6.1
3.0
4.5
4.4
7.6
5.0
3.2
3.0
3.3
2.5
2.6
2.2
2.4
1.8
2.3
4.1
1.1
1.3
1.6
1.5
2.4
2.8
2.1
3.9
2.3
3.1
1.8
3.7
3.6
2.6
1.5
2s
94.2
95.4
99.4
98.8
98.5
99.0
99.0
99.1
99.1
97.4
99.3
98.6
99.7
99.6
98.0
99.2
99.8
99.1
99.6
98.9
99.7
99.6
99.5
99.1
98.4
97.6
98.2
99.7
99.4
99.1
97.2
99.0
Ar* (%)
40
-------
-------
0 .1
0 .2
-------
------0 .1
------1 .3
-------
-------
-------
-------
-------
-------
0.04
-------
------0.12
-------
0.07
------0.07
-------
0.02
0.03
------0.04
0.04
0.10
0.05
-------
Ca/K
0.8
1.4
1.4
0.6
0.47
0.44
0.44
0.50
0.54
0.82
0.58
0.61
0.88
0.60
2s
------0.00004
------0.00006
0.00101
0.00055
------0.00011
0.00015
------0.00046
0.00045
0.00007
------0.00095
------0.00037
-------
0.00003
------0.00031
0.00064
-------
-------
-------
------0.00016
-------
-------
-------
------0.00005
Cl/K
0.00018
0.00007
0.00018
0.00008
0.00014
0.00009
0.00009
0.00006
0.00011
0.00008
0.00005
0.00002
0.00004
0.00007
0.00009
0.00006
2s
NUMBER 5
82
UV
75
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
Ar(atm)
36
VOLUME 45
83
UV
UV
UV
72
73
UV
71
74
UV
UV
UV
68
69
70
UV
UV
65
UV
UV
64
66
UV
63
67
UV
UV
UV
60
61
UV
59
62
UV
58
Sample C15, J ¼ 0.008191 0.000025
No.
Table 3: continued
JOURNAL OF PETROLOGY
MAY 2004
UV
UV
UV
UV
90
91
92
93
WM
WM
WM
WM
Description
UV
UV
UV
UV
UV
UV
IR
IR
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
94
95
96
97
98
99
100
101
102
103
104
105
1029
106
107
108
109
110
111
112
113
114
115
116
117
118
119
WM
WM on St
WM
WM
WM
WM
WM
WM
WM
WM(Bt)
WM
WM
WM
WM
WM
WM(Bt)
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
Sample C6, J ¼ 0.008197 0.000025
Total gas age
Laser
No.
Ar(Ca)
------------0.00004
------0.00037
-------------
0.00035
0.00023
0.00045
0.00007
-------
0.00060
0.00020
------0.00115
------0.00849
------0.00268
0.00058
0.00011
-------
0.00027
0.00009
0.00039
-------
-------
------0.00037
0.00022
0.00020
0.00025
0.00018
-------
0.00019
0.00044
0.00003
-------
------0.00013
-------
-------
------0.00002
-------
-------
-------
------0.00124
-------
------0.00024
-------
0.00032
0.00030
-------
0.00014
0.00076
------------0.00018
------0.00110
0.00010
------0.00011
-------
0.00001
0.00015
-------
0.00013
0.00023
0.00003
-------
0.00514
0.00162
-------
-------
------0.00037
Ar(Cl)
38
-------
-------
-------
37
0.00012
0.00010
0.00028
0.00007
0.00044
0.00024
Ar(atm)
36
11.643
6.417
6.971
8.703
6.652
4.781
5.783
3.476
5.720
4.983
2.911
3.528
0.2985
0.3662
0.2845
0.2000
0.2443
0.1450
0.2444
0.2072
0.1277
0.1494
3.830
6.237
0.1585
0.2638
0.4991
0.2727
3.968
3.735
4.108
2.962
2.137
6.595
2.740
1.877
3.318
3.811
3.505
4.266
0.1709
0.1579
0.1739
0.1256
0.0941
0.2707
0.1167
0.0775
0.1391
0.1594
0.1479
0.1793
8.555
0.3473
8.003
Ar(Tot)
10.268
9.596
40
0.3297
0.4212
0.3919
Ar(K)
39
317.7
314.0
303.4
309.0
315.0
318.2
317.3
320.2
311.2
315.3
313.5
313.5
315.5
314.3
315.8
310.0
306.8
2 .9
318.2
312.7
4 .9
7 .8
3 .7
3 .3
2 .9
5 .6
2 .8
4 .3
3 .2
2 .0
2 .3
3 .6
4 .5
3 .4
6 .7
3 .8
5 .5
4 .1
12
11
9 .0
6 .2
7 .8
3 .7
6 .0
2 .3
1 .8
2 .2
2 .9
2 .3
2s
303
319
317.2
318.9
317.6
317.3
319.3
325.2
327.2
330.9
322.2
326.4
Age
98.0
99.4
99.4
96.4
99.1
95.0
98.2
98.4
99.1
99.7
99.9
99.0
96.5
97.9
97.4
98.1
97.7
97.0
98.1
96.6
99.8
97.5
98.8
98.2
99.0
99.3
99.7
99.1
98.3
99.3
Ar* (%)
40
------0.01
-------
------0.06
-------
------0.02
0.001
------0.002
0.001
------0.01
-------
-------
------0.01
-------
-------
-------
------0.02
-------
0.06
0.02
-------
-------
-------
-------
Ca/K
0.13
0.07
0.05
0.035
0.027
0.06
0.016
0.11
0.07
0.05
0.06
2s
-------
------0.00066
-------
-------
------0.00005
-------
-------
-------
-------
-------
-------
------0.00016
-------
-------
------0.00086
------0.0012
-------
0.00040
------0.00052
0.00012
-------
-------
------0.00055
Cl/K
0.00018
0.00011
0.00014
0.00023
0.0005
0.00027
0.00025
0.00014
0.00008
2s
DI VINCENZO et al.
