Oxide and sulphide minerals in highly oxidized, Rb‐depleted, Archaean granulites of the Shevaroy Hills Massif, South India: Oxidation states and the role of metamorphic fluids

Abstract

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‐TiO2biotite 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 10−16bar to 820 °C and 10−11.5bar. 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 andnocoexisting 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 10−12.65bar 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, lowaH2Ofluids. It will be of interest to determine whether greatly Rb‐depleted granulites in other Precambrian terranes show similar highly‐oxidizing signatures.