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江南造山带西段岩浆作用特性

周金城, 王孝磊, 邱检生   

  1. 南京大学 地球科学系,内生金属矿床成矿机制研究国家重点实验室, 南京, 210093
  • 收稿日期:2005-12-20 修回日期:2005-12-20 出版日期:2005-12-20 发布日期:2005-12-20

The Characters of Magmatism in the Western Section of the Jiangnan Orogenic Belt

ZHOU Jin-Cheng, WANG Xiao-Lei, QIU Jian-Sheng   

  1. Department of Earth Sciences and State Key Laboratory of Mineral Deposits Research, Nanjing University, Nanjing 210093, China
  • Received:2005-12-20 Revised:2005-12-20 Online:2005-12-20 Published:2005-12-20

摘要: 在湖南浏阳南桥的中元古代冷家溪地层中发现了一种高度亏损、具N—MORB属性的玄武岩一辉绿岩。这种玄武岩的K2O特低,不相容的高场强元素,如Nb,Zr,Hf,Ti等和稀土元素的丰度都低于(少数元素接近)N-MORB的丰度,εNd(1271Ma)为6.86~8、98。在俯冲带附近N—MORB的发现为“九岭古岛弧”的存在提供了新的证据。湘西黔阳、古丈新元古代玄武岩-辉绿岩属碱性玄武岩.其微量元素特征近似于OIB,它的规模值得进一步调查。广西北部丹洲群中基性-超基性岩的TiO2,Ta及Nb的含量低,Hf/Th,Ti/Y和Ti/Zr等的比值也低,La/Ta和Th/Yb等的比值高,具有“弧玄武岩”特性。它们不具有CFB及OIB的地球化学特征,在构造环境判别图解上的标绘点集中在“火山弧玄武岩区”,少数样品位于“弧后盆地玄武岩区”。至今未发现具高温特征的高镁熔岩,也未见大规模放射性岩墙群。这些基性岩的规模极小(约100km^2),不具有地幔柱来源的镁铁质岩浆的特性。桂北及湘东北的新元古代强过铝(SP)花岗岩体与江西九岭和安敞许村、休宁、歙县等岩体具有相似的特征,它们沿江南造山带呈带状分布,其形成与华夏和扬子板块之间的碰撞事件有关。在全球不同时代的造山带中都有S-型花岗岩出露;地幔柱岩浆作用形成的“镁铁质大火成岩省”中,出现的少量花岗岩是非造山、板内或A-型花岗岩,而不会是S-型花岗岩。不能把S-型花岗岩作为地幔柱岩浆作用的证据。这些都说明“江南造山带”的厘定有充分的岩石学证据。

Abstract: The highly depleted basalts-diabases with N-MORB features have been found in the Mesoproterozoic Lenjiaxi strata from Nanqiao, Liuyang, Hunan province. K20 in the basahs are very low. The basahs and diabases show highly depleted geochemical features. Abundances of their incompatible HFSE, such as Nb, Zr, Hf, Ti , and REE are lower than ( some are close to) those of N-MORB. Their εNd( 1271 Ma) varies from 6.86 to 8.98. These basalts distributed along an ancient subducting zone provide a new evidence for ancient Jiuling island arc. The Neoproterozoic basalts and diabases from Qianyang and Guzhang, western Hunan, are classified as alkaline basahs. Their geochemical features of trace elements are similar to those of OIB. Their scale of distribution in the field should be further examined. The mafic-ultramafic rocks in Danzhou group from northern Guangxi have lower TiO2, Ta and Nb, and lower Hf/Th, Ti / Y and Ti / Zr ratios and higher La/Ta and Th / Yb ratios, Showing arc affinities. They did not show any CFB and OIB signatures. In tectonic discrimination diagrams, they are plotted in the field of “volcanic arc basahs”. Some samples are within “back-arc basalts” field. Both high-Mg lavas with high temperature features and giant radiating dyke swarms have never been found so far in the area. The exposed area of these mafic rocks in the field is very small ( about 100 km^2 ) without the characters of the plume-derived mafic magmas. The features of Neoproterozoic peraluminous S-type granites from northeastern Hunan and northern Guangxi provinces are similar to those of the Jiuling granites in Jinagxi province, and the Xiuning, Xucun and Shexian granites in Anhui province. They are distributed along the Jiangnan orogenic belt with zonal features. The generation of the S-type granites are related with the continent-continent collisional event between the Yangtze and Cathaysia blocks. Most S-type granites occur in the orogenic belts with various ages in the world. In the mafic large igneous province formed by plume magmatism, the granites associated with continental flood basalts are non-orogenic, intraplate and A-type granites rather than S-type. S-type granites should not be considered as the evidence of plume magmatism. It is suggested that the confirmation of “Jiangnan orogenic belt” has been supported by full petrological evidences.