METAMORPHIC EVOLUTION AND ARGON AGES
UV
UV
UV
UV
120
121
122
123
WM(Bt)
WM(Bt)
WM(Bt)
WM
Description
1030
IR
IR
IR
IR
IR
UV
UV
UV
UV
UV
UV
125
126
127
128
129
136
139
142
147
149
151
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
131
132
133
134
135
137
138
140
141
143
144
145
146
148
150
WM
WM(Bt)
WM
WM
WM
WM
WM
WM
WM
0.00186
0.00037
0.00453
0.00033
0.00023
0.00020
0.00057
0.00254
0.00018
0.00011
0.00013
0.00019
0.00016
0.00013
0.00034
0.00018
0.00005
0.00017
0.00010
0.00014
0.00058
0.00191
0.00006
0.00001
-------------
-------
------0.00017
0.00017
------0.00142
0.00194
0.00258
-------
------0.00002
------0.00016
------0.00029
0.00033
------0.00022
0.00007
------0.00021
0.00071
0.00098
0.00086
0.00023
0.00033
0.00116
0.00136
0.00141
0.00070
0.00043
0.00230
0.00660
0.00473
0.00111
------0.00639
------0.00099
-------
-------
0.00016
0.00039
0.00007
0.00017
0.00153
------0.00189
0.00015
0.00238
0.00050
0.00048
0.00013
0.00008
0.00048
0.02515
-------
-------
0.00025
0.00061
-------
-------
0.00011
0.00031
Ar(Cl)
38
0.07972
0.1475
0.2064
0.1248
0.1619
0.1910
0.1673
0.1687
0.1938
0.2501
0.1353
0.2036
0.3578
0.2477
0.03736
0.1789
0.1370
0.04989
0.1216
0.2326
0.1741
0.1506
0.1355
0.2165
0.4060
0.3136
0.2468
0.4062
0.1566
0.2106
0.1763
0.1501
Ar(K)
39
4.439
1.771
3.643
3.365
3.810
2.791
4.439
4.625
4.504
3.755
5.856
3.115
7.934
5.511
0.8872
4.039
2.889
1.054
2.630
4.954
3.701
3.395
2.839
4.432
8.506
6.776
5.180
8.584
3.661
4.854
4.160
3.555
Ar(Tot)
40
293.5
303.4
306.4
290.9
298.7
302.2
323.8
301.8
299.5
309.9
309.2
299.2
302.5
314.4
298.6
301.0
306
284.3
275
286.0
290.6
289.4
285.8
292.3
276.2
277
281.7
288.8
2.2
2.5
8.4
2.2
4.6
3.5
4.5
3.3
4.2
4.0
3.9
1.5
4.3
1.7
1.4
37
7.2
11
2.6
5.3
2.9
1.9
6.2
5.5
16
4.1
5.4
6.7
5.2
4.0
2.2
314.4
282.9
282.9
3.9
4.4
5.4
5.5
2s
307.0
310.6
317.0
309.4
Age
99.1
96.9
98.7
98.8
97.4
98.1
99.6
98.9
99.3
98.9
99.1
98.9
99.8
99.9
94.8
97.1
99.2
95.1
98.5
99.5
98.4
95.0
96.6
98.4
98.2
97.9
98.5
97.9
96.8
99.4
99.3
96.3
Ar* (%)
40
-------
0.024
0.023
------0.016
0.022
0.07
0.05
0.01
------0.058
0.019
0.003
0.014
------0.010
-------
-------
0.012
------0.03
0.05
0.03
0.003
0.001
0.014
0.004
0.115
-------
-------
-------
0.06
Ca/K
0.021
0.040
0.048
0.06
0.06
0.041
0.042
0.07
0.031
0.030
0.026
0.024
0.07
0.024
0.06
0.08
0.025
0.026
0.032
0.017
0.018
0.05
2s
-------
-------
0.00075
------0.00067
-------
------0.00006
------0.00061
------0.00134
0.00085
------0.00039
0.00120
------0.0026
0.0037
0.00265
0.00311
0.00097
0.00156
0.00339
0.00211
0.00283
0.00178
0.00067
-------
-------
0.00039
0.00132
Cl/K
0.00010
0.00017
0.00013
0.00012
0.00021
0.00007
0.00009
0.00103
0.0007
0.0006
0.00029
0.00039
0.00021
0.00065
0.00046
0.00028
0.00037
0.00025
0.00013
0.00014
0.00029
2s
NUMBER 5
WM
WM
WM
WM
WM
WM
WM
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
0.00039
0.00009
0.00593
0.00010
0.00044
-------
Ar(Ca)
37
Ar(atm)
36
VOLUME 45
Total gas age
UV
130
Total gas age
IR
124
Sample GFS280, J ¼ 0.008207 0.000025
Total gas age
Laser
No.
Table 3: continued
JOURNAL OF PETROLOGY
MAY 2004
Laser
Description
IR
IR
IR
IR
IR
UV
UV
IR
IR
IR
153
154
155
156
157
160
167
168
169
170
1031
UV
UV
UV
164
165
166
WM
WM
WM
WM
WM
WM
WM
WM
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
UV
IR
IR
IR
IR
IR
IR
IR
172
177
178
179
180
181
182
188
Total gas age
UV
171
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Sample RS5A, J ¼ 0.008186 0.000025
Total gas age
UV
163
UV
161
UV
UV
159
162
UV
158
Total gas age
IR
152
Sample RS2BA, J ¼ 0.008180 0.000025
No.
