Early Paleozoic Metamorphism In The Northern Margin Of The Qilian Block:Constraint To The Tectonic Evolution Of The Qilian Orogenic Belt

The Qilian orogenic belt is located between the Qilian block and the Alashan block and is considered to be a typical Early Paleozoic accretionary orogenic belt.The north margin of the Qilian block is an important part of the Qilian orogen and is of great significance in revealing the tectonic evolution process of the Early Paleozoic Qilian orogenic belt.Arc magmatism and concomitant metamorphism are typical features of accretionary orogenic belts and play great role in the growth and reconstruction of the continent.At the same time,the metamorphic rocks in the accretionary orogenic belt may make up of double metamorphic belts.The detailed study of the relevant metamorphic rocks can further clarify the structure and subducting polarities of the north Qilian ocean.In this paper,we combine with the result of previous studies on the tectonic units of the North Qilian orogenic belt,the magmatic rocks,high-pressure,low-temperature metamorphic rocks,and tectonic evolution process in the Early Paleozoic.This study focused on the less-researched complex in the northern margin of the Qilian block,and present an intergrated study of detailed field investigations,petrographic observations,mineralogy,geochemistry and geochronology on the metamorphic rocks and magmatic rocks widely distributed on the northern margin of the Qilian block.The Datong-Menyuan Complex,located in the northern margin of the Qilian Block,is composed dominantly of high-grade metamorphic rocks covered by Paleozoic-Mesozoic sedimentary sequence.The main rock types include gneiss,the garnet amphibolite,the garnet schist,the granite gneiss,the migmatite,and the marble.The felsic gneiss is mainly composed of garnet ?sillimanite + biotite + feldspar + quartz,and the metabasite consists of garnet + clinopyroxene +amphibole + plagioclase + quartz.The petrographic observation shows that the peak mineral assemblage of gneiss(AQ16-25-4B)is garnet + plagioclase + biotite + K-feldspar + ruti le + kyanite+ quartz + melt.The peak mineral combination of gneiss(AQ14-1-1.4)is biotite + plagioclase + K-feldspar + garnet + rutile + ilmenite + quartz + melt,corresponding to the peak temperature and pressure conditions of?795? and?8.9kbar.The peak mineral combination of gneiss(QL11-22-3.3)is garnet + biotite + sillimanite + plagioclase + K-feldspar + quartz + melt,the corresponding peak temperature and pressure is-720 0C and-6.4 kbar.The peak mineral assemblage of amphibolite(AQ 14-12-2.2)include garnet + plagioclase + hornblende + quartz,and the peak temperature and pressure conditions is?700? and?7.1 kbar.The P-T pseudosection and conventional geothermobarometry result shows that the gneiss and amphibolite underwent high amphibolite facies-granulite facies metamorphism.The anticlockwise P-T path of gneiss and garent amphibolite obtained using the garnet isopleths thermobaromety combined with phase equilibria modeling,reflecting that the rocks in the middle crust experienced high temperature metamorphism caused by the arc magmatism and subsequent thermal relaxation processes.The zircon U-Pb analyses result of five representive metamorphic rocks and four igneous rocks are all near 500Ma,indicating the Menyuan-Kekeli underwent similar 500Ma magmatic and metamorphic event.The HT/LP metamorphic rocks exposed in the Menyuan-Keikeli and the northern HP/LT metamorphic belt exposed in Baijingsi-Qingshuigou formed a paired metamorphic belt,and indicating the southward subduction of the north Qilian Ocean triggered the activity of mantle-derived magma and coeval metamorphic event.