GEMOC ARC National Key Centre

Ming Zhang1, Weiming Fan1,2, Suzanne Y. O'Reilly1 and William L. Griffin1
1. GEMOC Macquarie
2. Changsha Institute of Geotectonics, Chinese Academy of Sciences, Hunan, China

Geochemical studies of mantle xenoliths from early Paleozoic kimberlites in North China demonstrate that the subcontinental lithospheric mantle (SCLM) beneath the Sino-Korean Craton was then cold, refractory and about 200 km thick. In contrast, mantle xenoliths from late Tertiary basalts of the region suggest that the present SCLM is hot, relatively fertile and <80 km thick, consistent with geophysical data. This contrast indicates a removal of at least 120 km of old mantle root during the period. Systematic geochemical variations of mantle-derived magmas (136 ? 0 Ma) in the region can constrain the timing and processes for the SCLM destruction.
The mantle-derived rocks include high-Mg andesite (136-107 Ma), minette (103-91 Ma), olivine- and quartz-tholeiites (61-25 Ma), strongly alkaline basalts (16-0.7 Ma). The high-Mg andesites form a Sr-Nd isotope trend pointing to EM1 (87Sr/86Sr =0.7040-0.7049 and epsilon Nd=-5 to -14), while the minettes have very low epsilon Nd (-13 to -18) and variable high 87Sr/86Sr ratios (0.7056-0.7100), showing a trend between EM1 and EM2. Both of them have fractionated incompatible element patterns. The strongly alkaline basalts have Sr-Nd isotopic ratios (0.7032-0.7037 and +3.8 to +5.8) and incompatible element patterns indistinguishable from OIBs. The early Tertiary tholeiites are intermediate between the old and young rocks in terms of incompatible element signatures and Sr-Nd isotope ratios.
The temporal geochemical variations can be explained as the consequence of the diminishing role of the old SCLM beneath the Sino-Korean Craton in magma generation as it was destroyed during the late Cretaceous ? early Tertiary period. The SCLM destruction is related to some of the major tectonic events in the southeastern margin of the Eurasian plate.