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哀牢山西部晚古生代岛弧-弧后盆地系统——来自基性岩地球化学和年代学的证据
The Late Paleozoic arc-back arc system in western Ailaoshan: Evidence from geochemistry and geochronology of basic rocks
投稿时间:2019-08-12  
中文关键词:哀牢山西部  晚古生代  基性岩  陆缘弧  弧后盆地
英文关键词:western Ailaoshan  Late Paleozoic  basic rocks  continental margin arc  back-arc basin
基金项目:国家自然科学基金资助项目(91855206)
作者单位
徐文涛 中国地质科学院 地质研究所, 北京 100037 
刘福来 中国地质科学院 地质研究所, 北京 100037 
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中文摘要:
      哀牢山缝合带是东南亚古特提斯构造域重要的缝合带之一,带内广泛出露的晚古生代基性岩是研究其构造演化历史的关键,但它们的岩石成因和形成环境存在较大的争议,制约了人们对该地区古特提斯构造演化的理解。本文对哀牢山缝合带西部的基性侵入岩和基性火山岩进行了详细的野外地质调查、岩石学和地球化学研究,并结合已有的年代学和地球化学数据结果综合分析,目的在于查明它们的成因和形成环境,为进一步探讨哀牢山缝合带构造演化提供制约。锆石U-Pb年龄数据显示,五素和坝溜地区的基性火山岩形成于早二叠世(288~287 Ma),而大龙凯和雅轩桥地区的基性侵入岩和火山岩则形成于中二叠世(272~265 Ma)。地球化学数据显示,早期基性岩(早二叠世)稀土元素含量较高(ΣREE=59.75×10-6~135.08×10-6),明显富集轻稀土元素(LREE/HREE=2.64~3.32)和大离子亲石元素(Rb、Ba、Th等),显著亏损Nb、Ta和Ti等高场强元素,类似于岛弧岩浆岩,高的Ta/Yb(0.09~0.18)和Ce/Yb(6.59~9.03)值表明其属于钙碱性系列;晚期基性岩(中二叠世)具有较低的稀土元素总量(ΣREE=28.77×10-6~76.16×10-6),轻微富集轻稀土元素(LREE/HREE=1.75~2.55),相对早期基性岩更加富集Rb、Ba,亏损Nb、Ta、Ti等元素,较低的Ta/Yb(0.05~0.09)和Ce/Yb(3.58~5.97)值表明其属于拉斑质系列。上述研究表明,哀牢山缝合带西部的晚古生代基性岩具有由钙碱性系列向拉斑质系列演化的趋势,结合相关区域地质资料,认为哀牢山缝合带西部出露的基性侵入岩和火山岩记录了晚古生代与板片俯冲相关的多期演化过程:早二叠世,哀牢山支洋或弧后盆地向西发生洋-陆俯冲,俯冲板片脱水并导致地幔楔岩石部分熔融,在哀牢山西部五素-雅轩桥地区形成安第斯型陆缘弧;中二叠世,先存的大陆弧发生弧内伸展,逐渐演变为新的弧后盆地,形成类似于MORB和IAT的基性-超基性岩石组合。
英文摘要:
      The Ailaoshan suture (ALSS) is one of the important Paleo-Tethyan sutures in Southeast Asia, and the Late Paleozoic basic rocks in the ALSS are the key to reconstructing the Paleo-Tethyan evolution. However, the petrogenesis and tectonic environments of these basic rocks are controversial, and this controversy constrains researchers' understanding of the Paleo-Tetyan evolution. Based on field investigation, the authors studied petrological and geochemical characteristics and synthesized the geochronological and geochemical data of the Late Paleozoic basic rocks in the western part of the ALSS, with the purpose of investigating their petrogenesis and tectonic setting and further providing insights into the ALSS evolution. Zircon U-Pb data suggest that the Wusu and Baliu basic rocks were formed in Early-Permian (288~287 Ma), and Dalongkai and Yaxuanqiao basic rocks were formed in Mid-Permian (272~265 Ma). The products of Early-Permian magmatism are calc-alkaline in composition, and they have island-arc basalt (IAB)-like geochemical features with high rare earth element (REE) content (ΣREE=59.75×10-6~135.08×10-6), LREE/HREE ratios (2.64~3.32), Ta/Yb (0.09~0.18) and Ce/Yb (6.59~9.03) ratios, and slight enrichment in large ion lithophile elements (LILEs, e.g., Rb, Ba and Th) and depletion in high field strength elements (HFSEs, e.g., Nb, Ta and Ti), whereas the basic rocks from Mid-Permian magmatism are tholeiite, and are characterized by low REE content (ΣREE=28.77×10-6~76.16×10-6), LREE/HREE ratios (1.75~2.55), Ta/Yb (0.05~0.09) and Ce/Yb (3.58~5.97) ratios, and strong enrichment in LILEs and depletion in HFSEs. Combined with regional geological data, this paper proposes a two-pulse model related to the Late Paleozoic subduction in the western part of ALSS. During the Early-Permian, the westward subduction of the Ailaoshan branch ocean/back-arc basin beneath the east margin of Simao terrane induced partial melting of mantle wedge to form the Andean-type continental margin arc along the western Ailaoshan; subsequently (Mid-Permian), the collapse of continental margin arc formed a back-arc basin, and further produced basic-ultra basic rocks with the geochemical features of both MORB and volcanic arcs.
徐文涛,刘福来,2020,哀牢山西部晚古生代岛弧-弧后盆地系统——来自基性岩地球化学和年代学的证据[J].岩石矿物学杂志,(4):406~422.
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