evidence of magmatic co 2 -rich fluids in peraluminous graphite-bearing leucogranites from deep...

Contrib Mineral Petrol (1994) 117:111-123 �9 Springer-Verlag 1994

Maria Luce Frezzotti - Gianfranco Di Vincenzo Claudio Ghezzo - Ernst A.J. Burke

Evidence of magmatic C02-rich fluids in peraluminous graphite-bearing leucogranites from Deep Freeze Range (northern Victoria Land, Antarctica)

Received: 27 October 1993 / Accepted: l l March 1994

Abstract Fine-grained peraluminous synkinematic leuco-monzogranites (SKG), of Cambro-Ordovician age, occur as veins and sills (up to 20 30 m thick) in the Deep Freeze Range, within the medium to high-grade metamorphics of the Wilson Terrane. Secondary fibrolite + graphite intergrowths occur in feldspars and subordinately in quartz. Four main solid and fluid inclusion populations are observed: primary mixed CO2 + H20 inclusions + A12SiO5 _+ brines in garnet (type 1) ; early CO2-rich inclusions (_+ brines) in quartz (type 2) ; early C O 2 + C H 4 (up to 4 mol%) _+ H 2 0 inclusions + graphite + fibrolite in quartz (type 3); late C H 4 + C O 2 + N 2 inclusions and H20 inclusions in quartz (type 4). Densities of type 1 inclusions are consistent with the crystallization conditions of SKG (~ 750~ and 3 kbar). The other types are post-magmatic: densities of type 2 and 3 inclusions suggest isobaric cooling at high temperature (~ 700 -550~ Type 4 inclusions were trapped below 500~ The SKG crystallized from a magma that was at some stage vapour-saturated; fluids were CO2-rich, possibly with immiscible brines. CO2- rich fluids (__ brines) characterize the transition from magmatic to post-magmatic stages; progressive isobaric cooling (T< 670~ led to a continuous decrease offo~, entering in the graphite stability field; at the same time, the feldspars reacted with CQ-rich fluids to give secondary fibrolite + graphite. Decrease of Tandfo2 can explain the progressive variation in the fluid composition from CO2-rich to C H 4 and water dominated in a

Contribution within the network "Hydrothermal/metamorphic water-rock interactions in crystalline rocks: a multidisciplinary approach on paleofluid analysis". CEC program: Human Capital and Mobility

M.L. Frezzotti ([El) . G. Di Vincenzo �9 C. Ghezzo Dipartimento di Scienze della Terra, Via delle Cerchia, 3, 1-53100 Siena, Italy

E.A.J. Burke Instituut voor Aardwetenschappen, Vrije Universiteit, De Boelelaan 1085, NL 1081HV Amsterdam, The Netherlands

Editorial responsibility: J.L.R. Touret

closed system (in situ evolution). The presence of N2 the late stages indicates interaction with external metamorphic fluids.

Introduction

Several experimental studies have shown the fundamen- tal control of magma (silicate melt + fluids) composition, temperature, pressure and fo2 on the origin and crystallization of intrusive rocks (i.e. Andersen and Lindsley 1988; Whitney 1988; Ebadi and Johannes 1991). In petrologic investigations on granitic rocks, there has been a tendency to consider pure H20 as the dominant component in the fluid phase (e.g. Tuttle and Bowen 1958; Burnham and Ohmoto 1980; Whitney 1984), mainly because water activities can be calculated from compositions of primary mineral phases (i.e. presence of amphibole, phyllosilicates, etc.). There is increas- ing evidence, however, mainly from fluid and melt inclusion studies, that CO 2 may be a major constituent of magmatic fluids, especially in mid- to lower- crustal intrusives (e.g. Konnerup-Madsen 1977; Frost and Touret 1989; Wilmart et al. 1991). Non or weakly polar components, such as CO2, may not interact with minerals, but can eventually remain trapped as inclusions during the growth of primary minerals. Thus, fluid inclusion studies may give information about the composition of nat- urally occurring magmatic fluids, and about the crystallization dynamics. Since magmatic volatiles may be parental to magmatic-hydrothermal fluids responsible for metasomatic processes and eventually ore deposi- tion, fluid inclusion characterization is also useful to trace the cooling history of granitic rocks.

The present paper reports a fluid and mineral inclusion study in synkinematic mid-crustal peraluminous leucogranites of the Deep Freeze Range, northern Vic- toria Land, Antarctica. These high-silica granites are of special interest, as we have evidence that CO2-rich fluids were exsolved during the crystallization of the peraluminous magma. These fluids play an important role in the

112

, F2- 2 I "

6 ~ 7[~---~ 8 ~ 9 . ~ / 10./~

Fig. 1 General geological map after Biagini et al. (1991) of the Wilson Terrane showing location of investigated samples (black dots). 1 Mc Murdo Volcanics; 2 Ferrar Supergroup and Beacon Supergroup; Granite l larbour Intrusives: 3 Shoshonitic suite, 4 High-K calcalkaline suite, 5 Postkinematic peraluminous granites, 6 Mafic intrusives, 7 Synkinematic peraluminous granites; 8 Wilson Terrane metamorphic complex; 9 Main faults; I0 Main shear zones

genesis of the secondary sillimanite-graphite association observed in these rocks.

Geological setting The investigated synkinematic peraluminous leucogranites (SKG) outcrop in the Wilson Terrane, in the northern Victoria Land, Antarctica. The Wilson Terrane (WT - Fig. 1) represents the nearest structural unit to the East Antarctic Craton in northern Victoria Land (Stump et al. 1983). Palmcri et al. (1991) recognized a composite internal structure within the WT, identifying a cover sequence, which experienced a polyphase prograde low-P, high-T metamorphism (ranging from low-greenschist to high-grade am- phibolite facies with anatexis) during the Ross Orogeny, and an older polymetamorphic basement (consisting of migmatites and variously retrograded granulites). The geodynamic evolution of the WT is still under debate. Several tectonic models, spanning from strike-slip movements, continent-continent collision, to sub- duction related processes, have been proposed (for a discussion see Lombardo et al. 199 I).

During Cambro-Ordovician time, the WT experienced widespread syn-, late- and post-tectonic plutonism ("Granite t larbour

Intrusives" - Gun and Warren 1962), which formed a composite orogenic association including high-K calcalkaline and shoshonitic suites. The main field, petrographic, geochemical and isotopic data have been reported in Borg et al. (1986, 1987), Vetter and Tessensohn (1987), Ghezzo et al. (1989), Armienti et al. (1990b) and Biagini et al. (1991). The rock silica contents span from _< 50 to ~ 75 wt%. They are characterized by meta-alumi- nous and minor weakly peraluminous lithologies, and can be clas- sified as 1-type granites (Chappell and White 1974). Granite Har- bour ]ntrusives, in this region, have been dated by Rb-Sr on whole rocks and U-Pb on zircon at between 470 to 510 Ma (Adams 1986; Borg et a1.1986; and Armienti et al. 1990a). The sequence of their emplacement, however, has not yet clearly been defined. Armienti et al. (1990b) hypothesize, on the basis of geochemical and isotopic data, an origin by interaction of mantle and crustal components.

