The Early Paleozoic Tectonic Evolution Of Kaladawan Area,Northern Altyn Taugh

Kaladawan area is in the central of the ophiolitic melange of the northern Altyn Tagh. It is clamped by the NE Alytn Tagh strick-slip fault and EW Alytn northern margin fault, North tarim basin, south Qaidam basin and the qilian orogenic belt in the east, occupies the very important tectonic position.A number of medium-large-sized Fe,Au-Cu and Pb-Zn polymetallic deposits fonned in early Paleozoic were found in the study area in recent years, showing a good metallogenic prospects. So this paper choose the Kaladawan area as its target, to carry out the research theme about the early Paleozoic tectonic evolution for the master’s thesis work, which is of great significance on the identification of Paleozoic mineralization tectonic background, the establishment of the mineralization ore-controlling structure system and further prospecting work, but also for recognizing the early Paleozoic tectonic evolution of the northern Altyn.This paper carries out detailed geological observation, mainly select the Early Paleozoic volcanic-sedimentary strata, the intermediate-felsic rock geochemistry and tectonic deformation as the analytical work, supplemented by zircon SHRIMP U-Pb dating, and leads to the main achievements are as follows:1. Early Paleozoic stratigraphic section shows:the rock assemblages in the northern Kaladawan is dominated by intermediate-mafic rocks, eruption-overflow phase intermediate-felsic volcanics in the central and dark combination of fine-grained clastic sedimentary rocks which indicates a reducing environment of deepwater in the south. Zircon SHRIMP U-Pb ages of the intermediate-felsic rocks show these strata formed at (482.3±5.1)Ma～(517±7)Ma, belonging to the era of late Cambrian-early Ordovician.2. Geochemical studies show that:the intermediate-felsic rocks geochemically attributed calc-alkaline series, contains small amounts of alkaline rocks, mainly andesite-dacite-rhyodacite-rhyolite-trachy dacite assemblages,with an evolutionary trend from low potassium to high potassium, sodium to potassium and calc-alkaline to alkaline overall, similar to the volcanic rocks formed at an active continental margin. From north to south, the rock suites have a transition character that from I-type to S-type granitics magma evolution, reflects the fonned in the subduction zone at active continental margin.3. Preliminary analysis of the folds in Dapinggou area indicate that:It is the result of a continuous compression from north to south with characteristics of shaped continuous folds. Zircon SHRIMP U-Pb ages of granodiorites intrusived the core of the fold, determine the ages of(467.5±5.4)Ma-(477.1±4.7)Ma as its lower limite age, belonging to early-mid Ordovician; microfabric study shows the ductile-brittle defonnation flattening dominant simple shear defonnation based, with left-thrust characteristics, principal stress near the north-south, combined with the granites and mineralization age involved in the defonnation belts, preliminary determination of its activities with the upper limit of487Ma age, belonging to early Ordovician. Therefore we consider the folds and ductile-brittle deformation structures are the products of oceanic basin closure-syncollision stage in early Paleozoic.4. Polymictic Conglomerate strata which unconformably overlying the pillow basalt chip, Zircon SHRIMP U-Pb dating of the diorite which invasived the conglomerate indirect limit the upper set age of it as(458±5.4)Ma, belonging to the late Ordovician, implying that the ocean basin in Northern Alytn closed and the continent-continent collision begin.5.In previous work, based on the comprehensive study on volcanic-sedimentary strata, the intermediate-felsic rock geochemistry and tectonic deformation characteristics, combined with the geological data obtained, established the tectonic evolution model in early Paleozoic. Kaladawan area was located at an active continental margin in early Paleozoic, the oceanic crust subducted from north to south.The tectonic evolution history can be divided into three stage as the follows:early subduction-accretion (520～500Ma), subduction-sycollision (500～460Ma) and collision orogeny-post collisional (460～410Ma).