Mg, Sr, and O isotope geochemistry of syenites from northwest Xinjiang, China: Tracing carbonate recycling during Tethyan oceanic subduction

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• 作者：Shan Ke^a ; Fang-Zhen Teng^b ; ^{fteng@u.washington.edu" class="auth_mail" title="E-mail the corresponding author} ; Shu-Guang Li^a ; ^{lsg@ustc.edu.cn" class="auth_mail" title="E-mail the corresponding author} ; Ting Gao^a ; Sheng-Ao Liu^a ; Yongsheng He^a ; Xuanxue Mo^a
• 关键词：Magnesium isotopes ; Syenites ; Magmatic differentiation ; Carbonate recycling ; Tibetan and Pamir plateau
• 刊名：Chemical Geology
• 出版年：2016
• 出版时间：25 October 2016
• 年：2016
• 卷：437
• 期：Complete
• 页码：109-119
• 全文大小：1226 K

文摘

Magnesium isotopic compositions of igneous rocks could be potentially used to trace recycling of supracrustal materials. High-δ²⁶Mg granitoids have been previously reported and explained to reflect the involvement of surface weathered materials in their sources. Low-δ²⁶Mg granitoids, however, have not been reported. In this study, we report high-precision Mg isotopic analyses of Cenozoic alkaline syenites and syenogranites from the Kuzigan and Zankan plutons, northwest Xinjiang, China. The Kuzigan syenites were originated from the mantle metasomatized by recycled supracrustal materials, and the syenogranites are differentiated products of the syenites. Both syenites and syenogranites have δ²⁶Mg values (− 0.46 to − 0.26‰ and − 0.41 to − 0.17‰, respectively) significantly lighter than the mantle (− 0.25 ± 0.07‰, 2SD). No correlation of δ²⁶Mg with either SiO₂ or MgO is observed, indicating limited Mg isotope fractionation during alkaline magmatic differentiation. The low δ²⁶Mg of the syenites and syenogranites thus reflects a light Mg isotopic source. This, combined with high ⁸⁷Sr/⁸⁶Sr ratios (0.70814 to 0.71105) and negative correlation between δ²⁶Mg and δ¹⁸O, suggests that the magma source contains recycled marine carbonates. Modeling of the Mg-O-Sr isotopic data indicates that the recycled carbonate is mainly limestone with minor dolostone, suggesting that the metasomatism occurred at depths shallower than 60 to 120 km. Given that the plutons are located at the India–Eurasia collision zone, the carbonate recycling was most likely derived from the subducted Tethyan oceanic crust during the Mesozoic–Cenozoic. Our study suggests that the combined Mg, O, and Sr isotopic studies are powerful for tracing recycled carbonates and identifying their species in mantle sources.