In addition, strongly peraluminous granites (ASI values generally > 1.1) are also present and were identified in this area by Borg et al. (1986) and Vetter and Tessensohn (1987); they have been typified as S-type granites (Chappell and White 1974). The peraluminous lithologies have a narrow range in silica contents (between ~ 66 to 76 wt%); they are characterized by the occurrence of Al-rich biotites and muscovite + garnet _+ cordierite _+ andalusite. On the basis of field and petrographical data, Biagini et al. (1991) recognized synkinematic and postkinematic intrusives among thc peraluminous rocks.

In the Deep Freeze Range (Fig. 1), synkinematic granites outcrop as dykes, veins and sills (up to 20-30 m thick). Most of these bodies are semicomformable with respect to the main foliation of the country rocks, and some of the smaller ones are often folded. The SKG have a foliation parallel to the $2 foliation of the Ross orogeny. The rocks are two-mica Al-silicate leuco- monzogranites (SiO2 ~ 74%).with a strong peraluminous charac- ter (ASI = 1.12-1.22). Enclaves are rare, and are metasedimentary. The petrology, geochemistry and geochronology of the peraluminous granites have been investigated by Biagini et al. (1991), Tonarini et al. (1994), Turi et al. (1994). They propose for these rocks an origin from recycled crust involving metasedimentary rocks.

Analytical techniques Analyses of minerals (biotite and Fe-Ti oxides) were carried out on a Cambridge Microscan 9 at the Vrije Universiteit of Amster- dam. The operating conditions were: accelerating voltage 15 kV, sample current 25 nA and 10 s peak counting time. The FeO and F and C1 contents of the biotite (sample CF25) were determined in duplicate by titration and by ion-selective electrodes respectively, on selected samples. The H20 + content was obtained by the difference between LOI (loss on ignition at 1000~ after preheating at l l0~ of biotite separate) and F and C1 contents. The Fe20~ was calculated from the difference between the total FeO determi- nated by microprobe analyses and the FeO measured by titration. The structural formula of biotite was calculated on the basis of 24 (o, OH, F, el).

Fluid inclusion microthcrmometry was carried out by using both Chaixmeea (Poty et al. 1976) and Linkam heating-freezing stages. Calibration at high temperatures was perlbrmed using synthetic Merck substances of known melting point. The zero ~ point was determined with distilled water. The low-temperature calibration included measurement of the melting point of C C I 4 (Q3~ natural pure CO2 (-56.6~ ethylacetate (83~ and methylcyclopenthane @142~ The reproducibility of melting point measurements is _+ 0.2~ at a heating rate of 0.1~ Raman analyses were performed at the Vrije Universiteit in Am- sterdam with a Microdil-28 multichannel microspectromcter (Burke and Lustenhouwer 1987).

Fig. 2 A Photomicrograph showing secondary growth of fibrolite • graphite at grain boundaries and along intracrystalline fractures in feldspars (small arrows). Crossed nicols, magnification x78. Enlarged view with plane polarized light ( • 200) pin- pointing the contemporary growth of fibrolite and graphite. Plane polarized light. B Photomicrograph of thick section showing an inter- growth of ilmenite and fibrolite. Reflected light with plane polarized light ( x 125)

113

Sample description The investigated synkinematic peraluminous granites outcrop along the Wishbone Ridge (between the Campbell and the Priest- ley glaciers - Fig. 1) within the medium- to high- grade metamorphics (480-680~ 2.5~4 kbar - Palmeri et al. 1991). Four samples (CF 9, 10, 25; AM 62) have been selected for fluid and solid inclusion studies.

Granites are fine- to medium-grained equigranular leuco- monzogranites, characterized by a foliated fabric defined by biotite and feldspar alignments. Some microstructural features, such as undulatory extinction in large crystals and subgrain develop- ment in quartz, deformed feldspars with recrystallization along rims or in fractures, and kinked biotites, indicate post-magmatic deformation. Microstructures and field relationships with the metamorphic country rocks thus suggest that these leucogranites were emplaced during late deformational events and that the fabric evolution continued after crystallization in a dynamic regime.

The rocks consist of quartz (30-39 vol%), K-feldspar (26-30 vol%), plagioclase (23-33 vol%), biotite (3.0-3.4 vol%), muscovite

(2.9 3.2 vol%), sillimanite (0.4 1.3 vol%) and minor garnet (04).3 vol%) and andalusite (0-0.2 vol%). Accessory phases include ap- atite, zircon, monazite, ilmenite, graphite and minor pyrite, ilmenorutile and uraninite. Quartz occurs both as small subhedral grains enclosed within the feldspars (mainly plagioclase), and as large polycrystalline aggregates. Microperthitic K-feldspar (mi- crocline and minor orthoclase) is intergranular, but also shows subhedral forms. Plagioclase is euhedral to subhedral and is characterized by normal zoning (rims: An~o 1~; cores: An 12 2~). Myrmekite is common as isolated patches in the quartz-feldspar aggregates or as overgrowths along plagioclase crystal rims. Bi- otite (red-brown in thin section) is euhedral to subhedral, sometimes in intergrowths with white mica. In some cases, biotite is replaced by muscovite. Chloritization processes are uncommon. The biotite composition (sample CF25) is:

(Ki.9~ Nao.04) (Mgl.s4 Mno.,, Fell2.40 FeI"o.14 Tio.29 Al~.~8) (Si5.6s A12.32) O21.54 [(OH)1.75 F0.69 Clo.02].

In the Fe2+-FeS+-Mg diagram (Wones and Eugster 1965), this composition falls close to the QFM buffer.

