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    华南陆缘晚中生代造山及其地质意义

    Late Mesozoic orogenesis along the coast of Southeast China and its geological significance

    • 摘要: 提要:长乐—南澳带位于华南大陆的东南缘,是揭示晚中生代古太平洋板块与欧亚大陆相互作用和华南陆缘地质演化的一条关键性构造带。该构造带的T3-J含煤地层不整合下伏于下白垩统南园组之下,或以角闪岩相包体形式赋存于晚侏罗—早白垩世片麻状花岗岩体内。这些具有透入性混合岩化特征的副变质岩包体(T3-J)与围岩(片麻状花岗岩)发生过同变形,并且被未变形的白垩纪岩体/脉侵入。这些晚三叠—早侏罗世地层的变质和变形具有朝陆内(NW)方向减弱的特征。下白垩统南园组普遍发生褶皱和绿片岩相变质,并且被未变质的上白垩统石帽山群不整合覆盖。长乐—南澳带的主要岩石单元与构造特征表明华南陆缘在晚中生代经历过两期挤压(造山)。在第一期挤压(造山)过程中,T3-J陆相地层发生褶皱和变质。随后,这些经历了变形和变质的T3-J沉积地层与中-上地壳的其他岩石在早白垩世早期发生构造折返,其间伴随着大规模同构造岩浆作用(147~135 Ma)和区域性混合岩化。在第二期挤压过程中,早白垩世南园组发生褶皱和变质。华南陆缘110~100 Ma的岩浆杂岩、100~90 Ma的A型花岗岩和90~80 Ma的双峰式火成岩可能分别与这次挤压事件及其后的地壳伸展、减薄过程有关。因此,华南晚中生代两大岩浆旋回(165~120 Ma 和110~80 Ma)可能受控于同期的两大构造旋回(165~120 Ma和115~80 Ma)。每个构造旋回包括一个地壳受挤压增厚过程和一个随后发生的地壳伸展减薄过程。值得注意的是,华南沿海地区的晚中生代构造-岩浆作用与陆内同期构造-岩浆作用似乎具有可比性。

       

      Abstract: Abstract:The Changle-Nan’ao zone is situated along the southeastern margin of the Southeast China continent, hence it may hold a key to understanding the interactions between the Eurasian and Paleo-pacific plates and geological evolution of the coastal areas in Late Mesozoic. The T3-J1 sedimentary rocks in the coastal zone are unconformably overlain by Lower Cretaceous Nanyuan Formation or occur as amphibolite facies enclaves in late Jurassic-early Cretaceous gneissic granites. The metasedimentary rocks (T3-J1) enclaves with penetrative migmatization were deformed together with the wall rocks (gneissic granites) and intruded by insignificantly deformed Cretaceous granitic plutons or dykes. Metamorphism and deformation in the late Triassic to early Jurassic (T3-J1) metasedimentary rocks are characterized by cratonward (NW) weakening. The Lower Cretaceous Nanyuan Formation is characterized by extensive folding and greenschist facies metamorphism and unconformably covered by the Upper Cretaceous Shimaoshan Group, showing no signs of metamorphism. The major stratigraphic units and tectonics in the Changle-Nan’ao zone indicate that the continental margins underwent two stages of compression (orogenesis) in Late Mesozoic. During the first episode of orogenesis, the late Triassic to early Jurassic (T3-J1) continental sediments were folded and metamorphosed. Subsequently, the folded and metamorphosed T3-J1 sedimentary rocks were exhumed together with other rocks from the depth of middle and upper crusts in early Cretaceous, accompanied by large scale synkinematic magmatism (147-135 Ma) and regional migmatization. During the second stage of compression in Late Mesozoic, the Nanyuan Formation was folded and metamorphosed. The 110-100 Ma magmatic complex, the 100-90 Ma A-type granites, and the 90-80 Ma bimodal igneous rocks in coastal areas of Southeast China might be correlated with the middle Cretaceous crustal contraction and the subsequent crustal extensions, respectively. Consequently, the two Late Mesozoic magmatic cycles (165-120 Ma and 110-80 Ma) might be controlled by the two synchroneous tectonic cycles (165-120 Ma and 115-80 Ma). Each of the Late Mesozoic tectonic cycles includes a crustal contraction and a subsequent crustal extension. It should be noted that the Late Mesozoic tectonics and magmatism in the coastal areas of Southeast China seem to be comparable with those that simultaneously occurred in the hinterland of the continent.

       

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