Petrogenesis and tectonic settings of volcanic rocks of the Ashele Cu–Zn deposit in southern Altay, Xinjiang, Northwest China: Insights from zircon U–Pb geochronology, geochemistry and Sr–Nd isotopes

Abstract

The Early–Mid-Devonian Ashele Formation of the southern margin of the Chinese Altay hosts the Ashele Cu–Zn volcanogenic massive sulfide (VMS) deposit and consists of intercalated volcanic and sedimentary rocks that have experienced regional greenschist-facies metamorphism. We studied the petrography, zircon U–Pb geochronology, geochemistry, and Sr–Nd isotopes of dacites and basalts in order to understand the petrogenesis of these rocks and the regional tectonic evolution. Two dacites yielded LA-MC-ICP-MS zircon U–Pb ages of 402±6Ma and 403±2Ma. The dacites are calc-alkaline, and characterized by high Na2O/K2O ratios (3.6–9.3), and high Mg# values (47–63), enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in Nb, Ta, Ti, and P, and relatively positive εNd(t) values (+3.6 to +7.5), collectively suggesting a sanukitic magma affinity. The variations in the major and trace elements of the dacites indicate that Fe–Ti oxide, plagioclase, and apatite were fractionated during their petrogenesis. The basalts are tholeiitic, and are characterized by high Mg# values (66–73), and negative Nb and Ta anomalies. The geochemical characteristics of the basalts are similar to those of N-MORB. Those characteristics together with the positive εNd(t) values (+6.8 to +9.2) of the basalts, indicate that the precursor magma was derived mainly from an N-MORB-type depleted asthenospheric mantle in an island arc setting. The geochemical similarities between the basalts and dacites indicate that they both originated from a similar depleted mantle source via partial melting under different magmatic conditions in each case, possibly related to ridge subduction.