The southern Dharwar Craton of Karnataka and Tamil Nadu states in south India exposes a depth profile of Late Archean crust. Amphibolite facies felsic to intermediate Peninsular Gneiss of protolith ages 3.4-2.9 Ga with subordinate infolded supracrustal relics and the large Closepet Granite body give way southward to granulite facies (orthopyroxene-bearing) gneisses over a transition zone about 40 km wide, with mineralogic pressure indicators increasing from about 4 kbar (corresponding to about 15 km paleodepth) to about 7 kbar (corresponding to near 25 km paleodepth). Previous whole rock and zircon geochronology have established that amphibolite facies metamorphism, migmatization, and granite intrusion took place in the latest Archean at 2.53-2.51 Ga. However, some workers have considered the foliated granulites as products of an earlier metamorphism, distinct from the terminal Archean events. This work presents whole rock geochemical studies coupled with ion microprobe 207 Pb/ 206 Pb ages and Th/U ratios of zircons from banded granulites and orthopyroxene-bearing pegmatoids from three quarries at Chillapura, near Halaguru, Karnataka, at the southern end of the Closepet Granite and well into the granulite transition zone. The majority of the zircons, including all those from the pegmatoidal charnockites, show nearly concordant ages at 2.53 Ga and thus provide a firm temporal link with amphibolite facies metamorphism and granite emplacement at higher crustal levels. Moreover, we propose that the very low Th/U ratios of many zircons (≤0.001) could not reflect precipitation from siliceous magmas of the compositions of their host rocks but more probably record equilibration with a high U/Th metamorphic fluid. Two zircons from one banded granulite showed nearly concordant 207 Pb/ 206 Pb ages at 2.96 Ga and Th/U ratios at 0.6-1.0, compatible with primary crystallization from a mantle-derived magma with canonical Th/ ; these older zircons are believed to be relics of a widespread crustal accretion event that U ≈ 4 built much of the southern Dharwar Craton. A third cluster of slightly discordant ∼2.62 Ga zircons with intermediate Th/U (0.2-0.4) from one banded granulite may record a distinct migmatization event, perhaps premonitory to the more profound event at 2.53-2.51 Ga. No evidence is found of an older (Mid-Archean) metamorphic event, which could correspond to what has been termed "synaccretion metamorphism" at ∼2.95 Ga. However, the 2.53-2.51 Ga crystallization of zircons in the Chillapura granulites was nearly contemporaneous with the generation of juvenile tonalitic-trondhjemitic crust in the southern part of the Eastern Dharwar Craton. The Chillapura granulite facies metamorphism, together with amphibolite facies metamorphism and emplacement of the Closepet Granite at higher crustal levels, could record remobilization in the interior of the Dharwar Craton in response to an accretionary event at its southeastern margin.

Late Archaean Supracrustals of the Goa-Dharwar sector (GDS) are composed of a thick sequence of greywacke sequence with narrow intercalations of quartzite, BIF and carbonates. Mafic volcanics occupy the base of the sequence. The greywackes are predominantly tuffacious containing chlorite-sericite and hornblende. Arkosic variations containing biotite dominate the western part of the sector. Fine-grained variations occur as isolated narrow lenses within other types of greywackes. The conglomeratic greywackes are localized along the western and the eastern margins of the sector. All of the greywackes are all typically immature containing coarser clasts of mostly plagioclase (18-23%) and quartz (32-34%). Lithic fragments of felsic volcanic rocks are common. The matrix is dominated by mafic material. Biotite and amphibole are related to metamorphic recrystallization. Chlorite, sericite, epidote, carbonate and chert are products of the interplay of diagenesis and low-grade metamorphism. Fe-Ti oxide, sphene, apatite and zircon are usual accessories. But for slight enrichment in K 2 O, the major element chemistry of the GDS greywackes is similar to the chemistry of Late Archaean greywackes. They also compare in respect of V, Co, Hf contents, K Nd ratios, steep REE patterns with distinct LREE enrichment and HREE depletion. The GDS greywackes however are distinctly enriched in Rb, Ba, Sr, Th, U, Cu, Zr, Ce/Ce* and depleted in Cr, Ni, and Zn. The conglomeritic and biotite bearing verities contain considerable proportions of clasts derived from the basement tonalitic/granitic terrain. The common tuffacious greywackes containing hornblende and biotite-sericite however include only volcanic clasts and bear evidence of derivation from submarine weathering of predominantly felsic volcanics erupted on a large scale to form a magmatic arc in the later stages of geosynclinal deposition. Geochemical data suggest that the GDS greywackes were laid down in progressively changing basin geometry from a passive to active continental margin and island arc setting.

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