Magma Source of the A-Type Granite and Slab Break-Off in the Middle Segment of the Bangonghu-Nujiang Suture, Tibetan Plateau
详细信息
- 责任者：Qu Xiaoming (Institute of Mineral Resources ; Chinese Academy of Geoscience ; Beijing 100037 ; China) ; Xin Hongbo
- 刊名：Acta Geologica Sinica
- 出版年：2013
- 卷期：87(6)
- 关键词：A型花岗岩 ; A-type granite ; 西藏 ; Tibet
- 页码：p.759-772
- 图表：illus., 2 tables, 56 refs.
- 分类号：0801
- issnNo：ISSN0001-5717
- isbnNo：CN11-1951/P
摘要
During the geological investigation of last several years , agroup of A-type granitic plutons has been found in the middle segment of the Bangonghu-Nujiang suture.These granitic plutons occur as stocks with an exposed area of less than1km 2 and lithologically include two types of biotite granite and granodiorite porphyry , which intrude into Cretaceous sedimentary rocks.A systematic analysis of whole-rock element geochemistry and Nd , Sr , Pb isotope for these A-type granites indicates that they contain high SiO2 content varying from 68.62% ～75.36%.Moreover they have relatively high K2O+Na2O ( 8.03% ～ 9.37% ) and Fe 2 O 3T ( 0.86% ～5.39% ) and low Al 2 O 3 ( 12.76% ～15.54% ) content , appearing weak peraluminous and metaluminous features.Trace elements N-MORB-normalized spider diagram displays apparent enrichment in large ion lithophile element ( LILE ) Rb , Th , U , K , Pb with Ba and Sr of marked depletion.High field strenth element ( HFSE ) Nb , Ta and Ti are sharply depleted with Zr and Hf of relative enrichment.All of these are characteristic of A-type granite and different from arc granite.These granites have high REE contents ( ∑REE=122.37×10 -6 ～291.19×10 -6 , with an average of 201.31× 10 -6 ) .Light REE are rich relative to heavy REE ( LREE / HREE=4.89～9.58 , with an average of 5.93 ) and negative Eu anormalies are sharp ( δ Eu =0.14～0.54 , with mean value of 0.34 ), making their chondritenormalized rare earth element patterns appear slightly right-dipping V-type.Nd , Sr and Pb isotope results indicate that the A-type granites from the middle segment of the Bangonghu-Nujiang suture are rich in radioactive isotope.Their 87 Sr / 86 Sr and I Sr ratio values vary from 0.719842～0.786395and 0.706900～0. 710378 , respectively. 143 Nd / 144 Nd ratio values are from 0.512123～0.512392 , with large negativeε Nd values ( -3.37～ -10.34 ), reflecting important involvement of crustal components.Pb isotope composition 206 Pb / 204 Pb , 207 Pb / 204 Pb , 208 Pb / 204 Pb display ranges of 18.703～19.070 , 15.680～15.732and 39.121～39. 576 , respectively , which plot on the upper crustal line and above the orogenic evolution line in the plumtotectonic framework diagram of Zartman and Doe ( 1981 ) and show anormaly high enrichment components.The study points out that the A-type granites were derived from partial melting of enriched lithospheric mantle ( approximating EMⅡ ), which were metasomalized by sediment melt from slab during the oceanic crust subduction and later heat by upwelling asthenospheric materials through slab windows formed as a result of slab break-off during the postcollisional extension of the orogenic zone.
© 2004-2018 中国地质图书馆版权所有 京ICP备05064691号 京公网安备11010802017129号 地址：北京市海淀区学院路29号 邮编：100083 电话：办公室：(+86 10)66554848；文献借阅、咨询服务、科技查新：66554700 |