Geochemical characteristics of Late Carboniferous volcanic rocks in northern Tianshan, Xinjiang, and their geological significance

Abstract：Anjihai volcanic rocks in northern Tianshan are composed of pyroclastic rocks and lavas, which include andesite crystal tuff, andesite detritus crystal tuff, dacite welded tuff, basaltic andesite and rhyolite, thus being of calc- alkaline series. Major and rare earth element compositions of basaltic andesite and rhyolite are as follows： SiO2=51.03%,77.33%, TiO2=0.76%,0.16%, CaO=10.30%,0.43%, MgO=5.19%,0.89%, Na2O=2.10%,1.27%, K2O=1.28%,2.54%, ∑REE=369.1ppm,729.0ppm, (La/Yb)N=5.0,3.5, (La/Sm)N=2.7,2.4 and δEu=0.82,0.54. The primitive mantle normalized spider diagrams of volcanic rocks are characterized by depletion of HFSE (Nb, Ta, Ti and P) and enrichment of K, Rb, U, Th, Zr and Hf. One significant difference is that Sr and Ba display insignificant positive anomalies in basaltic andesite and distinct negative anomalies in rhyolite. Isotope geochemical characteristics of basaltic andesite and rhyolite show the following values： εNd(t)=0.2,0.1, (87Sr/86Sr)t=0.70680,0.70476, (143Nd/144Nd)t=0.52224,0.51225, (206Pb/204Pb)t=17.775,17.720, (207Pb/204Pb)t=15.790,15.826 and (208Pb/204Pb)t=38.240,38.876. LA-ICPMS zircon U-Pb dating of rhyolite yielded the age of 310±2 Ma. Rhyolite was fractionated from the magma of basaltic andesite. The primary magma of volcanic rocks occurred in continental intraplate stretching setting, derived from asthenosphere (or plume)-generated continental basaltic magma, which was contaminated by lithosphere as well as upper and lower continental crust. Crystal temperature and pressure of phenocryst clinopyroxene are 1176.4℃ and 4.13 kpa respectively, equivalent to the depth of 16.7km. It is thus inferred that pyroxene phenocrysts were crystallized slowly from contaminated continental basaltic magma in middle-upper crust, and subsequently moved quickly to the surface of the earth, due to continental crust stretching.