135~130 Ma：大别山第二次“去根”时间？

摘要

The Dabie complex is mainly composed of Early Cretaceous intrusive rocks and Triassic metamorphic rocks. On the four sides of the complex, there are four regional shearing belts: the Tancheng-Lujiang Fault, the Shangcheng-Macheng Fault, the Xiangfan-Guangji Fault and the Xiaotian-Mozitan Fault. Among them, the strike-slip directions of the Xiangfan-Guangji Fault and Xiaotian-Mozitan Fault were opposite in Early Cretaceous. The northern boundary (Xiaotian-Mozitan Fault) is a sinistral ductile shear zone, and the southern boundary (Xiangfan-Guangji Fault) is a dextral ductile one. In the inner part of the Dabie complex, low-angle mylonitic foliations, which were formed in Early Cretaceous, mostly dip toward SE or NW. Lineations on these later foliations also plunge toward SE or NW. Generally, the Dabie complex is characterized by southeastward extrusion and top-to-NW shearing along the orogen. These structures indicate that Late Mesozoic tectonic processes were very important for the later evolution of the complex. The deformation characteristics in the shear zone and the inner part of the Dabie complex suggest a low-angle exhumation along the orogen (SE-NW), which led to the exhumation of the HP/UHP metamorphic rocks and the synkinematic magmatic rocks to the Earth’s surface. Meanwhile, geochronological data indicate that, during Late Jurrassic-Early Cretaceous, the Dabie complex (or the northeastern margin of the Yangtze plate) underwent large-scale migmatization in 145~135 Ma, synkinematic magmatism in 145~135 Ma, and post-orogenic magmatism in 135~120 Ma. Therefore, Early Cretaceous exhumation recorded in the Triassic metamorphic rocks from the Dabie complex is not the continuation of the Endosinian event, but associated with the Yanshanian intraland orogeny and the subsequent extension. Though the starting time of the orogeny remains unclear, the available dating results for the undeformed plutons (130~120 Ma) and the new zircon U-Pb age of the synkinematic pegmatite dyke obtained by the authors provide better timing constraints on the end of the crustal deformation in Early Cretaceous along the Dabie orogen. In conclusion, the Dabie complex underwent Triassic collisional orogeny-extension, and Late Jurrassic-Early Cretaceous interland orogeny-extension two processes.%大别杂岩主要由早白垩世侵入岩和三叠纪变质岩组成。它的四周是四条区域性韧性剪切带：郯城-庐江断裂,商城-麻城断裂,襄樊-广济断裂和晓天-磨子潭断裂。其中,晓天-磨子潭断裂和襄樊-广济断裂在早白垩世具有相反的走滑剪切方向：北侧的边界断裂(晓天-磨子潭断裂)是一个左行剪切断裂,而南侧的边界断裂(襄樊-广济断裂)是一个右行剪切断裂。在大别杂岩内部,早白垩世低角度剪切面理的倾伏向以 SE向或 NW向为主。这些晚期剪切面理上的拉伸线理的倾伏向同样为 SE或 NW向。大别杂岩总体具有朝 SE 向挤出和顶部相对朝 NW向剪切的构造特征。这些表明晚中生代是该杂岩演化的重要阶段。该杂岩的边界断裂和内部构造特征指示其晚期抬升是沿造山带方向(SE-NW)以低角度方式进行的。这一过程直接导致高压-超高压变质岩和同构造岩浆岩被抬升至近地表。同时,年代学研究表明：大别杂岩(扬子板块东北缘地壳)在晚侏罗世-早白垩世经历大规模混合岩化的时间为145~135 Ma,同造山岩浆作用的时间为145~135 Ma,后造山火山-岩浆活动的时间为135~120 Ma。因此,该杂岩中三叠纪高压-超高压变质岩所记录的早白垩世抬升过程不是印支事件的后续,而是燕山期陆内造山及随后发生的伸展过程有关。尽管这一陆内造山事件的起始时间至今仍不确定,但大别山未变形岩体(130~120 Ma)的年代学研究结果和我们新测得的同构造伟晶岩脉的锆石U-Pb年龄(130 Ma)为早白垩地壳变形提供了良好的上限制约。这样,大别山经历了三叠纪碰撞造山和伸展,晚侏罗世-早白垩世陆内造山-伸展二次过程。