同构造岩浆流动与相对稳定环境下岩体侵位的差异性及其对华北北缘晚古生代晚期—早中生代构造演化的约束

    Difference of the syn–tectonic magmatic flow and granite emplacement under stable tectonic environment and its constrain on the Late Paleozoic to Early Mesozoic tectonic evolution in the northern margin of North China plate

    • 摘要: 花岗质岩浆岩是大陆地壳的重要组成部分,对华北北缘燕山地区构造变形、基本构造格架的分析已经做了大量研究,但对于花岗岩体侵位的动力学环境研究较少。与相对稳定构造环境下侵位的花岗岩不同,同构造岩浆流动通常发生在大陆边缘、造山带等构造活跃带,在强烈的构造应力场影响下,岩体特征、侵位过程、流动方向通常十分复杂。华北板块北缘在晚古生代期间有大量花岗质岩体侵入,其构造属性与成因分析为研究古亚洲洋的俯冲与华北板块破坏提供了关键证据。对华北板块北缘的大光顶岩体和盘山岩体从宏观到微观进行构造变形及岩石学特征分析,结合电子探针手段进行半定量的矿物学研究,并利用角闪石压力计来计算岩体形成的压力条件,得出岩体侵位的深度,探讨华北北缘不同动力学背景下的构造环境。大光顶花岗闪长岩在露头尺度上表现为大量轴面低角度北倾的无根褶皱,包体和捕虏体长轴平行于流动面理,显微镜下可见角闪石、黑云母等暗色矿物定向排列,具有典型的同构造变形特征,角闪石全铝压力计指示结晶压力为3.62~5.64 kbar,大致对应中地壳的深度(12.86~22.99 km),认为晚古生代(320~290 Ma)时期,华北板块北缘中下地壳重熔形成由北向南的同构造岩浆流动,同时也为中—深层次的应力传递提供媒介。中生代盘山花岗岩中的包体、捕虏体未发生变形,也无暗色矿物定向排列,岩体与围岩接触带发育热接触变质作用形成大理岩,属稳定构造环境下侵位的花岗岩。

       

      Abstract: Granitic magma constitute the important components of continental crust, a lot of researches have done on the structural deformation and fundamental framework of the Yanshan area on the northern margin of the North China plate. However, dynamic setting of granite intrusions was less studied. Different from granite intrusions in relatively stable tectonic setting, syn–tectonic magmatic flow usually occurs in active tectonic zones such as continental margin and orogenic belt, the characteristics, intrusion process and flow direction of syn–tectonic granite are usually complicated affected by strongest tectonic stress field. Large quantities of granitic magma intruded in the northern margin of the North China plate during the Late Paleozoic, and their tectonic properties and analysis provide crucial evidence for the study of subduction of Paleo−Asian Ocean and the destruction of the North China plate. By analyzing the structural deformation and petrographic characteristics of Danguangding pluton and Panshan pluton in the northern margin of North China plate from macro to micro, semi–quantitative mineralogical study was combined with electron microprobe, and the pressure conditions of pluton formation are calculated by using hornblende manometer to obtain the depth of granite intrusions, and the tectonic setting in the northern margin of the North China plate under different dynamic backgrounds is discussed. On the outcrop scale, the granodiorite shows a large number of rootless folds with a north axial direction and low axial plane angle, and the long axes of inclusions and xenoliths are parallel to the flow foliation. Under the microscope, dark minerals such as amphibole and biotite can be seen in directional arrangement, with typical syn–tectonic deformation characteristics. The crystallization pressure of the all–aluminum gauge of amphibole is 3.62~5.64 kbar. Corresponding to the depth of the middle crust (12.86~22.99 km), it is believed that during the Late Paleozoic (320~290 Ma), the middle and lower crust on the northern margin of the North China plate remelted to form a syn–tectonic magmatic flow from north to south, which also provided a medium for the stress transfer in the middle and deep layers. This indicates that the remelting of the middle and lower crust on the northern margin of the North China plate in the Late Paleozoic was formed by syn–tectonic magmatic flow from north to south, which formed the Daguangding pluton and also provided a medium for the stress transfer in the middle and deep layers. The inclusions and xenoliths in Panshan granite are not deformed in Mesozoic, and there is no directional arrangement of dark minerals, marble is formed by thermal contact metamorphism in the contact zone between pluton and surrounding rock, which belongs to granite intrusions in relatively stable tectonic setting.

       

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