Muscovite is generally secondary, as it is found as overgrowths

114

Table 1 Selected microprobe analyses of Fe-Ti oxides. The structural formulae have been calculated on the basis of 3 and 6 oxygens for ilmenite and ilmenorutile respectively (a primary, b secondary, c rim on secondary ilmenite, d discrete grain)

Sample no. CF25 CF25 AM62 CF25 CF25

Ilmenite Ihnenorutile a b a c d

TiO2 53.18 52.75 52.77 82.60 64.22 Nb2Os 0.59 0.63 0.30 11.10 24.68 AI203 0.09 0.07 0.07 0.62 0.17 Fe203 - - - 4.18 9.14 FeO 35.69 40.35 40.15 - - MnO 9.72 6.14 6.17 - 0.09 MgO 0.04 - - 0.03 0.05

Total 99.31 99.29 99.46 98.50 98.34

Ti 1.010 1.004 1.004 2.613 2.143 Nb 0.007 0.007 0.003 0.211 0.495 A1 0.003 0.002 0.002 0.031 0.009 Fe'" - - - 0.132 0.305 Fe" 0.753 0.840 0.849 - Mn 0.208 0.132 0.132 0.003 Mg 0.002 0.002 0.003

on feldspars, andalusite and sillimanite, or has grown along fractures. Andalusite found as isolated crystals generally enveloped by white mica, is considered to be magmatic. Almandine-spessartine with weak reverse zoning (rims: Aim48.6 56,3 PY4.4 3.4 8p46.3-38.6 Gro.7 1.7; cores: Almso.3_588PYsc~6.6 Sp43.743.3 Grl.0 1.9 - Biagini et al. 1991) occurs in small isolated euhedral crystals, sometimes showing "atoll-like" shapes. The composition of these garnets is different from garnets in high-grade metamorphic rocks: Alm65.3 60.7 PYl4.4-s.4 SPjT.o-27.s Gr3.3 3.9 and AlmTs.S4o.4 PY13.4-9.o SPs.1 8.8 Grz7 1.~ (respectively for core and rim - Palmeri and Ta- larico 1990). Both textural characters and mineral chemistry indicate a magmatic origin (e.g. Miller and Stoddard 1981).

One sample (AM 62) is characterized by minor coarse aggregates of muscovite and pale green biotite. These minerals, visible as spots in hand specimen, correspond probably to pseudo- morphous replacements of former cordierite crystals.

Fibrolitic sillimanite is common in all samples and forms mats of fine needles generally enveloped in white mica, in intergranular folia along grain boundaries or in bands in quartz and feldspar crystals (Fig. 2A). Microstructural relationships strongly indicate a secondary origin for fibrolitic sillimanite, similar to fibrolite with "disharmonious textures" described by Vernon and Flood (1977) in the Cooma Complex (New South Wales, Australia).

Graphite is a common phase in these granites and generally occurs as rounded or irregular tabular grains. Graphite with primary textures has never been found. Its distribution suggests a contemporary growth with fibrolitic sillimanite (Fig. 2A).

Ilmenite is the main opaque mineral; it occurs mainly as isolated euhedral crystals, sometimes associated with biotite; few minor irregular anhedral aggregates, characterized by ilmenorutile rims, are enclosed in fibrolite mats (Fig. 2B). Results of microprobe analyses for ilmenite with different textural relationships (Table 1), point out ilmenite-pyrophanite solid solutions with a manganese content ranging from 0.132 to 0.208 a. u. f. (atom per unit formula, on the basis of 3 oxygens), and without hematite contents. I1- menorutile is present as rims on secondary ilmenite and as discrete grains associated with sillimanite. The presence of ilmenite without hematite components, and the absence of coexisting magnetite suggest equilibration at lowfoz, probably at or below QFM (quartz-fayalite-magnetite) buffer (Price 1983; Whalen and Chap- pell 1988).

Fluid and solid inclusions

Textures and compos i t i on

In S K G , solid and fluid inclusions are observed mos t ly in quar tz , and, only in a few instances in garne t and feldspars. Two compos i t i ona l types d o m i n a t e : ca rbon ic inclusions + H20 , and unde r sa tu ra t ed aqueous inclusions. Inclus ions record different stages of a complex cool ing evolu t ion with: (1) a clear t iming between different popu la t ions ; (2) a consis tent associa t ion of different solid and fluid inclusions; (3) widespread t ranspos i t ion and decrepi ta t ion, especially in S K G emplaced in high- grade m e t a m o r p h i c rocks (CF 9, CF10, AM62).

Based on their compos i t i on and m o d e of occurrence, we dist inguish four types of fluid and solid inclusion popu la t ions (Table 2).

Primary mixed CO 2 + H20 inclusions + brines in garnet (type 1)

Two isolated three-phase (liquid CO2, v a p o u r CO2, liquid H20) inclusions were identified in the garnet core, in associa t ion with AI2SiO5 (Fig. 3A). They have r o u n d shapes, a small size (7-12 gm) and are d o m i n a t e d by CO2 in vo lume (volume fract ion of H 2 0 ~ 0.2).

M i c r o t h e r m o m e t r i c invest igat ions show tha t the carbonic par t of the fluid consists of pure CO2; temperatures of mel t ing (Tin) were recorded at - 5 6 . 6 ~ Inclu- sions homogen ize to the l iquid (ThL) at 29~ salinities for the aqueous pa r t have been calculated on the basis of

Table 2 Characteristics of selected fluid inclusions. (incl. inclusion, recryst, recrystallized, transp, transposed, subgr. subgrain, bound, boundary; clathr, clathrates)

Sample Host setting Incl. n. Type Distribution Composition Notes

CF25 o Grt primary 2 1 CF25k Qtz incl. in Grt 1 1 CF25a Qtz incl. in fs 3 2 CF25b Qtz strained 2 2 CF25bl Qtz strained 7 2 CF25b3 Qtz recryst. 29 3 CF25f Qtz strained 6 3 CF251 Qtz 2 4 CF251 Qtz 7 4

Isolated H20 + CO2 + NaC1 No C Isolated H20 + NaC1 Brine Cluster CO2 No C Transp. trail CO2 No C Evolved H20 + NaCI Brine Subgr. bound. CO2 + CH4 § Sil + C Transp. trail CO 2 + CH 4 + Sil + C Trail CH4 + CO2 + N~ + H20 incl. Trail H20 + NaC1 Clathr.

