Geochronology and geochemistry of Late Triassic.Early Jurassic granites in Moerdaoga area,NE China and its tectonic implications

In this paper,we present new zircon U-Pb dating and whole.rock geochemical data for the granites in Moerdaoga area, NE China,with the aim of constraining its formation time,petrogenesis and the regional tectonic setting. Zircon grains from the granites in the study area are euhedral-subeuhedral in shape,and display fine-scale oscillatory growth zoning,indicating a magmatic origin. Zircon U-Pb dating indicates that two stages of Mesozoic magmatism are recorded in the Erguna Massif,namely during the Late Triassic (similar to 216Ma) and Early Jurassic (201 similar to 195Ma). The Late Triassic and Early Jurassic granites in the study area exhibit similar geochemical features. They contain high concentrations of SiO2 (67. 09% similar to 77. 05%)and Al2O3 (12. 18% similar to 16. 83%),and low concentrations of Fe2O3T (0. 11% similar to 0. 30%). They are classified as subalkaline,high-K calc-alkaline,metaluminous to peraluminous series,and exhibit chemical affinities to the I-type granites. In addition,the granites in the Moerdaoga area are enriched in light rare earth elements (LREEs)and large ion lithophile elements (LILEs),and depleted in heavy rare earth elements (HREEs)and high field strength elements (HFSEs). These features are similar to the igneous rocks that formed in the subduction zone. Additionally,the formation of coeval calc.alkaline rock associations in the Erguna Massif further indicated that the Late Triassic to Early Jurassic granites formed at an active continental margin setting. Combined with the temporal-spatial distributions of Mesozoic igneous in NE China,we concluded that the formation of the Late Triassic to Early Jurassic magmatism was related to the southward-s subduction of the Mongol. Okhotsk Oceanic Plate