Coexisting Early Cretaceous arc-type and OIB-type mafic magmatic rocks in the eastern Jiangnan Orogen, South China Block: Implications for paleo-Pacific plate subduction

Abstract

Mafic magmatic rocks can provide important information about the nature of mantle source, material recycling, and mechanism of crust–mantle interaction. However, owing to the scarcity of such rocks, the nature and evolution of the Late Mesozoic mantle source beneath southeastern China have not been well constrained. Here, we present data on zircon UPb ages, whole-rock major and trace elements, and Sr–Nd–Pb isotopes for two types of simultaneous mafic intrusive rocks from the eastern Jiangnan Orogen (EJNO), with the aim of providing insights into their petrogenesis and the relationship of extensive Late Mesozoic magmatism in southeastern China and subduction of the paleo-Pacific plate. The Qukou diabases (~ 128 Ma) show a subalkaline affinity and have a mean Mg# value of 53.4. They exhibit arc-like trace-element distribution patterns, namely, enrichment in light rare earth elements (LREEs), Ba, Th, and U, and depletion in Nb, Ta, P, and Ti, as well as moderately radiogenic Sr and chondritic Nd isotopic compositions, with 87Sr/86Sr(i) = 0.7057–0.7063 and εNd(t) = −1.01 to 1.13. In contrast, the Yangboping gabbros (~ 125 Ma) belong to alkaline basalt and have a mean Mg# value of 56.8. They present OIB-like trace-element characteristics, show enrichment in LREEs, Sr, and Ba, and no depletion in Nb, Ta, Zr, Hf, and Ti, and possess highly radiogenic Sr and weakly nonradiogenic Nd isotopes of 87Sr/86Sr(i) = 0.7096–0.7114 and εNd(t) = −0.97 to −0.11. These geochemical features suggest that the Qukou diabases were generated by partial melting of a pyroxenitic source region, and this pyroxenitic mantle source was formed via the metasomatism by HFSE-depleted hydrous melts of subducted slab-derived terrigenous sediments, whereas the Yangboping gabbros were generated from a metasomatite that formed by metasomatic reaction between depleted MORB mantle and subducted slab-derived HFSE-enriched hydrous melts, in which the ratios of altered oceanic crust-derived melts to sediment-derived melts were 90:10–60:40. The differences in trace-element and isotopic compositions of these two types of mafic magmatic rocks suggest that their mantle sources were metasomatized by hydrous melts from different depths: the HFSE-depleted hydrous melts were from the shallow depths (<80 km), whereas the HFSE-enriched melts were from the greater depths (>200 km), indicating that the subducted oceanic slab would have founded/broken off to provide enough heat to melt the subducted oceanic crust. Consequently, we propose that the flat-slab subduction and slab break-off model of the paleo-Pacific plate was the most likely geodynamic mechanism for Late Mesozoic magmatism in the South China Block.