Granitoid Features And Fission Track Evidence On Tectonic Settings In Yemaquan Region,western Part Of East Kunlun Mountains

The Eastern Kunlun Mountain is located in the northern Qinghai-Tibetan plateau,and bounded to the north by the Qaidam Basin.The Yemaquan research region belongs to the western Qimantage area in the north of Kunbei fault,western part of Eastern Kunlun Mountain.Granitoids of late Paleozoic-early Mesozoic developed widely.Although it experienced complex geological evolution and significant mineralization,a few research works have been done because of the bad nature condition and inaccessibility in the plateau.This paper try to discuss granitoids petrotectonic assemblages based on geological and geochemical characteristics,and use apatite and zircon fission-track method to probe tectonic activity,thermal history and uplifting of Yemaquan area in Qimantage belt.Some results are concluded as following:Granitoids are mainly types in intrusive rocks in Ymaquan,there are diorite,quartz diorite,granodiorite,monzonitic granite,moyite,granite,granite porphyry in the research region.and they have similar geochemical characteristics.Granitoids mainly belong to high-potassium calc-alkaline type,with high potassium and high aluminum.Chondrite-normalized REE distribution pattern diagram of all intrusive rocks show negative Eu anomalies,indicating the rocks have similar formation.Granitoids have similar primitive mantle-normalized trace elements spider diagram;enriched in K,Rb,Th,U,Hf and depleted in Ba,Sr,Nb,Ti,P;which reflected that magma was dominated by crustal rocks.w(Rb)/w(Ba)and w(Rb)/w(Sr)in rocks are increased from early period to late period of every stages,showing that magma underwent high evolution.Granitoids are mainly I-type and A-type.The late Paleozoic-early Mesozoic orogenic cycle is composed of four stages:the formation and spreading of oceanic basin,the subduction,the syn-collision and post-collision.Samples were collected from granitoid outcrops and drill-core in Yemaquan region.This paper achieves 11 apatite and 12 zircon fission track dating results.The samples with ?2 values higher than 5%and we use the central age.For the samples with x2 values less than 5%,we use the ages decomposed by BINOMFIT program.The final ages can be divided into different groups that reveal multiphase tectonic-magmatic activities.The zircon fission track ages range from 237 Ma to 121 Ma that might be divided into 3 groups:237 Ma?220 Ma,199 Ma-162 Ma,139-121 Ma.The apatite fission track ages range from 155 Ma to 48 Ma that might be composed of 3groups:155 Ma?120 Ma,99 Ma?81 Ma and 56 Ma-48 Ma.These ages indicate the complex geological thermal events in this area very well.The zircon fission track ages 237 Ma?220 Ma,zircon ages 139?121 Ma and the apatite fission track ages 155 Ma?120 Ma and apatite ages 99 Ma?81 Ma represent northward subductions of Qiangtang terrane,Gangdese terrane and Himalayan terrane in Middle-Later Triassic,Late Jurassic and Early Cretaceous,Late Cretaceous respectively,resulting in collision-convergences with north-side terranes.The zircon ages of 199 Ma?162 Ma,164-163 Ma and the apatite ages of 56 Ma?48 Ma reflect post-orogenic stretching events in Early Jurassic,Middle Jurassic and Eocene respectively.There are distribution of coal measure strata in early-middle Jurassic and existence of pyrite,which indicate at that time the tectonic environment is extensional,consisting with synchronous granite rock's geochemical characteristic and relevant evidence of geophysical data.Four stages of thermal evolution history are revealed by apatite fission track modeling using the AFTSolve in this region.First stage from 200 Ma to 160 Ma was in the bottom temperature of apatite fission track anneal zone and belong to post-orogenic stretching environment.Second stage from 160 Ma to 120 Ma,reflects Gangdese terrane northward subduction during late Jurassic to early cretaceous and the closure of Bangong-Nujiang oceanic.Third stage of 120 Ma-13 Ma records slow cooling,and during that time,there are not intense press events.The last stage since 13 Ma indicates rapid cooling with temperature dropped 55 C.Some experts pointed the major uplift period for Qinghai-Tibetan plateau is from 13 Ma.The erosion ranges during the last three stages are 0.7 km,0.6 km and 1.6 km respectively and the erosion rates are 0.017 mm/a,0.006 mm/a,0.11 mm/a respectively.The cumulative amount of erosion is about 2.9 km.The average rock uplift value for the apatite samples is 5340 m.Thus the surface uplift extent are averaging 2440 m from Cretaceous in the research region.