Incipient Charnockite Research Articles

Carbon isotope compositions of fluid inclusions in charnockites from southern India

Models for the formation of crustal granulites, which include the metamorphism of anhydrous lithologies1,2, the extraction of hygroscopic granite melts3,4 and fluxing by CO25–7 constrain the thermal and geochemical evolution of the lower crust and its interaction with mantle-derived volatiles. Here we present abundances and carbon isotope compositions of CO2 in fluid inclusions in quartz and garnet from gneiss–granulite pairs from the incipient char-nockite quarries of southern India, determined by a stepped combustion/thermal decrepitation carbon–extraction technique, which quantifies problematic sample contamination and allows precise lithological control. The results show that the CO2 in incipient charnockite is more abundant and isotopically heavier than the CO2 in associated gneiss, providing evidence for the influx of CO2-rich fluids during their formation. The inferred carbon isotope compositions of these fluids are consistent with a mantle source, and at Ponmudi, the isotopic composition of coexisting graphite and CO2 implies fluid/rock volume ratios in excess of 0.15.

The Southern High-Grade Margin of the Dharwar Craton

The amphibolite-facies to granulite-facies transition at the southern margin of the Dharwar craton has been studied in the Krishnagiri, Satnur-Halaguru, and Kushalnagar areas of southern India. In all three areas the transition appears to be a progressive metamorphic overprint on cratonal gneisses and their mafic and metasedimentary enclaves, without major structural interruption. In the Kabbal-Satnur-BR Hills section of Karnataka, a high-grade charnockite massif with pronounced Rb depletion is the culmination of an apparently continuous increase of metamorphic grade southward. In this and the Kushalnagar areas, increase of paleopressure from near 6 to near 8 kbar with increasing grade indicates a depth-zone relationship of the amphibolite and granulite facies. Incipient charnockite replacing Peninsular Gneiss first appears along N-S shears parallel to the regional grain of the craton. Low-P(), high vapors were instrumental in the creation of orthopyroxene. Introduction from a deep source, either a decarbonating mantle, basaltic underplate, or deeply buried sediments, was facilitated by the N-S deformation system. A deformed continental margin or infracontinental basin in the latest Archean is a plausible setting for the metamorphism. Great crustal thickening, perhaps with entrainment of shelf or basin sediments, was effected by overthrusting, the record of which may be preserved in an early isoclinal foliation and in fold-interference patterns in the southern cratonal margin. Subsequent transcurrent shearing facilitated outgas-sing of the deep crust and upper mantle, and rising vapors transported heat, , and K to middle levels of a thickened crust, which resulted in anatectic melting. Eventual uplift and erosion exposed a 6-8 kbar paleopressure surface at the southern cratonal fringe. The weakened and segmented continental shelf or platform south of the present craton was prone to later remobilization and retrogression, in the manner of a typical "mobile belt." This view of the relationship of the Dharwar craton to the southern high-grade terrain provides no support for the concept that granulite facies terrains of the general aspect of the South Indian massifs everywhere underlie the interior of the craton. The 2-3 kbar terrain of the cratonal interior was not greatly thickened by overthrusting nor magmatic underplating during the 2.6 Ga high-grade event that affected the southern cratonal margin. It is, however, possible that granulite-grade roots of an older metamorphic cycle underlie the cratonal interior.

Evidence for fluid pathways through Archaean crust and the generation of the Closepet granite, Karnataka, South India

In the Archaean amphibolite—granulite facies transition zone of southern Karnataka anatexis of amphibolite facies Peninsular gneiss occurred. Based on cross-cutting relationships this melting event, which produced the Closepet granite, was contemporaneous with the formation of charnockitic rocks. This formation of charnockite took place initially due to channelized CO2-rich volatiles moving along an anastamosing network of shears and micro-fissures. These fluid pathways are now represented by brown veins of charnockite. Later, the fluid became more pervasive and resulted in areas of total inversion to charnockite. A lateral change along the fluid pathways from charnockite through incipient charnockite to either a thin granitic vein or a vein of reddening, can be traced through the country rocks. These different effects along the same, laterally continuous, vein are interpreted to represent the lateral change of volatile composition which, in front of the CO2-rich fluids, were increasingly H2O-dominated.

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 Fluid inclusion studies of rocks from the late Archaean amphibolite‐facies to granulite‐facies transition zone of southern India provide support for the hypothesis that CO2,‐rich H2O‐poor fluids were a major factor in the origin of the high‐grade terrain. Charnockites, closely associated leucogranites and quartzo‐feldspathic veins contain vast numbers of large CO2‐rich inclusions in planar arrays in quartz and feldspar, whereas amphibole‐bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed‐facies quarries contain no large fluid inclusions. Inclusions in the northernmost incipient charnockites, as at Kabbal, Karnataka, occasionally contain about 25 mol. % of immiscible H2O lining cavity walls, whereas inclusions from the charnockite massif terrane farther south do not have visibile H2OMicrothermometry of CO2 inclusions shows that miscible CH4 and N2 must be small, probably less than 10mol.%combined. Densities of CO2 increase steadily from north to south across the transitional terrane. Entrapment pressures calculated from the CO2 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‐feldspar‐quartz show that an associated fluid phase must have less than 0.35 H2O activity at the inferred P and T conditions, which agrees with the petrographic observations. High TiO2 content of biotite stabilizes it to lower H2O activities, and the steady increase of biotite TiO2 southward in the area suggests progressive decrease of aH2O with increasing grade. Oxygen fugacities calculated from orthopyroxene‐magnetite‐quartz are considerably higher than the graphite CO2‐O2 buffer, which explains the absence of graphite in the charnockites.The present study quantifies the nature of the vapours in the southern India granulite metamorphism. It remains to be determined whether CO2‐flushing of the crust can, by itself, create large terranes of largeion lithophile‐depleted granulites, or whether removal of H2O‐bearing anatectic melts is essential.