Abstract
Oxygen and hafnium isotopic compositions, measured in-situ on U-Pb dated zircon grains from Paleoproterozoic to early Cambrian successions in NW India have implication for regional crustal evolution and supercontinent cycles. Analyzed zircon grains have high Th/U ratios (>0.1), display strongly fractionated REE patterns, metamorphic overprint, and evidence of interaction with low temperature fluids. Their positive Ce and negative Eu anomalies preclude any Pb loss after zircon crystallization. The U-Pb age spectra (concordance between 90 and 110%) indicate prominent peaks at 2.6 to 2.4 Ga, 1.9 to 1.7 Ga, 1.6 to 1.5 Ga, 1.2 to 1.0 Ga and 0.9 to 0.7 Ga that coincide with the assembly and breakup of Precambrian supercontinents. The Hf model ages of zircon grains with mantle like δ18O values reveal continuous generation of the continental crust from 3.3 to 1.3 Ga in NW India with major episodes during 3.3 to 2.7 Ga and 1.7 to 1.5 Ga. These ages correspond well with the 3.4 to 2.9 Ga and 2.2 to 1.6 Ga age peaks recognized in detrital zircon populations from eastern Australia and North America, underlining the significance of these time brackets in continental crust generation during the global continental evolution. Magmatic episodes at 1.9 to 1.7, 1.2 to 1.0 and 0.9 to 0.7 Ga are considered to represent crustal reworking rather than juvenile addition and the former two phases correspond with periods of supercontinent assembly. However, a progressive depletion in 18O from supra-mantle to mantle values in the 1.7 to 1.5 Ga zircons, coupled with their mantle-like εHf(t) values, indicate at least some juvenile input. Moreover, the 1344 to 1120 Ma zircon grains with low δ18O (3.7–1.5‰) but high εHf(t) (+8.1− +1.9 with one exception of −2.5) values signify rapid reworking of mantle derived materials in an extensional setting during this period. The 0.9 to 0.7 Ga peak, corresponding to the fragmentation of Rodinia supercontinent, documents crustal reworking that is in contradiction to the generally considered juvenile crustal addition in extensional setting associated with supercontinent breakup.
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