Petrology and geochemistry of early Carboniferous volcanic rocks in the Xinyuan region of western Tianshan: Implications for magma sources

Abstract

Understanding the sources and processes that produce fossil continental arc magmas is still a challenging problem. To address this problem, here we perform detailed petrological investigations as well as whole-rock geochemical and SrNd isotopic analyses on the early Carboniferous volcanic rocks in the Xinyuan region. These volcanic rocks mainly consist of calc-alkaline basalt, trachy-andesite, and rhyolite, and show subduction-related geochemical and isotopic signatures. Petrographic observations and new geochemical data suggest that the rhyolite was generated by partial melting of juvenile crustal materials, while the basalt and trachy-andesite might undergo a series of open-system magmatic processes, such as fractional crystallization, crustal contamination, and magma replenishment and mixing. The primitive basaltic rocks with Mg# values of ~70 were least affected by crustal-level magmatic processes, and they could be used to constrain sub-arc mantle processes. The Nb/Yb, Zr/Yb, and Ta/Yb ratios of these basaltic rocks are similar to those of E-MORB, suggesting a relatively fertile mantle source prior to subduction, while their high Ba/Th, Sr/Nd, and Th/Nd ratios suggest a significant contribution of slab-derived components to the pre-subduction mantle wedge. Geochemical modelling based on trace elements and SrNd isotopic ratios indicates that the pre-subduction mantle wedge was modified by ~0.5% subducted sediment-derived fluids and ~2% bulk sediments. About 10% partial melting of such enriched mantle source could produce the early Carboniferous primitive magmas in the Xinyuan region.