| The East Kunlun Orogenic Belt(EKOB),as the western part of the Central China Orogenic Belt(CCOB),is located in the northern Tibetan Plateau.It is a key to understanding the tectonic evolution of the CCOB.Numerous igneous rocks crop out in this area,which is important record of the long and complex tectonic evolution of the EKOB.This study choose Pingchangliang and Dashagou plutons in the western segment of the EKOB and investigate the formation age,petrogenesis,tectonic setting and tectonic significance of these granitic rocks.According to these studies,together with the regional geology and sedimentation,the Late-Paleozoic to Early Mesozoic tectonic evolutionary history of the EKOB has been discussed.All above works allow us yield the following conclusions:1.The Pingchangliang pluton,outcropping as the irregular dyke and stock,is located in the south of the Central East Kunlun Fault in the west part of the EKOB.It consists of quartz diorite.LA-ICP-MS zircon U-Pb dating indicates that the formation age of the Pingchangliang pluton is 345±2.3 Ma(MSWD=0.25).Samples from the pluton show low Si02(51.98%-54.63%),total alkali(K2O+Na2O=4.16%-4.80%),Ti02(0.77%-0.99%)and P2O5(0.10%?0.25%)contents,high A1203(16.62%-18.01%),MgO(3.86%-5.47%)and CaO(6.80%-8.29%)contents,and belong to peraluminous high-K calc-alkaline series.They are also characterized by relatively enrichment in LREE,obvious high LREE/HREE ratios,insignificant negative Eu anomaly(?Eu=0.88-0.97),as well as the highly enriched LILE(e.g.,Cs,U,K and Pb)and depleted HFSE(e.g.,Nb,Ta,Zr,and Ti),indicating a subduction-related geochemical features.Accordingly,we suggest that the primary magma of the quartz diorite was derived from partial melting of mantle wedge which had been metasomatized by subduction fluids.Together with regional geology,we propose that the Pingchangliang pluton was formed in volcanic arc setting on active continental margin triggered by the northward subduction of A'nimaqing-Buqingshan Ocean.2.The Dashagou pluton,locating in the southwest of Qimantagh,west part of the EKOB,outcrops as the irregular dyke and stock,and consists of syengranite.LA-ICP-MS zircon U-Pb dating indicates that the formation age of the Dashagou pluton is 215±1.8 Ma(MSWD=0.51);Samples from the pluton show high SiO2(75.78%-79.08%),total alkali(K2O+Na2O=8.02%-8.66%),A1203(11.04%-12.68%)contents,low Ti02(0.04%-0.06%),CaO(0.34%-0.64%),MgO(0.04%-0.08)and Fe2O3t(0.95%-1.37%)contents,and belong to weakly peraluminous high-K calc-alkaline series.They have relatively enrichment in LREE,obvious high LREE/HREE ratios,and sharply negative Eu anomaly(?Eu=0.07-0.11).They are also enriched in Cs,Th,K and Pb in LILE as well as Nd,strongly depleted in Nb,Ta,Zr and Ti in HFSE as well as Ba.These samples also have high contents of Zr,Nb and Ga/Al,Y/Nb ratios,with A2-type affinity.Therefore,we infer that the syengranite from Dashagou pluton were derived from partial melting of the felsic materials in lower crust,which are heating by the mantle derived magma caused by the delamination.The Dashagou A2-type granitoid suggest the post-collsion extension setting of the EKOB during late Triassic period.3.Our new results,together with previous investigations,provide new insights on the tectonic evolution of the western segment of EKOB during Late Paleozoic-Early Mesozoic:Based on the per-Devonian tectonic evolution.The following is implying:A'nimaqing-Buqingshan ocean continued spreading and northward subduction during the Carboniferous.The pingchangliang pluton was derived from mantle wedge metasmatized by subduction fluids,as the products of continuous northward subduction of A'nimaqing-Buqingshan ocean;During the Permian-Middle Triassic,the continous northward subduction of oceanic crust formed a large scale of arc magmatic rocks.During Later period of Middle Triassic,the East Kunlun region was considered in a collision-related setting.In the early late Triassic time,the EKOB was in post-collison setting which formed the Dashagou pluton induced by the delamination. |
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