Nd isotopes of siliciclastic rocks from Tibet, western China: Constraints on provenance and pre-Cenozoic tectonic evolution

Nd isotope data from pre-Cenozoic siliciclastic rocks from Tibet are used to determine variations of source regions with time and hence tectonic history. On average, the T-DM and T-CHUR ages of the Tibetan samples are up to 1.83 +/- 0.36 Ga and 1.25 +/- 0.45 Ga, respectively, indicating that Tibet is underlain by Lower Proterozoic basement and has close affinity with the Himalayas. An Nd isotope crisis occurred at the Early Permian, with older samples defining a trend of slight decrease in epsilon(Nd) values with time while the Lower Permian rocks sharply increasing similar to 6.5 epsilon(Nd) units. This is related to the coeval widespread basaltic magmatism in Tibet, which could be responsible for Tibet rifting away from northern Gondwanaland. The central Qiangtang meta-sedimentary samples have distinct Nd isotope characteristics (epsilon(Nd)(0) -19.87 +/- 1.51; T-DM 2.07 +/- 0.13 Ga; T-CHUR 1.78 +/- 0.21 Ga) from the Songpan-Ganzi samples (epsilon(Nd)(0) -12.77 +/- 2.87; T-DM 1.49 +/- 0.18 Ga; T-CHUR 1.08 +/- 0.23 Ga), suggestive that these two groups were derived from different sources and that the former were not the equivalents of the latter that was underthrust from the Jinsa suture and may represent subducted continental crust. The Mid-Jurassic flysch rocks in northern Lhasa have typical upper crust Nd isotopic signatures and could have formed under a passive margin environment. Their Bangong counterparts, averagely 6.35 units higher in epsilon(Nd) values, unveil derivations from a likely magmatic arc along the northern Bangong margin. Basaltic pebble with low Nd content and rNd value from the Berriasian-Valanginian rocks in central Tibet, along with the samples with generally high epsilon(Nd) values, shows that the Bangong ophiolites could have been a main source of sediments during the earliest Cretaceous time. A significant transition of sediment sources occurred at the beginning of the Hauterivian Stage (similar to 120 Ma). Sedimentary basaltic tuff (epsilon(Nd)(t)2:27) in the Hauterivian-Lower Cenomanian samples in central Tibet, and common Nd signatures for input of mantle-like materials in these rocks, indicate that mantle-derived magmatic rocks could have constituted a significant source. The huge Hauterivian-Lower Cenomanian sediments in central Tibet are thus inferred to have been accumulated under a (back-arc) rifting setting. (C) 2007 Elsevier B.V. All rights reserved.