【作者】 赵磊；

【导师】 周喜文；

【作者基本信息】 中国地质科学院 ， 矿物学、岩石学、矿床学， 2012， 硕士

【摘要】 浙西南地区位于华夏陆块中北部,处于华夏陆块和江南造山带的交接部位,同时该地区也是欧亚大陆和太平洋板块相互作用的焦点位置。区域内呈“窗口状”出露于显生宙盖层中的早元古代八都群,是华夏陆块内部目前已知的分布范围最大、形成时代最古老、变质程度最深、地质演化最复杂的变质岩石地层。通过对八都群详细的野外地质工作,以及详细的岩石化学、矿物化学、变质演化和年代学的研究,主要取得了以下几个方而的新认识：在华夏地块古老基底中首次发现了具有“石榴石+正/反条纹长石+夕线石”特征组合的泥质麻粒岩,表明其曾经历广泛的区域麻粒岩相变质作用。含榴长英质片麻岩的原岩为以高Si富Al为特征的地壳重熔型过铝质花岗岩。夕线石榴黑云片麻岩的原岩为泥沙质沉积岩。石榴角闪斜长片麻岩和石榴二辉石麻粒岩的原岩类型相同,地球化学特征显示它们的原岩为安山岩-英安岩(花岗闪长岩)。两种岩石在主微量元素、稀土元素含量上的差别,证明在变质级别不同的变质作用中,元素迁移规律有一定差异。首次对三种典型变质岩：夕线石榴黑云片麻岩、石榴角闪斜长片麻岩和石榴二辉石麻粒岩进行了详细的变质演化研究。三种岩石的变质演化,均可以划分为三阶段的演化过程：早期进变质阶段(M1)、变质峰期阶段(M2)和峰后降压冷却阶段(M3)。夕线石榴黑云片麻岩的峰期阶段温压条件为：800-850℃、0.60-0.70Gpa；峰后降压冷却阶段温度压力估算结果分别为560-590℃、0.25-0.33Gpa；石榴角闪斜长片麻岩的峰期角闪岩相-低麻粒岩相变质阶段(M2)温度压力范围为：T=770-830℃,P=0.82-0.92Gpa,峰后的角闪岩相变质阶段(M3)温度压力范围为：T=580-630℃,P=0.52-0.73Gpa；石榴二辉石麻粒岩峰期麻粒岩相变质阶段(M2)温度压力估算结果：T=850-900℃,P=0.92-1.10Gpa,峰后的角闪岩相变质阶段(M3)温度压力范围为：T=650-700℃,P=0.58-0.65Gpa。三种典型变质岩的变质演化,都具有顺时针样式的PT轨迹,表现为峰期后同时发生降温、降压,但降压程度大于降温,反映了一种加厚地壳快速减薄的动力学过程。通过对八都群中富铝片麻岩、变酸性火成岩、变基性火成岩和大理岩等典型变质岩石开展锆石SHRIMP和LA-ICPMS U-Pb体系定年分析,确定其麻粒岩相变质作用发生的时代为18.5-18.8亿年。富铝片麻岩多数锆石具有2002±29Ma和2451±63Ma等两个时段的源区特征,表明其物质主要来源于古元古代以前的古老地体。变酸性岩的成岩时代1923±7.6Ma,在18.7亿年左右发生变质,含有20-28亿年的继承锆石核,最古老继承锆石年龄为2803±34Ma。浙西南八都群古元古代变质表壳岩所经历的麻粒岩相变质作用和区域广泛发育的S型和A型花岗质岩浆活动,表明其曾经历完整的造山作用过程,其成因很可能与全球1.8-2.1Ga哥伦比亚超大陆的聚合事件有关。更多还原

【Abstract】 The southwestern Zhejiang region is situated in the middle of the northern Cathaysia Block, which is the boundary of the Cathaysia Block and the Jiangnan orogenic belt, also it the focal area where the Eurasia and the Pacific plate interact with each other. The Paleoproterozoic Badu group outcrops as windows among the phanerozoic cover. By far, it is the oldest rock stratigraphic unit which has experienced the highest level of metamorphism and has the most complicated history of evolution and biggest outcropping area in the Cathaysia Block. Through a combined study of metamorphic evolution, geochemistry and geochronology, we get mainly these following several new findings.In this dissertation, we report pelitic granulite, with mineral assemblage of garnet, perthite and anti-perthite, sillimanite, for the first time that was discovered in the basement of Cathaysia block. This discovery suggests that the area experienced regional metamorphism of up to granulite facies.The protolith of garnet-bearing felsic gneiss is granite generated from the remelting of crustal materials, which is characterized by high content of A1and Si. The protolith of sillimanite garnet biotite gneiss is pelitic to sandy sedimentary rock. The protoliths of garnet amphibole plagiogneiss and garnet two-pyroxene granulite are of the same, which are both andesite-dacite rocks, as are shown by their geochemical characteristics. The slight differences in major and trace elements of the two meta-igneous rocks show that elements migrate differently in metamorphic processes of various metamorphic facies.We present here, for the forst time, the study results of metamorphic evolution of the three different rock types: sillimanite garnet biotite gneiss, garnet amphibole plagiogneiss and garnet two-pyroxene granulite. Three stages of metamorphic evolution can be recognized in all three rocks:early prograde metamorphic stage (M1), peak metamorphic stage (M2) and after peak retrograde metamorphic stage (M3). The PT conditions of the early prograde metamorphic stage are altered in all three metamorphic rocks and they record the PT conditions of only the two later metamorphic stages. For the peraluminous gneiss, the PT condition of the peak metamorphic stage(M2) is800～850℃,0.60～0.70Gpa; the PT condition of the after peak retrograde metamorphic stage (M3) is560～590℃、0.25～0.33Gpa. For the garnet amphibole plagiogneiss, during the peak metamorphic stage (M2), the PT condition is770-830℃,0.82-0.92Gpa; during the after peak retrograde metamorphic stage (M3), the PT condition is580-630℃,0.52-0.73Gpa. For the garnet two-pyroxene granulite, during the peak metamorphic stage (M2), the PT condition is850-900℃,0.92-1.10Gpa; the PT condition of after peak retrograde metamorphic stage (M3) is650-700℃,0.58-0.65Gpa.The three representative metamorphic rocks mentioned above all recorded PT paths of clockwise style. After peak metamorphic stage, cooling happenned as well as decompression. But the rocks decompressed to a greater extent than that of cooling. Such a process usually happens during the fast thinning of thickened crust.SHRIMP and LA-ICPMS zircon-U-Pb-dating of peraluminous gneiss, meta-acid-igneous rock, meta-mafic rocks and marble show the early precambrian metamorphism occoured around1.85-1.88Ga. Zircons of peraluminous gneiss show source rocks with the age of2002±29Ma and2451±63Ma, which suggests that the its protolith, the sediments, was from mainly paleoproterozoic terrains. The acid granite, which formed around1923±7.6Ma, experienced metamorphism at about1.87Ga. The zircons of the granite also contain inherited cores ranging from2.0-2.8Ga, with the oldest of2803±34Ma.The paleoproterozoic granulite facies metamorphism recorded in the supracrustal rocks of the Radu group in southwestern Zhejiang province and the widespread of S-and A-type granites in the region show that this area experienced the whole process of a paleoproterozoic orogeny, which might be resulted from the amalgamation of the supercontinent Columbia between1.8and2.1Ga around the world.更多还原