新疆阿尔泰南缘达拉乌兹双峰式火山岩年龄及岩石成因

Geochronology and genesis of the bimodal volcanic rocks in Dalawuzi from the southern margin of Altay,Xinjiang

阿尔泰南缘达拉乌兹一带出露了一套玄武岩和流纹岩频繁互层的双峰式火山岩组合。该套双峰式火山岩发生了绿片岩相变质作用,其基性端元——变质玄武岩和酸性端元——变质流纹岩具明显的变余斑状结构。利用LA-ICP-MS锆石U-Pb法获得流纹岩的喷发年龄365.6±1.6Ma和捕获的锆石年龄413.5±2.0Ma。变质玄武岩富TiO2、碱(富钠)(Na2O/K2O=4.43～19.33)和FeOT,贫MgO和CaO,轻微富集LILE和LREE,具弱的Nb、Ta和Ti的负异常,具有高于正常弧火山岩但低于OIB的HFSE元素(Nb、Ta、Zr、Hf)的含量,Eu负异常不明显。流纹岩具有高SiO2和全碱,具有低的TiO2、极低的MgO和FeOT,呈现明显的Ti、P、Sr、Ba负异常,Th、U、Pb的正异常,HFSE元素(Zr、Hf)和LREE略富集但Nb、Ta亏损的特征,并显示Eu的明显负异常(δEu=0.21～0.41)。它们均具有相似于N-MORB的较低的Pb同位素组成。玄武岩与流纹岩来自两个不同的岩浆房,玄武岩为受俯冲物质加入的亏损软流圈地幔部分熔融后并发生结晶分异作用的结果,流纹岩为新生的地壳物质受幔源岩浆加热熔融并与之混合后演化的产物。结合阿尔泰南缘地区的构造演化特征,推测该双峰式火山岩俯冲作用引起的局部拉张作用的产物。

LA-ICP-MS U-Pb zircon ages are reported for meta-rhyolites in the bimodal volcanic rocks from the Altaizhen Formation in Dalawuzi area at the southern margin of Altay orogenic belt. The zircons with well-defined oscillatory zoning from meta-rhyolite sample yielded mean 206Pb/238U ages of 365.6±1.6Ma and 413.5±2.0Ma. The age of 365.6±1.6Ma represent the eruption age of the volcanic rocks of the Altaizhen Formation,which indicated that the Altaizhen Formation was formed during the Middle to Late Devonian. The age of 413.5±2.0Ma,which are consistent with the eruption age of the Kangbutiebao silicic volcanic rocks and Early Devonian granite,can be interpreted as an age of captured zircon from Early Devonian magmatism. Geochemically,meta-basalts are rich in titanium,alkali (especially sodium),iron,LILE and LREE and poor in magnesium. They have higher contents of HFSEs (Nb,Ta,Zr and Hf) than those of typical IAB. Meta-rhyolites are generally characterized by rich silicon,alkali,and poor magnesium. They have relatively high U,Th and rather low Ba,Sr,Ti and P. They display obvious negative anomaly of Nb,Ta and Ti,obvious enrichment of LREE,noticeable negative Eu anomalies (δEu=0.21~0.41). Basaltic and rhyolitic rocks show similar Pb isotopic data to N-MORB. On the basis of regional geological and geochemical evidences,this paper concludes that the basalt was originated from depleted asthenosphere and subducted material (subducted slab,sediments and fluids),and rhyolite resulted from thermal and the new accretion materials and mingling between basaltic magma and felsic magma. The bimodal volcanic rocks are the products of partial extension after the subduction.