摘要:
雅鲁藏布江蛇绿岩带自萨嘎以西分为达巴—休古嘎布(南亚带)和达机翁—萨嘎(北亚带)两个亚带,但两者的成因和构造背景还不清楚。本文在研究北亚带加纳崩—错不扎基性岩脉的年代学和地球化学及对比南亚带的基础上,探讨了两个带的成因和关系问题。加纳崩辉长岩和错不扎辉绿岩呈脉状或长透镜状产在方辉橄榄岩中,宽1~3 m不等,走向北西。两者的锆石U-Pb年龄分别为(125.8±2.6) Ma和(127.0±0.5) Ma。岩石地球化学均具有高Si、Al、Na、Mg和低Ti、K、P的特征,属钙碱性玄武质成分。球粒陨石标准化曲线与N-MORB一致; N-MORB标准化蛛网图中显示Nb、Ta、Ti负异常,判断两者形成于大洋俯冲的弧前或弧后环境。对比前人研究,南亚带普兰、东波和休古嘎布蛇绿岩中的基性岩具有相同产状和时代(120~130 Ma),地球化学特征也同样显示形成于洋内俯冲带环境。结合两带基性岩的围岩地幔橄榄岩均具有弧前环境特征,初步认为南北蛇绿岩亚带可能是相同构造背景的大洋岩石圈残余。
Abstract:
The relationship and genesis of the Daba–Xiugugabu (Southern belt) and Dajiweng–Saga (Northern belt) ophiolitic subbelts divided from Saga in the western part of the Yarlung Zangbo suture zone remain unclear. This paper imposes some restrictions on the genesis of the southern and northern subbelts based on geochronology and geochemistry of the Jianabeng and Cuobuzha mafic dikes in the northern belt and a comparative study of rock associations of two belts. The Jianabeng gabbro and Cuobuzha dolerite intruded into harzburgite in the forms of NW-striking lenses and dikes 1 to 3 m in width. LA-ICPMS and SIMS in-situ analysis of zircon grains yielded (125.8±2.6) Ma and (127.0±0.5) Ma respectively. The mafic samples are characterized by high Si, Al, Na, Mg and low Ti, K, P values and plotted into the area of cala-alkali basalt series. Compared with mafic rocks from the Purang, Dongbo and Xiugugabu ophiolites in the Southern belt, the mafic dikes in the two subbelts share similar occurrences and zircon ages (120~130 Ma), showing N-MORB type chondrite-normalized REE patterns and noticeable Nb, Ta and Ti negative anomalies in N-MORB normalized spider diagrams. The above evidence indicates that the mafic rocks from two subbelts had a supra?subduction affinity and were formed in a fore-arc or back basin environment. Combined with fore-arc geochemical characteristics of their wall rock peridotite, the authors preliminarily hold that the two ophiolitic subbelts probably represent the Neo-Tethys ocean fragments in the same tectonic setting.
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