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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]

Divergent double subduction (DDS) plays an important role in facilitating tectonic processes such as ocean closure, accretion and amalgamation of magmatic arcs, and continental growth. However, DDS geodynamics remain poorly constrained, especially its evolution and the accompanying magmatism at the terminal stage. According to geological and geophysical data, the Meso-Tethys Ocean (also known as the Bangong–Nujiang Tethyan Ocean) in central Tibet has been considered a Mesozoic example of DDS, which provides a rare opportunity to evaluate the geodynamic processes of the terminal stage of a DDS system. Here, we present new geochronological, geochemical, and isotopic data of volcanic rocks from the Gerze area in the western segment of the Bangong–Nujiang suture zone (BNSZ). Combined with previous data, we found that the ca. 105 Ma magmatic rocks in the Gerze area are distributed not only along the two sides of the BNSZ but also as manifold types. On the northern side, the magmatism ca. 105 Ma varied, including A-type rhyolites, bajaitic latites, high-Nb basalts, N-MORB-type basalts, adakites and bimodal volcanic rocks, which originated from sources including a sinking oceanic slab, asthenospheric mantle, metasomatized mantle and overlying continental crust. The lithology on the southern side only includes A-type rhyolite and magnesia-rich andesite and adakitic intrusions. Spatial and compositional variations in ca. 105 Ma magmatism should have been the response to slab breakoff, which may have occurred only in the northern branch of the Meso-Tethys oceanic slab after ocean closure. Our results not only verify the existence of slab breakoff in the classical DDS model but also emphasize that breakoff can cause upwelling of the subslab asthenosphere, which leads to a complex magmatic response. [Display omitted] • Manifold magmatism from the Gerze area, central Tibet, is active at ca. 105 Ma. • The ca. 105 Ma Gerze magmatic rocks record a slab breakoff event. • Slab breakoff plays an important role in the divergent double subduction system. [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]