Fig. 3 A - F Photomicrographs of fluid inclusions in S KG: A Pri- mary three-phase mixed CO2 + H20 inclusions (empty arrows) + prismatic AlzSiO s inclusions (black arrows) in garnct. Scale bar = 20 I,tm. B Quartz inclusion within garnet. In the enlarged area ~ an isolated primary brine is observed (arrow). Scale bar = 20 gm. C Early secondary type 2 CO2-rich monophase (liquid) fluid inclusions in a quartz grain. One aqueous inclusion with decrepitation textures is observed (encircled area). It consists of liquid + a salt cube. Scale bar = 20 gm. D Early secondary type

3 CO2-rich monophase (liquid) fluid inclusions along an intracrystalline microfracture in a quartz grain. Note the association with fibrolitic sillimanite and graphite. Scale bar = 80 gm. E Trail of late secondary type 4 aqueous and carbonic fluid inclusions in a quartz grain. One mixed carbonic + aqueous inclusion is present (arrow). Inclusions are at different focal depths. Scale bar = 40 #In. F Intracrystalline trails of graphite + ilmenite + empty fluid inclusions in feldspars (arrows). Scale bar = 1 mm (v = vapour, s = salt, l = liquid)

116

Fig. 4 A Temperatures of ho- lo mogenization to the liquid phase (ThL) of CO2-ricb fluid inclusions. Type 2 inclusions 8 homogenize in a small temperature interval, while later 6 type 3 inclusions have a scat- ,-- tered temperature distribution 4 over a 40~ interval. B Final melting temperatures (7~n) of COz-rich fluid inclusions. 2 Type 3 inclusions always show a significant melting point de- pression relative to the CO2 triple point, indicating minor amounts of othcr compounds

0 -42 - 3 6 - 3 0 - 2 4 - 1 8 -12 -6

m ~

A

0 6 12 18 24 30

35-

30

25

20 [ ] Tm type 2

r- I Tm type 3 15-

10

5

-63 -62 -61 -60 -59 -58 -57 -56 Toc

B

clathrate melting temperatures at ~ 7-8~ and correspond to 3-5 wt% NaC1 eq. (Diamond 1992).

A single isolated brine fluid inclusion has been observed in a quartz inclusion (~ 150 gm) within garnet. The inclusion is 9 gm long and consists of liquid, gas and a daughter mineral phase, possibly halite (Fig. 3B). Due to poor optical conditions, microthermometric studies were not performed.

Early carbonic inclusions (+ brine rests ?) in quartz (type 2)

Clusters and/or transposed intragranular trails of biphase CO2 inclusions without visible water rims occur mostly in quartz grains within feldspars, and less commonly in the large polycrystalline quartz aggregates (Fig. 3C). These inclusions are frequent, but not found in every sample. They have a quite small size (5-10 gm) and a typical round shape. No graphite and/or fibrolite are associated. Quartz inclusions within feldspars are considered to have a better capacity of retaining early inclusions. Feldspars have preserved them during cooling, in a similar manner to fluids in quartz inclusions within garnet and pyroxene rigid minerals in high-grade metamorphic rocks (e.g. Touret 1987).

Temperatures of melting (Tin) range from --56.7 to --57.8~ (Fig. 4). Homogenization temperatures (7hL) range between 3 and 26~ (Fig. 4).

Raman microspectroscopy indicates these inclusions as pure CO2 and, only in a minority of cases, traces of methane (< 1 mole%) were detected. A sensible depres- sion (~, I~ of the CO2 melting point without significant amounts of additional gaseous species, has been indicated by several authors (Ramboz etal. 1985; Dubessy et al. 1989) as a result of CO2 clathrate formation at low temperatures. In this respect, it may be important to recall that some water may be present and be overlooked in the inclusions because of their regular shape and optical restrictions due to small dimensions.

A few evolved aqueous inclusions with irregular to "star-shaped" contours are found associated with type 2 inclusions (Fig. 3C). At room temperature, these inclusions consist of liquid + one isotropic Raman-inactive

solid phase, tentatively identified as NaC1, but no vapour bubble. They might represent the remnants of former brines, which underwent a complex post-trapping evolution with re-equilibration to higher fluid density (i.e. disappearance of the gas bubble). Inclusion post- trapping evolution, in fact, cannot be explained by simple decrepitation processes, as these must lead to a partial water leakage out of the inclusions (i.e. an increase of relative vapour bubble dimensions) (Sterner and Bod- nar 1989).

Early carbonic inclusions + graphite in quartz (type 3)

Monophase carbonic inclusions _+ water rims are the most common fluid inclusion observed in SKG. They are small (5-15 gm), round in shape, and in a few of them water is visible, concentrated in recesses of the microcavities. These inclusions are distinguished from type 2 carbonic inclusions by their ubiquitous association with graphite flakes of comparable dimensions (20- 40 ~tm), and subordinately with fibrolite needles and ilmenite grains (Fig. 3D). Type 3 inclusions are generally found lining subgrain boundaries in recrystallized quartz grains, and less commonly in short intragranular trails.

Figure 5 illustrates fibrolite mats + graphite grow- ing along plagioclase boundaries in SKG. The fibrolite mats extend into quartz (Fig. 5A), which shows local dynamic recrystallization (i.e. undulose extinction and subgrain formation), possibly as a result of cooling of the granite in a tectonically active regime. CO2-rich fluid inclusions (_+ H20) + C _+ AlaSiO5 _ ilmenite line new subgrain boundaries (Fig. 5B) an indication that recrystallization took place in the presence of a fluid phase. Note that graphite is present only outside the fluid inclusions. Traces of graphite within fluid inclusions have not been detected. This observation suggests that graphite was in equilibrium with the carbonic fluid phase, at the time of inclusion formation.

Melting temperatures (Tm) of type 3 carbonic fluid inclusions vary between -62 .2 and -57.0~ (Fig. 4). Raman analyses indicate that subordinate methane is always present, from traces up to 4 mol%. Only in one

Fig. 5A,B Schematic repre- sentation of a petrografic view in SKG showing textural relationships between secondary fibrolite, carbonaceous material and type 3 CO2-rich fluid inclusions: A Growth of fibrolite + graphite at plagioclase boundaries and along secondary intracrystalline trails. B Enlargement of quartz sub- grains lined by type 3 fluid inclusions, graphite and fibrolite (see text). (Dotted lines mi- crofi'actures, dashed areas subgrain boundaries)

]g"~o,

I "\"

A ' , ' \ , ' '

117

"~ %

o,w\

, o - 2 o •

2 ) ~

o ~ ~o o~~ (:}

B

inclusion higher CH4 amounts (10 mol%) have been detected. In this inclusion water is about 50% of the total volume. Homogenization temperatures (7hL) cover a 60 ~ interval, with a very scattered distribution from - 4 1 to 21~ (Fig. 4).

Secondary intracrystalline trails of small flakes of graphite + fibrolite grains + quartz _+ small (< 1 15gm) empty cavities (Fig. 3F) are observed in feldspars. They may represent an association similar to type 3 inclusions in quartz. Trails of empty fluid inclusions in feldspars have been observed in other plutonic rocks (e.g. Sybille Monzosyenite, Frost and Touret 1989) and are possibly the result of fluid loss from the inclusions during the post-magmatic evolution of feldspars (i.e. perthitization of K-feldspar, deformation).

