Study On The Kimberlitic Rocks, Nephelinites, And Carbonatites Within Tarim Large Igneous Province

This study repots the kimberlitic rocks, nephelinites, and magnesiocarbonatites exposed in the Wajilitage complex in the Tarim Large Igneous Province, to constrain the petrogenesis and the deep geological processes of the Tarim Large Igneous Province. Based on this study, several achievements and conclusions have been made.1. Here we report rutile SIMS U-Pb age of 270 Ma indicating that the nephelinites may represent the last phase of the TLIP magmatism, which is also confirmed by the field relation. Combined with the previous geochronology data, the duration of the Tarim Large Igneous Province is longer than previously considered(up to 30Ma).2. We conducted EMPA and LA-ICP-MS studies on the carbonate- and silicate-rich globules recognized within the Wajilitage kimberlitic rocks. The carbonate-rich globules are composed of pure calcites, while the silicate-rich globules are composed of calcite, garnet, phlogopite and diopside. Our new data suggest that the carbonate-rich globules most likely do not represent immiscible liquids, as the geochemical data are not consistent with the partition coefficients produced by the experimental studies. The presence of the carbonate-bearing globules indicates the carbonaterich nature of the Wajilitage kimberlitic magmas, which are most likely to be derived from an carbonated mantle source.3. We report Mg isotopes of the kimberlitic rocks, carbonatites and nephelinites, the results show that δ26Mg values range from-0.36‰~-0.75‰,-1.09‰~-0.68‰ and-0.35‰~-0.55‰, respectively, which are all ligher than the normal mantle value(-0.25‰), indicating their sources had been metasomatized by the early-middle Palaeozoic recycled sedimentary carbonates. The nephelinites display mantle-affinity Sr-Nd isotopes and crustal Mg-O signatures, the carbonatites show mantle Sr-Nd-O and crustal M-C isotopic compositions. The decoupling of isotopes provides important constraints on the mineral phases of the suducted sedimentary carbonates in convergent margins, which are subducted to deep domains in the mantle(>900km, or even to the core-mantle boundary), with phase transitions from calcite/dolomite to magnesite, and eventually to periclase/ perovskite. Our study shows that the carbonates occurred as magnesite in the sources of the nephelinites and periclase in the sources of the carbonatites.4. According to the field relation, the nephelinites cover the Devonian sedimentary directly without the Carboniferous, indicating the presence of the pre-erupted crustal uplift. The decomposition of the magnesite indicates a obvious thermal abnormality compared to the normal mantle geothermal. The thermal abnormality and pre-erupted crupted uplift provide more supports for the involvement of mantle plume in the Tarim Large Igneous Province.5. We proposed a plume-lithosphere model to be responsible for the important characteristics for the Tarim Large Igneous Province. Slight thinning of lithosphere is expected during the plumelithosphere interaction, and the variation in lithology of the Tarim Large Igneous Province could be distributed to the variable depth and degree of melting of the carbonated mantle, which are also different evolution stages of the plume-lithosphere model.