湘中盆地基底的时代格架:来自锡矿山碎屑锆石U-Pb年龄的证据

湘中盆地中部锡矿山晚古生代沉积地层中含有大量碎屑锆石,U-Pb同位素年龄显示这些锆石形成于3400~350Ma。在年龄谱系图上它们呈多个时间区段分布,反映出湘中地区对华南大陆所经历的印支运动之前的各期重要构造变革均有物质响应。根据不同时段锆石年龄范围,结合区域构造演化特征,厘定出湘中盆地的垂向物质组合从下向上主要包括:结晶基底(1.9Ga)、变质基底(1.9~0.54Ga)、微(或未)变质基底(0.54~0.41Ga)以及表层的沉积盖层(0.41Ga),并根据不同层锆石的年龄组合特征,以及岩层发育过程中对应的构造事件,对盆地基底-盖层框架中各物质层的形成环境进行了阐述。此外,通过对比煌斑岩与其围岩锆石的U-Pb年龄结构,结合野外地质特征以及前人的研究结果,得出该煌斑岩中锆石是熔体形成期间残留的锆石,而非其上升过程中捕获自围岩。由这些锆石大多形成于840~700Ma断定,锡矿山煌斑岩主要的物质来源是盆地的中上部变质基底,明确了煌斑岩熔体及与之相关流体获取物质补充的层位在基底-盖层框架中的位置。这些工作可为进一步探讨超大型Sb成矿作用与岩浆作用的耦合关系提供理论基础。

Chronlogical framework of basement beneath the Xianmgzhong Basin: Evidence by U-Pb ages of detrital zircons from Xikuangshan

The systematic LA-ICP-MS U-Pb dating on detrital zircons from Late Paleozoic sedimentary rocks in Xikuangshan mining area, Xiangzhong Basin, shows that their concordant ages ranging from 3400Ma to 350Ma. On the age spectrum, these zircons can be divided into several groups with different ages, indicating the existence of physical responses to major pre-Indosinian tectonic events in Central Hunan. According to age structure of zircons within different periods of time, together with regional tectonic evolution, the basement-cover framework of Xiangzhong Basin, from bottom to top, was established as crystalline basement (>1.9Ga), metamorphic basement (1.9~0.54Ga), low (or non) metamorphic basement (0.54~0.41Ga), and surface sedimentary cover (<0.41Ga). Combined age spectrum of these strata, as well as corresponding tectonic histories, formation environment of each material layer within the basement-cover frame also has been discussed. Moreover, by comparing age structure of zircons in lamprophyre with zircons in sedimentary rocks, and combined with field geological observations and previous studies, the majority of zircons within the lamprophyre were suggested as residual grains during the melt formation, but not xenocrysts captured from wall rocks during the melt (or dyke) ascending. Most of those zircons were crystalized in 840~700Ma suggest the lamprophyre melt formed at middle-upper metamorphic basement beneath this basin, and thus the position of material-providing layer for lamprophyre melt and other relative fluids is well definited within the basement-cover frame. These works will provide important evidences for further understanding about coupling relationship between magmatism and superlarge-scale Sb mineralization in Xikuangshan.