Lithospheric Thinning and Accretion in Mesozoic-Cenozoic in Eastern China: An Isotopic Study of Mantle Xenoliths

Wei-Ming FAN1) and M.A. MENZIES2)

1) Changsha Institute of Geotectonics, Academia Sinica, Changsha, Hunan, P.R. China

2) Department of Geology, Royal Holloway University of London, UK.

Mantle xenoliths collected at twenty-four localities along a >3000 km N-S traverse in eastern China have a very large range in isotopic composition (87Sr/86Sr = 0.701845-0.709560; 143Nd/144Nd = 0.514009-0.511409) indicative of an extremely heterogeneous shallow mantle beneath eastern China. However, the vast majority of xenoliths have Sr and Nd isotopic ratios that encompass the range observed in depleted mid-ocean ridge mantle (DMM) and in the sources of ocean island basalts (OIB). In general the lower lithosphere with an "oceanic" affinity prevails beneath eastern China. No systematic difference is observed in isotopic variability for the bulk of the shallow mantle beneath either Archaean or post-Archaean crust. The lithospheric mantle beneath the Cenozoic basins and Tanlu Fault and along the southeast China coast tends to be isotopically homogeneous and the most MORB-like xenoliths predominate in two regions around the Bo Sea (thin Archaean crust) and along the southeast China coast (thin post-Archaean crust).

The Sr and Nd isotopic variations of the shallow mantle beneath eastern China are possibly related to lithospheric tectonic-thermal evolution in Mesozoic-Cenozoic. The North China block-South China block collision in early Mesozoic possibly caused destruction of Pre-Mesozoic thick, cold and variously enriched lithospheric mantle. The lithosphere was further thinned because of extension in Cenozoic. Lowering of the lithosphere-asthenosphere boundary in late Cenozoic since weaker extension and thermal decay resulted in accretion of young lower lithosphere, which accounts for the predominance of peridotite xenoliths with isotopic characteristics similar to the sublithospheric sources. We conclude that the present-day shallow mantle beneath eastern China is a mixture of old (Archaean/Proterozoic) relics and a young (post-Tertiary) components, the latter being accreted in association with the Cenozoic continental extension.