Yushuai Wei

Geochemical and Isotopic Characterization of Middle Eocene Hybrid Magmatism in the Gangdese Belt (Tibet) and Its Ancient Indian Crustal Fingerprint

We present new geochemistry, geochronology, whole-rock Sr-Nd-Pb, and zircon Hf isotopic data from the Zhongba pluton in the western part of the Gangdese magmatic belt (GMB) and discuss its melt evolution within the collisional tectonic framework of southern Tibet. Our U-Pb zircon dating of the Zhongba plutonic rocks has constrained the timing of their emplacement at 38 Ma. The Zhongba granitoids are enriched in light rare earth elements, large-ion lithophile elements, and Pb and are depleted in high-field-strength elements. They show enriched Sr isotopes, low εNd(t) values, and fairly radiogenic Pb isotopic signatures. They have negative zircon εHf(t) values and ancient crustal model ages of 1483–1180 Ma. These geochemical and isotopic characteristics, combined with the literature data, suggest mixing of different magma types, derived from partial melting of juvenile southern Lhasa crust and ancient Indian continental crust, to produce hybrid magmas of the Zhongba granitoids. We propose that asthenospheric upwelling caused by the breakoff of the northward-subducting Neotethyan oceanic lithosphere provided the heat and material flux to trigger partial melting of the overlying Lhasa arc lithosphere and subducted sediments of the ancient Indian crust. Similar geochemical and isotopic signatures of the coeval Kailas and Zedong granitoids within the GMB indicate that this slab breakoff–induced mid- to late Eocene magmatism was a syncollisional and orogen-parallel magmatic event. The Zhongba pluton and its contemporaneous equivalents thus provide the geochronological and isotopic evidence for the earliest melt flux derived from the subducted continental crust and sediments of India into the melt regime of Cenozoic magmatism in southern Tibet.

Thermal history of the Jurassic marine sequences in the Qiangtang Basin, northern Tibetan Plateau: implication for the hydrocarbon preservation

The Jurassic marine sequences of the Qiangtang Basin in the northern part of the Tibet Plateau have the best petroleum exploration prospect. The prototype basin has undergone multi-stage and complicated tectonic events since the Cretaceous, especially the mid-Cretaceous tectonic event and the Cenozoic uplift of Tibetan Plateau. However, little information about the impact of these tectonic events on petroleum preservation has been done. In this paper, the burial and thermal histories modeling using vitrinite reflectance (Ro) are used to characterize the impact. The results showed that there is a regional falling of palaeotemperature in the Qiangtang Basin during the mid-Cretaceous indicating a regional tectonic event caused by the collision between Lhasa and Qiangtang blocks. However, the Cenozoic tectonic event caused by the uplift of Tibetan Plateau since 30 Ma was only recorded in the Southern Qiangtang Depression and not obvious in the Northern Qiangtang Depression, which indicates that uplift-denudation caused by the mid-Cretaceous tectonic event was extensive in whole Qiangtang Basin and more significant to the hydrocarbon accumulation than those in the Cenozoic. The event could damage the previously formed hydrocarbon traps and caused the leaking of cap-rocks for the dramatically denudation especially in the Southern Qiangtang Depression. We speculated that the Northern Qiangtang Depression has the lower thickness of denudation and barely influenced by the Cenozoic tectonic events and thus is primary exploration target for gas or gas hydrate.