中生代华北南缘带状岩石圈结构特征及其大陆形成演化意义

Mesozoic Zonal Lithosphere beneath the Southern Margin of the North China:Significance for Continental Formation and Evolution

大陆岩石圈根的形成与破坏是当前固体地球科学的重大研究课题之一.对独具时空特色的华北东部地块南缘信阳中生代火山岩中一系列包括下地壳镁铁质-长英质的麻粒岩、榴辉岩、变辉长岩、辉石岩和上地幔橄榄岩等岩石包体进行了系统的定深、定年研究,建立了华北中生代(～160Ma)多块体结合部位的组成和年龄呈带状结构的岩石圈几何模型,并在此基础上分析了形成的动力学过程.在华北南缘地表出露最老～2.85Ga的岩石之下的30km处(上部下地壳),由年龄为3.6～3.4Ga的长英质麻粒岩和辉石岩组成;更深处30～40km位置,则由具古元古代年龄(2.0～1.8Ga)的镁铁质-长英质麻粒岩和变辉长岩构成,其形成过程与华北东部地块与西部地块的碰撞有关,记录着全球性的哥伦比亚超大陆汇聚事件.Hf同位素数据显示在这次重要事件里,既有新生地幔物质加入,也有古老地壳(3.8～3.0Ga)组分的再熔融作用.在来自下地壳更深处的榴辉岩(40～45km)和上地幔橄榄岩(>45km),它们的主要年龄分别是古生代(440～260Ma)和早中生代(228～219Ma),记录着在显生宙不同时期扬子与华北碰撞的动力学过程.

The formation and destruction of the continental root is one of the most important going topics on the solid earth sciences. A series of deep-seated xenoliths, including mafic to felsic granulite, eclogite, metagabbro, pyroxenite and peridotite, from the Xinyang area with unique temporal and spatial meaning, at southern margin of the North China, were used to dating in ages and properties. The geometrical model of the zonal lithosphere structure in age and composition were suggested for the conjoint position of multi-blocks in Mesozoic（-160Ma）. Basing on the zonal lithosphere, the authors also discussed the dynamic processes for its formation. The exposed rocks up to ca. 2.85 Ga old are underlain by felsic granulites and rare pyroxe- nites with zircon ages of 3.6-3.4 Ga （to ca. 30 km depth）. Deeper （ca.30-45km） parts of the lower crust consist of highpressure mafic to felsic granulite and meta-gabbro, which give Paleoproterozoic（2.0-1.8Ga） zircon ages. Our data show the significance of underplating and vertical crustal growth in the Paleoproterozoic, which was related to the amalgamation of the eastern and western block of this eraton and a global （Columbia） supercontinent assembly. Hf-isotope data indicate that both juvenile material and remelting of older （3.8-3.0Ga） crustal rocks were involved in this important event. Paleozoic （440-260Ma） and Early Mesozoic （228-219 Ma） zircons are also found in xenoliths from the deeper part of the lower crust and the uppermost mantle （i. e. , eclogite and peridotite）. They are interpreted as reflecting geodynamie processes related to the continental collision between this craton and the Yangtze craton, respectively. The xenoliths from Xinyang diatremes thus record the growth and modification of the old （Paleo-Mesoarchean？） continental lithosphere by magma underplating during several tectonic events： assembly of the southern and northern parts of the Eastern Block in Neoarchean time, collision of the western and eastern blocks along the Trans-China orogen in the Paleoproterozoic, and subduction and collision of the Yangtze craton with the North China craton in early Paleozoic and Triassic times, respectively.