Analysis Of Jiermeng Granite Pluton Zircon U-pb Age And Tectonic Environments,South Qilian

A series of early Paleozoic intrusive rocks were exposed along the southern margin of the Qaidam,the study area is located in Jiermeng in the north of Qinghai Lake which exposes acidic intrusive rocks: granodiorite,monzogranite and a small amount of tonalite,they are all in the early Paleozoic strata.The age and genesis of the rock mass are of great significance to the tectonic evolution of south Qilian area in Early Paleozoic.This paper is based on field investigation,petrography,LA-ICP-MS zircon U-Pb chronology,whole rock geochemistry and Hf isotopic characterization to figure out the crystallization age and genesis type,to discuss the relationship between the formation of the early Paleozoic intrusive rocks and the tectonic evolution of the south Qilian.The age of six samples were measured by using a high-precision LA-ICP-MS zircon U-Pb dating,the ages of granodiorite are 458 Ma,439.6Ma,453.5Ma,the ages of the monzogranite are 457.5Ma,455.8Ma,the age of the tonalite is 448.4Ma,so Jiermeng pluton formed during the late Ordovician-early Silurian.Geochemical characteristics show that the Jiermeng granite plutons belong to peraluminous to strongly-peraluminous calc-alkaline to high-K calc-alkaline series of tonalite to monzogranite,and it both has the characteristics of I-type and S-type granite,it is I-S transitional granite.The spider diagram of trace element shows that it is enriched large ion lithophile elements like K?Th?U and loss of high field strength elements like Nb?Ta?P?Sr?Ti which is character of island arc granite.The diagram of chondrite-normalized rare-earth element pattern shows light rare earth enrichment,heavy rare earth relatively flat right "V" font pattern.Through the study of zircon Hf isotope of Jiermeng granite,the ?Hf(t)values of the two granite samples were distributed between-1.4 ~-12.0 and-7.8 ~-11.8 respectively,the range of TDM C(Ma)distributes between 1.50~2.17Ga(mainly between 1.70~2.17Ga)and 1.92~2.17 Ga respectively.The Hf isotopic composition of the former has a large variation,suggesting that it has mantle-derived magma,while the latter has a relatively stable Hf isotopic composition,suggesting that it is derived from the deep melting of the ancient crustal base.Based on the study of Jiermeng granite pluton in south Qilian,combining with the structural backgrounds,it is considered that the granite is closely related to the subduction of the south Qilian ocean,magma formed in the background from the subduction of the south Qilian ocean to the collision of two plates.Therefore,Jiermeng pluton has two types of granites formed in different backgrounds and stages in the same subduction zone.It is revealed that the south Qilian ocean was still under the tectonic environment of the northward subduction between 450 and 460 Ma,while the south Qilian ocean was in the closed stage at 439 Ma,and Jiermeng pluton in this paper was formed in the ocean-continental transitional stage.