Ion Microprobe U‐Pb Age Determinations on Zircon from the Late Archean Granulite Facies Transition Zone of Southern India

Abstract

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.

Pb–Pb zircon ages of Archaean metasediments and gneisses from the Dharwar craton, southern India: Implications for the antiquity of the eastern Dharwar craton

“…Clustering of zircon ages around 2.5 to 2.6 Ga in both these samples (table 2) appears to reflect the time of late Archaean granitoid magmatism reported for this block (see Balakrishnan et al 1999;Jayananda et al 2000). We also note that intermediate ages between 2.7 and 2.8 Ga obtained in a few zircons (table 2, Sakarsanahalli gneiss, #4-6) are similar to the metamorphic ages of rocks in the EDC reported earlier (Mojzsis et al 2003). The scatter in the age data most probably reflects the complex succession of geological events in the EDC.…”

“…Three analyses in a single grain (4) yielded a reproducible age of ∼3.27 Ga. Most of the ages obtained for the other grains are higher than the radiometric ages of ∼3 Ga reported previously for samples from this region (Friend and Nutman 1991;Mahabaleshwar et al 1995;Mojzsis et al 2003). Three analyses conducted on rim material yielded younger ages ranging from 2.7 to 2.8 Ga.…”

“…The spread of minimum ages of detrital zircons from the metasedimentary rocks is rather large (2.5-3.5 Ga; table 2) and suggests that the sediments have different provenance. A minimum age for the onset of deposition in this block may be inferred from the presence of 2.5 Ga zircons in granitoids and charnockites (Mojzsis et al 2003) that formed as a result of associated metamorphism presence of 2.5 Ga rims in our dataset (table 2, Sakarsanahalli quartzite data 1c). Nonetheless the ages of detrital zircons, obtained in this study, establish the presence of an epoch of crustal growth at ∼3.3 Ga prior to the voluminous emplacement of younger granitoids during 2.5-2.7 Ga.…”

Chemical and isotopic evidence for widespread Eoarchean metasedimentary enclaves in southern West Greenland

“…Powder splits of samples for combined zircon geochronology and in situ zircon oxygen isotope measurements were sieved at 6400 lm, processed through heavy liquids, cleaned in acetone and subsequently passed through a Franz magnetic separator following our usual procedures (Mojzsis et al, 2003b). Zircon yields were highly variable depending on the rock type (see EA-1).…”

“…All conventional U-Pb zircon geochronology was determined using the UCLA CAMECA ims1270 high-resolution ion microprobe under routine conditions (e.g., Mojzsis and Harrison, 2002a;Mojzsis et al, 2003b); a short summary is provided. The standard operating conditions for U-Pb analyses were a $6 nA O 2 À primary beam focused to a 25 lm spot; the ion microprobe was operated at a mass-resolving power of $6000.…”