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Abstract: U–Pb and Hf isotopes in zircon from the highly metamorphosed core of the Himalaya in the Garhwal region of India indicate that the ortho- and paragneiss units of the Greater Himalayan Sequence (oGHS and pGHS, respectively) are geochemical distinct from the older Lesser Himalayan Crystalline Sequence (LHCS). Zircon grains from Paleoproterozoic metasediment of the LHCS yield U–Pb crystallization ages from 1614±14Ma to 2679±14Ma with a single distinct peak in the age spectra at 1870Ma. εHf values from zircon in the LHCS range from 1 to −8. In contrast, zircon grains from the Neoproterozoic pGHS yield U–Pb crystallization ages from 481±10Ma to 2560±8Ma with a single distinct peak at 860Ma. εHf values from these zircons range from 13 to −21. The oGHS yields zircon grains with U–Pb crystallization ages from 416±2Ma to 2740±2Ma with a single distinct peak at 472Ma. These grains often exhibit older inherited cores ranging from ~800Ma to ~2740Ma with Ordovician rims (416Ma to 510Ma). The εHf values of zircon grains in the oGHS have two distinct modes: the ~860Ma inherited zircon cores have a similar range to the ~860Ma zircons from the pGHS (13 to −21), whereas the ~480Ma rims range from −7 to −11. The detrital zircon U–Pb age spectra from the pGHS in the Garhwal region differs significantly from the U–Pb age spectra in other parts of the range (Nepal and Bhutan). These differences indicate wide lateral variations in provenance for the various source regions of the pGHS. Comparisons of the U–Pb and Hf data from the Garhwal region with potential ~1800Ma source regions show overlap with metasediments of the Aravalli shield of India. In the pGHS the most juvenile εHf values overlap with those of the northern Arabian Nubian Shield in Israel. However, these rocks show little crustal contamination at ~800Ma, whereas the pGHS has significant crustal contamination at ~860Ma. The Ordovician oGHS has εHf values that match the average εHf values of the paragneiss implying that it was sourced from the paragneiss. Although the nature of this Ordovician igneous event is still poorly understood, it is clear that the Hf isotopes can provide further insight into the petrogenesis of the magmatic rocks associated with it. [Copyright &y& Elsevier]

Abstract: The Subhimalaya of NW India exposes a well developed marine to continental foreland sequence preserving sediments shed from the tectonically evolving Cenozoic Himalayan orogen. Detrital zircon U–Pb ages and their Hf isotopic compositions, of seven samples, were analyzed from the Subathu sub-basin foreland formations to constrain sediment provenance. The detrital zircon U–Pb age spectra yielded a similar pattern, for all foreland formations, with major peaks clustered at ~500–650Ma, 900–1000Ma and ~2500Ma, corresponding to (indistinguishable) Tethyan Himalaya and Higher Himalayan Crystalline sources and those at ~1800–1900Ma corresponding to lower a Lesser Himalayan arc source. Comparison of the foreland detrital zircon U–Pb age spectra with Himalayan lithotectonic source units, in conjunction with their Hf isotopes and previously determined sediment petrography and age constraints, suggest that the sediments to the flysch marine Subathu Formation (~65 to >41Ma) were sourced from a positive-relief area of the Tethyan Himalaya mixed with detritus from lower Lesser Himalaya and additional cratonic northern Indian Precambrian rocks. No Cretaceous to Eocene-aged detrital zircons, contributed by eroding Ladakh batholith north of the Indus Tsangpo suture zone, were recorded in the Palaeogene-aged foreland samples. The younger synorogenic continental Dagshai Formation (<30 to <22Ma) sediments were sourced from the very low-grade Tethyan Himalaya, whereas the alluvial-facies Kasauli Formation (<22 to 13Ma) began to receive sediments additionally from erosion of the medium-grade metamorphic rocks of the Higher Himalayan Crystalline and the lower Lesser Himalaya. The Lower Siwalik Formation sediments (~13–11.5Ma) were derived from the low-grade rocks of the Tethyan Himalaya, both the upper and lower Lesser Himalaya and additionally from the eroding high-grade migmatitic gneisses of the Higher Himalayan Crystalline. Thus, from the similar detrital zircon age spectra of the foreland formations, we infer that differences in the continental sediment detritus into the foreland basin reflect a straightforward sediment mixture of variable proportions from fixed Himalayan lithotectonic sources which progressively shifted towards the foreland with development of the orogen. [Copyright &y& Elsevier]

Abstract: Multiple subduction events occurred preceding the collision of Greater India with the Karakoram terrane that consumed the Neotethyan Ocean between Early Cretaceous and Eocene time. Zircon U–Pb dating and Hf isotope analyses of granitic rocks collected from the Karakoram terrane and Ladakh batholith have been conducted to constrain these geological processes. Undeformed Ladakh batholith granodiorite yielded zircon U–Pb ages of 68 and 64 Ma with ε Hf(t) values of +6.4 and +10.3, and three diorites near Leh and Darbuk, gave zircon U–Pb ages of 50.4–51.0 Ma and ε Hf(t) values of +7.4 to +8.9, indicating that the Paleocene–Eocene was the most important period of granitic magmatism that derived from a juvenile source. Three tonalite–granodiorite enclaves at Skyangpoche within the Karakoram batholith have zircon U–Pb ages of 101–103 Ma with ε Hf(t) values of −2.3 to −3.7, indicating generation of granitic rocks by partial melting of the Karakoram basement. Three samples of the Miocene Karakoram batholith, from around Darbuk, yielded ages of 16.6–19.1 Ma with ε Hf(t) values of −8.5 and −10.5, that indicate an input of ancient crust from the Indian plate. Granodiorite within the Karakoram metasedimentary sequence (zircon U–Pb age ∼74 Ma, ε Hf(t) of +5.4 from the Tangtse gorge) and granite around Muglib (zircon U–Pb age of 63 and 56 Ma, ε Hf(t) of +6.8 and +6.0) indicate similar geochronological and isotopic characteristics to those of the Ladakh batholith. Thus the time resolved Hf isotopic compositions of magmatic zircons from the granites in the Karakoram and Ladakh batholith show a large variation that indicate their derivation from both ancient and juvenile continental crusts, in active continental margin since at least the Mid Cretaceous. [Copyright &y& Elsevier]