青藏高原沱沱河地区晚泥盆世高铝玄武岩的发现及意义

Discovery and significance of the Late Devonian high-alumina basalt in the Tuotuo River region of the Qinghai-Tibet Plateau

在青藏高原腹地的沱沱河—开心岭地区的区域地质调查中新发现的高铝玄武岩,利用激光等离子体质谱法(LA-ICP-MS)测试的锆石206Pb-238U年龄(380.1±5.6)Ma(MSWD=0.21),表明其形成时代为晚泥盆世。玄武岩具有高Al2O3(平均20.16%)和低SiO2(平均48.32%)、低Na2O(平均2.29%)、低K2O(平均0.39%)、低TiO2(均值0.754%)、低Alk(均值2.68%)的特点,并有(低钾)拉斑玄武系列向钙碱性系列过渡的特征,CIPW标准矿物属石英拉斑玄武岩(Q含量2.77%～9.32%),高铝、低钾、贫碱的特点与典型的高铝玄武岩一致。岩石的稀土总量均一且总体偏低,ΣREE含量为32.69×10-6～41.01×10-6,稀土配分模式近水平,δEu为1.12～1.28,均值为1.2,具有较强的正Eu和Ce异常。岩石富集亲石元素LILE,亏损高场强元素,具有较高Sr、Ba、Ce正异常和Nb负异常,岩石化学特点及相关的一系列岩石化学图解均表明其为岛弧玄武岩,表明分布于昌都—兰坪地块的晚泥盆世高铝玄武岩形成于岛弧环境,它的发现为三江地区古生代火山地层的确定、古特提斯洋的演化提供了丰富的资料。

On the basis of the field geological survey,the author discovered the high-alumina basalt in Tuotuo river-Kaixingling area located in the central region of the Qinghai-Tibet Plateau.U-Pb isotopes of zircon from the basalt were determined by LA-ICP-MS and the causes of formation of the zircon were studied by using CL images.The 206Pb-238U dating from 12 zircon suggests an isotopic age of（380.1 ±5.6） Ma（MSWD=0.21）,which indicates that the basalt was the product of the Late Devonian.The basalt shows the characteristics of low SiO2（average 48.32%）,Na2O（average 2.29%）,K2O（average 0.39%）,TiO2（average 0.754%）,Alk（average 2.68%）and high Al2O3（average 20.16%）.Its chemical composition has the characteristics from low K Tholeiitic series to calc-alkiline series.The CIPW normative mineral of the basalt indicate that it belongs to quartz tholeiitic basalt（The contents of quartz was 2.77%-9.32%）.The characteristics of high Al,low K and Alk were similar to the typical high-alumina basalt.It had low total quantity of REE elements,ΣREE= 32.69×10-6-41.01×10-6.δEu was between 1.12 and 1.28 with the mean of 1.2;chondrite-normalized REE patterns of basalt approximate to the level,with positive anomaly for Eu and Ce.The rocks are enriched in large ion lithophile elements（LILE）,depleted in high strength field elements（HFSE）,with positive Sr,Ba and Ce but negative Nb anomalies.Based on these geochemical characteristics and combined with a series of rock geochemistry diagrams for the tectonic setting,the discovery suggested that the basalt was formed in island arc condition,determined that the volcano stratum was formed in the Eopaleozoic in the Northern Part of Qiangtang-Sanjiang area,and provided the information for studying the formation and evolution of the Paleo-Tethys Ocean.