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

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 Sr-Nd 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 subductionrelated 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 trachyandesite 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 similar to 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 Sr-Nd isotopic ratios indicates that the pre-subduction mantle wedge was modified by similar to 0.5% subducted sediment-derived fluids and similar to 2% bulk sediments. About 10% partial melting of such enriched mantle source could produce the early Carboniferous primitive magmas in the Xinyuan region. (C) 2020 Elsevier B.V. All rights reserved.