【摘要】：位于藏南的拉薩地體普遍發(fā)育晚漸新世-中新世埃達克質巖石,而雅魯藏布江縫合帶以南的特提斯喜馬拉雅帶中卻鮮有同類巖石的報道,關于他們的時空聯(lián)系上存在著爭議。本文報道了緊鄰縫合帶以南的群讓地區(qū)新發(fā)現(xiàn)的二長斑巖年代學和地球化學數(shù)據(jù),通過對其研究為藏南地區(qū)構造-巖漿活動及演化提供約束。通過全巖地球化學分析,群讓斑巖呈現(xiàn)為酸性(Si O2=66-68.3%),鉀質(K2O/Na2O=1.0-1.38,Na2O-2.0K2O),高鋁(Al2O3=15.85-16.20%),低鎂(Mg O=0.44-1.27%),低相容元素(Cr=14.81-24.62 ppm,Ni=8.79-12.28 ppm),相對虧損重稀土元素和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),無明顯負銪異常(δEu=0.73-1.11),明顯具有埃達克質巖的特征。獲得的鋯石LA-ICP-MS U-Pb加權平均年齡為10.02±0.33 Ma,與報道的特提斯喜馬拉雅以及拉薩地塊上的晚漸新世-中新世埃達克質巖時代一致。地化特征表明群讓斑巖可能來自增厚的鐵鎂質下陸殼的部分熔融,源區(qū)深度約為50 km,源巖巖性可能為石榴角閃巖或角閃巖。結合區(qū)域地質情況,表明群讓斑巖可能是晚中新世由藏南拆離系的活動造成鐵鎂質下地殼的減壓和脫水,部分熔融形成了埃達克質熔體,在10 Ma左右侵位形成了東西走向的巖脈。在斑巖中發(fā)現(xiàn)的大量的黑云母片麻巖捕擄體表明熔漿在上升過程中捕擄了上部的地殼物質。關于藏南晚漸新世-中新世埃達克質巖的巖石成因,普遍認為是由下部巖石圈的減薄作用引起形成的,本文首次提出群讓埃達克質巖是由藏南拆離系的活動而形成,為埃達克質巖巖石成因提供新的解釋,同時對藏南拆離系的活動時限提供約束條件。
[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 鹵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.
【學位授予單位】：中國地質大學(北京)
【學位級別】：碩士
【學位授予年份】：2015
【分類號】：P588.1;P597.3

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