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

Magnesium isotopic compositions of igneous rocks could be potentially used to trace recycling of supracrustal materials. High-delta Mg-26 granitoids have been previously reported and explained to reflect the involvement of surface weathered materials in their sources. Low-delta Mg-26 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 delta Mg-26 values (-0.46 to-0.26 parts per thousand and -0.41 to -0.17 parts per thousand, respectively) significantly lighter than the mantle (-0.25 +/- 0.07%, 2SD). No correlation of delta Mg-26 with either SiO2 or MgO is observed, indicating limited Mg isotope fractionation during alkaline magmatic differentiation. The low delta Mg-26 of the syenites and syenogranites thus reflects a light Mg isotopic source. This, combined with high Sr-87/Sr-86 ratios (0.70814 to 0.71105) and negative correlation between delta Mg-26 and delta O-18, 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. Published by Elsevier B.V.