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This article presents zircon U-Pb and Hf isotope data, together with the whole-rock major- and trace-element composition, of Early Carboniferous granitoids newly identified from the Jiacha and Langxian areas in the southern Lhasa terrane, southern Tibet. The Jiacha rocks are monzogranites that yield zircon U-Pb ages of 347-345 Ma and εHf(t) values from -5.4 to -4.9. The Langxian rocks are granodiorites with slightly older zircon U-Pb ages of 355-352 Ma and lower εHf(t) values from -6.8 to -6.5. Our data suggest that these granitoids were generated largely by reworking of Paleoproterozoic (TCDM=1.78-1.67 Ga) basement materials. In conjunction with literature data, it is further argued DM that the southern and central parts of the Lhasa terrane, separated by the Sumdo eclogite belt, should have been an integrated block before the late Paleozoic. Our study supports the notion that the Lhasa terrane was derived from the northern margin of Gondwanaland, in association with formation of at least two stages of Tethyan Ocean basins, now exposed as the Sumdo belt and the Indus-Tsangpo suture. [ABSTRACT FROM AUTHOR]

Uplift of the Tibetan Plateau and its influence on our global climate have been the focus of numerous studies. Miocene potassic to ultrapotassic volcanism is widespread in southern Tibet and has been generally attributed to convective removal of collision-thickened Asian lithosphere, which is also responsible for the uplift of the plateau. An implicit assumption of this model is the existence of a hydrous, metasomatized (i.e., phlogopite bearing) lithospheric mantle that remained after the convective thinning and was subsequently heated to form small-volume melts. If such a lithospheric mantle was present in the Miocene, it implies further change since that time, as seismic velocities indicate that cold and strong upper mantle occurs beneath the thick crust in southern Tibet. Here we describe peridotite xenoliths entrained in Middle Miocene ultrapotassic lavas from Sailipu, southern Tibet. The results suggest the existence of hot, highly metasomatized lithospheric mantle beneath southern Tibet during the Middle Miocene, and thus support the idea that convective thinning of the lithosphere was responsible for the uplift of the plateau. The relict mantle was later removed or squeezed northward by the underthrusting Indian continental lithosphere, which terminated magmatism in southern Tibet and played a role in creating the entire plateau. [ABSTRACT FROM AUTHOR]

Recent studies have indicated that an Andean- type orogen (Lhasaplano) developed on the Lhasa block in the Cretaceous during northward subduction of Neo-Tethyan oceanic lithosphere. When and how uplift of the Lhasaplano began, however, has remained controversial. This article integrates stratigraphic, sedimentological, tectonic, and provenance data from the latest marine to nonmarine strata in the Linzhou Basin to pinpoint the early topographic growth in southern Tibet. The Takena Formation mainly consists of lagoonal carbonates and mudstones yielding foraminiferal assemblages of Early Aptian age (ca. 123-119.5 Ma). The conformably overlying lower member of the Shexing Formation, mainly deposited in fluvial environments, was fed by volcanic and sedimentary rock fragments from the north Lhasa terrane. Clasts of the Gangdese arc to the south firstly appeared in the middle member and became dominant in the upper member of the Shexing Formation. By contrast, coarse grained, braided river facies occur in the uppermost part of the Shexing Formation, where detritus was mostly recycled from Paleozoic strata of north Lhasa, with minor volcaniclastic contribution from the Gangdese arc. Basin analysis indicates accelerating subsidence and sedimentation rates during deposition of Takena to middle Shexing strata (ca. 125-108 Ma), followed by steady subsidence during deposition of upper Shexing strata (ca. 108-96 Ma). Given this regional tectonic and sedimentary evidence, such an evolution is interpreted to reflect tectonic extension followed by thermal subsidence. Basin inversion and regional compression initiated during deposition of the uppermost Shexing strata (ca. 96 Ma), as indicated by active thrust faults and widespread accumulation of syntectonic conglomerates in the western part of the Lhasa block. This event marked the beginning of the Andean-type orogeny in southern Tibet. Such a paleotectonic evolution, from extension to tectonic inversion, is also documented in the Andes mountain range. It may be typical of the early stage growth of Andean-type active continental margins. [ABSTRACT FROM AUTHOR]