东天山晚石炭世大石头群流纹岩Sr-Nd-Pb同位素地球化学研究  

Sr-Nd-Pb isotope geochemistry of Rhyolite of the Late Carboniferous Dashitou group in eastern Tianshan.

作　　者：王银喜[1] 顾连兴[2] 张遵忠[2] 吴昌志[2] 李惠民[1] 杨杰东[1] 

机构地区：[1]南京大学现代分析中心,南京210093 [2]中国地质大学(北京)地质过程与矿产资源国家重点实验室

出　　处：《岩石学报》2007年第7期1749-1755,共7页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(No.40472042;No.40072075);国家重点基础研究发展规划项目(2001CB409802);国家九.五攻关305项目(No.96-915-05-03-01)合作研究成果;南京大学开放测试基金(0303Y302-2004)资助。

摘　　要：新疆大石头-色皮口地区位于博格达造山带东段北部。该区大石头群流纹岩 Rb-Sr 同位素等时线年龄为306.7±2.3Ma,是博格达裂谷闭合后区域隆升阶段的产物。这些流纹岩的ε_(Nd)(t)为+5.30～+6.40,(^(87)Sr/^(86)Sr)_i 为0.703289～0.703496,(^(206)Pb/^(204)Pb)_i 为18.037～18.425、(^(207)Pb/^(204)Pb)_i 为15.524～15.567、(^(208)Pb/^(204)Pb)_i 为37.198～37.810,因此其 Nd-Sr-Pb 同位素特征与博格达陆内裂谷伸展和沉降阶段形成的早石炭世七角井组玄武岩和流纹岩相近。七角井组与大量玄武岩伴生的少量流纹岩是由玄武岩浆分离结晶作用的产物,而大石头群中的大量流纹岩群仅与少量玄武岩伴生故是由玄武岩浆分离结晶的产物可排除大的可能性。该区流纹岩很可能是碰撞后的底侵玄武岩在地幔热量影响下发生重熔的产物。大石头群流纹岩正δ_(Nd)(t)值、负ε_(Sr)(t)值(低 Sr 初始值)和低 Pb 同位素比值表明博格达裂谷碰撞后的底侵幔源岩浆重熔的基性产物与碰撞前的七角井组玄武岩一样也来自亏损地幔。The Dashitou-Shepikou area belongs to the northern part of the eastern section of the Bogda orogen. Rhyolite of the Dashitou group gave a Rb-Sr age of 306.7 ± 2.3Ma, and thus is suggested to have been produced at the uplifting stage following the closure of the Bogda rift. With 8Nd （t） of ＋ 5. 30 - ＋ 6. 40, （ ^87 Sr/^86 Sr）i of 0. 703289 - 0. 703496, （^206 Pb/^204 Pb） i of 18. 037 - 18. 425, （ ^207 Pb/^204 Pb） i of 15. 524 - 15. 567 and （ ^208 Pb/^204Pb） i of 37.198 - 37.810, the rhyolite is comparable in Nd, Sr and Pb isotopes to the basalt and rhyolite of the Lower and Middle Carboniferous Qijiaojing formation that were formed during extension and subsidence of the Bogda intra-continental rift. Minor rhyolite accompanying a large amount of basalt in the Qijiaojing formation are though to have been produced by fractional crystallisation of the basaltic magma, but a large quantity of rhyolite accompanied by very minor basaltic rocks can precludes the big possibility of their formation by fractional crystallisation of basaltic magma. The rhyolite of the Dashitou group is likely to have been derived from an underplated basaltic body by remehing of this body in response to mantle heating. Highly positive 8Nd （t）, and negative εSr, （t） （low ^87Sr/^86Sr initial ratio）, and low Pb isotope ratios suggested that, like the pre-collision basalt of the Qijiaojing formation, the mantle magma underplated basaltic body by re-melting at the post-collision stage of the Bogda rift came from depleted mantle.

关 键 词：Nd-Sr-Pb同位素地球化学 碰撞后流纹岩 博格达 新疆 

分 类 号：P597.3[天文地球—地球化学] P588.141[天文地球—地质学]