内蒙古雅干地区晚石炭世白山组火山岩LA-ICP-MS锆石U-Pb年龄及其构造环境

弧火山岩对揭示俯冲过程中深部岩浆作用具有重要意义。内蒙古雅干地区白山组火山岩岩性主要为流纹岩、英安岩、流纹质晶屑凝灰岩、英安质晶屑凝灰岩。流纹岩LA-ICP-MS锆石U-Pb年龄为309±1 Ma、308±1 Ma及300±2 Ma，时代为晚石炭世。地球化学分析显示，白山组火山岩具有较高的SiO2（70.98%~75.45%）、Na2O（2.90%~5.23%）含量，总体相对富钠，Na2O/K2O平均值为1.57。样品富集Rb、K、U等大离子亲石元素，亏损Nb、P、Ti等高场强元素，显示轻稀土元素富集、重稀土元素亏损的右倾稀土元素配分模式，并且具有较明显的负Eu异常（δEu平均值为0.22），（La/Yb）N值为3.24~9.99，（La/Sm）N值为4.47~8.31。研究发现，白山组火山岩整体具有弧火山岩的地球化学特征，可能形成于大陆边缘弧构造环境，为晚石炭世古亚洲洋俯冲作用的产物。

Zircon LA-ICP-MS U-Pb dating and tectonic setting of volcanic rocks from Baishan Formation in Late Carboniferous in Yagan area,Inner Mongolia

Arc igneous rocks play the key role in tracing the magmatic process in the subduction zone.The Baishan Formation in the Yagan region is mainly composed of rhyolite, dacite, rhyolitic crystal tuff and dacitic crystal tuff.LA-ICP-MS U-Pb dating were conducted on zircons from the rhyolites, and the weighted mean ages are 309±1 Ma, 308±1 Ma and 300±2 Ma, suggesting that the volcanic rocks was erupted during Late Carboniferous.The rhyolites have high SiO2(70.98%~75.45%) and Na2O(2.90%~5.23%) contents, and all samples are relatively rich in Na and have the average Na2O/K2O ratios of 1.57.The volcanic rocks are relatively enriched in large ion lithophile elements, such as Rb, K and U, and depleted in high field strength elements, such as Nb, P and Ti.They show right-leaning chondrite-normalized patterns, which means enrichment in LREE and depletion in HREE, with(La/Yb) N ratios of 3.24~9.99, (La/Sm) N ratios of 4.47~8.31 and significant Eu negative anomalies(average value of δEu is 0.22).The geochemical characteristics of Baishan Formation volcanic rocks are similar to arc-related magmatism.These rocks are likely formed in a continental margin arc setting, and products of the subduction of the paleo-Asian ocean during Late Carboniferous.