Discovery of the Late Silurian volcanic rocks in the Dazhonghe area, Yunxian-Jinggu volcanic arc belt, western Yunnan, China and its geological significance
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摘要:
云县-景谷火山弧带大中河地区新识别出一套晚志留世中基性-中酸性火山岩组合，其LA-ICPMS锆石U-Pb年龄为421.2±1.2Ma和417.6±5.1Ma。该套火山岩具有富铝(12.73%~16.63%)、富钠(K2O/Na2O=0.56~0.99) 和高Mg#(46.0~50.0) 的特征，属于钙碱性系列岩石；同时富集轻稀土，Eu具有不同程度的弱亏损，亏损高场强元素(Nb、Ta、Ti)，具有正εNd(t) 值(3.86~4.39) 和较高的Th/Ta比值(15~17)，显示与活动大陆边缘岛弧型火山岩相似的地球化学性质。大中河晚志留世火山岩很可能是俯冲沉积物流体交代地幔楔物质部分熔融的产物，并在岩浆上升过程中经历了一定的分离结晶作用和浅部地壳物质的同化混染；结合区域同期(410~420Ma) 岩浆活动及相关的高压变质事件分析，应为原-古特提斯洋在早古生代末期向东俯冲消减作用的产物，从而为扬子陆块西部边缘晚古生代“三江”多岛弧盆系的形成演化提供了前锋弧发育的岩石学证据及其动力学机制。
Abstract:A suit of Early Paleozoic volcanic rocks was recognized from Dazhonghe area in Yunxian-Jinggu volcanic arc belt. LA-ICPMS U-Pb zircon analyses suggest that the volcanic rocks was dated at 421.3±2.3Ma and 418.8±3.5Ma. The volcanic rocks belong to calc-alkaline serie rocks, with characterized by high Al2O3(12.73%~16.63%), Na2O (K2O/Na2O=0.56~0.99) and high Mg#(46.0~50.0). Furthermore, they are similar to island arc volcanic rocks of active continental margin, with characterized by enrichment of LREE, weak negative anomaly of Eu, strong negative anomalies of HFSE (Nb, Ta, Ti), positive εNd(t) (3.86~4.39) and high Th/Ta (15~17). The Late Silurian volcanic rocks in Dazhonghe area were probably generated by fractional crystallization of parent magma or assimilation of shallow crustal materials in the process of magma ascent, which partial melting of the depleted mantle wedge by metasomatism with subduction deposit. Being study in combination with the regional magmatic activities and high-metamorphic events during the same period (410~420Ma), we suggest that the island arc volcanic rocks were resulted from the eastward subduction of Paleo-Tethys ocean, which provide direct petrology evidences of forward arc and its dynamic mechanism for the formation and evolution of the Late Paleozoic Sanjiang multi-island-arc basin system in the western edge of the Yangtze block.
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图 1
西南“三江”造山带南段构造略图(a) 和大中河地区地质简图(b)(据云南省地质矿产局，1983①修改)
Figure 1.
Tectonic sketch map of southern potion in Sanjiang orogenic zone (a) and simplified geological map in Dazhonghe area (b)
图 2
云县-景谷火山弧带大中河火山岩地质剖面图
Figure 2.
Geological section map of volcanic rocks in Dazhonghe area in Yunxian-Jinggu volcanic arc belt
图 3
大中河绿泥石化玄武安山岩露头(a) 和安山质凝灰熔岩单偏光镜下照片(b)
Figure 3.
Field photograph showing the chloritization of the basaltic andesite (a) and photomicrograph (-) showing the tuffaceous texture of the andesitic-tuff lava (b) in Dazhonghe area
图 4
云县-景谷岩火山弧带大中河火山岩锆石CL图像
Figure 4.
CL images of zircons from Dazhonghe volcanic rocks in Yunxian-Jinggu volcanic arc belt
图 5
云县-景谷火山弧带大中河火山岩锆石U-Pb年龄谐和图
Figure 5.
U-Pb concordia diagram of zircons from Dazhonghe volcanic rocks in Yunxian-Jinggu volcanic arc belt
图 6
云县-景谷火山弧带大中河火山岩Zr/TiO2-Nb/Y和Th-Co图解
Figure 6.
Zr/TiO2vs. Nb/Y and Th vs. Co diagram from Dazhonghe volcanic rocks in Yunxian-Jinggu volcanic arc belt
图 7
大中河火山岩REE配分模式和微量元素蛛网图解(标准化值据Sun and McDonough, 1989)
Figure 7.
Chondrite-normalized REE patterns and primitive mantle normalized trace element spider diagram for volcanic rocks in Dazhonghe area (normalization values after Sun and McDonough, 1989)
图 8
大中河火山岩Th/Ta-Yb和Hf-Th-Ta构造环境图解
Figure 8.
Rb-Hf-Ta and Hf-Th-Ta tectonic environments diagram for volcanic rocks in Dazhonghe area
图 9
大中河火山岩La-La/Yb图解
Figure 9.
La vs. La/Yb diagrams for volcanic rocks in Dazhonghe area
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