Zircon U-Pb Ages and Hf Isotopes of the Askot Klippe, Kumaun, Northwest India: Implications for Paleoproterozoic Tectonics, Basin Evolution and Associated Metallogeny of the Northern Indian Cratonic Margin

摘要

Throughout the Himalayan thrust belt, klippen of questionable tectonostratigraphic affinity occur atop Lesser Himalayan rocks. Integrated U-Pb ages, Hf isotopic, and whole rock trace element data establish that the Askot klippe, in northwest India, is composed of Paleoproterozoic lower Lesser Himalayan rocks, not Greater Himalayan rocks, as previously interpreted. The Askot klippe consists of 1857 ± 19 Ma granite-granodiorite gneiss, coeval 1878 ± 19 Ma felsic volcanic rock, and circa 1800 Ma Berinag quartzite, representing a small vestige of a Paleoproterozoic (circa 1850 Ma) continental arc, formed on northern margin of the north Indian cratonic block. Detrital zircon from Berinag quartzite shows εHf 1850 Ma values between —9.6 and —1.1 (an average of —4.5) and overlaps with εHf 1850 Ma values of the Askot klippe granite-granodiorite gneiss (—5.5 to —1.2, with an average of —2.7) and other Paleoproterozoic arc-related Lesser Himalayan granite gneisses ( —4.8 to —2.2, with an average of —4.0). These overlapping data suggest a proximal arc source for the metasedimentary rocks. Subchondritic εHf 1850 Ma values (—5.5 to —1.2) of granite-granodiorite gneiss indicate existence of a preexisting older crust that underwent crustal reworking at circa 1850 Ma. A wide range of εHf 1850 Ma values in detrital zircon (—15.0 to —1.1) suggests that a heterogeneous crustal source supplied detritus to the northern margin of India. These data, as well as the presence of a volcanogenic massive sulphide deposit within the Askot klippe, are consistent with a circa 1800 Ma intra-arc extensional environment.