摘要
在苏尼特左旗以南地区的中古生代造山带的混杂岩带内发现了以岩块形式出现的蓝片岩 ,其矿物组合为斜长石 +阳起石 +蓝闪石 +绿帘石 +榍石。蓝片岩化学成分说明由基性火山岩变质而成。矿物化学分析表明 ,蓝片岩中角闪石可分为钙质、钙钠质和钠质 3类。钙质闪石均为阳起石 ,钙钠质闪石为蓝透闪石和冻蓝闪石 ,钠质闪石为蓝闪石和青铝闪石及少量镁钠闪石。利用Na(M4 )和AlⅣ 的含量推测本区钙质、钙钠质和钠质 3类闪石的压力从 0 3～ 0 7Gpa ,表明蓝片岩相变质作用的压力约为 0 7GPa。用化学反应限定蓝片岩的形成温度为 2 0 0～ 3 75℃左右。4 0 Ar 3 9Ar同位素年代学测定蓝闪石的等时线年龄为 3 83± 1 3Ma(1δ)。这些结果进一步证实沿贺根山—苏尼特左旗南是一条中古生代的缝合线 ,其俯冲—碰撞的标志即为混杂岩带以及其中的蓝片岩。
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(M-4) and Al Ⅳ and metamorphic reaction, it can be estimated the P-T conditions of the blueschist being: T=200～375℃, P=0 7 GPa. 40 Ar/ 39 Ar dating on Na-amphibole was performed at the CENG Reactor of University of Montpellier, which yielded an intersect age of 383±13 Ma (1 σ , 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 40 Ar/ 39 Ar age can be used to constrain the upper limit for collision between the North China plate and the Mongolia microplate.
出处
《地质科学》
CAS
CSCD
北大核心
2001年第4期424-434,共11页
Chinese Journal of Geology(Scientia Geologica Sinica)
基金
中国科学院知识创新工程项目KZCX2 1 0 4资助
关键词
蓝片岩
中古生代
造山带
内蒙古
变质年代
岩石学
年代学
Blueschist,The middle Paleozoic orogenic belt,The south of Sunitezuoqi,Northern Inner Mongolia