Late aqueous and carbonic inclusions in quartz (type 4)

These inclusions typically occur along fractures and constitute by far the dominant inclusion association, in those SKG veins emplaced in the high-grade metamorphic rocks. Type 4 inclusions postdate unambiguously the other inclusion populations; fracturing is, in fact, a clear of changing from a plastic to a brittle deformation regime in quartz. Undersaturated aqueous inclusions, monophase carbonic inclusions _+ minor mixed ones coexist within single trails (Fig. 3E), which suggests that two immiscible fluid phases (a carbonic and an aqueous one) were present at the time of trapping. Aqueous inclusions dominate by a factor of 10. Graphite is absent.

Late type 4 carbonic inclusions do not always freeze completely but show the assemblage S(CO2) + L + V, on cooling down to -170~ On subsequent heating, phase transitions are typified as $2 and subordinate H4, following the classification proposed by Kerkhof (1988, 1990) (i.e. SLV ~ SL --+ L or SLV ~ SL ~ SLV ~ LV

L, respectively). Partial homogenization (SLV ~ SL) is between - 110 and - 93~ temperatures of CO2 dis- solutions (SL ~ L) are between - 7 5 and -62~ temperatures of homogenization (LV --, L) between - 4 9

and -53~ Gas composition determined by Raman microspectrometry indicates C H 4 dominated (53-47 mol%) fluids with variable COz (39-24 mol%) and N2 (9 20 mol%).

Associated late aqueous inclusions (type 4) have eu- tectic temperatures (Te) ~ 23~ corresponding to Na- C1 dominated solutions. Temperatures of ice melting (Tm) are between - 11 and - 8.3~ and all inclusions show the formation of gas hydrates (clathrates) that melt between 3.2 and 7.3~ Carbonic parts have very low densities. Salinities could not be determined from the clathrate melting as there was no liquid CO2 in the inclusions (Diamond 1992). However, a rough idea of the possible maximum salinity, can be obtained from the ice melting temperatures (Collins 1979). Upper lim- its for possible salinities are between 12 and 15 NaC1 eq wt%. Temperatures of homogenization are between 178 and 298~

Calculation of molar volume and isochores

The molar volume of type 1 primary CO2 + H20 inclusions in garnet is 44 cm3/mole with XCO2 = 0.4-0.5 (Bowers and Helgeson 1983a). Molar volumes of type 2 and 3 CO2-rich inclusions in quartz were calculated for pure CO2 systems according to Angus et al. (1976), since CH4, present, does not exceed 4 mol%. The density of type 2 pure or nearly pure CO2 inclusions in quartz ranges between 64 and 49 cm3/mole.

Type 3 CO2 + CH4 inclusions have an extremely wide density range, where the highest values around 58 cm3/mole overlap the lowest "V in type 2 inclusions, and continuously decrease to ~ 39 cm3/mole. The asymmetric shape of the histogram in Fig. 4 suggests that type 3 inclusions underwent re-equilibration processes after trapping (Touret 1987).

The CO2 + CH4 mixtures in fluid inclusions are metastable and tend to form graphite and H 2 0 at decreasing temperature, by a reaction of the form:

C O 2 @ C H 4 = 2C + 2 H 2 0

118

The absence of graphite in type 3 inclusions is an indication of the difficulty of nucleating "in situ', in the small volume of these inclusions. Metastable gaseous mixtures of CO2 and CH4 may be preserved on a wide temperature range, especially when compositions are different from stoichiometric proportions (e.g. Kreulen 1987). There is thus indication that the wide molar volume range among type 3 inclusions it is not related to variation in the composition of the starting fluids, but to post-entrapment modifications of a single generation of fluid inclusions by processes such as decrepitation, leakage, implosion, for which the relative effects are known to vary the original inclusion volume and adjust fluid densities (e.g. Sterner and Bodnar 1989; Bakker and Jansen 1991).

The densities of mixed CH4-CO2-N 2 inclusions have been determined combining the microthermometric data and the fluid compositions, plotted on CO2- CH 4 + N 2 polychoric 7h-X diagrams (Kerkhof 1990). Calculated molar volumes are low, around 40-48 cm3/ mole.

isochores for carbonic inclusion fluids were calculated using Hol loway's (1977, 1981) version of a modified Redlich-Kwong equation of state. Water-dominated inclusion fluids have been modelled using the computer program of Brown (1989), and the equation of state of Brown and Lamb (1989) and Zhang and Frantz (1987). Densities for aqueous fluids vary between 0.78 and 0.92 g/cm 3.

Graphite

There are at least 3 distinct occurrences of graphite in SKG: the first type (a) is observed in quartz, where graphite flakes of 20-100 pm form secondary trails and occur along subgrain boundaries, together with type 3 carbonic inclusions _+ fibrolite needles (Figs. 3D and 5B). The second type (b) consists of alignments of flakes with variable dimensions (from a few pm up to

100 ~tm) in the feldspars, along short intragranular fractures, in association with fibrolite, quartz _+ empty cavities (black arrow in Figs. 3F and 5A). The third type (c) consists of large crystals, up tol mm in size, in intergrowths with the fine-grained secondary mats of needles of flbrolitic sillimanite (Figs. 2A and 5A).

These observations confirm that carbonaceous material is always a secondary phase in SKG. Furthermore, the common association of graphite with fibrolite, suggest a contemporary growth of these two phases.

The Raman spectra of graphite (Fig. 6) show, in all analyses, the variation of peak ratios. The two peaks (at 1360 and 1580 cm -1) indicate that there are differences in crystallinity (e.g. Wopenka and Paster 1988, 1993). The typical Raman band for well crystallised graphite, in fact, occurs at ~ 1580 cm 2, and in less ordered carbonaceous material (i.e. lower degree of crystallinity), a second band appears at ~ 1360 cm ~ (Tuinstra and Koenig 1970; Nemanich and Solin 1979). Pasteris and

756

2 0

[ I I I I I I I

1 6 0 0 1 5 5 0 1 5 0 0 1 4 5 0 1 4 0 0 1 3 5 0 wavenumber (era "1)

Fig. 6a-e Raman spectra of the different types of carbonaceous material present in SKG: a, a' Type a graphite associated with type 3 fluid inclusions in quartz; a contains peak of intercalated CO2. b Type b graphite associated with opaques and fibrolite in short intracrystaUine trails in feldspars, e, e' Type c graphite in large grains associated with fibrolite mats. Spectra are plotted on the same scale both vertically and horizontally

Wopenka (1991) have shown that the degree of crystallinity depends on the temperature of formation of carbonaceous material. In a general way, the relative intensities of the two bands in the graphite observed in SKG, indicate a relatively good degree of crystallinity: spectra have properties (intensity ratio of peaks, half- maximum line-width) of graphite formed at ~ 500 600~

Interestingly, additional differences are observed between the different types of mechanically trapped graphite:

1. Graphite associated with type 3 carbonic inclusions (spectra a and a', Fig. 6) may contain adsorbed CO2 and CH4 molecules (note CO2 peaks, in spectrum a, Fig. 6). Intercalated fluid has the same composition as in type 3 fluid inclusions (CO2 96-99 tool%, CH4 1-4 mol%). This observation indicates precipitation of graphite from a supersaturated carbonic fluid phase.

