藏南群让埃达克质岩锆石U-Pb年龄、地球化学特征及其地质意义

发布时间：2018-12-09 15:35

【摘要】：位于藏南的拉萨地体普遍发育晚渐新世-中新世埃达克质岩石,而雅鲁藏布江缝合带以南的特提斯喜马拉雅带中却鲜有同类岩石的报道,关于他们的时空联系上存在着争议。本文报道了紧邻缝合带以南的群让地区新发现的二长斑岩年代学和地球化学数据,通过对其研究为藏南地区构造-岩浆活动及演化提供约束。通过全岩地球化学分析,群让斑岩呈现为酸性(Si O2=66-68.3%),钾质(K2O/Na2O=1.0-1.38,Na2O-2.0K2O),高铝(Al2O3=15.85-16.20%),低镁(Mg O=0.44-1.27%),低相容元素(Cr=14.81-24.62 ppm,Ni=8.79-12.28 ppm),相对亏损重稀土元素和Y元素(Y=12.23-17.18 ppm,Yb=0.78-1.05 ppm),高Sr(822.2-1386 ppm),具有高的Sr/Y和(La/Yb)N比值(Sr/Y=64.56-81.30、(La/Yb)N=95.1-138.4),无明显负铕异常(δEu=0.73-1.11),明显具有埃达克质岩的特征。获得的锆石LA-ICP-MS U-Pb加权平均年龄为10.02±0.33 Ma,与报道的特提斯喜马拉雅以及拉萨地块上的晚渐新世-中新世埃达克质岩时代一致。地化特征表明群让斑岩可能来自增厚的铁镁质下陆壳的部分熔融,源区深度约为50 km,源岩岩性可能为石榴角闪岩或角闪岩。结合区域地质情况,表明群让斑岩可能是晚中新世由藏南拆离系的活动造成铁镁质下地壳的减压和脱水,部分熔融形成了埃达克质熔体,在10 Ma左右侵位形成了东西走向的岩脉。在斑岩中发现的大量的黑云母片麻岩捕掳体表明熔浆在上升过程中捕掳了上部的地壳物质。关于藏南晚渐新世-中新世埃达克质岩的岩石成因,普遍认为是由下部岩石圈的减薄作用引起形成的,本文首次提出群让埃达克质岩是由藏南拆离系的活动而形成,为埃达克质岩岩石成因提供新的解释,同时对藏南拆离系的活动时限提供约束条件。
[Abstract]:Late Oligocene-Miocene Adakitic rocks are generally developed in Lhasa terrane in southern Tibet, while there are few reports of similar rocks in the Tethys Himalayan belt south of the Yarlung Zangbo River suture belt, and their temporal and spatial links are controversial. This paper reports the geochronology and geochemistry data of the two long porphyry discovered in the south of the suture zone, which provides the constraints for the tectonic-magmatic activity and evolution in the south of Tibet. According to the geochemical analysis of the whole rock, the porphyry is characterized by acid (Si O2O 66-68.3%), potassium (K _ 2O / Na _ 2O 1.0-1.38) Na _ 2O-2.0K _ 2O, high aluminum (Al2O3=15.85-16.20%), low magnesium (Mg O _ 2O _ (0.44-1.27%). Low compatible elements (Cr=14.81-24.62 ppm,Ni=8.79-12.28 ppm), relatively depleted heavy rare Earth elements and Y elements) (Y 12.23-17.18 ppm,Yb=0.78-1.05 ppm), High Sr (822.2-1386 ppm),) It has a high ratio of Sr/Y to (La/Yb) N (Sr/Y=64.56-81.30, (La/Yb) N95.1-138.4), no obvious negative europium anomaly (未 Eu=0.73-1.11), and has the characteristics of adakitic rocks. The zircon LA-ICP-MS U-Pb weighted average age is 10.02 卤0.33 Ma, which is consistent with the reported age of the Tethys Himalayas and the late Oligocene Miocene Adakitic rocks in the Lhasa massif. The geochemical characteristics indicate that the porphyry may originate from the partial melting of the thickened ferromannesite lower continental crust, and the source depth of 50 km, source rocks may be pomegranate amphibolite or amphibolite. Combined with the regional geological conditions, it is suggested that the porphyry may have been caused by the activities of the late Miocene detachment system in southern Tibet, resulting in the decompression and dehydration of the lower crust of ferromagnesite and partial melting of the adakitic melt. The emplacement formed east-west vein at about 10 Ma. A large number of biotite gneiss captives found in porphyry indicate that the magma captured the upper crust material during the rising process. As to the petrogenesis of the late Oligocene to Miocene adakitic rocks in southern Tibet, it is generally believed that they were formed by the thinning of the lower lithosphere. It provides a new explanation for the rock genesis of the adakite, and also provides the constraint conditions for the time limit of the activity of the southern Tibetan detachment system.
【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】：P588.1;P597.3

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