ALBERT

Notes: The Shevaroy Hills of northern Tamil Nadu, southern India, expose the highest-grade granulites of a prograde amphibolite facies to granulite facies deep-crustal section of Late Archaean age. These highly oxidized quartzofeldspathic garnet charnockites generally show minor high-TiO2 biotite and amphibole as the only hydrous minerals and are greatly depleted in the incompatible elements Rb and Th. Peak metamorphic temperatures (garnet–orthopyroxene) and pressures (garnet–orthopyroxene–plagioclase–quartz) are near 750 °C and 8 kbar, respectively. Pervasive veinlets of K-feldspar exist throughout dominant plagioclase in each sample and show clean contact with orthopyroxene. They are suggested to have been produced by a low H2O activity, migrating fluid phase under granulite facies conditions, most likely a concentrated chloride/carbonate brine with high alkali mobility accompanied by an immiscible CO2-rich fluid. Silicate, oxide and sulphide mineral assemblages record high oxygen fugacity. Pyroxenes in the felsic rocks have high Mg/(Mg+Fe) (0.5–0.7). The major oxide mineral is ilmenite with up to 60 mole per cent exsolved hematite. Utilizing three independent oxygen barometers (ferrosilite–magnetite–quartz, ferrosilite–hematite–quartz and magnetite–hematite) in conjunction with garnet–orthopyroxene exchange temperatures, samples with XIlmHm〉0.1 yield a consistent oxygen fugacity about two log units above fayalite stability. Less oxidized samples (XIlmHm〈0.1) show some scatter with indications of having equilibrated under more reducing conditions. Temperature-f (O2 ) arrays result in self consistent conditions ranging from 660 °C and 10−16 bar to 820 °C and 10−11.5 bar. These trends are confirmed by calculations based on the assemblage clinopyroxene–orthopyroxene–magnetite–ilmenite using the QUIlF program. In the most oxidized granulite samples (XIlmHm〉0.4) pyrite is the dominant sulphide and pyrrhotite is absent. Pyrite grains in these samples have marginal alteration to magnetite along the rims, signifying a high-temperature oxidation event. Moderately oxidized samples (0.1〈XIlmHm〈0.4) have abundant co-existing pyrrhotite, pyrite and magnetite. The most reduced granulite samples have pyrrhotite as the dominant sulphide with little or no pyrite and no coexisting magnetite. Chalcopyrite is a common accessory mineral of pyrite and pyrrhotite in all the samples. Textures in some samples suggest that it formed as an exsolution product from pyrrhotite. Extensive vein networks of magnetite and pyrite, associated principally with the pyroxene and amphibole, give evidence for a pervasive, highly oxidizing fluid phase. Thermodynamic analysis of the assemblage pyrrhotite, pyrite and magnetite yields consistent high oxidation states at 700–800 °C and 8 kbar. The oxygen fugacity in our most oxidized pyrrhotite-bearing sample is 10−12.65 bar at 770 °C. There are strong indications that the Shevaroy Hills granulites recrystallized in the presence of an alkali-rich, low H2O-activity fluid, probably a concentrated brine. It cannot be demonstrated at present whether the high oxidation states were set by initially oxidized protoliths or effected by the postulated fluids. The high correspondence of maximally Rb-depleted samples with the highest recorded oxidation states suggests that the Rb depletion event coincided with the oxidation event, probably during breakdown of biotite to orthopyroxene+K-feldspar. We speculate that these alterations were effected by exhalations from deep-seated alkali basalts, which provided both heat and high oxygen fugacity, low aH2O fluids. It will be of interest to determine whether greatly Rb-depleted granulites in other Precambrian terranes show similar highly-oxidizing signatures.

Notes: Oxide–sulphide–Fe–Mg–silicate and titanite–ilmenite textures as well as their mineral compositions have been studied in felsic and intermediate orthogneisses across an amphibolite (north) to granulite facies (south) traverse of lower Archean crust, Tamil Nadu, south India. Titanite is limited to the amphibolite facies terrane where it rims ilmenite or occurs as independent grains. Pyrite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade. Pyrrhotite is confined to the high-grade granulites. Ilmenite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade and occurring primarily as hemo-ilmenite in the high-grade granulite facies rocks. Magnetite is widespread throughout the traverse and is commonly associated with ilmenite. It decreases in abundance with increasing metamorphic grade. In the granulite facies zone, reaction rims of magnetite + quartz occur along Fe–Mg silicate grain boundaries. Magnetite also commonly rims or is associated with pyrite. Both types of reaction rims represent an oxidation effect resulting from the partial subsolidus reduction of the hematite component in ilmenite to magnetite. This is confirmed by the presence of composite three oxide grains consisting of hematite, magnetite and ilmenite. Magnetite and magnetite–pyrite micro-veins along silicate grain boundaries formed over a wide range of post-peak metamorphic temperatures and pressures ranging from high-grade SO2 to low-grade H2S-dominated conditions. Oxygen fugacities estimated from the orthopyroxene–magnetite–quartz, orthopyroxene–hematite–quartz, and magnetite–hematite buffers average 2.5 log units above QFM. It is proposed that the trends in mineral assemblages, textures and composition are the result of an external, infiltrating concentrated brine containing an oxidizing component such as CaSO4 during high-grade metamorphism later acted upon by prograde and retrograde mineral reactions that do not involve an externally derived fluid phase.