------0.00598
0.04191
0.02664
0.01863
-------
0.00193
0.00441
0.00391
0.00282
0.00144
-------
0.00249
0.00454
0.01845
0.01322
0.02062
0.01444
0.02775
0.01870
0.01957
0.00477
0.00419
-------
0.01644
0.30316
0.00012
-------
0.00005
0.00005
0.00027
0.00000
-------
-------
------0.02830
------0.03682
-------
0.00038
0.00015
0.00193
0.00229
------0.00211
0.00128
0.00065
0.00048
0.00040
0.00060
0.00060
0.00080
0.00100
0.00054
0.00047
0.00929
0.00475
0.00314
0.00029
0.00239
-------
------0.00831
0.00388
0.00352
-------
-------
0.00280
0.00627
0.00241
0.00079
------0.03142
0.00523
0.00643
0.00245
0.00233
0.00210
Ar(Cl)
38
0.00249
0.00116
------0.05321
Ar(Ca)
37
0.00139
0.00787
Ar(atm)
36
1.063
0.9595
1.143
1.010
1.739
1.025
1.131
0.3156
0.1435
0.04613
0.09158
0.3526
0.2305
0.4051
0.4608
0.09844
0.1839
0.6829
1.040
0.8739
0.4079
0.09762
1.168
0.8300
0.8389
1.046
0.7771
0.5327
Ar(K)
39
24.403
21.153
25.297
21.729
37.818
24.223
21.972
305.8
287.1
284.2
291.2
285.5
286.2
310.1
251.2
282.5
280.6
2.2
308.4
6.912
3.141
13
314
1.2
1.7
1.8
1.1
1.9
1.0
1.6
2.7
3.4
6.2
24
302
3.3
1.6
2.5
5.4
2.3
3.8
1.7
7.1
1.8
1.6
13
2.1
0.94
1.4
2.2
1.5
1.5
3.0
2s
2.248
308.9
310.0
302.9
312.6
307.9
306.6
257.2
285.2
296.2
295.9
265
258.8
305.21
289.2
280.6
282.4
304.7
238.1
Age
5.459
1.169
9.290
10.954
8.226
2.399
4.303
15.541
24.080
19.969
8.404
2.124
27.148
19.493
18.806
23.580
17.888
11.513
Ar(Tot)
40
Ar* (%)
98.2
94.5
96.7
97.3
96.5
96.9
93.9
94.5
93.8
87.9
94.7
97.8
96.7
97.4
97.3
93.3
96.7
95.3
82.3
89.0
94.1
91.5
96.9
90.5
91.8
91.9
97.7
79.8
40
-------
0.03
0.04
0.05
------0.01
-------
0.10
3.9
-------
-------
-------
------0.1
------0.17
-------
------0.01
------0.01
-------
-------
-------
------0.06
------0.19
Ca/K
0.19
0.06
0.05
0.09
0.33
0.6
0.8
0.24
0.29
0.07
0.14
0.36
2s
0.0109
0.0087
0.0114
0.0090
0.0101
0.0115
0.0109
0.0095
0.0184
-------
0.00015
0.0017
------0.00009
0.00041
0.00242
0.00052
0.00212
0.00145
0.00139
0.00179
0.0018
0.00190
0.00189
0.00175
0.00233
0.00170
0.00290
Cl/K
0.0010
0.0008
0.0010
0.0008
0.0008
0.0010
0.0009
0.0010
0.0023
0.0010
0.00007
0.00010
0.00005
0.00049
0.00012
0.00022
0.00013
0.00015
0.00020
0.0005
0.00017
0.00018
0.00017
0.00021
0.00016
0.00027
2s
DI VINCENZO et al.
METAMORPHIC EVOLUTION AND ARGON AGES
UV
UV
UV
UV
UV
UV
IR
IR
IR
IR
173
174
175
176
183
184
185
186
187
189
WM interm.
WM interm.
WM rim
WM rim
WM rim
WM rim
WM core
WM core
WM core
WM rim
Description
IR
IR
IR
IR
IR
IR
IR
IR
IR
IR
IR
IR
IR
IR
IR
IR
IR
IR
IR
IR
IR
IR
190
191
192
193
1032
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Bt
-------
-------------
0.00002
0.00011
0.00313
0.00691
------0.00263
0.00023
0.00417
0.00271
0.00026
0.00216
------------0.00459
-------
0.00099
0.00120
0.00025
0.00100
0.00080
0.00054
0.00051
0.00093
0.00081
0.00023
0.00033
0.00038
0.00080
0.00128
-------
0.00059
------0.00456
0.00362
0.00098
0.00106
0.00082
0.00088
0.00332
0.00310
0.00130
0.00199
0.00248
0.00142
0.00254
0.00236
0.00330
0.00213
0.00233
0.00343
0.00130
0.00253
0.00284
0.00167
0.00116
0.00133
0.00049
0.00101
0.00035
0.00017
0.00088
0.00118
0.00159
------0.00196
------0.00337
0.00217
0.00006
0.00013
-------
0.02275
0.19182
-------
-------
0.00070
0.00294
0.00015
0.00010
0.6306
0.9172
0.4869
0.5018
0.6421
0.3814
0.5437
0.5823
0.5644
0.5056
0.6172
0.6486
0.3686
0.5643
0.5729
0.5139
0.2847
0.4688
0.1690
0.3375
0.3232
0.1179
0.09982
0.3715
0.4115
1.053
0.5266
0.3687
0.2806
0.3262
0.3742
0.3219
Ar(K)
39
14.366
20.523
11.067
11.305
14.352
8.543
12.403
12.905
12.225
11.641
14.061
14.832
8.373
12.719
13.370
11.783
6.409
10.489
3.944
7.788
7.374
2.671
2.309
8.610
9.371
24.301
12.277
8.563
6.598
7.706
8.670
7.476
Ar(Tot)
40
304.5
298.9
306.0
303.3
299.2
302.2
306.2
295.5
289.9
308.3
307.7
304.6
298.5
298.3
309.8
303.4
295.2
297.2
308.4
311.0
309.0
303.4
312.3
308.2
312.2
302.1
301.9
310.7
308.9
316.0
317.9
307.6
309.7
Age
1.5
3.0
2.7
3.5
2.7
3.1
2.2
3.0
3.0
4.1
2.4
2.5
3.6
3.5
4.1
4.1
3.2
5.5
6.1
11.9
10.9
3.5
2.0
8.4
2.5
3.0
1.6
2.1
3.7
4.8
4.6
3.5
3.5
2s
Ar* (%)
98.3
98.1
99.1
99.0
98.3
99.2
98.8
97.8
98.0
98.6
99.4
98.0
96.5
97.2
97.8
97.3
96.2
97.5
97.3
99.3
99.7
98.5
99.6
99.7
97.8
96.4
98.4
98.2
99.4
99.6
98.0
98.5
40
-------
-------
------0.017
-------
0.006
0.009
0.001
0.001
0.015
------0.008
0.023
0.013
0.016
------0.015
0.007
0.02
0.003
------0.05
-------
-------
0.34
0.1
0.1
------0.11
------0.03
-------
Ca/K
0.023
0.024
0.022
0.013
0.012
0.009
0.016
0.026
0.015
0.014
0.015
0.031
0.031
0.07
0.05
0.6
0.25
0.12
0.7
0.18
2s
0.00332
0.00213
0.00168
0.00250
0.00244
0.00234
0.00294
0.00255
0.00369
0.00266
0.00238
0.00333
0.00222
0.00282
0.00312
0.00204
0.00295
0.00080
0.0044
0.00217
0.00310
0.0047
-------
-------
------0.00117
-------
-------
0.00035
0.00020
0.00117
0.00029
Cl/K
0.00019
0.00024
0.00035
0.00034
0.00019
0.00027
0.00024
0.00032
0.00029
0.00033
0.00035
0.00032
0.00029
0.00027
0.00028
0.00015
0.00029
0.00029
0.00043
0.00046
0.0007
0.0007
0.00011
0.00014
0.00014
0.00017
0.00011
2s
NUMBER 5
Bt
Bt
Bt
Bt
Bt
Ar(Cl)
0.00070
0.00015
38
------0.03167
0.02798
------0.00398
-------
Ar(Ca)
37
0.00064
0.00052
0.00012
0.00009
0.00058
0.00038
Ar(atm)
36
VOLUME 45
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Sample GFS294, J ¼ 0.008206 0.000025
Total gas age
Laser
No.
