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鱼洞子杂岩是秦岭造山带出露的重要太古宙基底,也是该地区最古老的复杂地质体。鱼洞子杂岩主要由变质地层(鱼洞子岩群)和规模不等的花岗质变质侵入体(花岗片麻岩)组成。各地质体间多以构造面理接触。鱼洞子岩群是重要的铁矿产出层位,铁矿及其赋矿围岩的形成时代一直没有可靠的依据。本次选择小沟山矿区磁铁石英岩分选出大量锆石。CL影像特征显示锆石普遍具有清晰的幔-边结构,幔部发育较清晰的震荡环带,显示典型的岩浆锆石特征;边部为灰白色后期变质环带。依据锆石CL影像特征,结合锆石成因分析,通过锆石微区U-Pb同位素LA-ICPMS(激光剥蚀等离子体质谱)测定,获得鱼洞子小沟山矿区磁铁石英岩岩浆锆石不一致线上交点年龄为2645±25 Ma(MSWD=1.2),207Pb/206Pb加权平均年龄为2645±24 Ma,二者极为一致,代表石英岩原岩的形成时代。同时获得变质锆石较好的不一致线上交点年龄为2555±24 Ma(MSWD=1.4),与207Pb/206Pb加权平均年龄2527±27 Ma在误差范围内较为一致,应为原岩发生变质形成磁铁石英岩的时代,是太古宙一次重要的区域变质事件信息的反映。
Yuandongzi complex is an important Archean basement exposed by the Qinling orogenic belt and is also the oldest complex geological body in the area. Yudongzi complex mainly consists of metamorphic strata (Yuandongzi group) and granitic metamorphic intrusive rocks (granitic gneiss) with different scales. More than the physical contact between the body texture. The Yudong Formation is an important iron ore production horizon. There has been no reliable basis for the formation of iron ore and its host rocks. The selection of small ditch mountain area magnetite quartz rock sorting a large number of zircon. The CL image shows that the zircons generally have a clear mantle-edge structure, the shale ring is clearer in the mantle, showing typical magmatic zircon characteristics and the marginal pale-gray metamorphic belt. According to the zircon CL image characteristics and the zircon genetic analysis, the magmatic zircons from the magnetite quartzite in the Xiaogoushan mining area of Yudongdingzi were determined by the U-Pb isotopic LA-ICPMS (Laser Ablation ICP-MS) The age of intersection is 2645 ± 25 Ma (MSWD = 1.2), and the weighted average age of 207Pb / 206Pb is 2645 ± 24 Ma. The two are very consistent and represent the formation age of the quartzite protolith. The mean coincident line age of metamorphic zircons was 2555 ± 24 Ma (MSWD = 1.4), which was consistent with the 207Pb / 206Pb weighted mean age of 2527 ± 27 Ma over the error range, which should be metamorphosed as a magnet The age of quartzite is a reflection of an important regional metamorphic event in the Archean.