大别造山带岩脉群锆石定年和暗色包体成因分析

发布时间：2018-10-25 14:37

【摘要】：大别造山带分布有大量NNE走向的岩脉,其岩石类型多样,包括石英正长斑岩、花岗斑岩、石英闪长岩以及辉绿岩等。区内广泛发育中生代岩浆岩,以中酸性为主,伴有少量的基性岩。岩脉切穿区内广泛发育的岩浆岩,为燕山期岩浆活动最晚的产物。通过锆石LA-ICPMS定年方法得到石英正长斑岩、石英闪长岩和花岗斑岩3种类型的四件岩脉样品的年龄分别为127.1±4.5 Ma、127.5±1.9 Ma、127.4±2.5 Ma和127.9±2.1 Ma,这些岩脉的形成时代较为集中,其形成时代为早白垩世,代表了大别造山带中生代岩浆活动幕的结束。岩脉群是郯庐断裂带左行走滑动力学体制下,岩浆沿剪张裂隙侵入形成,岩脉的形成时代指示郯庐断裂带南段中生代第三次左行走滑可能发生在127 Ma左右。大别造山带中生代石英二长岩和二长岩体中暗色包体广泛发育。这些暗色包体具有典型的岩浆结构,与寄主岩有着相似的矿物组合,但矿物粒度更细,暗色矿物含量更高,岩性主要为闪长岩质。部分包体中发育钾长石斑晶和眼球状的石英,以及针状磷灰石。三个包体和一个寄主岩的锆石U-Pb定年表明,暗色包体的形成年龄主要为133~129 Ma,石英二长岩为130 Ma左右,表明包体与寄主岩的形成时代在误差范围内一致,都为早白垩世。单矿物的电子探针分析得到,二者矿物化学组分相似,形成的温压条件相仿,且包体中存在地幔物质的加入。在地球化学方面,包体和寄主岩的主量元素组成在Hacker图解中呈现出明显的线性关系。二者都较富集轻稀土元素和大离子亲石元素,亏损高场强元素,稀土元素配分图和微量元素蛛网图的配分模式表现出相似的特点,说明包体和寄主岩之间存在成因联系。综合包体和寄主岩的岩石学矿物学特点、锆石U-Pb年龄和相关地球化学特征,本文认为大别地区发育的暗色包体是岩浆混合作用的结果。早白垩世幔源岩浆底侵,诱发下地壳物质的部分熔融,进而发生铁镁质岩浆和长英质岩浆的混合作用。
[Abstract]:The Dabie orogenic belt has a large number of veins with NNE strike, and its rock types include quartz syenite porphyry, granitic porphyry, quartz diorite and diabase. Mesozoic magmatic rocks are widely developed in the area, mainly intermediate acid with a small amount of basic rocks. The magmatic rocks widely developed in the area are the latest product of Yanshanian magmatic activity. Through zircon LA-ICPMS dating method, the ages of four types of quartz syenite porphyry, quartz diorite and granitic porphyry are 127.1 卤4.5 Ma,127.5 卤1.9 Ma,127.4 卤2.5 Ma and 127.9 卤2.1 Ma, respectively. The age of their formation is early Cretaceous. It represents the end of the Mesozoic magmatic activity curtain in the Dabie orogenic belt. Under the dynamic regime of left strike-slip in the Tanlu fault zone, the magma intruded along the shear-tensional fissure. The formation age of the vein indicates that the third left strike-slip occurred in the southern part of the Tan-Lu fault zone at about 127 Ma. Dark inclusions are widely developed in Mesozoic quartz monzonitic rocks and erzhang rocks in the Dabie orogenic belt. These dark inclusion bodies have typical magmatic structure and have similar mineral assemblages with host rocks, but the mineral granularity is finer, the dark mineral content is higher, and the lithology is mainly diorite. Potassium feldspar porphyry and eyeball quartz, as well as needle-like apatite, were found in some of the inclusions. The zircon U-Pb dating of three xenoliths and one host rock indicates that the age of dark xenoliths is about 130 Ma, which indicates that the ages of formation of the xenoliths and the host rocks are consistent within the range of errors and belong to the early Cretaceous. Electron probe analysis of single mineral shows that the chemical components of the two minerals are similar, the temperature and pressure conditions are similar, and the inclusion of mantle material exists in the inclusion. In terms of geochemistry, the principal element compositions of the xenoliths and host rocks show an obvious linear relationship in the Hacker diagram. Both of them are relatively rich in light rare earth elements and large ion lithophile elements, and are depleted of high field strength elements. The distribution patterns of rare earth elements and trace element cobwebs show similar characteristics, indicating the genetic relationship between the xenoliths and host rocks. Based on the petrological and mineralogical characteristics of the xenoliths and host rocks, zircon U-Pb age and related geochemical characteristics, it is considered that the dark inclusion developed in the Dabie area is the result of magmatic mixing. In the early Cretaceous mantle magma submergence induced partial melting of lower crust materials and then the mixing of ferromagnesian magma and felsic magma occurred.
【学位授予单位】：合肥工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P597.3;P587

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