Table 3: continued
JOURNAL OF PETROLOGY
MAY 2004
IR
IR
IR
IR
IR
UV
UV
212
213
214
215
216
217
229
1033
UV
UV
UV
UV
UV
UV
UV
UV
UV
UV
219
220
221
222
223
224
225
226
227
228
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
WM
Bt
Bt
Bt
Bt
Bt
Bt
Bt
Description
0.00392
0.00059
0.00148
0.00026
0.00027
0.00006
0.00026
0.00031
0.00020
0.00085
0.00077
0.00073
0.00059
0.00050
0.00032
0.00050
0.00061
0.00284
0.00053
0.00088
0.00080
-------
-------
------0.00534
-------
0.00190
-------
------0.00057
0.00489
0.00437
-------
0.00016
------0.00385
------0.00038
-------
-------------
-------
0.00115
0.00379
0.00199
0.00157
0.00621
0.00057
0.00247
Ar(Cl)
38
0.00433
------0.00580
-------
-------
Ar(Ca)
37
Ar(atm)
36
0.2679
0.4141
0.3917
0.3524
0.4217
0.4001
0.5201
0.6747
0.7098
0.5177
0.6154
0.2244
0.8638
0.3358
0.5486
1.0737
0.4070
0.5900
Ar(K)
39
Ar(Tot)
6.042
9.381
8.803
8.002
9.444
9.098
11.768
15.591
16.123
11.702
14.063
7.665
5.119
23.820
19.123
13.066
12.673
8.919
40
302.8
304.9
305.2
304.7
301.6
301.2
305.5
306.8
2.3
2.5
3.2
2.2
2.7
3.4
3.3
4.0
3.2
3.4
3.8
307.1
309.0
304.6
4.0
2.0
4.9
304.3
1.3
3.8
3.8
1.7
2.3
4.0
2s
301.5
302.7
299.0
303.0
309.3
296.6
290.3
296.2
Age
98.7
99.1
99.8
99.0
99.0
99.3
97.8
98.5
98.6
98.5
98.9
98.1
99.2
97.6
98.6
98.1
97.1
98.2
Ar* (%)
40
------0.04
-------
0.008
0.017
0.023
------0.018
-------
-------
0.016
------0.05
-------
-------
0.013
0.013
0.002
Ca/K
0.12
0.037
0.020
0.026
0.026
0.014
0.06
0.016
0.018
0.012
2s
Error on single ages does not include the uncertainty in the J value. Error on the total gas ages also includes the uncertainty in the J value.
Total gas age
UV
218
Total gas age
Laser
No.
-------
-------
-------
-------
------0.00103
0.00025
------0.00034
-------
-------
0.00373
0.00324
0.00180
0.00364
0.00276
0.00089
0.00263
Cl/K
0.00016
0.00012
0.00010
0.00047
0.00027
0.00043
0.00026
0.00032
0.00012
0.00029
2s
DI VINCENZO et al.
METAMORPHIC EVOLUTION AND ARGON AGES
JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 5
MAY 2004
30
number
50
C3
C15
40
total gas age:
325.5±1.8
total gas age:
323.6±2.2
20
30
20
10
10
number
15
20
10
5
10
GFS280
C6
total gas age:
303.4±2.2
total gas age:
314.4±2.2
8
number
6
6
4
RS2BA
2
RS5A
4
total gas age:
308.4±2.2
total gas age:
308.2±2.0
2
number
340
330
320
310
290
8
300
0
10
Age (Ma±2σ)
6
GFS294
4
total gas age:
304.9±2.3
2
340
330
320
310
300
290
0
Age (Ma±2σ)
Fig. 8. Frequency diagram of
40
39
Ar--- Ar in situ laser ages from white mica. Errors are 2s and include the uncertainty in the J value.
and presented in the frequency diagrams of Figs 8 and 9
(white mica and biotite, respectively). In addition, data
for selected areas are reported in Fig. 10 to illustrate the
intra-sample distribution of argon in analysed micaschists. Table 3 also lists the Ca/K and Cl/K ratios
from neutron-produced 39ArK, 38ArCl and 37ArCa.
Because of the low Cl and Ca contents of micas and
the small lasered sample volume, 38ArCl and 37ArCa
intensities were in most instances close to the detection
limits. Ca/K and Cl/K ratios derived from argon
isotopes are therefore affected by large uncertainties.
40
Ar---39Ar apparent ages for white mica of sample C15
(garnet zone) range widely, mainly from 320 Ma to
338 Ma (Fig. 8). The oldest ages were detected in
syn-D1 white mica within an albite porphyroblast and
in microlithons (Fig. 10a). Analyses along the main S2
1034
DI VINCENZO et al.
RS2BA
total gas age:
285.2±1.7
total gas age:
284.3±2.6
5
5
RS5A
8
GFS294
24
total gas age:
287.1±1.7
6
18
4
12
total gas age:
301.5±2.0
320
280
240
320
300
280
260
240
300
6
2
260
number
10
GFS280
10
number
METAMORPHIC EVOLUTION AND ARGON AGES
Age (Ma±2σ)
Age (Ma±2σ)
Fig. 9. Frequency diagram of 40Ar---39Ar in situ laser ages from biotite. Errors are 2s and include the uncertainty in the J value.
315.8±3.3
326.4±2.3
Grt
327.2±2.2
Grt
330.9±2.3
320.3±6.1
Pl
WM
324.0±6.7
330.8±3.7
324.0±2.3
322.9±3.9
322.9±2.4
320.6±2.6
324.6±3.0
324.6±2.8
333.5±1.8
323.9±1.9
323.1±2.2
325.7±3.9
Pl
325.9±1.1
323.4±1.3
336±22
315.1±2.6
318.3±2.2
318.7±4.2
322.9±6.9
(b) C3
(d) C6
315.5±2.3
315.3±3.6
313.5±3.4
307.0±3.9
303±12
WM
309.0±5.6
Qtz
315.8±4.5
316.6±5.7
331.4±4.9
334.3±5.9
326.7±5.0
330.3±2.6
321.2±2.6
252±33
263±108
(a) C15
322.1±4.4
320.7±2.4
BtChl
WM
317.5±5.0
330.5±4.4
317.9±2.3
292.5±7.9
324.5±2.8 267.2±9.2
328.3±3.8
321.4±3.6
Bt-Chl
325.3±3.0
330.4±3.3
339±49
337.8±2.3
325.9±1.5
328.0±1.6
324.6±3.3
323.9±4.5
332±12
335.3±4.5
325.2±3.6
330.0±6.0
320.2±1.9
273±256
332.5±3.3
338.6±6.1
(c) C3
318.3±3.8
322.4±2.3
330.6±1.5
Pl
Grt
319.4±9.2
323.2±1.4
WM
311.2±2.8
322.6±4.6
324.4±5.1
317.3±3.7
317.6±7.8
St
St
Grt
306.8±6.7
313.5±3.2
312.7±5.5
310.0±4.1
320.2±2.0
314.0±3.3
317.3±2.9
Bt-Chl
Grt
291±30
BtChl
Pl
318.2±4.3
318.2±2.9
317.2±9.0
314.3±3.8
318.9±6.2
319.3±6.0
319±11
303.4±4.9
St
Fig. 10. Back-scattered electron photomicrographs of some areas in the micaschists investigated by argon laserprobe. Errors are 2s and do not
include the uncertainty in the J value. Mineral symbols according to Kretz (1983), and: WM, white mica.
