New Zinc Isotope Constraints On The Origin Of Carbonatites From West Sichuan,China

Detailed C-O-Sr-Nd isotopes,trace elements,and Zn isotope data are reported on the Maoniuping carbonatite-syenite complex and minerals,which were formed in a continental collision zone.The?¹⁸O_V-SMOW?6.95‰-7.33‰?and?¹³C_V-PDB?-6.51‰--7.00‰?values of the Maoniuping carbonatites suggest that they have the isotope characteristics of“primary igneous carbonatites”.The similar formation ages,high(⁸⁷Sr/⁸⁶Sr)_t,low?_Nd values,and enrichment of REE for the spatially associated carbonatites and syenites indicate that they were from the same metasomatized mantle source.Combing with C-O-Sr-Nd isotopes,major and trace elements reported in previous studies,the Maoniuping carbonatites and syenites can have been formed via liquid immiscibility of CO₂-rich alkaline silicate magmas.Since zinc is a trace element in the mantle and zinc isotope composition of marine carbonate is significantly different from that of the mantle,Zn isotopes can be used to trace deep carbonate cycling in the sources of igneous rocks.Twelve samples drilled in the interval of 1 cm from the carbonate-syenite profile in a hand specimen scale were analyzed.The similar Zn isotope compositions of carbonatites?0.26±0.03‰-0.29±0.03‰?with syenites?0.23±0.03‰-0.28±0.03‰?suggest that they are from the same sources,and no significant Zn isotope fractionation has not been generated during liquid immiscibility.Beside the drilling samples,most of other syenites and carbonatites also have similar?⁶⁶Zn,which are coincident with that of the global basalts?0.28±0.05‰?.Some syenites have high?⁶⁶Zn values that are negatively correlated with Zn concentrations,possibly due to fractional crystallization of aegirine during the formation of syenite.Some carbonatites have high?⁶⁶Zn values and elevated Zn concentrations,which may reflect the incorporation of silicate minerals during the crystallization-evolution of carbonate magma.Overall,Zn isotopic compositions of the restored primary magmas of both carbonatites and syenites are consistent with that of the global basalts.Such Zn isotopic signatures suggest that there is no significant involvement of recycled carbonates into the mantle source of the carbonatites and syenites from Sichuan,and these rocks represent dominate mantle-derived magmas.