The extremely enriched mantle beneath the Yangtze Craton in the Neoproterozoic: Constraints from the Qichun pyroxenite

Abstract

To decipher the geochemical property of Precambrian subcontinental lithospheric mantle (SCLM) beneath the Yangtze Craton, a combined geochemical and geochronological study was performed on mafic-ultramafic rocks at Qichun in the northern Yangtze Craton. The major igneous rocks in this area are pyroxenite and serpentinite, with some small schist veins interlayered in them. The pyroxenite has low SiO2 of 43.19–54.19% but very high MgO of 19.65–29.36%, with Mg numbers of 69–80. It shows flat REE patterns, slightly negative Eu anomalies, low concentrations in LILE and most incompatible elements with peaks of Ba and Pb but a trough of Nb in the spidergram. The majority of pyroxenes and hornblendes from the pyroxenite show similar δ18O values of 5.1–6.2‰. Contemporaneous analyses of zircon U-Pb ages and Lu-Hf isotopes for a pyroxenite yield a crystallization age of 808±4Ma and ɛHf(t) values of −21.3±1.2. Therefore, the pyroxenite is of cumulative origin and was derived from partial melting of an enriched mantle source. The enriched signature in the Neoproterozoic SCLM beneath the northern Yangtze Craton would be acquired by reaction of the mantle wedge peridotite with felsic melts derived from partial melting of the ancient terrigenous sediment at the slab-mantle interface in a fossil oceanic subduction channel. On the compilation of all Neoproterozoic mantle-derived mafic igneous rocks in the northern Yangtze Craton, it appears that the SCLM beneath the Northern Yangtze Craton is heterogeneous in different places and evolved with time. It is inferred that the early Neoproterozoic is an important period for the enrichment of SCLM beneath the Yangtze Craton.