Late Mesozoic Magmatism And Tectonic Setting In Chaihe-moguqi Region, Central Great Xing’an Range

The Great Xing’an Range is tectonically located in the eastern part of Central Asian Orogenic Belt(CAOB) between the Siberian Craton in the north and the North China Craton in the south, where exposes a large area of Late Mesozoic volcanic and intrusive rocks. Its tectonic setting and geodynamic process is research focus for the geologists. In this paper, based on field investigation, we chose the granitic and volcanic rocks as the research object, using petrographic, geochronologic and geochemical methods to discuse the Late Mesozoic magmatic activity, petrogenesis and tectonic setting for Chaihe-Moguqi region, central Great Xing’an Range. The following conclusions can be obtained from this study.LA-ICP-MS zircon U-Pb dating results show that magmatic activities in Chai-Moguqi region, central Great Xing’an Range can be divided into three period including Early-Middle Jurassic(179~166Ma), Late Jurassic(157~145Ma) and Early Cretaceous(141~117Ma). In Early-Middle Jurassic, the magmatism is mainly intrusion. The zircon U-Pb age of granitic rocks is 179~166Ma, representing the magma activity time of Early-Middle Jurassic in central Great Xing’an Range. Late Jurassic magmatism includes both intrusion and volcanism. The zircon U-Pb ages of Late Jurassic granitic rocks, intermediate-mafic volcanic rocks and acidic volcanic rocks are 153~147Ma, 157~147Ma and 155~145Ma, respectively, defining the Late Jurassic magmatism at 157~145Ma. Early Cretaceous magmatism also includes both intrusion and volcanism. The zircon U-Pb ages of Early Cretaceous granitic rocks, early Early Cretaceous intermediate volcanic rocks, middle Early Cretaceous acidic volcanic rocks and middle Early Cretaceous intermediate-mafic volcanic rocks are 138~118Ma, 141~131Ma, 133~117Ma and 125~120Ma, respectively, defining the Early Cretaceous magmatism at 141~117Ma.The geochemical features show that the Late Mesozoic granitoids(including the Early-Middle Jurassic, Late Jurassic, and Early Cretaceous granitoids) were all derived from the partial melting of a juvenile lower crust, contaminated by a small amount of riched(metasomatised) mantle material or oceanic sediments at Middle Jurassic, and contaminated by a number of mantle material at Early Cretaceous.The Late Jurassic intermediate-basic volcanic rocks were originated from enriched lithospheric mantle, and contaminated by a certain extent of crustal material. The Late Jurassic acidic volcanic rocks, early Early Cretaceous intermediate volcanic rocks, and middle Early Cretaceous acidic volcanic rocks were derived from the partial melting of a juvenile lower crust, of which the Late Jurassic and middle Early Cretaceous acidic volcanic rocks were also contaminated by a certain of mantle material, and the early Early Cretaceous intermediate volcanic rocks might be affected by the fluid caused from earlier subduction event. The middle Early Cretaceous intermediate-basic volcanic rocks were derived from enriched ashospheric mantle and contaminated by a certain extent of lithospheric mantle or crustal material.The Late Mesozoic magmatism occured in Early-Middle Jurassic, and continued to middle Early Cretaceous in the Chaihe-Moguqi area, central Great Xing’an Range. The Early-Middle Jurassic granitoids formed in the oceanic crust subduction environment related to southward subduction of Mongol-Okhotsk oceanic plate beneath the Erguna Block. The Late Jurassic-early Early Cretaceous magmatism can be related to the lithospheric extensional setting after the final closure of the Mongol-Okhotsk Ocean and subsequent continental thickening. The middle Early Cretaceous magmatic rocks formed in regional extensional setting resulted from combined influences of subduction of the Paleo-Pacific Plate and closure of the Mongol-Okhotsk Ocean.