The Closure Of The South Tianshan Ocean

In this dissertation,we focus on the Dahalajunshan volcanic-sedimentary located at the lower part of Kekesu River,western Dahalajun Mountain,southern Tekes,southern Yili Basin.We studied on the outcrop,petrology,zircon U-Pb chronology,geochemistry,zircon rare earth elements and Hf isotope characters to discuss the evolution of the South Tianshan Ocean.Firstly,we divide the Dahalajunshan Formation into two part based on its lithology and distributing features: the lower part consists of clastic rocks and the upper part consists of volcanic rocks.In general,this formation represents a sedimentary-volcanic sequence.The overlying strata of the Akeshake Formation and underlying the Kushitai Group are both unconformity with the Dahalajunshan Formation.Secondly,we analyses the samples from lower part of the Dahalajunshan Formation with LA-ICP-MS zircon U-Pb dating.The youngest detrital zircons give a weighted mean 206Pb/238 U age of 367.4±4.2 Ma and the magmatic zircons give a weighted mean 206Pb/238 U age from 359.7±1.2 Ma to 356.1±2.4 Ma.Therefore,we suggest the lower part of the Dahalajunshan Formation formed during the late Devonian to early Carboniferous.Thirdly,the geochemistry of rhyolite shows significant arc-related volcanic rock characters with enrichment of U,Th and LILE,depletion of Ba,Sr and HFSE(Nb,Ta and Ti).Besides,with REE analyses of magmatic zircons and youngest detrital zircons,we can judge the geological setting with Th/Nb-Hf/Th pattern.All spots plot in arc-related/orogenic zone.Associated with all zircons with positive ?Hf(t)values,we conclud that the volcanic rocks were formed in the arc-related geological setting.Fourthly,we found there are detrital zircons with age groups of 380~360 Ma,460~430 Ma,505~480 Ma and Meso-to Neoproterozoic from conglomerates at the lower part of the Dahalajunshan Formation.Combined with the comparison of magmatic and detrital zircons age spectrum with adjacent areas,these ages are consist with the magmatic events occurred in the southern Yili-Central Tianshan Block.Furthermore,there are abundant zircons ages from 360 Ma to 355 Ma which are appeared in volcanic rocks from southern Yili-Central Tianshan Block.Consequently,we suggest the source of the Dahalajunshan Formation conglomerates are not only from the underlying strata but also adjacent areas.Above all,combined with 430~330 Ma ophiolite mélange reported in the South Tianshan Orogenic Belt,magmatic zircons from the Dahalajunshan Formation with age 360~355 Ma and arc-related background,(ultra)high pressure metamorphic rocks with ages of 320~300 Ma,regional unconformity and sedimentary facies,we suggest the south Tianshan Ocean closed at late Carboniferous(ca.310 Ma).