Decoupling of Mg–C and Sr–Nd–O isotopes traces the role of recycled carbon in magnesiocarbonatites from the Tarim Large Igneous Province

Abstract

The Tarim Large Igneous Province in NW China hosts numerous magmatic carbonatite dikes along its northern margin. The carbonatites are composed mainly of dolomite (90vol.%) and minor calcite (5vol.%), with apatite, barite, celestine, aegirine, monazite and bastnaesite as accessory minerals. The rocks correspond to magnesiocarbonatites with a compositional range of 13.73–19.59wt.% MgO, and 20.03–30.11wt.% CaO, along with 1.65–3.31wt.% total Fe2O3, 0.02–2.39wt.% SiO2 and other minor elements, such as P2O5, Na2O and K2O. These magnesiocarbonatites are characterized by extreme enrichment in incompatible elements with high total rare earth element (REE) contents of 372–36965ppm. The strontium [(87Sr/86Sr)i=0.70378–0.70386], neodymium [εNd(t)=+2.51–+3.59] and oxygen (δ18OV-SMOW=5.9‰–8.0‰) isotope values of these rocks are consistent with a mantle origin, whereas the magnesium (δ26Mg=−1.09‰ to −0.85‰) and carbon (δ13CV-PDB=−4.1‰ to −5.9‰) isotopes are decoupled from mantle values and reflect signature of recycled sedimentary carbonates. Global plate tectonic models predict that sedimentary carbonates in convergent margins are subducted to deep domains in the mantle, with phase transitions from calcite/dolomite to magnesite, and eventually to periclase/perovskite. The involvement of a mantle plume enhances the normal mantle geotherms and promotes decomposition reactions of magnesite. The decoupling of Mg–C and Sr–Nd–O isotopes in the mangesiocarbonatites provides insights on the origin of carbonatites, and also illustrates a case of interaction between mantle plume and subduction-related components.