Geochemistry Characteristics And Petrogenesis Of The Xinlin Area Cretaceous Granitic Rock,northern Da Hinggan Range

A mass of Phanerozoic granite and volcanics exist in Da Hinggan range, which are located in the east part of the central Asia Hinggan-Mongolia orogenic belt. This orogenic belt is composed of the Ergun block, Hinggan block and Songnen block from the northwest to southeast. The study area is located in the Ergun block, which bounded to the west by the Siberian craton, to the north by the Gonzha block, to the south by the central Mongolia block, and to the east by the Hinggan block along the Tahe-Xiguitu suture zone. This area has a complicated tectonic evolution history since Paleozoic because of the superposition of the Paleo-Asian Ocean, Mongolia-Okhotsk and Paleo-Pacific domains. A mass of Mesozoic magmatite exist in Da Hinggan Mountains, we studied the petrology, geochronology, Hf isotope and geochemistry of the Late Mesozoic intrusive rock in Xinlin town, northern Da Hinggan range.The petrology and microphysiography show the intrusive rock in Xinlin town is mainly composed of quartz diorite, adamellite, granodiorite and monzogranite. The zircon LA-ICP MS U-Pb dating of three intermediate rock show they were formed in Early Cretaceous, and their formation ages ranged from 141 Ma to 124 Ma.Comprehensive analysze with previous findings, we divide Early Cretaceous magmatic events into two stages: an early Early Cretaceous stage(145~130 Ma), and a late Early Cretaceous stage(125~123 Ma).The petrogeochemistry of six granitoid samples in Xinlin show similar major and trace elements compositions. They are rich in silicon [w(Si O2)= 61.78%~70.17%(average is 66.25%)] and kalium [w(K2O)=3.39%~4.37%], belong to high-kalium calc-alkaline series. The w(Na2O)/w(K2O) ratios range from 0.93 to 1.50(average1.20) and is sodic type rocks. w(Na2O+K2O)=8.04%~8.92%. w(Ca O)=1.45%~3.83%,w(Mg O)=0.49%~2.14%, Mg#=32.6~45.9(average 41.4). They are also riched in Al[w(Al2O3)=14.8%~16.64%] and the Al2O3/(Na2O+K2O) ratios are 1.21~1.38,Al2O3/(Ca O+Na2O+K2O) ratios are 0.86~1.00, belonging to metaluminous.rocks. The Xinlin granitoids are poor in ΣREE [ΣREE = 154.1×10-6 ~235.1×10-6]and are characterised by LREE enrichments with relatively flat HREE distributions and weak to moderately negative Eu anomalies(0.53 to 0.72, average 0.65). Trace element spidergrams for the Xinlin granitoids indicate enrichments in LILE(Th, Rb,Sr and K), and depletion in HFSE(Ti, Nb, Ta). The εHf(t) values of the Xinlin granitoid range from 2.1 to 6.2, and the TDM2 range from 1025 Ma to 786 Ma,indicating that the Xinlin granitoids was origrinated from melting of Meso-Neoproterozoic accreted juvenile crust. The Xinlin granitoid bear the distinctive major and trace element characteristics of C-type adakite, and were origrinated from partial melting of thicken lower crust, while the mantle heat energy and element may participate in this event.The petrology and lithogeochemistry show the Xinlin granitoid were formed in extensional setting. All the samples show the same rock association, major element and trace element composition with the post-collision granitoid. In these samples,three of them belong to granite, with A/CNK of 0.856~0.995(<1.1) and A/NK of1.21~1.38(>1). The granites contain hornblende and no muscovite or garnet,belonging to I-type granite. In consideration of regional tectonic evolution and the geochronology, we speculate they were formed in post-collision of Mongolia-Okhotsk orogenic belt, during transformation stage from compressional setting to extensional setting. This stage is characterized by decompression and calescence, lead to the orogenic belt element massively melting and finally turn in to granitic rock.