西天山乌孙山地区大哈拉军山组火山岩LA-ICP-MS锆石U-Pb年龄及其构造环境

摘要

西天山乌孙山地区大哈拉军山组由玄武岩、安山岩、英安岩、流纹岩及相应的火山碎屑岩组成,安山岩和流纹岩分布最广.LA-ICP-MS锆石U-Pb定年结果表明,火山活动喷发的安山岩与安山质晶屑凝灰熔岩分别形成于353.9Ma±6.5Ma和356.3Ma±4.4Ma,属于早石炭世早期.通过区域对比,西天山大哈拉军山组的火山岩浆作用显示从伊犁中天山板决南北缘向伊犁盆地内部逐渐变年轻的特点,且火山岩喷发时代差别不大(约40Ma).岩石地球化学研究表明,火山岩属钙碱性系列,富集轻稀土元素,相对亏损重稀土元素.中性火山岩富集大离子亲石元素(如Cs、Rb、Th、U),而相对亏损高场强元素,具有明显的Nb、Ta、Ti负异常,显示出岛弧火山岩的特征；酸性火山岩相对富集Rb、Th、U、Ta等元素,具有明显的Ba、Sr、P、Eu、Ti等元素的负异常.综合伊犁-中天山板块南缘的构造演化特征,认为大哈拉军山组形成于活动大陆边缘环境,产在板块俯冲-碰撞的最后阶段.%Volcanic rocks in the Dahalajunshan Formation of Wusunshan area in West Tianshan Mountains are composed of basalt, an-desite, dacite, rhyolite and relevant volcaniclastic rocks, with andesite and rhyolite being dominant. The LA-ICP-MS zircon dating results show that adesite and andestic tuff lava in the Dahalajunshan Formation from Wusunshan area yield concordant 206Pb/238U ages of 353.9 ±6.5Ma and 356.3 ±4.4Ma, implying that they were erupted at the early stage of Early Carboniferous. Regional correlation shows that the volcanic magmatism gradually became younger from the north and the south edge of Yili-Central Tianshan plate to Yili basin, and the eruption time span is small (about 40Ma). Therefore, the age of Dahalajunshan Formation at the south edge of Yili block should be confined between 363Ma and 326Ma, namely Late Devonian to Early Carboniferous and mainly in Early Carboniferous Tournaisian period-Visean period. The geochemical characteristics show that the volcanic rocks belong to the calc-alkaline series, enriched in LREE and depleted in HREE relatively. The primitive mantle -normalized trace element patterns show that intermediate volcanic rocks are slightly enriched in large-ion-lithophile elements(LILE) such as Cs, Rb, Th and U and relatively depleted in HFSE and have obvious negative Nb, Ta, Ti anomalies, and that acid rocks are characterized by Rb, Th, U, Ta enrichment and Ba, Sr, P,Eu,Ti depletion. In combination with the south edge of Central Tianshan tectonic evolution, the characteristics of volcanic rocks inDahalajunshan Formation suggest that they were probably formed in an active continental margin environment.