内蒙朝克山辉长岩中单斜辉石矿物化学特征及地质意义

Mineralogical Chemistry Characteristics and Geological Significance of the Clinopyroxene from Chaokeshan Gabbro,Inner Mongolia

内蒙贺根山带广泛发育古生代基性岩浆岩,为探讨其岩浆源区和岩石成因,进一步了解中亚造山带东段古生代构造背景,笔者对朝克山蛇绿岩进行了矿物学研究。矿物电子探针结果显示,辉长岩中单斜辉石均属于透辉石,既有碱性特征,也有拉斑特征,SiO2含量为50.75%~52.99%,具有高的Al2O3,含量为2.03%~3.77%,相对亏损轻稀土元素(La/Sm)_(N)=0.12~0.22和高场强元素(HFSE;Nb、Ta、Zr、Hf、Ti负异常),富集大离子亲石元素(LILEs),与辉长岩全岩的特征程度相一致,共同指示岩体母岩浆可能为亚碱性的拉斑玄武质岩浆向碱性玄武质岩浆演化的趋势。单斜辉石的平衡温度为1099~1242℃,平衡压力为1.5~6.4 kbar,深度为5~21 km,显示明显的深源特征。结合前人研究成果,笔者认为朝克山蛇绿岩形成于弧后盆地环境。

Paleozoic mafic magmatic rocks are widely developed in Hegenshan belt of Inner Mongolia.In order to explore the magmatic source and petrological genesis of these rocks and to further understand the Paleo-zoic tectonic setting of the eastern part of the Central Asian orogenic belt,the Chaokeshan ophiolites are studied by mineralogy.Mineral electron probe results show that the monocline in gabbro belongs to diopside,which has both alkaline and tholeitic characteristic,with SiO2(50.75%to 52.99%)and high Al2O3(2.03%to 3.77%),and are relatively enriched large ion lithophile elements,but depleted in light rare earth elements((La/Sm)_(N)=0.12~0.22)and in high field strength elements(e.g.Nb,Ta,Zr,Hf,Ti),consistent with the characteristics of the whole gabbro rock,indicating that the parent magma of the pluton may be subalkaline tholeitic magma to alka-line basalt magma evolution trend.The temperature and pressure of clinopyroxene are 1099~1242℃,1.5~6.4 kbar and 5~21 km respectively,suggesting obvious deep-derived characteristics.Combined with the previous research results,it’s suggest that the Chaokeshan ophiolites was likely produced in the back-arc tectonic setting.