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Abstract: Large charnockite massifs occur in the high-grade Southern Granulite Terrain (SGT) and Eastern Ghats Belt (EGB) crustal provinces of Peninsular India. Available geochronological data indicate that the magmatism is episodic, associated with distinct orogenic cycles in the different crustal domains. The geochemical data also indicate a change in composition from trondhjemitic at ∼3.0–2.9 Ga to dominantly tonalitic at ∼2.6–2.5 Ga to tonalitic-granodiorite-granitic at ∼2.0–1.9 Ga to dominantly tonalitic at 1.7–1.6 Ga to quartz monzonitic or tonalitic at ∼1.0–0.9 Ga to granodiorite-granitic at ∼0.8–0.7 Ga. The trondhjemitic and tonalitic end members are metaluminous, magnesian and calcic to calc-alkalic, characteristic of magnesian group charnockites. The granodioritic to granitic end members are metaluminous to slightly peraluminous, ferroan and calc-alkalic to alkali-calcic, characteristic of ferroan group charnockites. The quartz monzonitic end members are metaluminous to peraluminous, magnesian to ferroan and calcic to calc-alkalic, neither characteristic of the magnesian group nor of the ferroan group of charnockites. Based on the occurrence and difference in composition of the charnockite massifs, it is suggested that the charnockite magmatism registers the crustal growth of the Indian plate on its southern (SGT) and eastern (EGB) sides, along active continental margins by accretion of arcs. [Copyright &y& Elsevier]

The southwestern margin of the Eastern Ghats Belt characteristically exposes mafic dykes intruding massif-type charnockites. Dykes of olivine basalt of alkaline composition have characteristic trace element signatures comparable with Ocean Island Basalt (OIB). Most importantly strong positive Nb anomaly and low values of Zr/Nb ratio are consistent with OIB source of the mafic dykes. K-Ar isotopic data indicate two cooling ages at 740 and 530 Ma. The Pan-African thermal event could be related to reactivation of major shear zones and represented by leuco-granite vein along minor shear bands. And 740 Ma cooling age may indicate the low grade metamorphic imprints, noted in some of the dykes. Although no intrusion age could be determined from the present dataset, it could be constrained by some age data of the host charnockite gneiss and Alkaline rocks of the adjacent Prakasam Province. Assuming an intrusion age of ∼1.3Ga, Sr-Nd isotopic composition of the dykes indicate that they preserved time-integrated LREE enrichment. In view of the chemical signatures of OIB source, the mafic dykes could as well be related to continental rifting, around 1.3Ga, which may have been initiated by intra-plate volcanism. [ABSTRACT FROM AUTHOR]

A regional magnetic survey over a part of the Eastern Ghats Mobile Belt (EGMB) and southwest Krishna basin around Ongole Prakasam District, Andhra Pradesh (between 15° and 16°N lat. and 79 °45′ and 80°48′E long.) was carried out. The magnetic anomalies are generally noisy with no easily identifiable trends, particularly north of 15°21′N lat. Inversion of upward continued anomalies brought out the magnetic basement constituting the charnockites located at an average depth of 3 km. The basement is overlain by the khondalitic group of rocks and exhibits horst-graben-like structures developed due to faulting. The study shows that the charnockites extend south of Ongole also, and attributes the seismicity of the region to the faulted charnockitic basement. [ABSTRACT FROM AUTHOR]

A regional magnetic survey was carried out over an area of 8000 km² in Godavari districts of Andhra Pradesh, India, which is covered by the rocks of Eastern Ghat Mobile Belt (EGMB) viz., the Khondalitic series and Charnockites in the northern half and Permian to Mesozoic and Cenozoic sediments in the southern half, and forms a part of the Krishna-Godavari (K-G) basin. The survey brought out a strong NE-SW trending anomaly in the area covered by the rocks of Eastern Ghat Mobile Belt (EGMB), and a mild ENE-WSW trending anomaly in the area covered by the sediments of the Krishna-Godavari (K-G) basin. The NE-SW trending anomaly in the northern half could be attributed to the exposed/near surface Charnockite basement that has come closer to the surface as a result of Eastern Ghat Mobile Belt (EGMB) tectonics. Explanation of the mild ENE-WSW trending anomaly over the sediments of the Krishna-Godavari (K-G) basin required a faulted magnetic basement at depth downthrown towards the south. It is therefore concluded that the Charnockitic basement together with the Khondalite group of rocks which are folded and faulted during the different phases of tectonics of Eastern Ghat Mobile Belt (EGMB) extend into the Krishna-Godavari (K-G) basin and further, were involved in faulting during the phases of formation and sedimentation in the Krishna-Godavari (K-G) basin. [ABSTRACT FROM AUTHOR]

Charnockite rocks, a significant litho unit in Eastern Ghats Granulite Belt (EGGB), are characterized by the presence of dry mineral assemblage. The anhydrous conditions required for the formation of ganulites are believed to be controlled by the lowering of water activity through the influx of CO2-bearing fluids. Here we present the results of systematic petrographic and microthermometric studies characterizing the nature, composition, and density of CO2-rich fluid inclusions in charnockitic rocks from Gangarajumadugula. In this study, we have combined mineralogic thermobarometry with microthermometric data of the carbonic fluid inclusion population of high density in the charnockite samples from Gangarajumadugula area to substantiate desiccation of lower crustal environment producing anhydrous conditions in the Eastern Ghats Granulite Belt. Close-to-peak pressure-temperature calculations for core composition using convergence technique in present study indicate that these charnockites have experienced maximum P–T around 860 °C at 9 kbar, which is broadly consistent with independent P–T estimates inferred for Gangarajumadugula area. The reaction textures preserved by these charnockites and the thermobarometric data characterize decompression followed by cooling history. High-density carbonic fluid inclusions (1.08 g/cm3) in quartz provide potential evidence for the involvement of CO2-rich fluid during granulite facies metamorphism in EGGB. Occurrence of CO2-rich inclusions and elevated P–T conditions open up the possibility favoring the mechanism of vapour-deficient dehydration melting accompanied by CO2 infiltration to account for charnockite formation from precursor crustal rocks. Our study documents that fluid inclusions in high-grade metamorphic rocks have enormous potential in retaining worthwhile information of peak metamorphic conditions and understanding the metamorphic processes. [Copyright &y& Elsevier]

Discusses the origin of arrested charnockitization in the Chilka Lake area in Eastern Ghats, India. Definition of charnockite and its use; Field relation between larger bands and smaller patches of charnockites; Representative microprobe analyses of biotite, garnet and orthopyroxene from leptynite and arrested-type charnockite of Hatia quarry.