1035
JOURNAL OF PETROLOGY
VOLUME 45
foliation show a nearly continuous variation from 3202 24 Ma to 3335 18 Ma (Fig. 10 and Table 3). White
micas in sample C3 display a comparable continuous age
variation, but with the lower limit shifted toward slightly
younger apparent ages (3151 26 to 3363 30 Ma).
Ages younger than 315 Ma come from areas where white
mica is contaminated by biotite---chlorite interlayering.
Younger ages, not yet well-defined, were also detected
for biotite---chlorite interlayering along late shear bands
(Fig. 10b). Two IR spot-analyses on biotite yielded ages
of 3194 92 Ma and 299 11 Ma. The oldest white
mica ages were detected in syn-D1 white mica enclosed in
albite porphyroblasts (Fig. 10b and Table 3) or the inner
portions of mica sheaves (Fig. 10b and c). In contrast,
orthogneiss GFS280, collected within the same metamorphic zone but not far from the contact with an
300 Ma pluton, yielded significantly younger apparent
ages mainly ranging from 290 to 310 Ma (Fig. 8). It is
worth noting that in this sample most of the white mica
ages cluster at 300 Ma. One analysis carried out within a
glomeroblastic association yielded an older age (3238 42 Ma). Biotite apparent ages range from 2762 41 Ma
to 2923 55 Ma (Fig. 9), with a weak positive correlation
between age and radiogenic argon content (Table 3).
The white mica in sample C6 (staurolite þ biotite zone)
yielded argon ages mainly ranging from 310 to 320 Ma
(Fig. 8). Figure 10 shows that a millimetre-sized syn-D2
white mica flake exhibits a homogeneous intragrain
argon distribution; the weighted average of 10 UV analyses from a single grain yielded an age of 3174 14 Ma
and an MSWD of 043. The homogeneous spatial pattern at the single-grain scale argues against the importance of argon loss by solid-state diffusion. Younger ages
( 300---310 Ma) were detected where white mica is
replacing staurolite or, intergrown with biotite, garnet
(Fig. 10d).
Samples RS2BA and RS5A (sillimanite þ K-feldspar
zone) yielded comparable white mica ages of 300 to
320 Ma (Fig. 8). In sample RS2BA there is no clear
core---rim relationship within single flakes, whereas in
RS5A the youngest ages were observed along the rims
and the oldest in the inner portions (Table 3). Biotite
ages in both samples range widely from 240 Ma to
305---310 Ma (Fig. 9). Ages younger than 290 Ma are
the least radiogenic (Table 3) and come from areas where
biotite exhibits pronounced parting along the basal
cleavage and/or appears variably chloritized. Sample
GFS294 yields a narrow range of white mica apparent
ages (300---310 Ma; Fig. 8). Eleven 40Ar---39Ar analyses
yield a weighted mean of 3046 16 Ma and an
MSWD of 23, which attests to only a slight excess of
scatter. The age interval of the GFS294 biotite
( 290---310 Ma) is narrower than that of RS2BA and
RS5A biotite (Fig. 9). Most biotite ages overlap with
those of white mica.
NUMBER 5
MAY 2004
DISCUSSION
Tectono-metamorphic evolution and
microstructural---microchemical variations
in white mica
It is widely acknowledged that white micas from metamorphic rocks of greenschist to lower amphibolite facies
may not always be in equilibrium with each other or with
other constituent minerals of the rocks. White mica may
preserve chemical zoning, and/or single rock samples
may contain diachronous generations of white mica
with distinct chemical compositions. The preservation
of multiple generations of white mica is well documented
within mafic and felsic single rock samples of high-pressure (HP) and ultra-high-pressure (UHP) metamorphic
units. During recent decades, numerous studies have
shown that phengites, relics of the HP or UHP stages,
may survive subsequent retrogression at upper-crustal
levels at temperatures of 450---650 C (e.g. Heinrich,
1982; Saliot & Velde, 1982; Klemd et al., 1991; Nowlan
et al., 2000). Other studies have similarly shown that premetamorphic igneous relics (Frey et al., 1976) and detrital
features (Dietrich, 1983) in white micas survived metamorphism at 450---550 C. HP and UHP metamorphic
rocks are peculiar systems, which experienced very
extreme physical conditions characterized by limited
mass transfer and exceedingly sluggish reaction kinetics.
However, multiple generations of white micas have also
been preserved in metamorphic rocks that experienced
less extreme conditions. In a detailed investigation of
white micas in the metasediments of the Dalradian
block of the SE Scottish Highlands, Dempster (1992)
reported a large compositional range in micas within
single samples. Compositional variations were strongly
controlled by the structural age of white mica. The
studied micas, much like our samples, showed a marked
reduction in chemical variability starting from the staurolite zone. Dempster (1992) concluded that diffusioncontrolled exchange reactions in the studied white
micas had a relatively high ‘closure temperature’ (higher
than 500 C) below which the exchange of structureforming major elements was negligible; most of the
chemical variations preserved in white mica may reflect
equilibria controlled by deformation recrystallization.
Microtextural and microchemical data from our study
indicate that samples from the garnet zone contain diachronous white mica generations that refer to different
P---T---deformation stages of tectono-metamorphic evolution. The preservation of different compositional features
in white micas with distinct microstructural ages indicates
disequilibrium and suggests that the later syn-D2 generation of white micas developed under physical conditions
that did not allow major-element exchange reactions.
The preservation of high-celadonite and low-paragonite
compositions, and the lack of significant chemical zoning
1036
DI VINCENZO et al.
