Geochemical and chronologic characteristics of ultramafic rocks in Gaxian ophiolitic mélange of the Da Hinggan Mountains and their geological significance

The Gaxian ophiolitic melange belt is located in the middle of Xinlin-Gaxian-Jifeng ophiolitic melange belt.This ophiolite mainly consists of ultramafic and mafic rocks, mixed with metamorphic fine clastic rock matrix in the tectonic block form.This paper focuses on the systematic petrology, geochemistry and chronology of ultramafic rocks.The result shows that, in the ultramafic rock, the average value of SiO₂ is 44.87%, the average value of Mg^# is 87.06, and the zircon U-Pb age is 649.6±4.2 Ma, suggesting Neo-proterozoic.The curve of REE is of flat type, which shows MORB's transition to OIB.Geochemical data of the sample show depletion of Nb, Ta, Ti and enrichment of Th, U, Zr, Hf, exhibiting characteristics of the arc magma.In the Nb/Yb-Th/Yb tectonic environment discrimination diagram, it is located at the intersection of E-MORB and OIB.A comparison with the rock association of the regional tectonic and geochemical characteristics shows that the magma source area of the Gaxian ultramafic magma may be an enriched mantle replaced by subductive fluid, belonging to the SSZ type ophiolite, and its emplacement mechanism might have been related to the continuous intra-ocean subduction.