1. 北京大学地质学系, 北京, 100871; 2. Department des Sciences de la Terre, Universitéd' Orléans, Orléans 45067, France; 3. 中国科学院地质与地球物理研究所, 北京, 100101
PRIMARY STUDY ON PETROLOGY AND GEOCHRONONOLOGY OF BLUESCHISTS IN SUNITEZUOQI, NORTHERN INNER MONGOLIA
Xu Bei1, Jacques Charvet2, Zhang Fuqin3
1. Department of Geology, Peking University, Beijing 100871; 2. Department des Sciences de la Terre, Université d’Orléans, Orléans 45067 France; 3. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100101
This paper reports new data of petrography and geochronology of the blueschists from the middle Paleozoic orogenic belt between the North China plate and Mongolia microplate.The orogenic belt comprises five tectonic units: 1) melange belt,2) foreland thrust belt,3) molasse and littoral basin,4) arc diorite series,and 5) syn-collision granitoid series.The melange belt extends about 150 km in a NEE-striking from Erdaojing,Chaganwula to Honggeer.The well-exposed melange varies in width from 5 km in the west to 2 km in the east and consists of various blocks and heterogeneously deformed matrix in Chaganwula area.The most common texture in the melange belt is the matrix-supported blocks.These blocks include dolomite,quartzite,ultramafic and mafic rocks,marble and blueschist.The blueschist,in very limited amounts,observed as isolated blocks about 20 m wide and 40 m long,occurs as blocks of melange in the south of Naomuhunni area.The matrix consists of meta-sandstones,meta-volcanic rocks,and schists containing greenschist facies mineral assemblage.Deformation and schistosity are heterogeneous. The blueschist shows a mineral assemblage of feldspar+amphibole+actinolite+epidote+sphene.A representative chemical composition of the blueschist suggests the protolith being basic rocks.44 microprobe analyses of the amphibole from the blueschist were obtained by using the GEOL 733 electron microprobe,which suggest the amphibole belonging to Ca-,Ca-Na- and Na-amphiboles.The Na-amphibole can be classified into glaucophane and crossite,with the exception of one magnesioriebeckite.According to the contains of Na(M4M) and AlⅣ and metamorphic reaction,it can be estimated the P-T conditions of the blueschist being: T=200～375℃,P=0.7 GPa.40Ar/39Ar dating on Na-amphibole was performed at the CENG Reactor of University of Montpellier,which yielded an intersect age of 383±13 Ma (1σ,MSWD=4.4) by using least-squares linear regression with correlated errors.We accept the isochron age as a respective cooling age of amphiboles during erosion and interpret the blueschist blocks as the product of high P/low T metamorphism in a subduction zone between the North China plate and Mongolia microplate.This 40Ar/39Ar age can be used to constrain the upper limit for collision between the North China plate and the Mongolia microplate.