Zircon geochronology of Xingxingxia quartz dioritic gneisses: Implications for the tectonic evolution and Precambrian basement affinity of Chinese Tianshan orogenic belt
星星峡石英闪长质片麻岩的锆石年代学:对天山造山带构造演化及基底归属的意义

The Tianshan orogen is the major component of the Central Asian Orogenic Belt(CAOB).There has been a continued debate on the derivation of the old Precambrian crustal basements and Paleozoic tectonic divisions and evolutions of the orogen.In this paper we present petrological characteristics,zircon U-Pb ages and Hf isotope compositions as well as the whole rock geochemistry of the quartz dioritic gneisses from west of the Xingxingxia Town.Based on the mineral assemblies,their protoliths are speculated to be quartz dioritic.Zircon grains from two collected samples have similar internal structures identified on the cathodoluminescence(CL) images,which is characterized by CL-dark,homogeneous rims surrounding the oscillatory zoning cores with rare inherited inner cores.Zircon U-Pb dating results indicate that their protoliths were formed in Early Paleozoic time at ～ 425Ma,and were metamorphosed during Early Carboniferous at approximately 320 ～ 360Ma.The inherited zircon cores yield a wide age range of ca.1381 ～ 1743Ma.The crystallization zircons from the protolith show positive and varying εHf(t) values(from 0.9 to 17.8),and the inherited zircon cores give tDM2 model ages of 1.54Ga to 2.44Ga.The quartz dioritic gneisses are characterized by marked positive Rb,Ba,Th and K anomalies and depletion in Nb,Ta and Ti on the primitive mantle-normalized spider diagrams.Through integration of these new data with the previous results of Precambrian basements,high-grade metamorphic rocks,ophiolitic mélanges and igneous rocks from the Tianshan orogen,we suggest that:1) the Precambrian crustal basements in the Tianshan orogen don’t have much affinity with the Tarim Block,and are inferred to be formed through a magmatic arc accretionary orogen along the margins of Baltica during the Mesoproterozoic;2) the eastern segment of Central Tianshan was an active continental margin during the Early Paleozoic,due to the subduction of the South Tianshan Ocean;3) the closure of the South Tianshan Ocean may be synchronous in its eastern and western segments.