|Title:||Ion microprobe U‐Pb age determinations on zircon from the Late Archean granulite facies transition zone of southern Karnataka|
|Authors:||S.J. Mojzsis, T.C. Devaraju, and R.C. Newton|
|Publication:||India. Jour. Geol., v. 111, p. 407‐425.|
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 207Pb/206Pb 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 207Pb/206Pb 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/U ≈ 4; these older zircons are believed to be relics of a widespread crustal accretion event that 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.