Secular geochemical variations of the Lower Cretaceous siliciclastic rocks from central Tibet (China) indicate a tectonic transition from continental collision to back-arc rifting

Fifty-five Lower Cretaceous siliciclastic rocks from central Tibet were analyzed for major and trace elements, in an attempt to evaluate their provenance and the type of tectonic setting in which they were deposited. The spatial-temporal variation of both sediment maturity indices (SiO2/Al2O3, La/V, and Zr/TiO2) the ratios of immobile trace elements (Cr/Th, Th/Sc, Th/Co, La/Sc, and La/Co), and SiO2, K2O/Na2O, Eu/Eu*, (Gd/Yb)(n), and ferromagnesian contents, displays that the Berriasian-Valanginian (BV) sediment could have been derived from northern orogenic source areas, whereas the Hauterivian-Lower Barremian (HB) sediment could have been closely related to the syndepositional volcanism in the northern Lhasa block itself. The Na2O/Al2O3-K2O/Al2O3 relations also place approximately half of HB samples in the field of volcanic rocks of back-arc rifting origin, whereas little volcanic clasts exist in the BV rocks. There are distinct signatures that the mafic/ultramafic materials in the BV rocks were derived from oceanic island basalts (ophiolites) in the Bangong-Nujiang suture. Therefore, the collisional orogen due to the Qiangtang-Lhasa collision should have played a key role in supply of the sediments during BV time. Bimodal sedimentary basaltic and rhyolitic tuffs, with respective distinguished geochemical features, exist in the HB sequence. Therefore, a distinct tectonic transition from continental collision to back-arc rifting in central Tibet is believed to have occurred during Early Cretaceous time. Extreme depletion of Na of most Lower Cretaceous samples compared to the upper crust, and their high average chemical index alteration (CIA) values and Th/U ratios, point out the source rocks formed in tropical climatic regions with little tectonic uplift, although the weathering degree of the source rocks for HB samples could likely have been affected by the syndepositional volcanism. During Middle Cretaceous time, the entire southern Eurasian margin could have been characterised by a series of back-arc rift basins (including those in Kohistan and Ladakh). (C) 2004 Elsevier B.V. All rights reserved.