Petrological, Geochemical And Petrogenesis Study Of The Early Permain Volcanic Rocks In Qunjisayi, Wester Tianshan

In the Qunjisayi area, western of Awulale, west Tianshan, Xinjiang, there are a lotof Early Permian volcanic rocks, which is an important ore rock for copper-irondeposits. It is considered to be very important to determine the lithologic characterand space distribution features of these set of volcanic rock, to study its age, process,tectonic setting and evolutionary mechanism. Through these studies, the author wantsto know the late paleozoic tectonic setting of the region and to provide some base datarelated to ore deposit origin.Based on the results of previous research, the author selected a appropriatesection intend to carry out a systematic research of field geological observation andpetrography for the Wulang Formation. At the same time, LA-ICP-MS U-Pb dating,bulk and trace element analysis and Sr-Nd isotope analysis were carried out todiscuss the source and petrogenesis of Wulang Formation, in order to illuminate theearly Permian volcanics’ tectonic setting and deep dynamics mechanism oflithosphere in the northern of Yili Plate. We derived the following conclusion:(1)The early Permian Wulang Formation is a set of continentalintermediate-basic or felsic volcanic rocks and Clastic rocks. The composition ofrocks includes andesite, dacite, rhyolite, olivine basalt, felsite, volcanic breccia,ignimbrite and tuff. Wulang Formation volcanic rocks are mostly calc-alkalic rockseries, partly tholeiite rock series, which possess typical bi-model volcanic features,and regarded a formation of volcanic hydrothermal type copper deposits.(2)The Wulang Formation has an accurate Zircon U-Pb age. The LA-ICP-MSU-Pb age of andesite simple(B504-7) is300±18Ma, the rhyolite simple of B203-2and B016-2are290.9±3.3Ma and296.3±3.1Ma respectively. The basalt simple ofB204-1can’t forma concorde age. According to the field observation and formationdescription, the author recognized that the probable eruption age of WulangFormation volcanic rocks are between291-300Ma.(3)The date of petrology and geochemistry suggested that the qunjisayi basalt aresub-alkaline tholeiite series, trachyandesite and rhyolite are calc-alkalic rock series,which formed in the post-collision environment. The intermediate-basic mafic magmaof bi-model volcanic series likely come from a lithospheric mantle source whichmetasomatismed by the subducting plate fluid, the rhyolite are remelting basalticmagma in the condition of new lower crust upper amphibolite facies, after a different degrees of separation and crystallization.(4)The εNd(t) of qunjisayi basalt and trachyandesite are (+5.1-+6.9) and(+4.5-+4.9) respectively, the εNd(t) of qunjisayi rhyolite are (+1.8-+5.8), whichsuggested an similar depleted magma source. The geochemistry date showed thequnjisayi basalt and trachyandesite came from the same source, which different fromrhyolite, they have different Nd depleted mantle model age(T2DM).(5)Qunjisayi bi-model volcanic rocks was considered to be the product ofcollision magmatic activity. According to previous studies, When the north Tianshanocean crust Under subducting south to Yili plate, it is possible that the subductionplate “break off” and cause a stretched background overlying lithosphere platebetween291Ma-300Ma. The author thinks that plate “break off” theory can be used toexplain the deep formation mechanisus of early Permian volcanic rock in awulaledistricts. The underplating of the mantle-derived magma triggered by the break offevent caused melting of lithospheric mantle which metasomatismed by the subductingplate fluid, formed the Qunjisayi basalt and andesite by eruption. On the other hand,the underplating magma led to remelt of basaltic rocks of lower crust upper part ormiddle crust lower part, formed acidic magma, following Qunjisayi rhyolite byeruption, result in a simultaneous bi-model volcanic eruptions in the study area(6)Qunjisayi Wulang Fomation volcanic magma of western area of Awulaleoriginated from the mantle source, contained a certain content of Cu metal. Althoughthe volcanic rock itself could not formed a direct deposit, mineralization wouldhappen by the late affect of tectonic magmatic hydrothermal to migrate the metalelements which in these rich ore formation. There are so many type of latehydrothermal deposits in the western of Awulale area. At the same time, the diabaseand quartz porphyry mineralization in the mining area gradually suggesting the localmantle magma formed an magma which enrichment of the metal in the late period.