Geochemical and Isotopic Variations of Three Basalt Groups in the Early Permian Tarim Large Igneous Province (NW China): Implications for Plume–Lithosphere Interaction

Abstract:

Several lines of geological, petrological and geochemical evidence have supported that the Early Permian Tarim Large Igneous Province (LIP) in the Tarim cratonic block of northwestern China were generated by a mantle plume. However, the over 200,000 km² Tarim continental flood basalts, as the dominant part of the Tarim LIP, show little geochemical and isotopic features similar to those plume-derived intrusive rocks in this region. This is mainly because that their parental magmas were more or less contaminated by the thick crust during ascending. Modeling by trace element and Nd isotopic compositions further suggest that the three basalt groups (Groups 1a, 1b and 2) in the Tarim LIP have experienced variable degree of crustal contamination (i.e., Group 1b > Group 1a > Group 2). After eliminating the effect of crustal contamination, the widespread Group 1 basalts (including both Groups 1a and 1b) would have relatively uniform ε_Nd(t) values of ca. −1.7. This indicates that they were more likely to be produced by partial melting of some enriched mantle components in the sub-continental lithospheric mantle (SCLM) beneath the Tarim block, probably due to conductive heating that resulted from an incubating mantle plume. The Group 2 basalts, on the other hand, were only found in a small region but display a relatively higher and wider range of ε_Nd(t) values roughly between −1.7 and 0.8 (if without crustal contamination). This may suggest that during the generation of Group 2 basalts, the upwelling mantle plume not only provided an enormous amount of heat, but also continuously injected isotopically depleted plume components into the isotopically enriched magma source region in the SCLM. The source isotopic heterogeneity of three basalt groups and other various Tarim LIP rocks (e.g., picrites, ultramafic–mafic intrusive rocks and syenitic rocks), with their ε_Nd(t) values varying between ca. −5 and 5, may correlate with the plume–lithosphere interaction during the generation of the Tarim LIP.