Fluid inclusions in rocks from the amphibolite‐facies gneiss to charnockite progression in southern Karnataka, India: direct evidence concerning the fluids of granulite metamorphism

Abstract

Abstract. Fluid inclusion studies of rocks from the area suggests progressive decrease of the late Archaean amphibolite-facies to granulitewith increasing grade. Oxygen fugacities calcufacies transition zone of southern India provide lated from orthopyroxene-magnetitequartz are support for the hypothesis that C0,-rich H,Oconsiderably higher than the graphite C0,-0, poor fluids were a major factor in the origin of the buffer, which explains the absence of graphite in high-grade terrain. Charnockites, closely asthe charnockites. sociated leucogranites and quartzo-feldspathicThe present study quantifies the nature of the veins contain vast numbers of large C0,-rich vapours in the southern India granulite metainclusions in planar arrays in quartz and feldspar, morphism. It remains to be determined whether whereas amphibole-bearing gray gneisses of C0 Microthermometry of CO, inclusions shows that miscible CH, and N, must be small, probably less than 10 mol. %combined. Densities of CO, increase steadily from north to south across the transitional terrane. Entrapment pressures calculated from the CO, equation of state range from 5 kbar in the north to 7.5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750"C, in quantitative agreement with mineralogic geobarometry. This agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions.Calculations on the stability of the charnockite assemblage biotite-orthopyroxene-K-feldsparquartz show that an associated fluid phase must have less than 0 . 3 5 H 2 0 activity at the inferred P and T conditions, which agrees with the petrographic observations. High TiO, content of biotite stabilizes it to lower H,O activities, and the steady increase of biotite TiO, southward in

Infiltration‐driven dehydration and anatexis in granulite facies metagabbro, Grenville Province, Ontario, Canada

“…This is particularly disadvantageous for granulite facies rocks, which have been shown in numerous studies to have variable and low aHz0 (e.g. Phillips, 1980;Hansen et al, 1984;Lamb & Valley, 1984). In particular, aHZO along the vapour-absent dehydration-melting reaction Hbd + Qtz = Opx + Cpx + PI + L (dotted line in Fig.…”

Metamorphic evolution of the Sierras de San Luis, Argentina: granulite facies metamorphism related to mafic intrusions

“…Granulite facies terranes are usually characterized by low water activities, which could result either from a CO 2 dominated¯uid or under¯uid absent conditions (Lamb and Valley, 1988;Hansen et al, 1984;Edwards and Essene, 1988;Bhattacharya and Seb, 1986). The widespread occurrence of migmatites suggest that water activity in the¯uid phase was high (X H 2 O b 0X4) at the beginning of granulite facies metamorphism (emplacement of the ma®c intrusion), was reduced at the peak of metamorphism to values corresponding to a X H 2 O of 0.2±0.5.…”

Fluids in geological processes

“…Sevigny and Ghent, 1989;Perchuk et al, 1985;Hansen et al, 1984;Riciputi et al, 1990;Aranovich et al, 1987) have estimated the activity of water in the fluid as 0.5-0.7 for the amphibolite facies and as 0.2-0.3 for the granulite facies. For simplicity, we take values of 0.6 and 0.3, respectively.…”