Formation Age And Geochemical Characteristics Of Jiyuan Pluton In Northern Da Hinggan Moutains And Its Tectonic Implications

Jiyuanlinchang area is located in the north Da Hinggan Mountains.This area,which is located between Erguna massif and Xing'an massif,is ideal area to study Mongolia-Okhotsk tectonic domain and Circum-Pacific tectonic domain.Because the mesozoic magmatic rocks distributed in Da Hinggan Mountains area is influenced by the Mongolia-Okhotsk tectonic domain or the Circum-Pacific tectonic domain,or the common influence of the Mongolia-Okhotsk tectonic domain and Circum-Pacific tectonic domain and the final closure time of Mongolia-Okhotsk ocean are uncertain,the Mesozoic tectonic evolution of northern Da Hinggan Mountains Area is disputed all the time.The problems mentioned above limit the understanding of the Mesozoic tectonic evolution in northern Da Hinggan Mountains Area.So,we chose the granitic rocks as the research object,using petrographic,geochemical and geochronologic methods to discuss the Mesozoic magmatic activity and tectonic setting for northern Da Hinggan Mountains.The results of LA-ICP-MS Zircon dating indicate the magmatic activities in Jiyuanlincahng area are divided into three stages:early Jurassic,late Jurassic and early Cretaceous.Jiyuan pluton is the largest pluton in the study area.We chose two granite samples for LA-ICP-MS Zircon dating.The zircon from these samples all display fine-scale oscillatory growth zoing and have high Th/U ratios,implying a magmatic origin.This indicates that the dated ages could represent the emplacement timing of the granites.The U-Pb Age of these samples are 180.7�1.3Ma and 174.2�0.5Ma,suggesting that the Jiyuan pluton was emplaced in the early Jurassic.The main mineral compositions of granodiorites are plagioclase,quartz and Hornblende.The accessory minerals include magnetite,Sphene and apatite.The field geological characteristics show that the granitediorite intrude into the Moergenhe formation,unconformably covered by the volcanic rocks of Manketouebo formation and Baiyingaolao formation.The main mineral compositions of monzogranites are perthite,plagioclase and quartz.The field geological characteristics show that the monzogranites intrude into the Moergenhe formation and granodiorites,unconformably covered by the volcanic rocks of Baiyingaolao formation.The results of geochemical indicate all the samples of jiyuan pluton are characterized by hign silicon and alkalis,and low calcium,magnesium and titanium.The rare earth element patterns show that these granites are enrich in light rare earth elements and also strongly enriched in large ion lithophile elements?e.g.Rb,U,K?.They are depleted in high field strength elements?Nb,Ta,P and Ti?and Ba,Sr?LILE?.The petrogeochemical fetures are likely to the granites whose magma sources are from crustal material.Furthermore,in the harker diagram,they have linear variation trend between the major elements and SiO₂.In the discrimination diagrams,all the samples fall into the A-type granitoids.Nevertheless,the Zr contents and the zircon saturation temperature is lower than A-type granitoids,suggesting that early Jurassic granites belong highly fractionated I-type igeous rocks.The granite samples are located at post-collision area,representing the post-collision environment.Combined with these characteristics and regional geologic condition,it is suggested that the primary magma of Jiyuan pluton in northern Da Hinggan mountains is related to the continent-continent collision setting after the closure of the Mongolia-Okhotsk Ocean.It also defines the closure time of the Mongolia-Okhotsk Ocean.