Volcanic rocks of Dahalajunshan Formation extensively occur along Kekesu River area in Western Tianshan Mountains. The authors studied zircon geochronology and geochemistry of the intermediate and basic lavas exposed in this area. Generally speaking, the section can reflect the progressive extension of the sedimentary—volcanic sequence from continental fades to marine fades until large-scale volcanic eruption. LA-ICP-MS zircon U-Pb analyses suggest that the basaltic andesites of Dahalajunshan Formation were formed at 358Ma±3Ma,probably resulting from magmatic activities that occurred at the beginning of the Early Carboniferous period. Moreover, chondrite—normalized REE patterns of the calc-alkaline volcanic rocks are characterized by LREE enrichment and weak negative Eu anomalies. Primitive mantle —normalized multi —element patterns are characterized by obvious enrichment of large ion lithophile elements (LILE) and depletion of high field strength elements(HFSE) as well as the existence of negative anomalies of Nb, Ta and Ti, similar to typical geochemical features of the continental rift —related volcanic rocks influenced by the involvement of crustal materials. The Zr-Zr/Y tectonic setting discrimination diagrams also suggest that they might have been formed in an intraplate setting. Combined with the previous work on sequence stratigraphy of volcanic-sedimentary rocks, the authors hold that the volcanic rocks from Dahalajunshan Formation were formed in a rift environment related to the intense extension of the lithosphere after the final closure of the Paleozoic Tianshan Ocean.%大哈拉军山组在西天山地区广泛分布,其建组剖面沿科克苏河上游出露,剖面总体反映出由陆相向海相转变的递进伸展、直至大规模火山喷发的沉积-火山喷发序列.LA-ICP-MS锆石U-Pb同位素测定结果表明:大哈拉军山组玄武安山岩形成于358.9Ma±2.3Ma,属早石炭世初期岩浆活动的产物.岩石地球化学特征显示此套以钙碱性为主的火山岩具有弱的负Eu异常和轻稀土元素富集的球粒陨石标准化分配样式,微量元素原始地慢标准化图解明确显示有大离子亲石元素富集和高场强元素的亏损,呈现出Nb-Ta-Ti的负异常,与受地壳混染的大陆裂谷火山岩地球化学特征较为相似.Zr-Zr/Y环境判别图也表明其可能形成于板内伸展环境.结合科克苏河地区大哈拉军山组火山-沉积地层的层序特征,认为研究区大哈拉军山组火山岩应形成于板内裂谷环境,同天山古生代洋盆最终闭合后区内岩石圈剧烈拉伸作用密切相关.
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