摘要
布嘎寺正长岩体是目前冈底斯乃至青藏高原地区唯一含似长石及碱性暗色矿物的新生代碱性正长岩体,主要岩性为霓辉正长岩。碱性正长岩体与新生代钾质熔岩密切共生,Ar-Ar法测得正长岩的年龄为27.14Ma±0.05Ma,略早于熔岩。正长岩在化学成分上高钾、富碱,K2O+Na2O>11%,K2O>8%,K2O/Na2O>2,里特曼指数σ>9,属超钾质的碱性系列岩石。岩石富集大离子亲石元素、生热元素和轻稀土元素,贫Sr、Ba,具弱的Eu负异常。岩石地球化学特征、O同位素组成、黑云母化学成分等显示其成岩物质来源于富集的上地幔。渐新世碱性正长岩与中新世钾质火山岩构成了该区碰撞后的构造岩浆组合,是在印度-欧亚板块碰撞引起的岩石圈加厚及高原隆升的基础上,岩石圈拆沉、软流圈上涌导致富集的岩石圈地幔部分熔融所形成的。
The Bugasi syenite in Coqen area, Tibet, is the only Late Cenozoic feldspathoid- and alkaline dark mineral-bearing alkali syenite ever known in the Gangdise zone and even on the Qinghai-Tibet Plateau. This syenite body, which is closely associated with Cenozoic potassic lavas, has been found mainly as aegirine-augite syenite. ^40Ar-^39Ar plateau dating of orthoclase from the syenite yielded an age of 27.14±0.05 Ma, which is slightly older than the age of its surrounding lava. The chemical composition is characterized by rich alkalis with K2O＋Na2O〉1l% and high contents of potassium with K2O〉8%, K2O/Na2O〉2 and the Rittmarm Index σ〉9. These suggest that the rock belongs to the ultrapotassiic alkaline series. The syenite is enriched in large ion lithophile elements, radioactive heat-generating elements, high field strength elements and LREE and depleted in Sr and Ba and has weak negative Eu anomalies. The geochemistry, oxygen isotopic composition and biotite composition of the syenite suggest that its rock-forming material was derived from the enriched upper mantle. The Oligocene alkali syenite and its associated Miocene potassic volcanic rocks in the Coqen area constitute a post-collisional tectono-magmatic association. On the basis of the lithospheric thickening and uplift of the Tibetan Plateau due to India-Eurasia collision, such a magmatic association formed by partial melting of the enriched lithospheric mantle caused by lithospheric delamination and asthenospheric upwelling.
出处
《地质通报》
CAS
CSCD
北大核心
2007年第4期417-426,共10页
Geological Bulletin of China
基金
中国地质调查局<1:25万措勤县幅区域地质调查>项目(编号:H45C001001)成果
关键词
碱性正长岩
地球化学
构造环境
西藏
alkali syenite
geochemistry
tectonic environment
Tibet
