Abstract:
The Lhasa block of Tibetan Plateau is one of the most important regions in revealing the collision between India and Asia. The entire processes of collision, from the Tethyan oceanic plate subduction to the subduction of Indian continental slab, were all recorded in the collisional and postcollisional igneous rocks in the Lhasa block. In this paper, three types of collisional and postcollisional igneous rocks in Lhasa block were discussed, following the identification of three main geochemical reservoirs in the lithospbere beneath the Tibetan plateau by viewing the updated Sr-Nd isotopic dataset. Three diverse geochemical reservoirs could be identified by their Sr-Nd isotope features: (1) the North Tibetan Plateau Geochemical Province (NTPGP, 87Sr/86Sr = 0. 707101 ~ 710536, εNd = -2 ~ -9, tDM = 0. 7 - 1.3Ga), which has a stable, homogeneous reservoir since ~42Ma, with a narrow range of isotopic ratios, as revealed by the widely spreading potassic rocks in Qiangtang, Hoh Xil, and West Kunlun belts; (2) the Neo-Tethyan mantle reservoir represented by the remnant oceanic lithosphere preserved in the Yarlung Zangpo ophiolite, with a range of 87Sr/86 Sr from 0.703000 to 0.706205, and a range of εNd from + 7.8 to + 10; (3) Himalayan continental crust component revealed by the basement and granitoids from Tethyan-, Higher and Lesser Himalayas, having the highest Sr and lowest Nd isotopic ratios among the three reservoirs mentioned above, and relatively older Nd model ages (εNd = -12 ~ -25, 87Sr/86Sr = 0.733110 ~ 0.760000, tDM = 1.9 ~ 2.9Ga.). Based on the proportion and interaction among the above-mentioned three geochemical components (reservoirs), three geochemical types of collisional and postcollisional magmatism can be recognized, each of which bears special implications for the deep processes and evolution of the Lhasa block. The first type is Lhasa block inherent type showing similar Sr-Nd composition and strong affinity to NTPGP, namely, the Lhasa block should belong to NTPGP. The second is Tethyan oceanic crust affinity type of igneous rocks (including the I-type Gangdese granitoid batholiths, Linzizong volvanics, adakitic ore-bearing porphyries, etc.), which is related to the subduction and recycling of the Tethyan oceanic crust, and relevant Cu-Mo-Au mineralization. The third one is the so-called Himalayan type represented by the ultrapotassic volcanics in western Lhasa block, which exhibit a mixture trend between the two components of NTPGP and Himalayan continental crust. Therefore, the Sr-Nd data of ultrapotassic rocks will be an important evidence for the subduction of Indian continental slab beneath southern Tibet, in consistent with some geophysical models.