Notes: Summary ¶Pressures and temperatures are estimated for the high grade portion (Region D) of the Bamble granulite facies terrane using barometers based on the assemblages almandine-grossular-ferrosilite-anorthite-quartz and pyrope-grossular-enstatite-anorthite-quartz and a garnet-orthopyroxene Fe2+-Mg KD exchange thermometer based on the experimental data of Lee and Ganguly (1988). Both barometers utilize the same thermochemical data base (Berman, 1988, 1990) and the same garnet (Berman, 1990) and orthopyroxene mixing models (Wood and Banno, 1973; Sack and Ghiorso, 1989) which are internally consistent both with each other and the Berman (1988, 1990) data base. Thus any disagreement between the two barometers is dependent only on the mixing models chosen. When Fe2+-Mg mixing in orthopyroxene and garnet is taken to be ideal, pressures from the two barometers are found to be in poor agreement with PAlm-Fs (Pavg=9.2+/−1.0 kb (1σ)) consistently greater than PPyr-En (Pavg=7.5+/−1.2 kb (1σ)) by 1.4 to 2.4 kb per sample. If the non-ideal orthopyroxene mixing model of Sack and Ghiorso (1989) is used and their internally consistent value for mixing on the Fe2+-Mg join in garnet is incorporated in the Berman (1990) garnet mixing model, PAlm-Fs and PPyr-En are now in good agreement with the mean pressures for PAlm-Fs and PPyr-En at 7.1+/−0.9 kb (1σ) and 7.0+/−1.1 kb (1σ) respectively. Individual pressures per sample are generally within +/−1 kb of each other which is within the error range of either barometer. The mean temperature is found to be 793+/−58 °C (1σ) which is in good agreement with a mean temperature of 795 °C obtained from titaniferous magnetite-ilmenite thermometry (Harlov, 1992) as well as the stability fields of sillimanite and Mg-rich cordierite.

Notes: Abstract Reaction textures, fluid inclusions, and metasomatic zoning coupled with thermodynamic calculations have allowed us to estimate the conditions under which a biotite–hornblende gneiss from the Kurunegala district, Sri Lanka [hornblende (NMg=38–42) + biotite (NMg=42–44) + plagioclase + quartz + K-feldspar + ilmenite + magnetite] was transformed into patches of charnockite along shear zones and foliation planes. Primary fluid inclusion data suggest that two immiscible fluids, an alkalic supercritical brine and almost pure CO2, coexisted during the charnockitisation event and subsequent post-peak metamorphic evolution of the charnockite. These metasomatic fluids migrated through the amphibolite gneiss along shear zones and into the wallrock under peak metamorphic conditions of 700–750 °C, 5–6 kbar, and afl H2O=0.52–0.59. This resulted in the formation of charnockite patches containing the assemblage orthopyroxene (NMg=45–48) + K-feldspar (Or70–80) + quartz + plagioclase (An28) in addition to K-feldspar microveins along quartz and plagioclase grain boundaries. Remnants of the CO2-rich fluid were trapped as separate fluid inclusions. The charnockite patches show the following metasomatic zonation patterns: – a transition zone with the assemblage biotite (NMg= 49–51) + hornblende (NMg = 47–50) + plagioclase + quartz + K-feldspar + ilmenite + magnetite; – a KPQ (K-feldspar–plagioclase–quartz) zone with the assemblage K-feldspar + plagioclase + orthopyroxene (NMg=45–48) + quartz + ilmenite + magnetite; – a charnockite core with the assemblage K-feldspar + plagioclase + orthopyroxene (NMg = 39–41) + biotite (NMg=48–52) + quartz + ilmenite + magnetite. Systematic changes in the bulk chemistry and mineralogy across the four zones suggest that along with metasomatic transformation, this process may have been complicated by partial melting in the charnockite core. This melting would have been coeval with metasomatic processes on the periphery of the charnockite patch. There is also good evidence in the charnockitic core that a second mineral assemblage, consisting of orthopyroxene (NMg= 36–42) + biotite (NMg=50–51) + K-feldspar (Or70–80) + quartz + plagioclase (An28–26), could have crystallised from a partial melt during cooling from 720 to 660 °C at decreasing afl H2O from 0.67 to 0.5. Post-magmatic evolution of charnockite at T 〈 700 °C resulted in fluids being released during the crystallisation of the charnockitic core. These gave rise to the formation of late stage rim myrmekites along K-feldspar grain boundaries as well as late stage biotite, cummingtonite, and carbonates.