Recycling of oceanic crust from a stagnant slab in the mantle transition zone: Evidence from Cenozoic continental basalts in Zhejiang Province, SE China

Abstract

Cenozoic continental basalts from Zhejiang Province, southeast China are tholeiitic to weakly alkalic in composition, with moderate MgO contents (6–11wt.%) and an average Mg# of 62. They display typical OIB-like trace element features, including enrichment in most incompatible elements, both LILE and LREE, and negative K, Pb, Zr, Hf anomalies. In particular, they are characterized by high Fe/Mn (73±5), La/Yb (19±6) and Nb/Ta (18.8±0.4) ratios, which can be attributed to the presence of residual clinopyroxene, garnet and rutile in the mantle source. Based on these minerals, the following hybrid source rocks are hypothesized: garnet pyroxenite/eclogite and peridotite. Clinopyroxene–liquid thermobarometry indicates clinopyroxene crystallization temperatures of >1257°C. This is higher than the assumed temperature at the base of the sub-continental lithospheric mantle (SCLM) (~1220°C) beneath Zhejiang, thus the magmas were presumably derived from the asthenosphere. Some typical geochemical features such as negative K, Pb anomalies, positive Ba, Sr, Nb, Ta anomalies and the extremely high Os isotopic signatures, suggest participation of EM-like mantle sources, indicative of ancient subducted oceanic crust. (87Sr/86Sr)i (0.7037–0.7046) and 143Nd/144Nd (0.512832–0.512990) isotope ratios point to the presence of mixed components in the source region, i.e., DMM, EM1 and EM2. Recent seismic tomographic images of the mantle beneath Zhejiang suggest the presence of a subducted slab of oceanic lithosphere in the transition zone. Based on the combined geophysical and geochemical evidence, we propose that the major source of the Zhejiang basaltic magmas was the ancient subducted oceanic slab in the transition zone with an EM-like signature. The other magma sources include depleted asthenospheric peridotite possessing a DMM-like signature. The dynamics of this upwelling hybrid magma was apparently related to westward subduction of the Pacific plate underneath the eastern Asian continent. This process may have triggered the widespread Cenozoic volcanism related to the lithospheric thinning in East China in the Mesozoic and Cenozoic.