METAMORPHIC EVOLUTION AND ARGON AGES
in white mica relics enclosed in syn-D2 mica sheaves
suggest that chemical exchange between white micas,
even those in direct contact, was very limited. Estimated
P---T conditions suggest a slight rise in temperature and a
decrease in pressure from the D1 to D2 deformational
events. This P---T evolution, typical of thermal relaxation
after a homogeneous thickening stage (England &
Thompson, 1984), agrees with the following petrological
constraints: (1) both syn-D1 garnets (within albite porphyroblasts) and syn-D2 garnets in micaschists display
growth zoning; (2) the grossular content in syn-D2 garnets
decreases from core to rim (Carosi & Palmeri, 2002) and
plagioclase compositions change from albite in the porphyroblasts to oligoclase in the syn-D2 matrix (i.e. the
grossular/anorthite ratio decreases; Spear et al., 1991).
Besides the absolute rise in temperature from the D1 to D2
deformational phases, it is important to note that rock
samples from the garnet zone, at least starting from
the baric peak, evolved entirely in the stability field of
almandine-rich garnet coexisting with chlorite and biotite. In summary, petrological data suggest that the early
formed white micas (syn-D1) escaped re-equilibration
during the thermal peak (syn-D2). This implies that the
effective critical temperature for the celadonite and paragonite exchange in studied white micas exceeded 500 C,
in agreement with estimates for other areas with comparable P---T regimes. The exchange of major elements,
however, does not simply depend on a fixed temperature,
but probably results from the complex interplay between
extrinsic (e.g. strain, circulation of fluids, thermal history)
and intrinsic (e.g. rock and mineral composition, grain
size) variables, which may vary from sample to sample.
In this respect it is important to note that orthogneiss
GFS280, from the garnet zone, preserves only rare white
mica relics compatible with the medium-pressure D1
event. White mica in this sample, in contrast to that in
micaschists, mainly re-equilibrated at upper-crustal
levels. Additionally, lower paragonite and higher Ti contents differentiate the main low-Si population observed in
this sample from the white micas along the main S2
foliation in micaschists C3 and C15. These findings suggest a lithological control (i.e. quartz–feldspathic vs pelitic
compositions) or, taking into account the diffuse alteration of this sample, the intervention of additional processes that gave rise to a somewhat different T---fluid
regime after the D2 deformational phase. The closeness
of the orthogneiss to a 300 Ma granite may be an important factor.
In the staurolite þ biotite zone, microtextural features
such as the increase in quartz inclusions from the core to
rim of garnet and the growth zoning of almandine
(increase in pyrope and decrease in both spessartine and
grossular from core to rim; Carosi & Palmeri, 2002) suggest that syn-D2 garnet grew during a rise in temperature
and decrease in pressure (Crawford, 1977; Tracy, 1982).
The presence of staurolite coexisting with biotite suggests
temperatures above 550 C at 08 GPa (Fig. 7), and the
preservation of zoning in garnet indicates a peak temperature below 650 C (Tracy, 1982). This temperature
interval agrees with our estimates of 590---625 C.
Under these metamorphic conditions white micas do
not retain any chemical record of the earlier D1 stage.
The effective critical temperature for cation exchange
was therefore overstepped in this metamorphic zone.
Based on the above arguments, the critical temperature
for the exchange of structure-forming major elements in
the studied white mica was 4500 C to 5570 C,
assuming that the remaining extrinsic variables (strain
and fluid activity) were nearly constant from the garnet
zone to the staurolite þ biotite zone.
Samples from the sillimanite þ K-feldspar zone were
affected by higher temperatures (T up to 750 C; Palmeri,
1993) at pressures of 06---07 GPa, under which anatexis
processes developed. The high-grade conditions were
sufficient to totally recrystallize earlier white micas.
White micas from this metamorphic zone are chemically
distinguishable from those of the lower-grade zone by
their rather uniformly low paragonite and celadonite
compositions, and high Ti contents irrespective of the
rock type.
Time constraints from geological and
previous geochronological data
Geological and stratigraphical data indicate that the
Variscan continental collision in Sardinia was preceded
by a passive margin stage (Carmignani et al., 1994). The
maximum age for the beginning of the collisional stage
can be constrained by the upper age of the sediments
deposited onto the passive margin and then deformed by
the Variscan tectono-metamorphic events. According to
Carmignani et al. (1994), the presence of Upper Devonian
and Lower Carboniferous (Tournaisian) sediments in the
External Nappes of southern Sardinia dates the collisional stage to 345 Ma. However, taking into account
that only the Upper Devonian (Middle Famennian) is
palaeontologically documented in the marbles of the
Internal Nappes (Pili & Saba, 1975; G. Bagnoli, personal
communication, 2003), the Variscan continental collision
and concomitant Barrovian metamorphism cannot have
commenced in north---central Sardinia earlier than
360 Ma. Furthermore, taking into account the time
required to change from a passive margin setting to a
collisional one and the theoretical time lag ( 20 Myr)
before elevation of a thickened crust commences
(Thompson et al., 1997), the above geological constraint
agrees with the actinolite 40Ar---39Ar age of 345 4 Ma
(Del Moro et al., 1991) from a metagabbro and with the
Rb---Sr whole-rock age of 344 7 Ma (Ferrara et al., 1978)
obtained from a banded migmatite with trondhjemitic
1037
JOURNAL OF PETROLOGY
VOLUME 45
leucosomes. In interpreting the meaning of the latter
345 Ma age, one should take into account that the
composition of the analysed migmatite indicates that it
underwent metamorphic differentiation (Carmignani
et al., 1992). This would imply that the metamorphic
stage recorded by this sample preceded the melting reaction involving the breakdown of white mica. Alternatively, assuming that trondhjemitic leucosomes formed
through water-fluxed melting, one should take into
account that they represent the earliest appearance of
melt during a prograde metamorphism of a thickening
orogenicbelt(Pati~
noDouce&Harris,1998).The 345 Ma
age could therefore be close to the collisional stage or
represent the beginning of exhumation. This interpretation agrees with the conclusion of Pin & Paquette (2002),
who inferred that the Early Carboniferous was the oldest
limit for the beginning of the Variscan continental
collision in the Massif Central (France). In summary,
geological constraints and available geochronological
data suggest the following temporal evolution for the
study area: (1) the maximum age for the beginning of
the collisional stage in the Internal Nappes is 5360 Ma
( 345 Ma, age of the thickening stage or beginning of
exhumation in the High-Grade Metamorphic Complex);
(2) the second intrusive cycle of late- and post-tectonic
granitoids took place at 310---290 Ma and plutonism
ended at 280 Ma; (3) the closure ages of muscovite
and biotite from intrusive rocks are 305---290 Ma and
295---275 Ma, respectively. These data can be further
used to temporally constrain the development of deformational events: both the D1 and D2 phases took place
later than 360 Ma but earlier than the emplacement of
the widespread late-tectonic plutonism, which postdates
the D2 structures, i.e. earlier than 310 Ma.
