Petrology, fluid inclusions and metamorphic history of Bhopalpatnam granulites, central India

Abstract

The Bhopalpatnam granulite belt (BGB) flanks the Pranhita–Godavari (PG) rift basin with ages ranging from 1.6 to 1.9 Ga, representing a major Mesoproterozoic thermal regime in central India. We present here new results on the metamorphic P–T conditions from garnet-bearing enderbitic charnockite and pelitic gneisses, and document the fluid regimes from detailed fluid inclusions studies in corundum and quartz in corundum-bearing gneiss and garnet–sillimanite–rutile gneiss, respectively, from this granulite belt. A peak pressure–temperature window of 8–9.5 kbar and 720–800 °C is defined by phase equilibrium considerations on garnet–clinopyroxene–plagioclase–quartz assemblages in garnet–bearing charnockite. The garnet–plagioclase–sillimanite–quartz assemblages in pelitic gneiss yield slightly lower P–T estimates of 660–720 °C at 4.7–6.5 kbar. Primary fluid inclusions in corundum comprise monophase carbonic inclusions with melting temperatures varying from −56.6 to −57.4 °C, indicating that the dominant part of the trapped fluid is pure CO 2. The inclusions homogenize into the liquid phase at temperatures in the range of −18.5 to −5.9 °C (1.024–0.961 g/cm 3). Fluid inclusions in quartz from garnet–sillimanite–rutile gneiss correspond to two categories; Group I (primary) and Group II (pseudosecondary). Both the groups show melting temperatures in the range of −55.9 to −62.9 °C, suggesting the presence of traces of other volatiles (CH 4/N 2) in addition to CO 2. The homogenization temperature of Group I inclusions ranges from −46.3 to −0.1 °C, the extreme low homogenizations indicating very high-density (up to 1.140 g/cm 3) for the carbonic fluid. Group II inclusions show higher homogenization temperatures of up to 16.2 °C. The estimated CO 2 isochore for primary inclusions in corundum and quartz intersects the peak P–T condition of the BGB derived from mineral phase equilibria. We therefore, infer that CO 2 was the dominant fluid species that was trapped at or near the peak metamorphic conditions in BGB. The metamorphic and fluid regimes, as well as the exhumation history of this granulite belt are thus constrained from a combination of petrologic and fluid inclusion studies.