东天山阿奇山-雅满苏成矿带富钠火山岩年代学和矿物学特征

Geochronology and mineralogical characteristics of Na-rich volcanic rocks from Aqishan-Yamansu belt in Eastern Tianshan

本文通过对东天山阿奇山-雅满苏成矿带内石炭系火山岩研究,认为其为富钠火山岩系(细碧角斑岩系)。下石炭统雅满苏组(C1y)富钠火山岩中斜长石主要为钠长石,少量更长石,并识别出两种类型钠长石(早期的Ab1和晚期的Ab2)的存在。Ab2不仅在形态和结构上与Ab1相异,而且在元素含量变化上也相异:从Ab1到Ab2,表现出Al2O3和CaO降低、Na2O升高、An值降低和Ab值升高的趋势。上石炭统土古土布拉克组(C2tgt)富钠火山岩中斜长石亦为钠长石和少量更长石,但总体上土古土布拉克组火山岩中斜长石较雅满苏组火山岩中斜长石具更低的Na2O含量和更高的An值。原位LA-ICP-MS微量元素特征显示,Ab2相对于Ab1具有较大微量元素变化范围、较高的Al、Sr、Ca、Ba、Y、Rb和较低的LREE/HREE、δEu,而且Ab2含有更高的有利成矿元素(Fe、Co、Ni、Cu、V、Zn),表明在Ab2形成过程中,更多富钠流体、更多成矿元素参与其中,这和Ab2常与磁铁矿等金属矿物共生的显微特征相一致,揭示了Ab2可能和带内的蚀变交代及矿化有关。SHRIMP锆石U-Pb年代学研究获得康古尔南地区土古土布拉克组富钠安山岩年龄为313. 9±5. 8Ma,表明阿奇山-雅满苏成矿带在晚石炭世亦发育富钠火山岩。阿奇山-雅满苏成矿带内海相富钠火山岩与矿床在空间上和时间上具有耦合关系,指示找矿勘查中应重视富钠火山岩,开阔了找矿思路。

The Carboniferous volcanic rocks in the Aqishan-Yamansu metallogenic belt,Xinjiang,NW China,are sodium-rich volcanic rocks( spilite-keratophyre series). Plagioclases in the sodium-rich volcanic rocks of the Yamansu Formation( C1y) are albites with a small number of oligoclases. Two types of albite( the early Ab1 and the late Ab2) were identified. The Ab2 is not only different from Ab1 in morphology and texture,but also in element contents: from Ab1 to Ab2,Al2O3 and CaO decreased,Na2O increased,An values decreased and Ab values increased. Plagioclases in the sodium-rich volcanic rocks of the Tugutu Bulak Formation( C2tgt) are also albites with a small number of oligoclases. However,the plagioclases of the Tugutu Bulak Formation generally have lower Na2O contents and higher An values than that of the Yamansu Formation. The trace element characteristics of in situ LA-ICP-MS show that Ab2 has a larger range of trace elements,higher Al,Sr,Ca,Ba,Y,Rb,lower LREE/HREE and δEu than Ab1. Moreover,the Ab2 has higher favorable metallogenic elements contents,such as Fe,Co,Ni,Cu,V and Zn,which indicates that more sodium-rich fluids and more ore-forming elements are involved in the formation of Ab2. These are consistent with the microscopic characteristics of the coexistence of metallic minerals( such as magnetite) and Ab2,which reveals that Ab2 may be related to alteration and mineralization in the Aqishan-Yamansu metallogenic belt. The SHRIMP zircon U-Pb dating of andesite from Tugutu Bulak Formation in the southern Kangguer area is 313. 9 ± 5. 8 Ma. It can be inferred that the sodium-rich volcanic rocks also formed in the Late Carboniferous. There is a spatial and temporal coupling relationship between marine sodium-rich volcanic rocks and deposits in the belt,which reminds that attention should be paid to the sodium-rich volcanic rocks in the direction of prospecting and exploration.