MAGNESIUM AND OXYGEN ISOTOPIC COMPOSITIONS OF ALKALINE ROCKS IN NORTHWEST XINJIANG, CHINA: IMPLICATIONS FOR ORIGIN OF LOW-D26MG GRANITOIDS

Magnesium isotopes of granitoids could be potentially used to trace recycling of supracrustal materials. High-d²⁶Mg granitoids have been previously reported which are attributed to the involvement of sedimentary rocks in the magma sources. Low-d²⁶Mg granitoids, however, have been rarely reported and their origin remains unclear. In this study, we report high-precision (±0.06‰ for d²⁶Mg, 2SD) Mg isotopic analyses for alkaline syenites and granites as well as their magnesian minerals from the Kuzigan region of northwest Xinjiang, China. All syenites have d²⁶Mg values (-0.46 to -0.28‰) which are significantly lighter than the mean of the silicate Earth and igneous rocks (-0.25 ± 0.07‰). The granites representing the products of fractional crystallization of the syenites also have low d²⁶Mg values (-0.41 to -0.19‰) similar to those of the syenites. Furthermore, d²⁶Mg values of syenites and granites show no correlations with SiO₂, MgO, FeO and CaO concentrations, indicating limited Mg isotope fractionation during crystal-melt differentiation of alkaline granitic magmas. This is consistent with the behavior of Mg isotopes during differentiation of I-type and A-type granitic magmas, although there is significant inter-mineral Mg isotope fractionation between co-existing aegirine-augite and biotite in the syenites and granites (D²⁶Mg_Agt-Bt= -0.15 to -0.67‰).

Given the absence of Mg isotope fractionation during alkaline magmatic differentiation, Mg isotopic signatures of the Kuzigan syenites and granites reflect a low-d²⁶Mg source. These syenites and granites also have high radiogenic lead isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 18.183-18.820) and heavy oxygen isotope ratios (d¹⁸O = 8.5-11.6‰), indicating the recycling and involvement of supracrustal materials in their source. There is a negative correlation between d²⁶Mg and d¹⁸O in the syenites and granites, which strongly suggests that the recycled materials are most likely carbonate, because carbonate has extremely light Mg and heavy O isotopic compositions relative to the silicate Earth and the crystalline crust. The combined use of Mg and O isotopic ratios is therefore a powerful tool to trace recycling of sedimentary carbonates in magma sources.