Lithospheric thinning accompanying mantle lateral spreading, erosion and replacement beneath the eastern part of North China: evidence from peridotites.

Abstract

We have compared the mineral chemistry of peridotites from xenoliths in volcanic rocks of different eruptive ages in the North China block to those from the Sulu ultra-high pressure (UHP) metamorphic terrane,south of the block. Refractory peridotites are the major rock types in both the Paleozoic North China kimberlitic xenoliths and the early Mesozoic Sulu UHP terrane. These types of peridotites constitute the majority of Pliocene Hebi basaltic xenoliths, and they are also found in late Mesozoic (~100 Ma) Fuxin basalts. In contrast, fertile peridotites are the major xenoliths in the late Mesozoic Fuxin basalts and,especially,in Miocene basalts from areas within the Tanlu Fault zone (e.g. Shanwang) and its adjacent area (e.g. Qixia). The spatial and temporal distributions of the refractory and fertile lithospheric mantle indicate the co-existences of different nature of mantle in some localities (such as Hebi and Fuxin). This suggests that Mesozoic-Cenozoic lithospheric thinning was accompanied by mantle replacement. The subduction of the Yangtze plate may have caused lateral spreading of the cratonic lithospheric mantle in the early Mesozoic. The thermal disturbance resulting from the subduction of a Pacific Ocean plate gave rise to the strong eroding of the cratonic mantle by asthenospheric materials in the late Mesozoic-Oligocene. Both the earlier lateral spreading and the younger strong erosion of the cratonic mantle brought about major thinning of the lithosphere. These processes collectively accomplished mantle replacement and lithospheric thinning in the eastern part of North China. A mantle weak zone in the lithosphere and the trans-lithospheric Tanlu fault functioned as channels for asthenospheric materials eroding the existed mantle. In addition, slight lithospheric thickening occurred during the Neogene-Quaternary in response to the cooling of upwelling asthenosphere.