Archean crustal evolution of the Aravalli Banded Gneissic Complex, NW India: Constraints from zircon U-Pb ages, Lu-Hf isotope systematics, and whole-rock geochemistry of granitoids

摘要

The Aravalli Banded Gneissic Complex forms a block of Archean gneisses and granitoids in the southern domain of the Aravalli orogen in NW India. Results of U-Pb-Hf isotope analyses on zircon grains combined with whole rock geochemical data reveal that the gneissic complex is made up of TTGs, which intruded during the Paleoarchean (3310 Ma) and Neoarchean (2563-2548 Ma), and granite-granodiorite-quartz diorite emplaced between 2545 and 2485 Ma. Zircon xenocrysts additionally point to Meso- and Neoarchean magmatic episodes at 3100-3000, 2880-2800, 2690 and 2660-2630 Ma. Ages from zircon rims in conjunction with previously published Sm-Nd isotope data suggest metamorphic events at ca. 2450 and 520 Ma. Medium-HREE characteristics and near chondritic epsilon Hf-3.31 (Ga) of -0.4 suggest formation of the Paleoarchean TTGs by partial melting of a deep-seated mafic crust, derived from CHUR-like mantle immediately prior to the TTG formation. In contrast, the high-HREE characteristics and subchondritic epsilon Hf2.56-2.55 Ga from -3 to -8 of the Neoarchean TTGs suggest partial melting at relatively shallow depth of a younger oceanic crust, which was derived from a depleted mantle source at < 3.1-3.4 Ga. The Neoarchean granitoids (2540-2485 Ma) with epsilon Hf-t from -8 to +2 in magmatic and xenocrystic zircon were likely originated by internal reworking of a heterogeneous felsic crust, consisting of Paleo-, Meso- and Neoarchean rocks. The Neoarchean quartz diorite (ca. 2545 Ma) represents a hybrid rock, which resulted from mixing of enriched mantle-derived mafic melts (epsilon Hf-2.55Ga = -3.4) with felsic melts formed by anatexis of Paleoarchean crust (epsilon Hf-2.55Ga = -11.7). Comparison of our new data from the Aravalli Banded Gneissic Complex with those of the adjacent Bundelkhand Craton indicate that both cratonic nuclei underwent different evolutionary histories during the Archean.