Geochronology, geochemistry and genesis of Na-rich volcanic rocks of the Zhaibeishan copper deposit in Eastern Tianshan Mountains

Marine Na-rich volcanic rocks of Yamansu Formation belong to calc-alkaline series and are dominated by basaltic andesitic porphyrite, andesite, dacite and rhyolite. These volcanic rocks are characterized by low MgO (0.51%-5.93%, averaging 2.54%), FeO (0.54%-6.39%, averaging 2.84%) and TiO₂ (0.09%-1.10%, averaging 0.58%) values, high Al₂O₃ (12.23%-17.75%, averaging 15.2%; A/CNK=0.79-1.42, averaging 1.11), and Na₂O (Na₂O/K₂O 7.30 on average). A majority of plagioclases in the Narich volcanic rocks consist of albite with less oligoclase. Chondrite-normalized REE patterns show the fractionation between LREE and HREE ((LREE/HREE)_N=3.68-9.00). The volcanic rocks are also rich in LILE (Th, U, Rb) and depleted in Nb, Ta, Sr, Ti, P. The SHRIMP zircon U-Pb dating yielded (337.6±3.3) Ma. Based on a comparative study of geochemical characteristics and inherited zircon age, the authors hold that Yamansu Na-rich volcanic rocks were formed in an island arc setting with continental basement and were the products of reaction between erupted magma and convective seawater during the subduction of oceanic plate in Early Carboniferous.