2. Spectra of graphite associated with flbrolite grown on feldspars and fibrolite (spectra b and c, Fig. 6) have their ~ 1580 cm -1 band down-shifted to values as low as 1565 cm 1. This suggests compositional variations from pure carbon, due to the presence of intercalation compounds. Low wave numbers may indicate donor intercalant (i.e. metals) (Wopenka and Pasteris 1988); microprobe analyses in the largest mm-sized graphite flakes, indicate that up to 3 wt% silicon is present. The analyses were obtained on parts of the carbonaceous material showing no A1 in order to avoid signal contam- ination from the surrounding feldspar.

Wopenka and Pasteris (1993) have observed similar low peak positions of graphite in specimens from the andalusite zone; in their publication however, they leave the phenomenon unexplained. They may be right as the 3 wt% Si in itself is not sufficient to warrant the strong down-shift of the graphite peak.

5000 Pbar

Magmatic evolution

The identification of magmatic stage fluids in plutonic rocks requires some structural control, as only the most favourable cases will associate fluid inclusions with magmatic stage evolution (i.e. association with silicate melt inclusions, Touret and Frezzotti 1993). Textural evidence suggests that the fluids of type 1 inclusions were trapped during the growth of garnet and were not affect- ed by the overall post-magmatic evolution of the granites. Though garnet in SKG is not very abundant, it is indisputably a magmatic phase. For these reasons, the primary COz + H20, and brine inclusions in garnet (type l) were formed at maglnatic stages, during the emplacement and crystallization of SKG.

Unfortunately, the extremely limited number of such inclusions prevents us from establishing precise time relationships between mixed CO2 + H20 and brine inclusions. However, it should be pointed out that brines and mixed CO2 + H20 inclusions are not found associated. This may indicate that those two fluids were not trapped at the same time; since brines are present in a mineral inclusion within garnet, riley may have been trapped at an earlier stage than the COa-rich fluids. This suggests that water-poor CO2-saturated fluids coexisted with the silicate melt, which progressively exsolved CO2 oia cooling. Water activities of magmatic fluids calculated from inclusion data are ~ 0.5-0.6.

Similar magmatic fluids dominated by CO2 and brines, are also known from the Sybille monzosyenite in the Laramie intrusive complex (Wyoming, USA) (Frost and Touret 1989), where they occur as CO2 inclusions and solid salt inclusions in feldspar and quartz.

We can estimate P-Tconditions of crystallization for SKG by combining magmatic CO2 + H20 -]- NaCI fluid compositions and densities with the estimated pressures of the enclosing metamorphics. Conditions of peak metamorphism in the WT wall-rocks range around 3-3.5 kbar (Palmeri et al. 1991). The calculated isochore of primary CO2 + H20 + NaC1 fluids (d = 0.71 g/cm 3) indicates trapping temperatures of

750~ at 3 kbar (Fig. 7). Since the water-saturated "solidus" temperature of peraluminous granites is at less than 610~ (Huang and Wyllie 1975), the present P - T estimates agree with the presence of significant amounts of CO2 in the magmatic fluid phase. The main effects of a CO2 diluent lowering H;O activity include, on cooling, an earlier volatile saturation due to low CO2 solubility in silicate magmas, and a considerable increase of "solidus" temperatures (Eggler and Kadik 1979; Ebadi and Johannes 1991). The solidus curves of water-saturated peraluminous granites and of Qz-Ab-Or-H20- CO2 system, for aH~o = 0.5, experimentally determined (Huang and Wyllie 1975; Ebadi and Johannes 1991), are reproduced in Fig. 7. Fluid inclusion data are consistent with crystallization of a vapour-saturated magma at 3 kbar, 750~ and an~o ~ 0.5.

4000

30o0

2000

1000

0 200 300 400 500

Discussion

119

600 700 800 TOe 900

Fig, 7 P-Tevolution for synkinematic peratuminous granites as derived from fluid-inclusion studies, lsochores for the different types of fluid inclusions are reported (.solid lines), labelled with densities in g/cm 3 (Holloway 1981; Bower and Helgeson 1983b; Zhang and Franz 1987). Estimated emplacement conditions for SKG from enclosing metamorphics are indicated (stippled box). So ldns hues for the gramte + flmds system at aH;o = 1 and an2o = 0.5, arc from Whitney (1988) and Ebadi and Johannes (1991), respectively. Texturally primary CO2 + H20 type 1 inclusions have densities of ~ 0.71 g/cm3; these densities together with the fluid composition (XH20- 0.5-0,6) arc in agreement at 3 kbar with the solidus in reduced water activity. The gray band indicates the densities for type 2 C02-rich fluid inclusions, which were trapped at the transition to post-magmatic stages. The do~- ~ed hand indicates densities for type 3 methane-bearing CO2-rich fluid inclusions associated with graphite. The extremely wide fan suggests conditions of fluid inclusion density modifications after trapping: Late aqueous and Cll4-dominated carbonic type 4 inclusions define an immiscibility box (dashed area). Stars indicate the probable cooling path for SKG based on fluid inclusion studies (see text)

Post-magmatic evolution

The different isochore bands point substantially to an isobaric cooling (starred path, Fig, 7). There are indica- tions that the earliest trapped fluids in quartz (type 2 inclusions) have the lowest densities. Their isochores in- tersect the P-Tcondition of crystallization (gray band, Fig. 7), and are compatible with the transition from magmatic to sub-magmatic stages. We do not know if at least part of type 2 inclusions formed at the magmatic stage, but they certainly indicate that CO2-rich fluids characterize the transition to the post-magmatic evolution. These fluids are similar to type 1 magmatic fluids; also in this case, minor brines might be present.

The selective loss of brine inclusions and the preser- vation of the carbonic ones, might be related to the isobaric cooling of these rocks. It has been summarized recently by Touret (1992), that the post-trapping changes of fluid inclusions depend on the relative AP between fluid pressure in inclusions and confining pressure at given temperature. In isobaric cooling paths, carbonic fluid inclusions with 1"1at isochores will be better preserved than the steep-isochore aqueous fluids, which form high angles with the P-Tuplift path.

