南秦岭地区晚三叠世老城岩体的形成时代、地球化学特征及岩石成因

发布时间：2018-05-31 09:34

本文选题：老城岩体 + 花岗岩　；参考：《中国科学技术大学》2017年硕士论文

【摘要】：老城花岗岩体出露于南秦岭宁陕地区,与五龙花岗岩体构成南秦岭早中生代花岗岩的重要组成部分。岩体的不同岩性单元均含有大量暗色闪长质岩浆包体。锆石U-Pb年龄分析结果显示,寄主花岗岩形成年龄为219～213Ma,其包体的锆石结晶年龄为211 Ma,同期侵入的闪长岩结晶年龄为214Ma,同属于早中生代五龙岩体群印支期岩浆作用中期产物。岩石地球化学分析显示寄主岩体的主体为花岗闪长岩和石英二长岩,具有低硅,弱过铝质、富碱和高Mg#(43～67),高Sr/Y,高La/Yb的地球化学特征,属于高钾钙碱性Ⅰ型埃达克质花岗岩。暗色包体主要为闪长质成分,具有低SiO2(53.64%～62.40%),富碱和高Mg#(63～72)的地球化学特征,属于钾玄岩岩石系列。闪长岩岩体具有低SiO2(52.60%～55.26%),富碱和高Mg#(54～63)的地球化学特征,属于钾玄岩岩石系列。暗色包体及闪长岩与老城花岗岩体相比具有相对较低的初始87Sr/86Sr同位素(0.7043～0.7058),以及较高的εNd(t)值(-0.7～-4.3)和高Mg#,显示形成包体及闪长岩的原始岩浆可能源于幔源玄武质岩浆。全岩元素地球化学,Sr-Nd同位素地球化学特征显示,寄主岩石和包体之间发生过明显的成分交换,使这些成分在二者中大体达到平衡。对比南秦岭基底岩石的初始Sr-Nd同位素比值,推测老城花岗岩岩浆源区主要是新元古代基性下地壳物质。包体和寄主岩石的关系显示,岩浆混合可能为基性岩浆注入到已经开始结晶的酸性岩浆,两种岩浆发生不完全混溶,形成了老城花岗岩体和暗色基性包体。
[Abstract]:Laocheng granitic body emerged in Ningshan area of South Qinling Mountains, and Wulong granite body formed an important part of early Mesozoic granite in South Qinling Mountains. The different lithologic units of the rock mass contain a large number of dark diorite magmatic inclusions. The results of zircon U-Pb age analysis show that the host granites are formed at a age of 219- 213Ma.The zircon crystallization age of the xenoliths is 211Ma. the crystallization age of the intrusive diorite is 214Ma. it is also a product of the Indosinian magmatism in the early Mesozoic Wulong Group. The petrogeochemical analysis shows that the host body is dominated by granodiorite and quartz monzonite, which is characterized by low silicon, weak peraluminous, alkali rich and high Mg#C 4367-D, high Sr-r-Y and high La/Yb, and belongs to high potassium calc-alkaline type I adakitic granite. The dark inclusion is mainly diorite with the geochemical characteristics of low Sio _ 2, 53.64, and 62.40, rich in alkali and high mg, #y6372and belongs to the series of kaliacite rocks. The diorite rock body has the geochemical characteristics of low Sio _ 2 ~ (2 +) 52.60 ~ 55.26m, alkali rich and high mg ~ (2 +) #Li _ (54) ~ (63), and belongs to the series of kaliacite rocks. The dark inclusion and diorite have relatively lower initial 87Sr/86Sr isotopic values of 0.7043 ~ 0.7058 and higher 蔚 -0.7 ~ (-4.3) and high mg #3 than Laocheng granite, indicating that the primitive magma forming the xenoliths and diorites may have originated from the Black Tortoise magma derived from the mantle. The isotopic geochemical characteristics of element geochemistry of the whole rock show that there has been a significant exchange of components between host rocks and inclusions, which makes these elements generally balanced between the host rocks and the inclusions. By comparing the initial Sr-Nd isotopic ratios of the basement rocks in the South Qinling Mountains, it is inferred that the source of the Laocheng granite magma is mainly the Neoproterozoic material of the lower crust. The relationship between the inclusions and host rocks indicates that the magmatic mixing may be the injection of the basic magma into the acidic magma which has already begun to crystallize. The two magmas are not completely miscible and form the Laocheng granitic body and the dark basic inclusion body.
【学位授予单位】：中国科学技术大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P588.1;P534.51

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