Geochemical and isotopic evidence for Mesozoic-Cenozoic transformation of subcontinental lithosphere mantle beneath North China

Abstract

Whether secular or episodic compositional change of SCLM related to major geological events has been a hot debate. This problem can only be viewed by multidisciplinary approach to delineate the variation of structure and composition of the SCLM with time in certain regions. This study would emphasize the Mesozoic-Cenozoic transformation of geochemical and isotopic characteristics of SCLM beneath North China, where has been well known as an ancient continental block and is very different from other typical cratons in terms of its intensive seismicity, high heat flow and extensive magmatism since Mesozoic. A number of geochemical studies of SCLM beneath North China have shown an affinity to its oceanic counterpart. The multiple isotope tracers, show mainly depleted signatures and young Os model age, which are the characteristics opposite to their geological and tectonic settings. Geophysical data suggest a thin, hot and modified continental lithosphere in the region However, very little has been known about the Mesozoic SCLM in the region. A geochemical survey of Mesozoic volcanic rocks has shown that the tectonic domains strictly control the source characteristics of these volcanic rocks. The Sr-Nd diagrams demonstrate a common trend of migration of source signature: it varied from enriched EM I and EM II components in the Late Jurassic-Early Cretaceous to less enriched or primitive signature in the Late Cretaceous, and then to depleted at the end of Cretaceous and Early Tertiary, Furthermore, the systematic studies on the granulitic xenoliths suggest that they were derived from multiple geochemical reservoirs and SHRIMP U/Pb zircon dating reveals that Mesozoic is the major episode for the genesis of the xenoliths, and implies that the extensive interaction between lower crust and mantle would have happened episodically, coupled with geological events recorded in the overlying crust. The thinning of lithosphere beneath eastern part of North China has been proposed since mid-1980 s. This study indicates that a typical cratonic SCLM no long exist since Late Mesozoic when a regional mantle upwelling occurred, resulted in thermal erosion, chemical or mechanical modification of the lower crust/upper mantle, accompanied by lateral spreading of the SCLM and rifting/basin development in the shallow level. This catastrophic event would have removed large amount of ancient lithosphere, and were modified by juvenile input from the underlain mantle as evidenced in 60 80 Ma, the mantle-derived magmas become unambiguously depleted in isotopic signature. The mantle upwelling is proposed to be related to the global dynamical overturn in the mantle and constrained by local tectonic boundaries.. This study is supported by NSF China (49733110) and RGC to HKU 7110/97P.