Permian basaltic rocks in the Tarim basin, NW China: Implications for plume-lithosphere interaction

摘要

There are large areas of Permian basaltic rocks in the Tarim basin (PBRT) in northwestern China. Precise Ar-Ar dating of these rocks revealed an eruption age span of 262 to 285Ma. Most of the PBRT is composed of alkaline basaltic rocks with high TiO_2 (2.43%-4.59%, weight percent), high Fe_2O_3+FeO (12.63%-17.83%) and P_2O_5 (0.32%-1.38%) contents. Trace elements of these rocks have affinities with oceanic island basalts (OIB), as shown in chondrite normalized rare earth elements (REE) diagrams and primitive mantle normalized incompatible elements diagrams. The rocks show complex Sr-Nd isotopic character based on which they can be subdivided into two distinct groups: group 1 has relatively small initial (t=280Ma)~(87)Sr/~(86)Sr ratio (~0.7048) and positive εNd(t) (3.42-4.66) values. Group 2 has relatively large initial ~(87)Sr/~(86)Sr ratio (0.7060-0.7083) and negative εNd(t) (from -2.79 to -2.16) values. Lead isotopes are even more complex with variations of (~(206)Pb/~(204)Pb)t, (~(207)Pb/~(204)Pb)t and (~(208)Pb/~(204)Pb)t ranging from 17.9265 to 18.5778, 15.4789 to 15.6067 and 37.2922 to 38.1437, respectively. Moreover, these two groups have different trace elements ratios such as Nb/La, Ba/Nb, Zr/Nb, Nb/Ta and Zr/Hf, implying different magmatic processes. Based on the geochemistry of basaltic rocks and an evaluation of the tectonics, deformation, and the compositions of crust and lithospheric mantle in Tarim, we conclude that these basaltic rocks resulted from plume-lithosphere interaction. Permian mantle plume caused an upwelling of the Tarim lithosphere leading to melting of the asthenospheric mantle by decompression. The magma ascended rapidly to the base of lower crust, where different degrees of assimilation of OIB-like materials and fractionation occurred. Group 1 rocks formed where the upwelling is most pronounced and the assimilation was negligible. In other places, different degrees of assimilation and fractionation account for the geochemical traits of group 2.