Geochronology and genesis of volcanic rocks in Beitashan Formation at the northern margin of the Junggar, Xinjiang
新疆准噶尔北缘北塔山组火山岩年龄及岩石成因

LA-ICP-MS U-Pb zircon ages and geochemical data of whole rocks are reported for mafic volcanic rocks(basalts) from the Beitashan Formation at the northern margin of the eastern Junggar.The zircons with well-defined oscillatory zoning from augitophyre sample yielded mean 206Pb/238U ages of 380.5±2.2Ma,which are interpreted as the eruption age of the volcanic rocks of the Beitashan Formation.All basalt samples belong to subalkali tholeiite magma.They display obvious negative anomaly of Nb,Ta and Ti,obvious enrichment of LREE and LILEs(Ba,Sr,K,U and Pb) characteristics.They have positive εNd(t) values(+6.84～+12.28) and low Sr isotopic compositions(initial 87Sr/86Sr values=0.703835～0.704337),indicating a deplete mantle source.The regional geology and geochemical evidence indicate that these mafic volcanic rocks were generated at island arc tectonic setting and have similar magma sources.The primary magmas may have been derived from asthenosphere mantle and mantle wedge metasomatized by fluids and melts released from the subducted slab.During the cooling of magmas,they underwent crystallization differentiation of pyroxenes and plagioclase.The different volcanic rocks(eg.basalt,augitophyre,picrite and plagioclase-basalt) were likely resulted from different primary magma contents and differentiated in different magma chamber.