东天山库姆塔格—沙泉子晚石炭世火山-沉积岩相学地质地球化学特征与构造环境

Lithofacies,geological and geochemical characteristics and tectonic setting of Late Carboniferous volcanic-sedimentary rocks in the Kumtag-Shaquanzi area,East Tianshan

通过对本区晚石炭世沉积岩和火山岩的岩相学类型及其特征研究,划分出山前洪积相(河流相)、水下三角洲沉积相、海岸潮坪相、浅海相、火山沉积岩相、火山喷溢相、火山喷发岩相、热水沉积岩相、火山热水蚀变岩相和矽卡岩相等10种不同岩相和相应的亚相,这种特征属大陆挤压收缩体制下活动大陆边缘上的火山-沉积岩相学组合类型。垂向相序结构表明在热水沉积岩相上下均有较强的火山活动,为成矿提供所需的热能。在库姆塔格晚石炭世压陷盆地中,热水沉积岩相的纵向相序结构为热水角砾岩亚相→菱铁矿岩亚相/石膏岩亚相→铁白云石岩亚相→碧玉岩亚相→铁白云石岩亚相,这种组合是寻找“库姆塔格型”大型石膏菱铁矿矿床的岩相学标志。本区火山-沉积岩相总的沉积序列显示向上由浅变深→由深变浅趋势,沉积物源主要来源于华力西期岛弧带剥蚀区及其南侧(中天山)基底隆起带;火山喷发仍形成发生在浅海环境中,具有大陆挤压收缩体制下(中天山造山带)后陆盆地沉积特点;晚石炭世原型沉积盆地可能为后陆盆地,有利于大型石膏菱铁矿矿床保存。

Ten hthofacies and their sublithofacies, including piedmont diluvial facies （fluvial facies）, subaqueous delta facies, tidal fiat facies, neritic facies, volcanic-sedimentary facies, volcanic outpouring facies, volcanic eruption facies, hydrothermal deposition facies, volcanic hydrothermal alteration facies and skarn facies, are recognized based on the lithofacies types and characteristics of Late Carboniferous sedimentary and volcanic rocks in the East Tianshan, Xinjiang. These characteristics indicate that the association of volcanic-sedimentary lithofacies formed on active continental margins in a continental compressional regime. The structure of vertical facies sequence indi- cates the strong volcanism occurred above and below the hydrothermal depositional facies, which provided heat needed for mineralization. In the Late Carboniferous Kumtag compressional basin, the vertical hthofacies sequence structure of the hydrothermal depositional facies is hydrothermal breccia subfacies→siderite rock/gypsum rock subfacies→ankerite rock subfacies→jasper rock subfacies→ankerite rock subfacies. This association is the hthofacies indicator for looking for large Kumtag type gypsum-siderite deposits in the East Tianshan. The general depositional sequence of the volcanic-sedimentary facies in the study area shows the trend of upward deepening facies sequence and then upward shallowing facies sequence. The provenance of clastic rocks was mainly the erosion area of the Variscan island arc zone in the north and the Central Tianshan basement uphft in the south. However, subaqueous volcanic eruption still took place in a neritic environment, showing the sedimentary characteristics of a hinterland basin in a continental compressional regime （Central Tianshan orogenic belt）. The Late Carboniferous prototype sedimentary basin is hkely to be a hinterland basin, which is favourable for preservation of large gypsum-siderite deposits.