新疆伊犁北部石炭纪火山岩地球化学特征及其地质意义

Geochemical characteristics and tectonic significance of Carboniferous volcanic rocks in the northern part of the Ili Block,Xinjiang

石炭纪火山岩广泛分布于西天山伊犁陆块周缘,其岩性复杂,并大多与陆源碎屑岩伴生或互层。其中伊宁阿希、尼勒克和那拉提3个剖面的火山岩岩石地球化学特征显示,这些火山岩属于钙碱性系列,由玄武岩、粗玄岩、玄武质安山岩、玄武质粗面安山岩、安山岩、流纹岩、英安岩和粗面岩组成。稀土元素含量较高,轻稀土元素较重稀土元素富集。这些火山岩亏损高场强元素Nb,Ta,Zr等,而富集大离子亲石元素Th,Rb等,因而这些岩石的形成与板块俯冲有关。微量元素地球化学图解进一步表明,这些岩石形成于大陆岛弧环境。与之共生的早、中石炭世浅海—滨海相沉积地层以及伊犁陆块北缘巴音沟—莫托沙拉沟晚泥盆世—早石炭世蛇绿岩带的存在,表明伊犁陆块北缘在石炭纪时其环境为活动大陆边缘,石炭纪火山岩的形成可能与晚泥盆世—中石炭世早期北天山洋盆向南的俯冲作用有关。

Carboniferous volcanic rocks are widespread at peripheries of the Ifi block of the West Tianshan. They are complex in lithology and mostly are associated or alternate with terrigenous clastic rocks. Geochemical study of volcanic rocks from the Axi, Nileke and Nalati sections in Yining suggests that these rocks belong to the calcalkaline series composed of basalt, trachybasalt, basaltic andesite, basaltic trachyandesite andesite, rhyolite, dacite and trachyte. They have relatively high REE concentrations and are enriched in LREE relative to HREE, and in addition, they display a pronounced enrichment of LILE such as Th and Rb relative to HFSE such as Nb, Ta and Zr. Therefore the formation of these rocks is related to plate subduction, the trace element geochemical diagram further indicates that these rocks formed in a continental island-arc environment. The Early-Mid Carboniferous neritic-littoral sediments associated with them and the existence of the Late Devonian-Early Carboniferous Bayan Gol-Motuoshala ophiolite belt suggest that the Carboniferous calc-alkaline volcanic rocks on the northern margin of the Ili block are likely to have erupted in an active continental-margin environment and probably resulted from the subduction of the North Tianshan oceanic basin southward beneath the Ili block during the Late Devonian-Mid Carboniferous.