Early Neoproterozoic meta-gabbro (~925 Ma) from the Lhasa terrane, Tibetan Plateau and its geological significance

In the past decades, a lot of studies have been focused on the amalgamation between the Indian and Eurasian continents in the Tibetan Plateau during the Cenozoic. However, little attention was paid to the Precambrian evolution of the Tibetan Plateau though it has the same important scientific significance to those of the Cenozoic. Numerous Precambrian rocks occur in the Lhasa terrane such as the Nyainqentanglha and Nyingchi Groups, and they are commonly considered to be the Precambrian basement of the Lhasa terrane. The Nyainqentanglha Group is one of the most important Precambrian rocks in the Tibetan Plateau, and it plays a key role in understanding the Precambrian tectonic evolution of the plateau. The Nyainqentanglha Group is composed of a suit of tectonic slices with different sizes and ages, including meta-sedimentary, meta-volcanic and meta-basic-acid intrusive rocks. In this paper, we reported new zircon U-Pb ages and Hf isotope, and whole-rock major and trace element compositions for meta-gabbro from the Nyainqentanglha Group in the Ren Co area, north-central Lhasa terrane, Tibet. Zircon LA-ICP-MS and SIMS U-Pb dating yielded weighted mean 206Pb/238U ages of 930.9±7.3 and 925.2±6.2 Ma, respectively. Zircons from the meta-gabbro exhibit pudding, fan-shaped or slight zoning structure and high Th/U ratios (0.73-31.6), indicating that these zircons are of magmatic origin. Thus these ages represent the crystallization age of the protoliths of meta-gabbro. This is the oldest ages of the mafic magmatic rocks in the Lhasa terrane, as well as within the Tibetan Plateau. The meta-gabbros contain low SiO2 concentrations (47.83wt%-49.32wt%) and show the sub-alkaline basalt features in the SiO2 versus Zr/TiO2 diagram. The chondrite-normalized rare earth element (REE) and primitive-mantle-normalized trace element multi-element patterns indicate that these rocks can be divided into two groups. The REE and trace element patterns of both groups are similar to those of N-type mid-ocean ridge basalts (N-MORB), meanwhile Group A samples have relatively higher REE and high field strength element (HFSE) contents than those of the Group B samples. The high field strength element ratios (Ce/Zr, Th/Yb, Th/La, La/Yb, La/Sm, La/Nb, Zr/Nb, Hf/Nb, and Y/Nb) of both group samples can be comparable with those of N-MORB. In addition, the meta-gabbros also display some features of island arc tholeiite (IAT), and Group B samples show some subduction affinity in the geochemical discrimination diagrams. Furthermore, zircons from meta-gabbro have clearly positive εHf(t) values (8.26-13.7) and young zircon Hf model ages (TDM=933-1205 Ma), suggesting that the protoliths of meta-gabbros are derived from a long-term depleted mantle source. These features indicate that meta-gabbros are probably the remnant of an Early Neoproterozoic oceanic crust. According to the published data, it is still unclear for the age of the Nyainqentanglha Group. Recently, zircon U-Pb dating showed that its ages had a large variation ranging from Early Neoproterozoic (ca. 897 Ma) to Cenozoic (ca. 59 Ma). The discovery of ca. 925 Ma meta-gabbro give us new constrain on the timing of the Nyainqentanglha Group. Thus the Nyainqentanglha Group should be disassembled in the future, and the Precambrian evolution of the Lhasa terrane will be clearer. © 2016, Science Press. All right reserved.