Contribution from ancient subducted slab to the Emeishan Large Igneous Province: Constraints from the petrogenesis of mafic intrusions in the western Guangxi area, South China

Several mafic intrusions in the Bama and Longlin areas, western Guangxi, consist primarily of gabbros and di-abases. These mafic rocks have LA-ICP-MS zircon U-Pb isotope ages ranging from 260 Ma to 264 Ma, which is contemporaneous with the eruption timing of the Emeishan large igneous province. All studied rocks have OIB-like chondrite-normalized rare earth element patterns [(La/Yb)N = 6.2-9.8], but negative Nb-Ta-Zr-Hf anomalies in the primitive mantle-normalized trace element diagrams. These mafic samples have slightly depleted zircon Hf isotope signatures (& epsilon;Hf(t) from-1.83 to 7.39 for Bama intrusions and -0.85 to 6.26 for Longlin intrusions), indicative of relatively depleted mantle sources. However, the slightly enriched & epsilon;Nd(t) values from-1.38 to-0.72 and negative Nb-Ta anomalies of these samples, which is similar to arc-related magmas, and relatively lower & epsilon;Hf(t) values of these samples compared with that of ELIP rocks may be consistent with the subduction-related domains in the mantle regions. Considering the above features for the mafic rocks, the enriched source may from the newly subducted slab (such as the Paleozoic Paleotethys oceanic slab) rather than the Neoproterozoic subducted slab. In comparison to a great number of LIPs in the world, such as Siberian Traps and Karoo-Ferrar LIP, they also have a similar evolution of the mantle plume over the subduction processes. This coincidence signifies the Paleotethys oceanic subduction process may wet the mantle source beneath the western margin of the Yangtze craton and disturb the thermal balance of the deep mantle, resulting in the generation of the Emeishan mantle plume.