吉林安图海沟岩群的解体：地质与锆石年代学证据

对吉林安图地区原划为中元古代构造地层的海沟岩群进行的野外地质工作和锆石 SHRIMP U-Pb 年代学工作表明， 海沟岩群不同岩组的变质程度和形成时代都存在明显差异。 以条带状含铁建造为特征的四岔子岩组受到高级变质作用改造， 变质程度为高角闪岩相并伴有深熔。 侵入 BIF 铁矿的奥长花岗质片麻岩给出的形成时代为 2 554 ± 3 Ma 或更老， 变质时代为 2 528 ± 11 Ma， 表明四岔子岩组应当老于 2 554 Ma。 结合区域对比， 推测其为新太古代变质表壳岩。 而东方红岩组虽然变形作用强烈， 但仅为绿片岩相变质。 该岩组中的变流纹岩形成年龄为 178 ± 2 Ma， 表明其为早侏罗世火山岩， 受到后期韧性变形作用的改造。 新资料表 明原划的海沟岩群需要解体， 其中既包括太古宙变质基底， 也包括受到变形影响的中生代地层。 太古宙地质体在古生代末-中生代初古亚洲洋闭合及造山后构造作用下， 呈岩片出露在年轻地质体之间。 本文的奥长花岗质片麻岩变质增生边和变质新生锆石具有异常高的 Th/U 比 值， 推测其和变质过程中高 U 矿物的形成有关。 高 Th/U 比变质锆石和低 Th/U 比岩浆锆石的实例表明：对锆石成因进行判别时， Th/U 比值只能作为一个辅助手段谨慎使用。

The disintegration of Haigou complex in Antu County,Jilin Province: Evidence from geology and zircon SHRIMP U-Pb dating

The Geology and zircon SHRIMP U-Pb dating of the Haigou complex in Antu County, Jilin Province which was proposed to be Mesoproterozoic strata in previous researches was reported in this article. The new data suggest that the different units in the Haigou complex show variable metamorphic grades and formation ages. The Sichazi Formation which is characterized by banded iron formation show upper-amphibolite facies metamorphism and anatexis. A trondhjemite gneiss which intruded into the BIF in the Sichazi Formation yielded formation age of 2 554 ± 3 Ma and metamorphic age of 2 528 ± 11 Ma. This age suggests that the Sichazi Formation is older than 2 554 Ma and is proposed be Neoarchean supracrustal rocks according to the correlation with other areas. However, the Dongfanghong Formation show strong deformation, but only show greenschist facies metamorphism. One meta-rhyolite in Dongfanghong Formation yielded formation age of 178 ± 2 Ma. This age suggests that the rhyolite is an Early-Jurassic volcanic rock with younger deformation. New data suggested that the Haigou complex is not suitable for a lithostratigraphic unit and should be disintegrated and redefined since it includes both Archean basement and deformed Mesozoic strata. The Archean rocks occurred as tectonic slices among the Phanerozoic rocks during the closure of Paleo-Asian Ocean at the end of Paleozoic-beginning of Mesozoic in the northeastern margin of North China Craton and post-orogen tectonic events. The metamorphic zircons which occur as either single grains or overgrowth rims surrounding magmatic cores in the dated trondhjemitic gneiss yielded very high Th/U ratio, and was suggested to be related to the formation of high-U minerals during metamorphism. The examples of high Th/U metamorphic zircons and low Th/U magmatic zircons suggested that the Th/U ratios of zircons only can be used as an assistant tool but not a critical tool during determining the origin of zircons.