华北东部大陆地幔橄榄岩组成、年龄与岩石圈减薄

对比分析了华北东部地块陆下岩石圈地幔橄榄石Mg#值和单斜辉石的REE配分形式。报道了汉诺坝和鹤壁橄榄岩中不同产状硫化物的激光MC-ICPMS原位Re-Os年龄和信阳橄榄岩中锆石的U-Pb年龄和信阳橄榄岩锆石的U-Pb年龄。在这些资料基础上,进一步讨论了华北东部岩石圈中、新生代时的减薄机制。原位分析在揭示岩石圈深部过程的细节上,有比全岩分析更大的优越性,并揭示出了在华北深部有中元古代(14亿年)和新元古代(7～8亿年)热活动的记录。岩石圈拆沉作用不能很好地解释古老难熔地幔、过渡型地幔和新生饱满地幔并存的事实;同时,单纯的熔体-橄榄岩相互作用也难以解释中、新生代岩石圈的减薄过程和新生地幔单斜辉石中出现强烈LREE亏损现象,即历史复杂的克拉通岩石圈向历史明显简单的“大洋型”地幔的转换。因此,华北东部岩石圈减薄包括地幔伸展、熔-岩作用、侵蚀置换等复杂过程。这些过程可能包括:(1)早中生代时,扬子地块向北俯冲碰撞所引起华北岩石圈的熔/流体交代富集作用、地幔伸展和受扰动软流圈物质上涌并侵蚀被改造了的岩石圈;(2)晚中生代—古近纪,因太平洋俯冲的热扰动致使软流圈物质进一步的强烈侵蚀作用引起岩石圈的巨大减薄;(3)晚第三纪以来的软流圈热沉降作用所带来的小幅度岩石圈增厚过程。岩石圈先大幅减薄、后小幅增厚实现了最终的地幔置换和岩石圈整体的减薄过程。喷发时代为100Ma的阜新玄武岩所捕获的橄榄岩主体是饱满的,说明华北东部部分地区在此之前曾有过地幔置换作用。

Age and composition of continental mantle peridotites and implications for the lithospheric thinning, eastern North China

Olivine-Mg# values and diopside-REE patterns of mantle peridotites from the.North China were systematically studied. In situ LAM-MC-ICPMS Re-Os isotopes of sulfides in the Hannuoba and Hebi peridotitic xenoliths and U-Pb ages of zircons in the Xinyang peridotitic xenoliths are also reported. Based on these data, we discuss the thinning mechanism of the lithosphere during Mesozoic-Cenozoic times beneath the eastern North China Craton. In situ analysis has an advantages over the whole-rock analysis in revealing deep processes of lithosphere in detail, and of exploring the possible existence of thermal events in the Mesoproterozoic (1.4 Ga) and in the Neoproterozoic (0.7-0.8 Ga) beneath the deep-seated lithosphere of the North China Craton. The coexistence of different types of lithospheric mantle, including refractory, transitional and fertile, cannot be well interpreted by the mechanism of lithospheric delamination. Simple melt-peridotite interaction is also difficult to interpret the lithospheric thinning processes in the Mesozoic and Cenozoic. Thus, complex processes such as extension, melt-rock interaction, asthenospheric erosion and mantle replacement must have been involved during the lithospheric thinning. These processes may be summarised as follows: (1) northward subduction and subsequent collision of the Yangtze Craton in the early Mesozoic, resulted in the North China lithospheric mantle experiencing metasomatism or modification (through melt-rock interaction) by fluids/melts derived from the subducted Yangtze continent, lithospheric mantle extension and asthenospheric erosion. (2) Asthenospheric upwelling related to the subduction of the Kula and the Pacific plates during the late Mesozoic-Paleogene eroded the remaining lithospheric mantle,and resulted in considerable thinning of North China lithosphere. (3) Asthenospheric cooling in Neogene times slightly lowered the lithosphere-asthenosphere boundary, creating newly accreted lithospheric mantle and achieving a thinning lithosphere as a whole. The fertile peridotite xenoliths in the Fuxian basalts erupted at 100 Ma indicate that the early mantle replacement beneath the eastern North China Craton took place in part of eastern North China beforehand.