Neoproterozoic protracted arc basaltic magmatism in the southern margin of the Yangtze Block, south China: New constraints from mafic-ultramafic intrusive rocks

Abstract

Mantle plume activity and subduction are two popular geodynamic models that have been used to explain the widespread Neoproterozoic basaltic magmatism in the Jiangnan Orogenic Belt along the southern margin of the Yangtze Block, South China. We have evaluated the competing hypotheses using new geochronological, petrological and geochemical data of two mafic–ultramafic intrusions in the Yuanbaoshan (YBS) area of this orogenic belt, namely the YBS south and west intrusions. Zircons from the YBS south intrusion yielded a U-Pb age of 835 ± 3 Ma. This new age and the previously reported age for another mafic–ultramafic intrusion situated < 10 km to the northwest (the YBS west intrusion) reveal a protracted (12–20 Myr) episode of mafic magmatism in a very small area. Such temporal-spatial relationship is more consistent with the style of subduction-related magmatism than that of mantle plume-related magmatism. The YBS south and west intrusions are characterized by arc-like geochemical features such as light REE enrichments and pronounced negative Nb-Ta anomalies in whole rocks. Olivine Mn/Fe and Ca contents, clinopyroxene AlZ/TiO2 and Cr-spinel TiO2/Al2O3 of these intrusions are similar to those of some undisputed subduction-related mafic–ultramafic intrusions elsewhere in the world. Zircons from the YBS south intrusion have mantle-likeHf-O isotope compositions, with εHf(t)varying from + 3.3 to + 8.7 and δ18Oof5.32 ± 0.29‰. The whole rock samples yielded εNd(t) from + 5.2 to + 8.8 and (87Sr/86Sr)i from 0.7034 to 0.7051 for the YBS south intrusion, and εNd(t) from + 1.7 to + 5.2 and (87Sr/86Sr)i from 0.7036 to 0.7051 for the YBS west intrusion. The Sr-Nd isotope compositions of these intrusions are similar to those of global Cenozoic arc basalts, but are also consistent with mantle plume-derived magma with < 5% of crustal contamination. However, the lack of correlation between Th/Nb and εNd(t) or (87Sr/86Sr)i for the intrusive rocks is inconsistent with the contamination model. The results from this study support for a popular view that northward oceanic subduction beneath the southern margin of the Yangtze Block took place in the Neoproterozoic.