Petrogenesis of carbonatites in the Luliangshan region, North Qaidam, northern Tibet, China: Evidence for recycling of sedimentary carbonate and mantle metasomatism within a subduction zone

Abstract

Carbonatitic magmatism in subduction zones provides extremely valuable information on the cycling, behavior and storage of deep carbon within the Earth. It may also shed light on insights into crust-mantle interaction and mantle metasomatism within subduction zones. Origin of carbonatite has long been debated: all hypotheses need to reflect the different mineral assemblages and geochemical compositions of carbonatites and their diverse tectonic settings. Here we present a petrological, geochronological, geochemical and isotopic study of carbonatite bodies associated with orogenic peridotites, which occur as stocks or dykes with widths of tens to hundreds of meters in the Luliangshan region, North Qaidam, northern Tibet, China. On the basis of modal olivine (Ol) content, the studied samples were subdivided into two groups: Ol-poor carbonatite and Ol-rich carbonatite. Zircon grains from the Ol-poor carbonatite show detrital features, and yield a wide age spectrum between 400 Ma and 1000 Ma with a pronounced peak at ca. 410–430 Ma. By contrast, oscillatory zoned zircons and inherited cores show two relatively small Neoproterozoic age peaks at ca. 920 and 830 Ma. Zircon grains from the Ol-rich carbonatite sample are also distributed in a wide spectrum between 400 and 1000 Ma, with a pronounced peak at ca. 440 Ma and a slightly inferior peak at ca. 410 Ma. The oscillatory zoned zircons and inherited cores exhibit a smaller Neoproterozoic age peak at ca. 740 Ma. The pronounced peaks ranging from 430 to 410 Ma are consistent with the deep subduction and mantle metasomatic events recorded in associated ultramafic rocks.Both groups of carbonatites are characterized by enrichment of light rare earth elements (LREEs) with high (La/Yb)N values and pronounced negative Eu anomalies. They show high 87Sr/86Sr values (0.708156–0.709004), low 143Nd/144Nd values (0.511932–0.512013) and high δ18OV-SMOW values (+17.9 to +21.3‰). This geochemical and isotopic evidence suggests that these carbonatites were derived from remobilized sedimentary carbonate rocks. We propose that the primary carbonatite magma was formed by partial melting of sedimentary carbonates with mantle contributions. Sedimentary carbonates were subducted into the shallow upper mantle where they melted and formed diapirs that moved upwards through the hot mantle wedge. The case presented provides a rare example of carbonatite originating from sedimentary carbonates with mantle contributions and relevant information on the mantle metasomatism within a subduction zone.