article i nfo The Lhasa Terrane in southern Tibet has long been accepted as the last geological block accreted to Eurasia before its collision with the northward drifting Indian continent in the Cenozoic, but its lithospheric architecture, drift and growth histories and the nature of its northern suture with Eurasia via the Qiangtang Terrane remain enigmatic. Using zircon in situ U-Pb and Lu-Hf isotopic and bulk-rock geochemical data of Mesozoic-Early Tertiary magmatic rocks sampled along four north-south traverses across the Lhasa Terrane, we show that the Lhasa Terrane has ancient basement rocks of Proterozoic and Archean ages (up to 2870 Ma) in its centre with younger and juvenile crust (Phanerozoic) accreted towards its both northern and southern edges. This finding proves that the central Lhasa subterrane was once a microcontinent. This continent has survived from its long journey across the Paleo-Tethyan Ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the Qiangtang Terrane to the north and with the Indian continent to the south. Zircon Hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks (e.g., ~50-90%, 0-70%, and 30-100% to the Mesozoic magmatism in the southern, central, and northern Lhasa subterranes, respectively). We suggest that much of the Mesozoic magmatism in the Lhasa Terrane may be associated with the southward Bangong-Nujiang Tethyan seafloor subduction beneath the Lhasa Terrane, which likely began in the Middle Permian (or earlier) and ceased in the late Early Cretaceous, and that the significant changes of zircon eHf(t) at ~113 and ~52 Ma record tectonomagmatic activities as a result of slab break-off and related mantle melting events following the Qiangtang-Lhasa amalgamation and India-Lhasa amalgamation, respectively. These results manifest the efficacy of zircons as a chronometer (U-Pb dating) and a geochemical tracer (Hf isotopes) in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics.

The Lhasa Terrane: Record of a microcontinent and its histories of drift and growth

Extremely light K in subducted low-T altered oceanic crust: Implications for K recycling in subduction zone

The >2000 km Indus-Yarlung Tsangpo suture zone (IYSZ) is composed of the Neo-tethys oceanic remnants, flysch units and related continental rocks, which has been regarded as the boundary between the Eurasian and Indian terranes. Among the ophiolitic complexes, the Purang ophiolite is the biggest massif in the IYSZ, and many studies have been conducted on this ophiolite. However, previous studies have mainly focused on harzburgite, clinopyroxenite and dunite. Field observations show that mafic dykes were emplaced within the Purang ophiolite. However, petrogenetic evolutions of those mafic dykes are poorly understood. In this study, we present new LA-ICP-MS zircon U–Pb dating results, whole-rock geochemistry and Sr–Nd–Hf isotope analyses for microgabbro, gabbro and dolerite dykes from the Purang ophiolite of the southwestern IYSZ, respectively. Three samples yielded zircon U–Pb ages of 144.2 ± 2.1 Ma, 127.9 ± 2.3 Ma and 126.5 ± 0.42 Ma, suggesting two different phases of magmatic activities distinctly. Whole-rock geochemical results suggest that the gabbro samples show alkaline features marked by enrichments of light rare earth elements (LREE) and large-ion lithophile elements (LILE), as well as Nb–Ta elements, suggesting an oceanic island basalt-like (OIB-like) geochemical affinity. However, the dolerite and microgabbro samples demonstrate sub-alkaline characteristics with normal mid-oceanic ridge basalt-like (N-MORB-like) geochemical features. Three distinct mafic dykes show significant Rb element depletion. The geochemical data and Sr–Nd–Hf isotopic features suggest that the microgabbro and gabbro rocks were derived from a depleted mantle that had been metasomatized by partial melts of sediments and enriched slab-derived fluids. The dolerite was also originated from a depleted mantle marked by significantly depleted Sr–Nd–Hf compositions, which was not influenced by enriched slab-derived fluids and sediments contamination during subsequent evolution. The isotope and geochemical data and tectonic diagrams suggest a tectonic transition from a within-plate to a mid-oceanic ridge basalt-like (MORB-like) setting during the period from ca. 144 Ma to 127 Ma. Combined with regional background and this study, we propose that these mafic dykes were formed in an oceanic back-arc basin setting. Additionally, integrated with previous studies, we suggest that the geodynamic evolution of the southwestern and central parts of the Neo-Tethys oceanic basin is comparable in Early Cretaceous.

Geochronology and petrogenesis of the mafic dykes from the Purang ophiolite: Implications for evolution of the western Yarlung-Tsangpo suture zone, southwestern Tibet

Geosphere, February 2020, v. 16, no. 1, 407-434, doi:10.1130/GES02154.1, Supplemental Materials. Includes whole rock geochemical, zircon U-Pb dating and Hf results of gabbrodioritic samples, as well as detrital zircon U-Pb ages and Hf isotopes of Langjiexue Group sandstones in Tethyan Himalaya from this study and many other published papers. Detailed analytical methods and metadata, as well as additional figures.

/pdf/identification-of-a-new-source-for-the-triassic-langjiexue-2cvkuaqkwj.pdf

Identification of a new source for the Triassic Langjiexue Group: Evidence from a gabbro-diorite complex in the Gangdese magmatic belt and zircon microstructures from sandstones in the Tethyan Himalaya, southern Tibet

Neoproterozoic magmatism in the northern margin of the Yangtze Block, China: Implications for slab rollback in a subduction-related setting

Petrogenesis of Late Cretaceous mafic enclaves and their host granites in the Nyemo region of southern Tibet: Implications for the tectonic-magmatic evolution of the Central Gangdese Belt

Back-arc basin evolution in the southern Lhasa sub-terrane, southern Tibet: Constraints from U-Pb ages and in-situ Lu-Hf isotopes of detrital zircons

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Petrogenesis of Late Cretaceous mafic enclaves and their host granites in the Nyemo region of southern Tibet: Implications for the tectonic-magmatic evolution of the Central Gangdese Belt

Back-arc basin evolution in the southern Lhasa sub-terrane, southern Tibet: Constraints from U-Pb ages and in-situ Lu-Hf isotopes of detrital zircons