IDENTIFICATION OF THE LATE TRIASSIC FUYUANGOU INTERMEDIATE-BASIC VOLCANIC ROCKS IN HEILONGJIANG PROVINCE: Geological Implication

The Fuyuangou intermediate-basic volcanic rocks, composed of gray-grayish green basalt andesite, pyroxene basalt and dacite pyroclastic rocks, is a newly identified informal mapping unit in the western Mohe City of northwest Heilongjiang Province. The LA-ICP-MS zircon U-Pb dating results yield the weighted mean age of andesite of 245±3 Ma, and the minimum ²⁰⁶Pb/²³⁸U concordia age of magmatic zircon is 201±2 Ma. Besides, the weighted mean age of dacitic volcanic breccia near the crater is 201±1 Ma, indicating that it was formed in Late Triassic. The lithogeochemical characteristics show that the Fuyuangou intermediate-basic volcanic rocks, belonging to calc-alkaline series, are enriched in LILEs(Ba, K and Sr) and LREEs, and depleted in HFSEs(Nb, Ta, Ti and P) and HREEs, with the geochemical features of arc magmatic rocks. The determination of the volcanic rocks suggests an intermediate-basic volcanic magmatic activity event in Late Triassic in the northern Daxinganling Mountains. Combined the characteristics of the volcanic rocks in the area with regional research results, it is considered that the formation of the volcanic rocks is related to the southward subduction of Mongol-Okhotsk oceanic slab.