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中朝克拉通北部早前寒武纪变质作用演化的三种主要样式及其地质动力学

卢良兆 徐学纯 董永胜   

  1. 长春科技大学地球科学学院
  • 收稿日期:1998-03-20 修回日期:1998-03-20 出版日期:1998-03-20 发布日期:1998-03-20

THREE MAIN STYLES OF EARLY PRECAMBRIAN METAMORPHIC EVOLUTION IN NORTHERN SINO-KOREA CRATON AND THEIR GEODYNAMICS

Lu Liang-zhao, Xu Xue-chun, Dong Yong-sheng   

  1. Changchun University of Science and Technology
  • Received:1998-03-20 Revised:1998-03-20 Online:1998-03-20 Published:1998-03-20

摘要: 中朝克拉通辫啊早前寒武纪变质作用演化及其PTt轨迹主要有三类样式:第一类以内蒙古集宁-怀安太古宙高级区为代表,PTt轨迹表现为顺时针形式,峰期温压高达800℃-850℃,0.90GPa-1.00GPa,峰期后为大幅度等热减压,反映了板块碰撞造山带的陆壳成倍构造增厚和后期拉张减薄机制;第二类以冀东-辽西太古宙高级区为代表,PTt为逆时针形式,早期增温为主,峰期温压均较高,峰期后为近等压冷却过程,它们它们反映地壳因岩浆底侵而增厚的动力学机制;第三类以辽东-吉南早元古代变质地带为代表,PTt轨迹亦为逆时针,但持征是早期增压明显。维之以近等压增温,峰期为中低压,峰期后近等压冷却。这类样式反映了较稳定陆块壳内裂陷区的闭合过程。

Abstract: The northern portion of Sino-Karea Craton consists of four independent blocks and each includs several metamorphic terrains or belts. Three styles of early Precambrian metamorphic evolution and their PTt-D paths have been distinguished in these terrains. The first style was found in Jning-Huaian Archaean high-grade terrain of Inner Mongolia. This district is characterized by widespread Al-rich pars-gneiss (Khondalite suite) and some hypersthene plagioclase gneisses of granulite facies. They were subjected to five episodes of tectonic deformation and six stages of metamorphism from 2.6Ga to 1.9Ga. The mineral paregenesis and transformation of Al-rich gneisses in the whole process of metamorphism show a trend of Ky-Sil+Grt+Bt+Kfs-- Crd士Opx-- And. The metamorphic PTt-D path of this terrain clockwise with T= 800℃ ~ 850℃ , P=0.90 GPa~ 1.00 GPa in its peak stage and a typical isothermal decompression(ITD)process in post-peak stage. It reflects double thickening of crust by geodynamic process such as A-subduction and tectonic stacking in a collisional belt, followed by tectonic thinning and uplifting in the later stage. The second style has been established in eastern Hebei-western Liaoning Archaean high-grade terrain, which is mainly composed of late Archaean grey gneiss (TTG intrusions) and other granitic rocks with Subordinate volcano-sedimentary supracrustal sequences and some layered basic intrusions. The minera1 evolution of basic rocks in granulite-facies belt is, characterized by assemblage of Opx+Cpx+Hb+Grt+pl in its peak metamorphism with T= 800~ 850℃ , P=0.70~ 0.80GPa, and extensive development of Grt corona around 0px or Cpx during its post-peak stage. All data give rise to a counter clockwise PTt-D path with a nearly isobaric cooling process at its post-peak stage. It indicates the tectonic environment of vertical crustal thickening by magmatic accretion. The third style of PTt-d path was constructed in eastern Liaoning-southern Jilin early Proterozoic metamorphic belt. It was a rifted ensialic depression within the Jiao-Liao Massif during the midddle stage of early Proterozoic with the formation of sedimentary sequences of shallow sea and continental shelf, by the end of early Proterozoic, they were suhjected to three deformational episodes and five stages of metamorphism with the development progradal zones and a typical thermal anticline. Its axis area is characterized by high amphibolite-facies metamorphism. The mineralogic evolution in its A1-rich rocks is And-- Sil+ Grt+Crd- Ky. Its PTt-D loop is also counter clockwise, but it is distinguished from the second style by rapid raising pressure in its early stage and succeeded by a nearly isobaric heating process and lower pressure(O.5O~ 0.60(3Pa)in peak stage. Such PTt-D path may be genetically related to the closing process of a rifted depression within a rather stable continental massif.