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Links between continental subduction and generation of Cenozoic potassic–ultrapotassic rocks revealed by olivine oxygen isotopes: A case study from NW Tibet

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Tibetan Cenozoic syn-collisional potassic–ultrapotassic igneous rocks provide unique insights into the processes and origins of metasomatism in the upper mantle, as well as continental subduction and plateau uplift. Crustal recycling in the magma source of the Tibetan potassic–ultrapotassic rocks has been well-documented. However, the nature of the metasomatic agents and the timing of mantle metasomatism are still disputed. Oxygen isotopes are a powerful tracer for identifying any recycled crustal material in the mantle due to the significant fractionation caused by surface water–rock interaction. Here we present an integrated in-situ study of oxygen isotopes and the major/trace elements of olivine in the Cenozoic potassic–ultrapotassic rocks and mantle xenoliths from the western Kunlun area of the northwestern Tibet. Olivines from mantle xenoliths have oxygen isotope compositions that range from elevated δ18O values (5.83 ± 0.78–5.97 ± 0.40‰) to values (5.09 ± 0.56–5.10 ± 0.46‰) that are indistinguishable from typical mantle olivine values of 5.18 ± 0.28‰ (Mattey et al., Earth Planet Sci Lett 128:231–241, 1994). Elevated olivine δ18O values reflect mantle metasomatic processes by an 18O-rich agent, although a few olivine rims show higher δ18O values caused by reactions with host lavas. In contrast, olivine oxygen isotope compositions of potassic–ultrapotassic rocks are higher than typical “mantle” values and those of the mantle xenoliths (6.51 ± 0.74–7.52 ± 0.24‰). There is no sign of crustal contamination, and fractional crystallization did not change the isotopic compositions of the studied potassic–ultrapotassic igneous rocks, thus their high-δ18O signature reflects the nature of the primary melts and their source region. Enrichments in olivine 18O and clinopyroxene Sr isotopic compositions, as well as the enriched trace element compositions of clinopyroxene and orthopyroxene in the mantle xenoliths, indicate that they have been highly metasomatized by silicate materials with a minor carbonate component from recycled Indian continental crustal component, and the enriched lithospheric mantle is the magma source of potassic–ultrapotassic rocks. High olivine δ18O values are a common feature of Tibetan Cenozoic potassic–ultrapotassic rocks, consistent with the mantle metasomatic agents that were derived directly from recycled continental crust material via Indian continental subduction. Our study reinforces the profound link between continental subduction, mantle processes and generation of Tibetan Cenozoic potassic–ultrapotassic rocks.

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We are grateful to Associate Editor Daniela Rubatto, Angus Fitzpayne and an anonymous reviewer for their highly constructive suggestions, which greatly improved the contents and clarity of this article. This study was supported by the National Natural Science Foundation of China (grants 41722205 and 41673033), the Xinjiang Major Science and Technology project (202101679), the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (2019QZKK0702) and the CAS ‘Light of West China’ Program (2018-XBYJRC-003). This is contribution 3180 from GIG-CAS.

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Correspondence to Gong-Jian Tang.

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Communicated by Daniela Rubatto.

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Tang, GJ., Wyman, D.A., Wang, Q. et al. Links between continental subduction and generation of Cenozoic potassic–ultrapotassic rocks revealed by olivine oxygen isotopes: A case study from NW Tibet. Contrib Mineral Petrol 177, 53 (2022).

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