Isochores for mixed CO; + C H 4 (-k H 2 0 ) fluids associated with graphite cross the proposed P-Tpath at

120

temperatures below 700~ and seem to form a P - T -lo "cont inuum" with type 2 CO2 (_+ brines) fluids (dotted band, Fig. 7). The presence of graphite in equilibrium with the fluids indicates ac = 1 at these temperatures. -15

The extremely wide isochore fan defined by type 3 fo2 fluid inclusions is due to post-entrapment partial re- equilibration in the host quartz. The main modifications -20 that may occur in COa 4-_ H20 inclusions after trapping, during isobaric cooling include: (1) density increase due to internal underpressure conditions (implosion: Stern- -25 er and Bodnar 1989); (2) density decrease due to selective H20 leakage, resulting in CO2-rich inclusions (Bakker and Jansen 1991). In SKG, density increase in -30 the inclusions to adjust to the new P and T, is most likely. Re-equilibration should also have been accompa- nied by a volume reduction of the inclusion cavities (i.e. internal underpressure). Otherwise, we have to admit that isobaric cooling continued below 200~ which is largely unrealistic (see Fig. 7).

Late type 4 fluids define a P - T immiscibility box around 300-450~ and 1.5-2.8 kbar (dashed area, Fig. 7). It is likely that these fluids record decompression in the cooling history of SKG. Thus, below 500~ the 5000 change of chemical equilibria of the buffered system obar brought to SKG a water-dominated fluid regime. The

4000 carbonic parts of the fluid became CH4-dominated (53- 47 tool%) with subordinate COz (39 24 tool%) and N2 (9 20 mol%) contents. 3ooo

400 500 600 700 800 900 1 ~ 0 Toc

Fig. 8 T-Jo ~ diagram at P = 3 kbar locating the oxygen fugacities calculated from type 3 and type 4 inclusions. Temperatures were derived from isochores in Fig. 7. Arrows indicate a possible cooling evolution along a QFM type buffer, for investigated granites (see text). Oxygen fugacities were calculated according to French (1966). Equilibrium constants for gases from Ohmoto and Kerrick (1977); fugacity coefficients from Saxena and Fci (1987)

Fluid origin and evolution: T-fo2 constraints

In modeling the fluid-rock interaction in SKG, the ques- tion arises about the origin of the different fluid inclusion populations. Two disparate explanations may be postulated: fluids in SKG represent distinct infiltration episodes from the wall metamorphics (open system), or they result from the "in situ" evolution of a single magmatic fluid phase at decreasing T in a buffered system (closed system).

The fluid phase composition depends on theJo2 control by mineral phases at decreasing temperatures, and can be modelled using the equilibria constant for gaseous compounds (French 1966). Combinat ion of chronological, microthermometric and compositional criteria of fluids, and the occurrence of graphite allow us to reconstruct T-fo 2 evolution. At a given P and Tin a closed C-H-O system, fugacities of the different gas species are in fact fixed, if the precise fluid composition and the activity of graphite are known. In particular, for a c = 1 (graphite in equilibrium with fluid),jo 2 is fixed, if the relative C O 2 / C H 4 ratios in the carbonic fluid are known (see Ohmoto and Kerrick 1977; Dubessy 1984; Wilmart et al. 1991). Therefore, oxygen fugacities were calculated for type 3 and 4 inclusions, using equilibrium constant for gases of Ohmoto and Kerrick (1977) and the fugacity coefficients after Saxena and Fei (1987a,b) (Fig. 8). In so doing, temperatures were assumed from the isochores in Fig. 7.

2000

1000

0 300 400 500 600 700 800 T~ 900

Fig. 9 P-T-fo 2 cooling path and fluid-rock interaction in the investigated peraluminous granites, at the inferred QFM buffer. Growth of secondary fibrolite over feldspars appears to be related to circulation of CO2-rich methane-bearing fluids with low H20 contents, between 670 and 550~ in the graphite stability field (see text). Graphite stability curve (heavy solid line) indicates graphite unstable at magmatic stages and during the early stages of cooling, as also indicated by fluid and solid inclusions (type 1 and 2, see text). Type 3 CO2-rich methane-bearing fluids in equilibrium with graphite (dotted area) may result from simple cooling (below 670~ of type 2 pure-CO 2 fluids (gray area) on the proposed fo2 buffer. Dashed lines represent isopleth lines for CH 4 contents (in mol%) in the carbonic part of QFM buffered C-O-H fluids in presence of graphite. Calculated P-Tconditions for CO 2- CH4 chemical equilibria fall within the P-Tareas defined by fluid inclusions with identical compositions. The dotted bar represents a schematic chemical boundary between the high-temperature CO2 fluid regime and the low-temperature H20 fluid regime. (Thick solid line graphite stability curve, solid line muscovite stability curve with amo = 0.5, after Bower and Helgeson, 1983a). Solidus haplograni(e H20 + COa, after Ebadi and Johannes (1991). P-Tareas for different generations of fluid inclusions are from isochores in Fig. 7. Calculations for the C-O-H fluid system at the different P-T-fo x conditions were done after French (1966), with equilibrium constants from Ohmoto and Kerrick (1977) and fugacity coefficients from Saxena and Fei (1987). AlaSiO ~ triple point from Holdaway (1971)

Very little can be said aboutfo~ at emplacement and crystallization conditions in SKG. However, some indi- rect information can be derived from the composition of magmatic fluids in garnet. As primary graphite has never been found in SKG and primary fluids are CO2 + H20 mixtures, we infer that at crystallization conditions (~ 3 kbar and 750~ fo2 should have been >_ the QFM buffer, above the graphite-saturated field (arrows in Fig. 8). Anfo 2 close to the QFM buffer is also suggested by composition of the mineral phases.

A comparison between Raman analyses and P-T-fQ constraints for type 3 inclusions in equilibrium with graphite is shown in Fig. 9. In a closed system on a QFM buffer, an equilibrium transition from magmatic to sub-magmatic stages (below 750~ along the proposed cooling path (stars in Fig. 9) will cause a circulation of CO2-dominated fluids in the absence of graphite. Pure CO2 type 2 fluid inclusions indicate a progressively CO2-richer fluid regime at solidus conditions.

