Search

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]

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]