Reconstruction Of Protoliths Of The Low-grade Metamorphic Rocks In Zhalantun-Arongqi Area

The survey area is in the border of Inner Mongolia Autonomous Region and Heilongjiang province, it is belongs to Zalantun City , Arongqi City and Molidawa City. The tectonic setting is in the eastern of the Junggar-mongolia-Hinggan folded zone. Because of in the joined position between the Siberia plate and the Huabei plate, it is the accretion zone of Siberia plate which restricts its evolution in tectonic history. The geologic background of the area is very complex, and the viewpoints about its tectonic history are different.The subject of research in this paper is the low-grade metamorphic rocks in Zhalantun-Arongqi Area which was belong to proterozoic or Cambrian System. Because the metamorphic rocks has had a greenschist facies low-rank metamorphism generally, just depending on analysing the characteristic of occurrence and the rock assemblage to reconstruct the protoliths is very difficult. So, I select some typical samples to do the geochemical rock survey, basing on the geological survey and the microscopical observation of rocks sections.The methods of using to reconstruct the protoliths mostly in this paper are, analyzing the characteristic of the major elements, calculating the of ratio of the key elements, calculating the DF value, calculating the Niggli eigenvalue , pointing the eigenvalue in the diagram of (al+fm)—(c+alk)—Si and the (al-alk)—c, and using the diagrams of lg(SiO₂/Al₂O₃)—lg(Na₂O/K₂O)and(Na₂O+K_<sub>2O)—SiO₂, and so on. In order to analyse the source of fragmental material, I using the diagrams of K₂O/Na₂O-SiO₂ , SiO2/Al₂O₃-K₂O/ Na₂O, La-Th-Sc and the Th-Sc-Zr/10.Lastly, I arrive at some conclusions by the study of the characteristic of the rock assemblage and the geochemistry.1 I divide the low-grade metamorphic rocks in Zhalantu-Arongqi Area into three rock groups. One group is the Zhalantun-Arongqi metasedimentary rocks; the second group is the Xiangyangyu metavolcanites; And the other group is the Molidawa metasedimentary rocks.2 The Zhalantun-Arongqi metasedimentary rocks bassets at Wobei village, Changfa village, Liujia village in Zalantun City and Honghualiangzi town, Xiangyangyu reservoir in Arongqi City, and the main rock types of this group of metamorphic rocks are calcite chlorite quartzschists, chlorite albite quartzschists and micaschists. The rocks have a low-rank metamorphism generally, and the metamorphic facies belong to the low greenschist facies. And they often have a strong deformation. The protoliths of them are a group of continental margin clastic rocks which contain a few volcanoclastic materials. The sources of fragmental materials are the Paleozoic greywacke, and the structural setting of it is the active continental island arc.3 The Xiangyangyu metavolcanites bassets at Xiangyangyu reservoir in Arongqi City, the main rock types of is a group of intermediate metavolcanites. The rocks have a low greenschist facies metamorphism generally, and they often have a strong deformation. The structural setting is continental island arc.4 The Molidawa metasedimentary rocks bassets at Wangbabozi Hill in Molidawa City. The main rock types are andalusite schists, metasandstones and garnet schists,and so on. The rocks have a low amphibolite facies metamorphism. The protoliths of them are a group of continental margin clastic rocks which contain a few intermediate-basicty volcanoclastic materials. The sources of the fragmental materials are the Paleozoic greywacke and the sandstones of craton. The structural setting evolved from the active continental island arc to the passive continental margin.5 The research of the ages about the zircon in the clastic rocks (Wei JIN, XianLI YANG, 2007) indicate that ,the lower age limit of the Zhalantun-Arongqi metasedimentary rocks are O3-S1, and the one of the Molidawa metasedimentary rocks are D1. Combining it with the returns of this paper, indicate that the structural setting of this area is the continental island arc in O3-S1 to D1, and evolve to the passive continental margin in D1. This return is consistent with the stratigraphy. In this area,the Ordovician is the volcanic-sedimentary rocks of the continental island arc, and the Silurian is the continental margin clastic rocks in most, the volcanic activity became weak. island arc...