Interpretation of
40
Ar---39Ar data
The key issue involved in the interpretation of 40Ar---39Ar
mineral data is whether apparent ages (1) reflect crystallization ages or (2) reflect cooling below the Tc, or (3) are
meaningless as a result of incorporation of excess argon.
Regarding points (1) and (2), there is still no general
consensus on the argon-retentive properties of white mica
and, more in general, on the effective role of thermal
activated solid-state diffusion or recrystallization processes
in controlling argon transport. Studies on eclogite-facies
metamorphic rocks have shown that celadonite-rich
white micas, relics of the HP stage, may yield
40
Ar---39Ar ages that overlap with those obtained by
more retentive dating systems (e.g. Schm€adicke et al.,
1995; Di Vincenzo et al., 2001; Jahn et al., 2001; compilation by Glodny et al., 2002). This evidence has been
indifferently used to either affirm high argon-retentive
properties of white micas [following hypothesis (1)] or
infer a very rapid cooling rate [following hypothesis (2)].
NUMBER 5
MAY 2004
Although evidence of argon transport controlled by
volume diffusion processes, namely, argon isotope gradients independent of crystal-chemical variations and compatible with Fickian behaviour, has been inferred for
white micas from various areas (e.g. Hodges et al., 1994;
Reddy et al., 1996), other studies have highlighted a close
link between the mobility of radiogenic argon and that of
structure-forming major elements. Temperature was not
considered the main factor controlling the rate of argon
transport in these natural samples; argon mobility was
instead attributed to mineral reactivity (Chopin &
Maluski, 1980; Hammerschimdt & Frank, 1991; Hames
& Cheney, 1997; Di Vincenzo et al., 2001). Most importantly, those studies documented incomplete age resetting
for white micas, which experienced metamorphic
overprinting at temperatures of 4425---450 C and
5600 C; that is, above the commonly quoted Tc
(350---400 C).
Bulk theoretical Tc for a given grain size, mineral
geometry and cooling rate may be derived using
Dodson’s formula (Dodson, 1973), provided that the
appropriate diffusion parameters are known. However,
experimental diffusion data on white micas are scarce
and the possible compositional effect, inferred from
crystal-chemical data (Dahl 1996), has not yet been
experimentally investigated. It is important to recall that
the closure temperature concept assumes that: (1) migration of an isotope (solute atom) occurs by thermally activated volume diffusion through a homogeneous crystal
structure (i.e. isotope transport obeys Fick’s second law);
(2) diffusing isotopes are efficiently removed at the grain
boundary (i.e. the concentration of that isotope at the
grain boundary is always zero). Complications in the
application of the closure temperature concept to natural
samples may arise from intrinsic variables such as mineralogical complexities, even at the scale of a single grain
(i.e. coexistence of diachronous multiple generations),
which produce a temporally and spatially inhomogeneous crystal structure. Additionally, externally controlled variables may also represent potential pitfalls,
namely, complicated thermal histories deviating from
linear cooling, heterogeneous repartitioning of strain,
and fluid-absent or fluid-present conditions, which may
all influence intrinsic variables. Interpretation of
40
Ar---39Ar mineral data should therefore be sustained
by detailed microscale investigations of structure, texture
and chemistry.
40
Ar---39Ar laser ages of white micas from this study
show large inter-sample and intra-sample variations that
correlate with metamorphic grade and microtextural and
microchemical features. In particular, micaschists from
the garnet zone, which preserve relict syn-D1 celadoniterich white micas, retain the oldest 40Ar---39Ar ages. Ages
as old as 335---340 Ma fall within the age interval defined
by different dating techniques and are compatible with
1038
no. of analyses
8
no. of analyses
no. of analyses
6
no. of analyses
DI VINCENZO et al.
METAMORPHIC EVOLUTION AND ARGON AGES
8
C3
7
6
Grt zone
5
4
3
2
1
0
2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45
C15
5
4
Grt zone
3
2
1
0
2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45
GFS280
8
6
C6
6
Grt zone
4
4
St+Bt zone
2
2
0
2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45
0
2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45
5
15
12
3
2
RS5A
18
RS2BA
4
9
Sil+Kfs zone
6
Sil+Kfs zone
1
3
0
2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45
0
2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45
White mica - Si (a.p.f.u.)
7
GFS294
6
5
4
3
2
Sil+Kfs zone
1
0
2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45
White mica - Si (a.p.f.u.)
Fig. 11. Histograms of Si (a.p.f.u.) in white mica from samples of the different metamorphic zones.
independent geological constraints. This argues against
the importance of excess argon in the studied rocks.
Furthermore, ages of 335---340 Ma are geologically
compatible with the time of maximum thickening in the
Internal Nappes and may closely approximate the white
mica crystallization age during the D1 phase. Conversely,
only ages 320 Ma were detected in samples that lack
syn-D1 white mica relics. It is important to note that the
age clustering at 315---320 Ma for most syn-D2 white mica
in micaschists from the garnet zone and staurolite zone is
older than the emplacement age of calc-alkaline granitoids and the closure ages of both muscovite and biotite in
these granitoids (Fig. 2). This finding, along with
the observation that syn-D1 white mica began to retain
radiogenic argon much earlier than 320 Ma and the
inter-sample consistency of 40Ar---39Ar ages, suggests
that the 315---320 Ma interval most probably represents
the end of the chemical re-equilibration (including neocrystallization) of white mica at upper-crustal levels
during the D2 phase. A thorough comparison of EMP
analyses with argon data for all the investigated samples
reveals that 40Ar---39Ar laser ages are remarkably consistent with microtextural and microchemical data:
(1) micaschists C3 and C15 from the garnet zone, which
preserve distinct, early-formed, high-celadonite populations (Fig. 11), also retain old ages; (2) when compared with C15, sample C3 shows the mode of the
low-celadonite population at lower Si contents (Fig. 11)
and yields younger ages (Fig. 8); (3) white micas in orthogneiss GFS280 (garnet zone) contain only rare phengite
relics and mainly yield argon ages younger than 320 Ma;
(4) in the sillimanite þ K-feldspar zone, white micas from
1039
VOLUME 45
samples RS2BA and RS5A display a larger chemical
variability and wider age interval than sample GFS294
(Figs 8 and 11). There are some intra-sample inconsistencies between chemical data and argon ages derived
from samples in the garnet zone. The bimodal distribution of celadonite content in micaschists C15 and C3
(Fig. 11) is not reflected in the age frequency, which
conversely exhibits a roughly single mode with a pronounced spread at 320---330 Ma (Fig. 8). Orthogneiss
GFS280 preserved rare celadonite-rich relics but did not
retain ages as old as 335---340 Ma. Such discrepancies
may arise from the significant difference between the
spatial resolution of EMP data and that of 40Ar---39Ar
laser analyses ( 2 105 mm2 and a few micrometres
depth vs 36 103 mm2 and several tens of micrometres depth). Complex small-scale mineralogical heterogeneities, such as white micas aligned along the S2
foliation with heterogeneous celadonite content or biotite--chlorite interlayers, may have been averaged by the
argon laserprobe.
