西藏桑日县帕南岩体岩石学、地球化学、地质年代学研究及构造背景探讨

Petrology, geochemistry and tectonic setting of the Panan intrusion in Mamen, Sangri,Tibet

关键词:  帕南岩体  二长花岗岩  岩石地球化学  锆石U-Pb年龄  平板俯冲

Key words:  Panan intrusion  adamellite  rock geochemistry  zircon U-Pb age  horizontal subduction

摘要:

帕南岩体位于西藏桑日县马门地区，主要由二长花岗岩、二长岩组成。本文通过锆石U-Pb年代学、岩石地球化学和Sr-Nd同位素等研究，探讨其构造背景和岩石成因。LA-ICP-MS锆石U-Pb定年表明，帕南岩体的成岩年龄为80.91±0.48Ma，形成时代为晚白垩世。帕南岩体地球化学特征表现为富钠、高钾、高铝、低钛，属准铝质高钾钙碱性-钾玄岩岩石系列。稀土分馏较明显且富集轻稀土（（La/Yb）_N=8.88~11.86），具负Eu异常，相对富集大离子亲石元素，亏损Nb、Ta、P、Ti等高场强元素。Sr-Nd同位素显示，样品具较高的Pb、Sr、Sm、Nd含量，偏低的I_Sr值（0.70442~0.70486），正εNd（t）值（+1.99~+2.73）。综合帕南岩体的地质、地球化学和年代学特征，认为帕南岩体形成于晚白垩世新特提斯洋向北俯冲背景下的岛弧环境，可能是新特提斯洋俯冲角度变缓进入了平板俯冲阶段之后，由陆壳发生部分熔融产生的正常岛弧岩浆形成，同时也预示着109～80Ma的大规模岩浆活动在该区的落幕。

Abstract:

The Panan intrusion is mainly composed of monzogranite and monzonite. It is calc-alkaline potassic basanite rock. The zircon LA-ICP-MS U-Pb dating of the Panan intrusive rocks yields a crystallization age of 80.91±0.48Ma, indicating the Panan intrusion was formed in Late Cretaceous. Geochemically, Panan intrusive rocks are rich in large ion lithophile elements and poor in high field-strength elements. The REEs of the rocks are obviously fractionated, with a (La/Yb)_N=8.88~11.86 value and a negative Eu anomaly. The Sr-Nd isotopic compositions indicate that the Panan intrusion is characterized by high Pb, Sr, Sm and Nd contents, a low I_Sr value (0.70442~0.70486), and a positive ε_Nd(t) value (+1.99~+2.73).Based on petrological, geochemical and chronological characteristics, the Panan intrusion may be formed from the normal island arc magma by the partial melting of continental crusts in an island background during the northward subduction of Neo-Tethys oceanic plate. With the development of the subduction angle of the Neo-Tethys ocean slowed down into almost zero degree in the Late Cretaceous, the large-scale magmatic activities in 109-80 Ma ended in the studied areas soon.