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The Mid-Late Mesozoic magmatic arc in southern Qiangtang block was associated with the long-lasting subduction of the Bangong-Nujiang Tethyan and the subsequent Lhasa-Qiangtang collision. The detailed processes associated with the transition from oceanic lithosphere subduction to lithosphere delamination remain unclear. This paper reports zircon U-Pb age, major and trace element and Sr-Nd isotopic data for the Cretaceous two-phase magmatic rocks in the Shuanghu region of the southern Qiangtang block located in central Tibet. Zircon dating on the Qushenla volcanic rocks yields a 206Pb/238U age of 114 ± 4 Ma. The Abushan volcanic rocks comprise rhyolites. Zircon dating on two rhyolites yields a 206Pb/238U age of 74 ± 1 to 76 ± 1 Ma. In contrast with the Qushenla volcanic rocks, the Abushan rhyolites exhibit features typical of high-fractionated I-type granitoids, high K calc-alkaline and shoshonite, and peraluminous, suggesting that the Abushan rhyolites formed in post-collision setting. The Qushenla volcanic rocks have slightly higher initial 87Sr/86Sr (0.70466–0.70537), and εNd(t) (1.71–2.89) in comparison with the Abushan rhyolites (87Sr/86Sr = 0.70453–0.70491, εNd(t) = 1.63–2.35). The Qushenla volcanic rocks were most likely derived from the mixing of mantle-derived components and crustal material. And the crust assimilation and fractional crystallization played a role during the ascent of magma. The Abushan volcanic rocks derived from magmatic mixing between the crustal materials and mantle-derived components, and undergoing extensive fractional crystallization. Taking into account the regional tectonic and magmatic data, we suggest that the transition from the oceanic lithosphere subduction to lithosphere delamination occurred during the Late Cretaceous in central Tibet. It evolved through three stages: 125–100 Ma, late subduction of Bangong-Nujiang Tethys; 100–87 Ma slab break-off of subduction Bangong-Nujiang Tethys; and 87–73 Ma lithosphere delamination. [ABSTRACT FROM AUTHOR]

The subduction of the Bangong–Nujiang Ocean is a long-standing problem in the study of the geological evolution of the Tibetan Plateau. Based on recent data acquired from the Abushan volcanic rocks in the Qiangtang Terrane, we will discuss the relationship between the Cretaceous volcanism and the evolution of the Bangong–Nujiang Ocean. The Abushan volcanic rocks are mainly composed of trachy-andesites, dacites, and rhyolites. Zircon U–Pb dating constrains the time of emplacement as the Middle Cretaceous (102.6 ± 1.6–96.1 ± 2.4 Ma). Major element geochemistry shows that the volcanic rocks belong to the high-K calc-alkaline and calc-alkaline series. All of the volcanic rocks are enriched in light rare earth elements (LREE) and some large ion lithophile elements (LILE, Rb, Th, and U) and are depleted in heavy rare earth elements (HREE) and some high field strength elements (HFSE, Nb, Ta, and Ti). The geochemical characteristics of the Abushan volcanic rocks are correlated with those of the volcanic arc rocks in the subduction zone. The petrogenesis of the Abushan volcanic rocks suggests that they were derived from the partial melting of the relict subducted oceanic crust combined with the input of oceanic sediments. Oceanic crust will melt upon reaching its hydrous solidus in the amphibolite facies after plate collision; this induces the formation of the Abushan volcanic rocks. Our data, compared with the previous studies on volcanism in the Lhasa Terrane, leads us to propose that the Bangong–Nujiang Ocean crust was subducted beneath the Qiangtang Terrane and underwent bidirectional subduction during the Cretaceous. [ABSTRACT FROM AUTHOR]