RECYCLING OF OCEANIC CRUST FROM A STAGNANT SLAB IN THE MANTLE TRANSITION ZONE: EVIDENCE FROM CENOZOIC CONTINENTAL BASALTS IN ZHEJIANG PROVINCE, SE CHINA

The Cenozoic intra-plate basalts of eastern China display many features typical of ‘OIB–type’ lavas. On the basis of recent seismic tomographic images for eastern China from 25° to 41°N, a high-velocity slab has been identified in the mantle transition zone (depths of 410-660km) thought to be a fragment of the subducted Pacific plate. Late Cenozoic basaltic volcanoes lying above the stagnant slab are distributed throughout eastern China. We have taken petrological and geochemical analysis for the Cenozoic basalts in Zhejiang Province (SE China) to pursue the correlation between such volcanos and the extent of the subducted slab.

The Zhejiang basalts 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. The CS-MS-A projection shows that most Zhejiang basalts appear to have formed between L+Gt+Cpx and L+Ol+Cpx+Gt cotectic at pressure ~3GPa. Based on these minerals, the following hybrid source rocks are hypothesized: garnet pyroxenite/eclogite and peridotite.

The isotopic features imply a three-component mixing of DMM, EM1 and EM2. Different lithologies in the ancient (>1 Gyr) subducted oceanic slab, such as oceanic crust including pelagic sediments, cumulated mafic rocks and modified lithospheric mantle, could provide both EM1-like and EM2-like signatures in the source material of the Zhejiang basalts. Depleted continental asthenospheric mantle with depleted isotopic signatures should represent a DMM end-member.

Subduction of the relatively young and hydrous Pacific slab into the mantle transition zone would potentially cause partial melting of the oceanic slab (eclogite/pyroxenite + sediments) with a low melting point. These melts would migrate upward due to the buoyancy difference and through the weakened asthenospheric mantle, thereby obtaining their DMM component. The overlying continental lithosphere may have served as a passive mechanical barrier to continuous upwelling of the magmas. East China was in an extensional tectonic setting during the Cenozoic, rifting of the continental crust would have allowed further ascent of the magmas and eruption at the surface.