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Deep Geodynamics of Yanshain Movement

WU Fu-yuan, SUN De-you, ZHANG Guang-liang, REN Xiang-wen   

  1. College of Earth Sciences, Changchun University of Science and Technology, Changchun 130061, China
  • Received:2000-09-20 Revised:2000-09-20 Online:2000-09-20 Published:2000-09-20

Abstract: The eastern China was characterized by intensive magmatic ac tivities, structrual deformation, basin formation and mineralization in the Mesozoic, whic h wa s described as the Yanshain Movement Wong (1927). Although a lot of studies have been conducted in the past 70 years on the Yanshain Movem ent, a warmly debate still exists about its geodynamic background, such as the sub duction and back-arc extension of Pacific plate, continental -continental collisi on, sheared strike-slip continental margin, impacting and lithosph eric thinning , etc. A comprehensive study about the mineralogical, petrological and Sr-Nd isotopic features of Cenozoic basalts and their mantle xenoliths has indicated t hat the eastern China in Yanshain Period was characterized by lithospheric delaminat ion, especially the direct contact of asthenospheric mantle with the crust. Most of the pre-existed ancient lithospheric mantle had been delaminat ed, the present lithospheric mantle was formed since the Late Mesozoic. According to the presen t st udy, this lithospheric removal is probably related to the subduction of Pacific Oceanic Plate. The early stage (170 Ma or so) of subduction of eastern oceanic p lat e thickened the lithosphere in the eastern China, which resulted in the mantle d elamination in 160 and 145~120 Ma duo to the gravity instability of thicken ed lithosphere. Therefore, the nature of Yanshain movement is the thinning, even disappearing of lithospheric mantle beneath eastern China. This phenomenon is r are in the world and was reported only in the Basin and Range of western United States. Du ring the period of lithospheric removal, the asthenosphere upwelling and heating , and the underplating of mantle-derived magma in the lower cr ust resulted in th e intensive Late Mesozoic magmatism-tectonism-minera lization of Yanshain Movement in China.   This research also suggests that the present lithospheric mantle beneath e astern China is juvenile, and does not correspond to the old ages found in the o verlyi ng crustal rocks. This implies a mantle-crust decoupling, which is inconsistent with the results of present researches on the major Archean Cratons in the world. Th erefore, further investigation should be conducted to study the mechanism of th is process.