西藏加查—郎县地区辉长岩岩石地球化学特征及形成背景

发布时间：2018-06-13 19:14

本文选题：雅江缝合带 + 辉长岩　；参考：《中国地质大学(北京)》2015年硕士论文

【摘要】：在雅江缝合带南侧加查到郎县一带产出有一套走向近东西的基性-超基性岩带,与雅鲁藏布江蛇绿岩带平行共生,基性-超基性岩带中主要为辉长岩辉绿岩,普遍发生蚀变,本文对加查到郎县一带的辉长岩进行了详细的野外观测,选择典型的野外露头采集了样品。整个研究区的辉长岩地球化学特征基本相同,系统的岩石学、地球化学分析和研究表明该辉长岩的Si02含量均45.80%-50.89%,与正常的洋中脊玄武岩SiO2含量(N-MORB,48.77%)相当,Mg0的含量在4.23%～7.8%,平均为5.74%。Mg#为41.31～63.55平均为48.85,低于原生岩浆范围,推测其在岩浆演化过程中存在与其他物质的混染作用。轻稀土和重稀土元素比值(LREE/HREE)的含量范围在2.08到8.09之间,平均为3.4,轻重稀土无明显分馏。σEu变化范围在0.72～1.15,平均值在0.98,说明岩石存在轻微的Eu负异常或无异常,未发生明显的斜长石分离结晶作用。从微量元素原始地幔标准化蛛网图可以看出,样品曲线分布位置和走势与E-MORB(富集型地幔)类似,说明岩浆可能来源于富集地幔,也有可能是来源于正常的亏损地幔,在岩浆的形成或者演化过程中收到了外来溶液的影响,使得样品相对的富集轻稀土。由SHRIMP锆石U-Pb年龄表明该辉长岩形成于55.5～56Ma,与雅江洋的闭合时间一致。由此本文认为当大洋板块俯冲到最终阶段,因重力等作用,大洋板块与其南的喜马拉雅地块之间发生断裂,软流圈减压发生部分熔融,形成超基性基性熔浆,俯冲作用持续断裂位置向北移动至增生楔之下,同时雅江洋闭合两侧碰撞,其熔浆底侵上升,不同程度地与地壳物质发生混染,在此条件下形成这套基性岩岩脉。
[Abstract]:On the south side of the Yagang suture belt, there is a set of basic-ultrabasic rock belt with the direction of east-west, which is parallel to the Yalu Zangbo ophiolite belt, and the basic-ultrabasic rock belt is mainly gabbro diabase, which is generally altered. In this paper, a detailed field observation of gabbro in Longlong County is carried out, and a typical outcrop is selected to collect samples. The geochemical characteristics of gabbro in the whole study area are basically the same. Geochemical analysis and study show that the content of SiO2 in the gabbro is 45.80-50.89, which is equivalent to the normal SiO2 content of mid-oceanic ridge basalt (N-MORBN 48.77g).) the content of Mg0 is 4.2377.8G, with an average of 5.74g 路Mg# 41.3163.55, 48.85, which is lower than that of the primary magma. It is inferred that the magma is mixed with other materials during the evolution of magma. The content range of LREE / HREE is between 2.08 and 8.09, with an average of 3.4, and there is no significant fractionation of REE. The variation range of 蟽 EU is 0.72 ~ 1.15 and the average value is 0.98, which indicates that there are slight EU negative anomalies or no anomalies in rocks. There was no obvious segregation and crystallization of plagioclase. It can be seen from the trace element primitive mantle-normalized spider web that the distribution of the sample curve is similar to that of E-MORBB, indicating that the magma may originate from the enriched mantle or from the normal depleted mantle. In the process of magma formation or evolution, the samples are relatively enriched with LREE due to the influence of external solutions. The shrimp zircon U-Pb age indicates that the gabbro was formed at 55.5 ~ 56 Ma, which is consistent with the closing time of the Yajiang Ocean. It is concluded in this paper that when the ocean plate subducts to the final stage, the fault occurs between the oceanic plate and its southern Himalayan block due to the action of gravity, and the subduction of the asthenosphere occurs partial melting, forming ultrabasic melts. The subduction continues to move northward below the accretive wedge, and at the same time the two sides of the Yajiang ocean close collide with each other, and its melting-bottom transgression rises, and mixed with the crustal material to varying degrees, the basic rock vein is formed under this condition.
【学位授予单位】：中国地质大学(北京)
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P588.124

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