Petrology and geochemistry of Cenozoic intra-plate basalts in east-central China: Constraints on recycling of an oceanic slab in the source region

Abstract

Cenozoic mafic rocks in Jiangsu and Anhui Provinces, east-central China are chiefly basanites and alkali olivine basalts with subordinate tholeiites, which were erupted in three stages; Paleogene, Neogene and Quaternary. The rocks become increasingly alkaline as they become younger. On a primitive mantle-normalized multi-element plot, these lavas exhibit typical OIB-like trace element patterns, including enrichment in most incompatible elements (LILE and HFSE) and negative K and Pb anomalies. The compositions of the mafic rocks indicate that they were derived from a mantle source mainly containing clinopyroxene and garnet, most probably a mixture of pyroxenite/eclogite and peridotite. A mineral equilibrium projection shows that all the mafic magmas were produced at pressures of 3–4GPa, implying an asthenospheric origin. Their positive Ba and Sr anomalies and relatively high 87Sr/86Sr ratios suggest derivation from an EM1-type mantle source. However, poor correlations between 87Sr/86Sr and 143Nd/144Nd indicate an isotopically heterogeneous source for the magmas, including DMM, EM1 and EM2, representing mantle peridotite, recycled ancient oceanic crust and seafloor sedimentary rocks, respectively. Variable correlations between 87Sr/86Sr and 143Nd/144Nd ratios, CaO–MgO contents and Eu/Eu* and Ce/Ce* anomalies with rock type imply that marine sediments (plus variable amounts of oceanic crust) and peridotites were the dominant source lithologies of the basanites, whereas recycled oceanic crust (pyroxenite/eclogite) was the main source of the weakly alkaline basalts. This hypothesis is supported by seismic tomographic images of the mantle beneath the region, which show the presence of a stagnant subducted slab in the mantle transition zone. Thus, we propose a petrological model in which a hybrid magma column originated from the mantle transition zone and assimilated some of the overlying peridotite during upwelling, to become the parental magmas of these mafic rocks. This process may relate to the documented lithospheric thinning in East China in the Mesozoic and Cenozoic.