Geochemical and Sr–Nd–Hf–O–C isotopic constraints on the origin of the Neoproterozoic Qieganbulake ultramafic–carbonatite complex from the Tarim Block, Northwest China

Abstract

The Qieganbulake ultramafic–carbonatite complex located within the northeastern margin of the Tarim Block of Northwest China hosts the world's second largest vermiculite deposit. Field observations, radiometric dating results and Sr–Nd–Hf isotopes reveal that the parental magmas of the carbonatite and ultramafic rocks are cogenetic and formed synchronously at ~810Ma. They are characterized by unusually enriched Sr–Nd–Hf isotopic compositions (ISr=0.70570–0.70762, εNd(t)=−7.7 to −12.5, and εHf(t)=−6.7 to −12.9), indicating that the parent magmas were derived mainly from a subcontinental mantle source that had been metasomatised by subduction processes. Higher δ13C (−3.65 to −4.11‰) values compared to primary magmatic carbonate (−8 to −4‰) argue for incorporation of recycled inorganic carbon derived from subducted oceanic crusts. The carbonatites and clinopyroxenties define two distinct differentiation trends, which suggest that liquid immiscibility rather than crystal fractionation controlled the petrogenetic process. Pyroxenities have clearly higher apatite ISr and δ18O values than coexisting carbonatites, indicating involvement of crustal components during their emplacement. The Qieganbulake complex is closely associated in time and space with the mid-Neoproterozoic Rodinia breakup event triggered by mantle plume activities in the Tarim Block. Therefore, the mantle plume likely induced partial malting of, and likely mixed with, the metasomatized subcontinental lithospheric mantle, to form the Qieganbulake ultramafic–carbonatite complex.