摘要
对出露于班公湖-怒江缝合带西段聂尔错和拉果错地区的火山岩开展了锆石U-Pb定年和岩石地球化学测试,旨在查明该区火山岩的形成时代、岩石成因及构造环境。锆石U-Pb定年结果显示,聂尔错地区流纹岩及拉果错地区英安岩分别形成于112.5Ma和112.3Ma,与区域上大规模展布的早白垩世岩浆作用时代一致。全岩主量、微量元素特征显示,英安岩与流纹岩样品均属于钙碱性系列,且明显富集Th、U,亏损Nb、Ta、Ti等高场强元素。地球化学特征指示英安岩起源于增厚的下地壳部分熔融,而流纹岩是下地壳熔体经历广泛结晶分异作用的产物。研究认为,西藏中部早白垩世晚期大规模岩浆作用形成于碰撞后伸展背景,其深部动力学机制可能与北向俯冲的班公湖-怒江洋壳的板片断离有关。
In order to ascertain the formation age, rock genesis and tectonic setting of volcanic rocks in the Nie'er Co and Laguo Co areas of the western Bangong Co-Nujiang suture zone, the authors mainly carried out zircon U-Pb dating and petrogeochemical analysis of volcanic rocks. The zircon U-Pb dating results show that the rhyolites in the Nie'er Co area were formed at 112.5 Ma, and the dacite in the Laguo Co area was formed at 112.3 Ma, respectively, consistent with the age of the magmatism extensively distributed in the study area. The characteristics of the main trace elements of the whole rock show that both the dacite and rhyolite samples belong to the calc-alkaline series, and are obviously enriched in Th, U, and depleted in high field strength elements such as Nb, Ta and Ti. Geochemical characteristics indicate that the dacite originated from the partial melting of the thickened lower crust,whereas rhyolite was formed by the extensive crystallization differentiation of the lower crust melts. The authors hold that the largescale magmatism in the late Early Cretaceous in central Tibet formed a background after collision, and the deep dynamic mechanism was probably related to the slab break-off of the northward subduction of Bangong Co-Nujiang oceanic floor.
引文
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