The fluid evolution on a QFM buffer is in accordance with the appearance of carbon around ~ 670~ in the fluid system (Fig. 9). Hence, the transition from type 2 pure-CO2 to type 3 inclusions, which outlines the appearance of C H 4 and C, is the consequence of decreasing fo2 along the path in Fig. 8, all other parameters being constant. The C H 4 content in the CO2 can be explained in terms of temperatures. At QFM and 3 kbar, 1 to 4 mol% C H 4 is observed in the carbonic part of the fluid corresponding to temperatures of 670 580~ in accordance with isochores for fluid inclusions (compare C H 4 isopleths 1 and 3 with dotted isochore band in Fig. 9), and thermometry of graphite (600-500~

At temperatures between 600-550~ the nature of C-O-H fluids in equilibrium with graphite at an fo2 equal to QFM, changes from carbonic fluids to aqueous-fluids containing minor C H 4 and traces of CO2. This variation in fluid regimes is also observed in our case, with the appearance of type 4 immiscible aqueo- carbonic fluids (compare CH 4 isopleths 60 and 70 with dashed area in Fig. 9). As the calculated oxygen fugacities in the present work for type 4 fluids are in equilibrium at 450-480~ for pressure of 2 3 kbar with a QFM buffer, we believe that also late type 4 inclusions are internally derived. The overall late hydrothermal evolution occurred at extremely low oxygen fugacities, within the stability field of graphite.

In summary, our explanation is that the different variations in fluid inclusion compositions at decreasing temperatures, from pure CO2 without detectable CH4, to water-dominated with C H 4 and minor CO2, represent a continuum of compositions in the C-O-H volatile system during isobaric cooling from 750 to 400~ in a QFM-buffered system. As fluid compositions are consistent with the thermodynamic equilibration along the inferred P-T-fo; evolution (Fig. 9), the early carbonic fluids (type 2) must be of magmatic origin. At later stages, below 500~ some interactions with the enclosing metamorphics have occurred, witnessed by the presence of N2 in the fluid. However, their equilibrium with the

121

proposed P-T-fo~ evolution indicates that the fluid composition was controlled by the buffering capacity of the granites.

Fluids and fibrolite formation

From mineral and fluid inclusion textures it is evident that the mineral association of fibrolite, quartz, graphite + opaques represents a single retrograde episode during cooling of SKG in a tectonically active regime; and that these secondary phases were formed in domains that were originally channelways for fluids.

The fluid-inclusion investigations have shown that secondary mineral phases began to grow over feldspars in association with precipitating graphite, during the early stages of cooling below temperatures of about 670~ when the fluid composition (i.e. appearance of CH4 in the carbonic part of the fluid, variation of aH2o) encounters the C saturation curve (heavy black line in Fig. 9). The system fluid + graphite is chemically active in reacting feldspars (i.e, presence of intercalated silicon in graphite), while only mechanically trapped in quartz.

Growth of fibrolite, quartz, graphite _+ opaques should have been completed above ~ 550~ upon entering the higherfi~2o environment (compare the stability curve of muscovite for aH2o = 0.5, with the fluid regime in Fig. 9), although there is evidence, namely some intergrowths offibrolite and muscovite, indicating that the process continued below these temperatures.

The actual reactions involve release of Na, K and Si into the aqueous part of the fluids. The ubiquitous presence of recrystallized quartz may testify redeposition of Si from the fluid. However, late type 4 fluids circulating at low temperatures have relatively low salinities (below 12 15 NaCI eq. wt%). Tentative explanations include progressive increase of XH20 in the fluid, at decreasing temperature (~ 0.9 at 450~ late muscovite formation acting as a sink for K, or a combination of these two.

Formation of secondary fibrolite by fluids in intrusive rocks has been proposed by different authors (e.g. Pitcher and Read 1963; Vernon and Flood 1977; Ver- non 1979; Kerrick 1987, 1990); they all postulate acid Al-rich magmatic fluids as possible metasomatic agents. The evolution presented above implies that the metasomatic fluids in SKG are C 0 2 - H 2 0 mixtures, released during crystallization of the peraluminous magma. The absence of a major external fluid influx from the wall- rock metamorphics, at high temperatures, is in agreement also with the observed mineralogical equilibria. An inflow of metamorphic fluids would have eventually led to hydration of igneous silicates. Petrographic evidence shows that muscovite is late with respect the high- temperature post-magmatic evolution; the growth of this mineral is consistent with the proposed closed system model indicating that muscovite is stable only below 570~ at the onset of the water-dominated fluid regime (Fig. 9). In addition, the presence of graphite in all investigated SKG indicates a similar fluid-rock evolution in distinct granite bodies, as should be expected

122

in a closed system evolution of magmatic fluids at similar P- T-fo ~ conditions.

Conclusions

The present results give information on magmatic fluids in granitic rocks. Magmatic fluids in equilibrium with silicate melts are often purely aqueous with different amounts of dissolved chlorides (up to hydrosaline melts, see Frezzotti 1992). But, in some cases, CO2 is a major constituent, especially in mid-crustal intrusives. It therefore will influence considerably the crystallization dynamics and the overall cooling history of these rocks.

It appears that the magmatic evolution in SKG and the overall subsolidus fluid-rock interactions are the di- rect result of a combination of melt and fluid compositions, Tandfo2. Present data suggest that "second boil- ing" (magmatic immiscibility caused by crystallization, Burnham and Ohmoto 1980) was not the main mecha- nism for volatile saturation. Due to the low solubility of CO2 melts (Kadik and Eggler 1974), SKG experienced early volatile saturation, and higher crystallization temperatures than water-saturated granites. Magmatic CO2-rich fluids, circulating through the intrusive rocks, participated also in the observed "carbothermal" evolution.

The effects of the presence of CO2 in m a g m a t i c fluids on the geochemis t ry of grani t ic systems have no t yet fully been clarified. They mus t be of critical i m p o r t a n c e for unde r s t and ing the behav iou r of m i n o r and trace ele- ments in grani t ic systems, especially those which are cons idered as " immobi le" in pure ly aqueous fluids.

A c k n o w l e d g e m e n t s This work is part of the "Programma Nazionale di Ricerche in Antartide" founded by ENEA, the Ital- ian Agency for Energy Research, through a joint research program on Antarctic Earth Sciences with the University of Siena. We thank W. Lustenhouwer for microprobe analyses on opaques and carbonaceous material. We acknowledge J.M. Huizenga for his computer facilities on the C-O-H fluid system. Thoughtful and constructive reviews by T. Hansteen, A.M. van den Kerkhof, C.A. Ricci and J.L.R. Touret are very much appreciated. Facilities for microprobe and Raman analyses were provided by the Vrije Uni- versiteit of Amsterdam and by NWO, the Netherlands Organiza- tion for Scientific Research.

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evidence of magmatic co 2 -rich fluids in peraluminous graphite-bearing leucogranites from deep...

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