Abstract:

The Permian era is the most important stage during the thermal-tectonic evolution of the Tarim basin. During this stage, huge volumes of mafic and mid-acid magmas were formed. Geochemical inversion of ultramafic rocks from Wajilitag, NW Tarim basin, indicates that these rocks (including the breccias and bindings) are composed of residual magma, olivine and pyroxene with different ratios. The breccias are mainly composed of olivine and residual magma with ratio up to 72∶28. The MgO and FeOt contents of the equilibrium magma (corresponding to olivine with F_o=0.82) are 9.06% and 11.77%, respectively, and it was produced by primary magma MgO=11.46% and FeOt=12.12%) under 7.3% fractionation of olivine (F_o=0.82). The primary magma was originated from low degree partial melting (<5%) of garnet bearing mantle peridotite instead of the high Mg magma. The original melting temperature was 1 150-1 250℃, and the pressure was 2.2 GPa, corresponding to 70 km deep. Combined with previous studies, it’s concluded that the Permian magmatism in Tarim basin was probably formed by the interaction of asthenosphere and lithosphere that was triggered by the upwelling of the asthenospheric mantle, and there was no direct petro-geochemical evidence to support the mantle plume hypothesis.

Key words: ultramafic rocks, geochemistry, primary magma, Permian, Tarim basin