藏南夏瓦地区基性脉岩锆石U-Pb定年、地球化学特征及地质意义

Zircon U-Pb dating, geochemistry characteristics and tectonic implications of mafic dykes in the Chaiwa area, Southern Tibet

西藏夏瓦地区位于特提斯喜马拉雅构造带中东部,广泛发育大量近东西向的基性脉岩,这些基性脉岩对于研究该区域地质构造演化具有重要的意义。本文对夏瓦地区基性脉岩开展了系统的岩石学、年代学、地球化学等研究。夏瓦基性脉岩的岩石类型以辉绿岩和辉长玢岩为主。锆石U-Pb年代学揭示基性脉岩结晶年龄为146~145 Ma。夏瓦基性脉岩具有低SiO_(2)(47.20%~50.54%)和高Mg^(#)值(39.78~53.79)特征,富集Ti、Fe、P元素,属碱性系列,富集轻稀土元素(LREE),高场强元素(Nb、Ta、Zr、Hf、Th)相对富集,显示出似OIB的地球化学特征,指示夏瓦基性脉岩来源于软流圈地幔的部分熔融,形成于大陆边缘裂谷背景下。结合区域大火成岩省基性岩的发育,认为夏瓦地区基性脉岩是大陆边缘裂谷背景下Kerguelen地幔柱作用的产物。夏瓦基性脉岩的结晶年龄虽然早于Kerguelen地幔柱活动的峰期(132 Ma),但可能属于地幔柱峰期之前的小规模岩浆活动。

The Chaiwa area of Tibet was located in the middle eastern part of the Tethys Himalayan structural belt,and a large number of nearly East-West trending mafic dykes were widely developed.These mafic dykes were of great significance for studying the geological structural evolution in this area.This paper has carried out systematic petrology,chronology,and geochemistry studies on the mafic dyke rocks in the Chaiwa area.The rock types of mafic dykes in Chaiwa area were mainly diabase and gabbro porphyrite.Zircon U-Pb geochronology revealed that the crystallization ages of mafic dykes are 146~145 Ma.The basic dykes in Chaiwa were enriched in Ti,Fe,and P elements,belonged to alkaline series and enriched in LREE,showing the feather of low SiO_(2)(47.20%~50.54%)and high Mg^(#)(39.78~53.79),which were relatively enriched in high field strength elements(Nb,Ta,Zr,HF,Th)and were similar to the geochemical characteristics of the OIB.These geochemical characteristics indicated that the Chaiwa mafic dykes were originated from the partial melting of the asthenosphere mantle and were formed in continental margin rift setting.Combined with the development of mafic rocks in regional volcanic province,it is considered that the mafic dykes in the Chaiwa area were the products of the Kerguelen mantle plume under the back ground of continental margin rift.Although the crystallization age of the Chaiwa dyke was earlier than the peak period of Kerguelen mantle plume activity(132 Ma),it might belong to the small-scale magmatic activity before the peak period of mantle plume.