Late Paleozoic Volcanism In Hongshashan Area,Balikun County,Xinjinag And It's Tectonic Setting

The study area of this thesis is located in the Yemaquan volcanic island arc zone of the Eastern Junggar orogenic belt,which is an important accretional orogen of the Central Asian Orogenic Belt.During the Late Paleozoic,the study area was involved in a continuous and intensive volcanism,with a series of intra-ocean and intracontinental volcanic eruptions occurred in Devonian-Permian.The tectonic background of these volcanic activities is the subduction and closing of the Kalamaili Ocean,as well as the subsequent collision,orogeny and the post-orogenic collision.The research of this thesis has been focused on the the Late Paleozoic volcanics in the Hongshashan area of Balikun County,Xijiang.By detailed field survey,key geological section survey,as well as microscopic thin section analysis,the field distribution,extension direction,scale,occurrence,rock association characteristics and petrographic feature of the volcanic rocks in the study area have been carefully studied.Meanwhile,the author has also completed geochemical analyses and isotopic dating of the volcanics samples to probe into their tectonic environments and ages of formation.Furthermore,an inversion of the tectonic evolution in Late Paleozoic in the study area has been conducted.Below are the preliminary conclusions:1.There are Devonian volcanics of Wulusubasitao Formation,Kaxiweng Formation and Jiangzierkudou Formation in the study area,which have characteristics of low K,Na and high Fe,Mg,similar to calc-alkalic and theoilitic igneous rocks series.In REE geochemistry of the volcanics,the La N/Yb N values have the tendency of increasing gradually from a plane type and the ?Eu are of no depletion to weak depletion.Among trace elements,weak affinity high field strength elements(HFSE)as Ta and Nb are of relatively low depletion.The geochemistry of the volcanics indicates that the Kalamaili Ocean of Eastern Junggar had started its north subduction since Middle Devonian,indicating an apparent volcanic island arc environment related to subduction and metasomatism mantle.Into Jiangzierkudou Formation of Late Devonian,its tectonic setting evolved gradually from an ocean island arc environment to a continental active marginal arc,representing the Kalamaili Ocean's gradual subduction and destruction.2.There are outcropped Early Carboniferous volcanics of Heshantou Formation and Jiangbasitao Formation in the study area,which have slightly weakened eruptions.The volcanics have characteristic of being calc-alkaline to sub-alkaline and slightly aluminous.Their La N/Yb N values of REE are somewhat higher than that of the Late Devonian formations,with an obvious inclining to the righ.The ?Eu are of weak depletion to depletion.The trace elements of Nb,Ta and Ti start to deplete apparently,indicating the tectonic environment had also changed into a continent marginal arc from an island arc environment.Moreover,in the neighboring area,typical molassic deposits has been found.Hence,it is concluded that the Kalamaili Ocean of Eastern Junggar was closed during Late Devonian to Early Carboniferous and the closing time is about between 377Ma—340Ma ago.3.There are outcropped the volcanics as Bataiyineishan Formation of Late Carboniferous and Kalagan Formation of Middle Permian in the study area,which show relatively intensive eruptions with predominant characteristics of continental lava and explosive facies.The volcanics have characteristics of high K and Na,similar to subalkaline to alkaline rocks.Their La N/Yb N values of REE are obviously higher,remarkably inclining to the right and the ?Eu are more depleted.Their trace elements are obviously rich in large ion lithophile elements(LILE),while Nb and Ta are still in remarkable depletion.This feature may have been inherited from the volcanic island arc volcanics.Combined with the geological background of the study area,it is believed that the area had been in the intracontinental orogenic environment since Late Carboniferous.The volcanics of this period have characteristics of A-type granite in an extensional environment.Hence,the Eastern Junggar area had evolved into the postcollision-stretching evolution stage since Late Carboniferous.