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孙载波,曾文涛,周坤,吴嘉琳,李龚健,黄亮,赵江泰.昌宁-孟连结合带奥陶纪洋岛玄武岩的识别及其构造意义——来自地球化学和锆石U-Pb年龄的证据[J].地质通报,2017,(10):1760-1771.
昌宁-孟连结合带奥陶纪洋岛玄武岩的识别及其构造意义——来自地球化学和锆石U-Pb年龄的证据    点此下载全文
孙载波1  曾文涛2  周坤1  吴嘉琳1  李龚健3  黄亮1  赵江泰1
1. 云南省地质调查院, 云南 昆明 650216;
2. 云南省地质环境监测院, 云南 昆明 650216;
3. 中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083
基金项目:云南省科技领军人才培养计划项目《西南“三江”叠合成矿作用与成矿预测》(编号:2013HA001)和中国地质调查局项目《西南三江有色金属资源基地调查》(编号:121201010000150007)
DOI:
摘要:
      西南三江昌宁-孟连构造带传统被认为是晚古生代古特提斯洋的缝合带。在该构造带中段铜厂街蛇绿混杂岩东侧勐勇-芒红一带,新识别出一套近南北向展布的灰绿色气孔-杏仁状玄武岩、安山玄武岩和硅质岩、浅变质泥质粉砂岩组成的火山-沉积岩系。对杏仁状玄武岩样品进行LA-ICP-MS锆石U-Pb定年,获得其206Pb/238U年龄加权平均值为449.3±8.4Ma(MSWD=3.5),表明该套火山岩喷发于奥陶纪。元素地球化学特征表现为富铝(12.11%~15.04%)、富钠(K2O/N2O=0.03~0.44)、高Mg#值(45.9~61.7),同时,该套火山岩中MgO=6.78%~12.34%,平均为9.43%,Cr=58.6×10-6~636×10-6,平均为310.3×10-6,Ni=57.4×10-6~410×10-6,平均为202.1×10-6,Nb=18.9×10-6~32.8×10-6,平均为25.06×10-6,具有轻稀土元素强烈富集的"直线状"稀土元素配分模式,(La/Yb)N=5.25,(Ce/Yb)N=4.42,(Ce/Sm)N=1.68,稀土元素总量随全碱含量增大有升高的趋势,表明其为亚速尔型洋岛。总体上,该套火山岩具碱性OIB(洋岛玄武岩)的特征,可能是大洋板内热点前部熔融的产物。亚速尔型洋岛玄武岩的出现代表了特提斯洋盆内洋岛发育早期阶段的物质记录,为重新认识滇西南昌宁-孟连地区特提斯主洋盆开阔多岛洋的格局提供了丰富的岩石学依据。
关键词:亚速尔型洋岛  碱性玄武岩  地球化学  LA-ICP-MS锆石U-Pb定年  奥陶纪  昌宁-孟连结合带
Identification of Ordovician oceanic island basalt in the Changning-Menglian suture zone and its tectonic implications:Evidence from geochemical and geochronological data    Download Fulltext
SUN Zaibo1  ZENG Wentao2  ZHOU Kun1  WU Jialin1  LI Gongjian3  HUANG Liang1  ZHAO Jiangtai1
1. Yunnan Institute of Geological Survey, Kunming 650216, Yunnan, China;
2. Yunnan Institute of Geological Environment Monitoring, Kunming 650216, Yunnan, China;
3. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
Fund Project:
Abstract:
      The Changning-Menglian tectonic belt has been traditionally known as one of the Paleo-Tethys suture zones in the Sanji-ang orogenic domain, Southwest China. This study focused on the newly found volcano-sedimentary rocks consisting of vesicular and amygdaloidal basalts, andesitic basalt, siliceous rock, and meta-mud siltstone in the Mengyong-Manghong area located to the east of the Tongchangjie ophiolitic rocks in the central Changning-Menglian suture zone. LA-ICP-MS zircon U-Pb dating reveals that the 206Pb/238U weighted mean age of the amygdaloidal basalt is 449.3±8.4Ma(MSWD=3.5), suggesting that this suite of volcanic rocks was emplaced in Ordovician. The basaltic rocks are rich in Al2O3 (12.11%~15.04%), show high K2O/N2O ratios of 0.03~0.44 and high MgO (6.78%~12.34%, 9.43% on average) with high Mg# of 45.9~61.7, and Cr, Ni, Nb values of 58.6×10-6~636×10-6, 57.4×10-6~410×10-6 and 18.9×10-6~32.8×10-6 (310.3×10-6, 202.1×10-6 and 25.06×10-6 on average) respectively. They have flat chondrite-normalized REE patterns with low (La/Yb)N (averagely 5.25), (Ce/Yb)N (4.42 on average) and (Ce/Sm)N (1.68 on aver-age). The ΣREE value of the basaltic rocks increases with the increasing of the total alkaline, which suggests that the rocks are simi-lar to the Azores-type oceanic island basalts (OIB). Generally, this suite of volcanic rocks is alkaline with OIB affinity, which might have resulted from the melting of the head of the hotspot in the Intra-oceanic plate. The new discovery of the Azores-type OIB in the Changing-Menglian records the product of OIB activity in the Early Paleozoic evolution of Tethys Ocean, which provides im-portant basic information for further understanding of the composite arc-basin system in the Tethyan domain of Southwest China.
Keywords:Azores-type oceanic island  alkaline basalt  geochemistry  LA-ICP-MS zircon U-Pb dating  Ordovician  ChangningMenglian suture zone
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