No abstract

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 À9.6 and À1.1 (an average of À4.5) and overlaps with εHf 1850 Ma values of the Askot klippe granite-granodiorite gneiss (À5.5 to À1.2, with an average of À2.7) and other Paleoproterozoic arc-related Lesser Himalayan granite gneisses ( À4.8 to À2.2, with an average of À4.0). These overlapping data suggest a proximal arc source for the metasedimentary rocks. Subchondritic εHf 1850 Ma values (À5.5 to À1.2) 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 (À15.0 to À1.1) 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.

We integrate U–Pb zircon geochronology and εNd(0) values with field mapping to determine which tectonostratigraphic units are represented to the north, south and within the Almora klippe in Kumaun, NW India. Rock in the Almora klippe and north of the Main Central thrust (MCT) have Neoproterozoic (c. 900 Ma) detrital zircon ages, coupled with similar εNd(0) (−7.6 to −11.8) values, suggesting that these two units are the same tectonostratigraphic unit, and that the Almora klippe is the southern continuation of the MCT or another thrust in the Greater Himalayan thrust system. However, north of the Almora klippe, detrital zircon age populations establish the presence of Palaeoproterozoic rock instead of the previous interpretation of Neoproterozoic rocks. These Palaeoproterozoic Lesser Himalayan (LH) rocks are carried by the Ramgarh–Munsiari thrust (RMT) dipping south and folded underneath the klippe. South of the klippe, the RMT carries both the less metamorphosed Palaeoproterozoic and Neoproterozoic LH rocks, in disagreement with the idea that only Neoproterozoic–Cambrian LH rocks are present south of Almora klippe. These results suggest that previous cross-sections in Kumaun are incorrect and must be re-evaluated.Supplementary material:U–Pb zircon geochronological data table is available at http://www.geolsoc.org.uk/SUP18777.