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The palaeogeographic locations of ancient continental blocks in the Tibetan Plateau, especially of micro-continental blocks during the Neoproterozoic supercontinent cycle, remain enigmatic. To clarify the petrogenesis of Neoproterozoic granitic gneisses and the early evolution of the Amdo micro-continent, we conducted an integrated study of the petrography, geochronology, and geochemistry of biotite monzonitic gneisses from the Precambrian metamorphic basement. The biotite monzonitic gneiss sample yielded a weighted mean zircon U–Pb age of 820 ± 11 Ma, indicating an early Neoproterozoic magmatic event. All gneiss samples show enrichment in light rare-earth elements and depletion in high-field-strength elements in N-MORB-normalized spider diagrams, which suggests that their precursor magma was generated in a subduction-related active continental margin. In addition, these rocks have mostly positive εHf(t) values (−1.0 to 7.0) and corresponding crustal model ages of 1.3 − 1.8 Ga. Zircon Hf isotopes and whole-rock geochemistry indicate that the gneisses were derived from partial melting of juvenile crust material. Recently published age spectra of detrital zircons from different blocks suggest that the Amdo micro-continent had a closer affinity to the Lhasa terrane than to the Qiangtang terrane prior to and during the break-up of the Rodinian supercontinent. However, the gneisses are geochemically distinct from back-arc magmatic rocks in the Lhasa terrane and similar to coeval Andean-type magmatic rocks in blocks that constituted the northwestern margin of Rodinia (including Madagascar, Seychelles, and Malani). This study therefore provides new constraints on the Neoproterozoic palaeogeographic location and evolution of the Amdo micro-continent. [ABSTRACT FROM AUTHOR]

The late Paleozoic tectonic evolution of the Lhasa terrane and subduction of Sumdo Paleo-Tethys ocean remains controversial due to limited data and variable Permian magmatism in central and southern Lhasa. We report new zircon U–Pb ages and Hf-isotope ratios as well as whole-rock major element, trace element, and Sr–Nd isotope data from middle Permian (266 ~ 257 Ma) arc-related magmatism, including high-Mg quartz diorite and trondhjemite in the Tangjia-Sumdo area. The quartz diorite has high SiO2 (53.92–57.90 wt.%), MgO (3.69–8.03 wt.%) and low TiO2 (0.60–1.01 wt.%) contents. It is enriched in the large ion lithophile elements (LILEs, e.g. Rb, Ba, Sr) and depleted in the high field strength elements (HFSEs, e.g. Nb, Ta, Ti). We interpreted these chemical features, in combination with uniform whole-rock (87Sr/86Sr)i (0.70375–0.70452), εNd(t) (+4.48 – +5.55), and zircon εHf(t) (+11.2 – +13.7) values, as indicative of dioritic melt derived by the interaction of fluids expelled from the subducting slab with the overlying mantle wedge in an island arc setting. The trondhjemite has high SiO2 (71.16–73.52 wt.%), Al2O3 (14.92–16.24 wt.%) and K2O + Na2O (7.34–8.27 wt.%) contents, is enriched in the light rare-earth elements (LREEs) and is depleted in Th and heavy rare-earth elements (HREEs). These rocks are characterized by positive εNd(t) (+5.91 – +6.68) and εHf(t) (+12.0 – +16.1) values and may have been derived from partial melting of subducted oceanic crust. Based on the elemental and isotopic composition of the quartz diorite and trondhjemite, we suggest that middle Permian magmatism occurred in an intra-oceanic subduction setting in the Sumdo Paleo-Tethys ocean. The Sumdo Paleo-Tethys ocean was probably a small ocean basin within the eastern Lhasa terrane and not a vast ocean basin that completely separated central Lhasa from the southern Lhasa subterrane. [ABSTRACT FROM AUTHOR]

• The newly discovered Middle Permian granite dikes are adakitic in composition. • The adakitic magma was generated by partial melting of subducted oceanic slab. • The granite dikes were related to the subduction of the Sumdo Paleo-Tethys Ocean. The Sumdo Paleo-Tethys Ocean was the southernmost part of the Paleo-Tethys Ocean, located on the Qinghai–Tibet Plateau. However, the evolution of the Sumdo Paleo-Tethys Ocean and the origin of eclogites in the Sumdo area of Tibet are still matters of debate. Here we report newly discovered adakitic granite dikes from the Sumdo area of the central Lhasa terrane. The rocks have high SiO 2 , Al 2 O 3 , and Sr contents, high Sr/Y and La/Yb ratios, enrichment in light rare-earth elements (REEs), and depletion in Y, Yb, Th, and heavy REEs. They are characterized by low 87Sr/86Sr (0.703678–0.704427) and high 143Nd/144Nd (0.512825–0.512904) ratios, and positive ε Nd (t) (+6.41 to +7.38) and ε Hf (t) (+13.8 to +15.7) values. Zircon U–Pb dating yielded ages of 269.1 ± 4.1 and 266.4 ± 2.0 Ma for the rocks. Our data indicate that the Sumdo adakitic granite dikes were derived from partial melting of subducted oceanic slab, with the magma being influenced by slab-derived fluids and slight interaction with peridotite in the mantle wedge during ascent. By combining our new geochemical data with results of previous studies, we infer a subduction event involving the Sumdo Paleo-Tethys Ocean during the middle Permian, which led to the formation of the Sumdo adakitic granite dikes and the Sumdo, Jilang, and Xindaduo eclogites. [ABSTRACT FROM AUTHOR]

Intra-oceanic subduction is a fundamental process on Earth, the study of which can improve understanding of plate tectonic processes and the history of continental growth. Several studies of the intra-oceanic subduction of the Meso-Tethys Ocean have been published in recent years, particularly relating to the Bangong–Nujiang suture zone (BNSZ) and the Shiquanhe–Nam Tso mélange zone (SNMZ) in central Tibet. Here, we report newly discovered occurrences of Jurassic diorite (161.2–160.6 Ma) and plagiogranite (161.0 Ma) from the western part of the central SNMZ. The diorite samples have moderate SiO 2 and high MgO contents and Mg# values; high Ba/Th and Ba/La ratios; low K 2 O, TiO 2 , and REE contents; low (La/Yb) N , (La/Sm) N , and Th/Yb ratios; and positive ε Hf (t) values, similar to high-Mg diorite formed by partial melting of a subduction-modified mantle wedge. The plagiogranite intrudes gabbro of the ophiolite suite and has high SiO 2 and Na 2 O contents; low Al 2 O 3 , K 2 O, P 2 O 5 , TiO 2 , and total REE contents; and clear negative Eu anomalies, similar to shear-related oceanic plagiogranite, which can be formed by partial melting process of gabbro in the high temperature shear zone near the expansion center. The formation processes of Asa high-Mg diorite and shear-related plagiogranite are consistent with initial subduction within the Meso-Tethys ocean. Based on these data and those for sedimentary and Jurassic magmatic rocks of the region, we propose that, during subduction, the Northern Lhasa terrane was formed from a collage of oceanic island arcs, oceanic plateaus, and accretionary complexes. • Intermediate-acid magmatism in the SNMZ, central Tibet. • High-Mg diorites and shear-type plagiogranite with Late Jurassic ages (161–160 Ma). • The Jurassic intra-Meso-Tethys oceanic subduction event (165–160 Ma). • The generation of the Northern Lhasa terrane by accretion of intra-oceanic arcs. [ABSTRACT FROM AUTHOR]