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引用本文:夏林圻 夏祖春 徐学义. 北祁连山奥陶纪弧后盆地火山岩浆成因[J]. 中国地质, 2003, 30(1): 48-60.
XIA Lin-qi, XIA Zu-chun, XU Xue-yi. Magmagenesis of Ordovician back-arc basins in the Northern Qilian Mountains[J]. Geology in China, 2003, 30(1): 48-60(in Chinese with English abstract).
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北祁连山奥陶纪弧后盆地火山岩浆成因
夏林圻 夏祖春 徐学义
西安地质矿产研究所,陕西 西安 710054
摘要:
提要: 本文对北祁连山早古生代弧后盆地熔岩的岩石地球化学研究结果加以报道。样品的分布将南部弧后盆地拉伸最早阶段发育的岛弧裂谷化区和北部的弧后海底扩张区联系起来。熔岩的岩相学和地球化学特点反映了拉伸方式的改变,北部是典型的弧后盆地基性熔岩,向南则逐渐向岛弧熔岩过渡。海底扩张区以玻质(现已脱玻化)、 少斑基性熔岩为特征,长英质熔岩和斑状基性熔岩产于南部岛弧裂谷化区。成熟岛弧部分(Y<20×10-6, TiO2<0.60%, Th/Yb>0.60)和弧后扩张区(Y>20×10-6, TiO2>1.0%, Th/Yb<0.60)在地球化学上相互有别。从由海底扩张形成的弧后盆地基性熔岩,向南经过逐渐与岛弧岩石相似的熔岩,直至裂谷区最南部的岛弧熔岩,它们的地球化学成分显示逐渐的变化。这种变化反映了弧后盆地形成过程中弧后盆地之下地幔对流方式和熔体产生作用的改变:从初始岛弧裂谷之下由消减板片俯冲引起的地幔下沉,转变为弧后海底扩张带之下的地幔上隆。早期岛弧裂谷阶段,裂谷轴捕获了岛弧岩浆流,从而使得喷出的熔岩在成分上与岛弧熔岩无法区分;随着弧后拉张继续,弧后盆地变宽,岛弧岩浆流逐渐离开裂谷轴,最终产生一个似洋中脊的减压熔融系统——弧后盆地岩浆系统。
关键词:  弧后盆地熔岩  岛弧熔岩  岩石地球化学  岩浆成因  北祁连山
DOI:
分类号:
基金项目:国土资源部“九五”资源与环境攻关项目(95-02-004)资助。
Magmagenesis of Ordovician back-arc basins in the Northern Qilian Mountains
XIA Lin-qi, XIA Zu-chun, XU Xue-yi
Xi'an Institute of Geology and Mineral Resources, Xi,an 710054, Shaanxi, China
Abstract:
Abstract: This paper reports the results of a petrogeochemical study of lavas in lower Paleozoic back-arc basins in the Northern Qilian Mountains. The distribution of samples links regions of back-arc seafloor spreading to the north with regions farther south where a progression of island arc rifting styles occurred in the earliest stage of back-arc basin extension. Petrographic and geochemical characteristics of lavas reflect the change in style of extension, with typical back-arc basin basic lavas in the north passing southward to arc lavas. Glassy (devitrified now), minor-porphyritic basic lavas characterize regions of seafloor spreading. Felsic lavas and porphyritic basic lavas occur in the southern, island-arc rifting regions. Geochemically, mature arc portions (Y<20×10-6, TiO2<0.60%, Th/Yb>0.60) are distinguished from regions of back-arc spreading (Y>20×10-6, TiO2>1.0, Th/Yb<0.60). Samples show progressive marked changes in geochemical compositions, from back-arc basin basic lavas formed by seafloor spreading southward through lavas that increasingly resemble arc lavas to arc lavas that are indistinguishable from arc lavas encountered in the southernmost portion of the rift. These changes reflect the changes in the mechanism of mantle convection and processes of melt generation beneath the evolving back-arc basin in the Northern Qilian Mountains: from mantle downwelling beneath incipient island arc rifts caused by slab subduction to mantle upwelling beneath zones of back-arc seafloor spreading. The rift axis captured the arc magmatic flux in the early island arc rift stage, so that the lavas erupting from this part of the rift cannot be compositionally distinguished from arc lavas. As back-arc extension continued and the back-arc basin widened, the arc magmatic flux moved progressively away from the rift axis, finally generating a mid-ocean ridge-like decompression melting system-back-arc basin magmatic system.
Key words:  back-arc lava  arc lava  petrogeochemistry  magmagenesis  Northern Qilian Mountains