Genesis Of Early Late Paleozoic Granitic Rocks Of Chaihe-Moguqi Area In Central Great Xing'an Range

The Chaihe-Moguqi area in the middle of the Great Xing'an Range is the front area of the subduction of the Xing'an block from the Paleo-Asian ocean.The genesis of the granitic rocks in the early Late Paleozoic is of great significance to the tectonic background of the eastern end of the Central Asian Orogenic Belt.This thesis presents systematic research on the granitic rock of Chaihe-Moguqi area in the central of Great Xing'an Range.Based on the field investigations,the mineralogy and petrology,zircon LA-ICP-MS U-Pb geochronology,whole-rock geochemistry analysis are carried out.We finally determine the formation age of the early Late Paleozoic granitic rocks Chaihe-Moguqi area in the central of Great Xing'an Range,clarify the petrogenesis and the tectonic setting of the early Late Paleozoic granitic rocks in the study area,and reveals the tectonic evolution process between Xing'an block and Songnen block in the early Late Paleozoic.The granitic rocks in the Chaihe-Moguqi area we research can be divided into two types: granitic mylonite in the Moguqi area and syenogranite in the Chaihe area.Zircon U-Pb isotopic chronology shows that the granitic mylonite in Moguqi area is formed in Early Devonian-Early Carboniferous(398~351 Ma),and the syenogranite in Chaihe area is formed in Late Carboniferous(316~303 Ma).The petrogeochemical characteristics show that the granitic mylonite in the Moguqi area and the syenogranite in the Chaihe area have different petrogenesis and tectonic setting.The granitic mylonites in the Moguqi area are rich in Si O2(68.20~77.90 wt.%),alkali K2O(2.81~4.72 wt.%),Na2O(3.31~4.95 wt.%),TFe2O3(1.34~3.72 wt.%),low Mg O(0.22~0.97 wt.%)and Ca O(0.37~2.18 wt.%).The REE distribution diagram shows that the rocks are enriched in LREE and depleted in HREE(LREE/HREE = 4.88~9.64),with obvious negative Eu anomaly(?Eu =0.03~0.13).The samples are enriched in large ion lithophile elements(e.g.,Rb,Ba,K,Th and U),and depleted in high field strength elements(e.g.,Nb,Sr,Ti,Ta and P).The magma shows the characteristics of T-type granite and is the product of island arc magmatic activity under subduction background.The Late Carboniferous syenogranite in Chaihe area is characterized by high Si O2(67.04~75.88 wt.%)and alkali((K2O+Na2O)= 7.40~10.26 wt.%).The REE distribution pattern is characterized by enrichment of LREE and depletion of HREE(LREE/HREE =2.09~11.09),with obvious negative Eu anomaly(?Eu = 0.06~0.49).The rocks are enriched in large ion lithophile elements(LILEs,e.g.Rb and K),and depleted of high field strength elements(HFSEs,e.g.Ta and Ti)to varying degrees.The magma shows the characteristics of A-type granite and is the product of partial melting of crustal material under the extensional tectonic background after collision.Combined with regional tectonic evolution,the Early Devonian-Early Carboniferous(405~352 Ma)Paleo-Asian ocean subducted continuously along the Heihe-Zhalantun area under the Xing'an block in the Chaihe-Moguqi area,and the subduction gradually intensified.During the Early Carboniferous(346~335 Ma),the ancient Asian Ocean subducted the Xing'an block and Songnen block.As early as the late Early Carboniferous-Late Carboniferous(335~298 Ma),the collision between Xing'an block and Songnen block has already occurred.After the collision,the crust thickened vertically and then the post-orogenic extension occurred.