Argon data from biotite also yielded a large time span
(420 Ma), which is not paralleled by significant compositional variations. In all investigated samples, biotite
ages are generally younger than those of white mica from
the same sample. This observation agrees with the widely
acknowledged lower retentive properties of biotite with
respect to those of coexisting white mica, sustained both
by experimental data and on theoretical grounds (Dahl,
1996). However, as stressed by Dahl (1996), this pattern
may arise irrespective of isotope loss mechanism (diffusion, recrystallization, alteration, etc.). Although it is difficult to draw straightforward conclusions because of the
lack of biotite argon data along the whole north---south
transect, biotite ages seem to be very sensitive to secondary alteration processes. This interpretation is supported
by the observation that younger ages mainly come from
areas where biotite is visibly chloritized or characterized
by pronounced parting along the basal cleavage, and by
the negative correlation between apparent ages and
atmospheric argon contents (Fig. 12). This finding agrees
with the outcome of other studies (e.g. Roberts et al.,
2001; Di Vincenzo et al., 2003), which demonstrated
that even incipiently altered biotite may yield younger
apparent ages with correspondingly higher atmospheric
argon contents.
The most important finding of this study is that microtexturally and microchemically earlier generations of
white micas in micaschists of the garnet zone preserve
older ages that can be referred to the D1 deformational
event. This is surprising, as the whole tectonometamorphic evolution, starting from the baric peak,
developed at temperatures under which white mica is
generally considered open to argon isotope exchange.
This means that both texturally and chemically resolvable syn-D1 and syn-D2 white micas should have recorded
NUMBER 5
MAY 2004
315
300
Age (Ma)
JOURNAL OF PETROLOGY
GFS280
RS5B
RS2BA
GFS294
285
270
255
240
0
5
10
40Ar
(atm)
15
20
%
Fig. 12. Argon ages vs atmospheric argon content (%) of biotite data.
comparable ages. It is important to note that old ages
were detected not only in white mica enclosed in albite
porphyroblasts, which could be considered armoured
relics, but also within the mica sheaves aligned along
the S2 foliation where petrographic investigation revealed
the presence of celadonite-rich relics.
This is not the first report of white mica that experienced metamorphic overprinting and escaped argon
isotope resetting even when reheating exceeded the
commonly quoted Tc. Previous studies, however, dealt
with rock samples that experienced very extreme physical
conditions (i.e. HP or UHP conditions, Chopin &
Maluski, 1980; Giorgis et al., 2000; Di Vincenzo et al.,
2001), or metamorphic overprinting under lower-grade
conditions (425---450 C, Wijbrans & McDougall, 1986;
425---525 C, Hames & Cheney, 1997), and/or were
based on bulk sample analyses (e.g. Wijbrans &
McDougall, 1986; Andriessen, 1991; Hammerschmidt
& Frank, 1991). Possible explanations for the survival of
argon isotope relics in early-formed white mica include:
(1) episodic thermal overprinting of short duration
(Wijbrans & McDougall, 1986); (2) the grain boundary
network was not free of argon (i.e. argon concentration at
the grain boundary was 40) because of external buffering (fluid flow advecting argon in solution; Wheeler,
1996) or internal buffering (limited mass transfer and
sluggish reaction kinetics owing to extreme physical conditions; Foland, 1979; Kelley & Wartho, 2000); (3) generally limited argon mobility in the absence of chemical
re-equilibration processes, i.e. mobility of radiogenic
argon was strictly coupled with that of structure-forming
major elements, with the implication that petrographic
relics ensure isotopic inheritance (Villa, 1998). Whereas
short-lived thermal overprinting is clearly not compatible
with the tectono-metamorphic evolution of the studied
rocks [hypothesis (1)], the latter two hypotheses should
be carefully addressed. However, a straightforward
1040
DI VINCENZO et al.
METAMORPHIC EVOLUTION AND ARGON AGES
discussion of these issues is complicated by the lack of
detailed information on argon partitioning between fluids
and K-rich minerals and, more in general, on the behaviour of Ar in the crust. The role of additional and
concomitant variables, such as composition and pressure,
which may have enhanced argon retention (Dahl, 1996)
in the studied white micas, appears of secondary importance. Celadonite-rich syn-D1 white mica and celadonitepoor syn-D2 white mica represent a disequilibrium
sub-assemblage; texturally and chemically resolvable
white micas grew at different times, and argon ages conform to white mica structural ages. We stress that the study
area is a natural setting in which petrographic and argon
isotope relics are preserved in rock samples that did not
experience very extreme physical conditions, but metamorphism of intermediate thermal gradient (Barroviantype), evolved under lower amphibolite-facies conditions
and experienced a protracted tectono-metamorphic evolution. As such, results from the present study may have a
broader significance. We conclude that: (1) mineral reactivity (recrystallization) was the most important factor
controlling the rate of argon transport in the studied
white micas; (2) the assumption that, in the absence of
recrystallization, white mica in the studied metamorphic
rocks retained radiogenic argon only below the commonly quoted Tc, would have led to a significant underestimation of the actual argon-retentive properties of
white mica. Argon retention in syn-D1 white mica
from the garnet zone was high enough to survive the
syn-D2 thermal climax, which occurred at a temperature
above 500 C.
ACKNOWLEDGEMENTS
We are grateful to G. Giorgetti (Dipartimento di Scienze
della Terra, Siena) and F. Olmi (IGG-CNR, Firenze) for
support during SEM investigation and EMP analyses,
respectively. The journal reviews of T. Dempster,
K. Hammerschmidt and I. M. Villa, and the editorial
handling of K. Bucher are acknowledged. The 40Ar---39Ar
laserprobe facility was funded by ‘Programma Nazionale
di Ricerche in Antartide’ (PNRA). Research was financially supported by ‘Consiglio Nazionale delle Ricerche’
(CNR).
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