摘要
在北秦岭造山带核部秦岭岩群内发育两类不同产状的斜长角闪岩。一类与秦岭岩群中的大理岩紧密共生,呈规模较大的似层状或较小块体产于大理岩内,另一类则呈密集岩墙群型式侵入于秦岭岩群南段二云母石英片岩中。不同的产出状态表明两类斜长角闪岩的成因和时代存在显著差异,也具有不同的地质构造意义。地球化学上,两类不同产状的斜长角闪岩的原岩皆为玄武岩质的。侵入秦岭岩群二云母石英片岩中的斜长角闪岩墙群形成于板内拉张环境,SHRIMP锆石U-Pb测年揭示该类斜长角闪岩形成于晚奥陶世(449±11Ma),Sr-Nd同位素特征显示其岩浆源区为亏损地幔源区,^(40)Ar/^(39)Ar热年代学研究显示该类斜长角闪岩的角闪岩相变质作用发生于石炭纪末期(301.3±6.4Ma)。地球化学和Sr-Nd同位素特征显示与秦岭岩群大理岩共生的似层状或块状斜长角闪岩的形成环境与侵入云母石英片岩中的斜长角闪岩墙群的存在显著差异,其形成于洋岛(OIB)或海山环境。^(40)Ar/^(39)Ar热年代学研究显示该类斜长角闪岩于晚二叠世(258.1±5.7Ma)发生了角闪岩相变质作用。不同性质、形成和变质时代、相似的变质作用等特点表明,秦岭岩群中的两类角闪岩分属不同性质的构造块体,秦岭岩群高级变地质地体可能是一个构造拼合地体。斜长角闪岩岩墙为晚奥陶世(449±11Ma)侵入秦岭岩群云英片岩中的基性岩墙群,是北秦岭晚加里东期后造山期热收缩而致的地壳伸展或岩圈拆离减薄的产物。与秦岭岩群大理岩共生的斜长角闪岩则可能是洋隆体的基性喷出岩+碳酸盐岩帽组合,是中二叠世(312～260Ma)期间构造移置而来的外来块体。
There are two types of amphibolites within the Qinling Group, north qinling orogenic belt. One is layer-liked or blocked amphibolites involved in marbles, another is the dikes Of amphibolites intruding into mica-schist, indicative of significant differences of petrogenesis and time between two types of amphibolites. Geochemistry shows that the Protolithes of two types of amphibolites are basaltic. The dikes of amphibolites were formed in a tectonic setting of the within-plate, and isotopes Of Strontium and neodymium indicate that magma of those dikes sourced from depleted mantle. SHRIMP U-Pb analyses for zircons show that the dikes were formed in the Late Ordovician, ca. 449 +/- 11 Ma, and (40)Ar/(39) Ar dating results reveral that those dikes underwent amphibolite facies metamorphism at the end of the Carboniferous, ca. 301.3 +/- 6.4Ma. In comparison, layer-liked or blocked amphibolites involved in marbles of the Qinling Group show the geochemical kind isotopic affinity with oceanic island basalls, or seamount basalts. (40)Ar/(39) Ar dating results show that this type of amphibolite underwent amphibolite facies metamorphism in the Late Permian, ca.258.1 +/- 5.7Ma, later about 40Ma than the dikes. Difference of tectonic affinities and timings, and similar metamorphism and ages suggest that two types of amphibolites in the Qinling Group are belonged to different tectonic blocks, and then assembled together to constructed a high-graded metamorphic terrane, that is the Qinling Group. In conclusion, this paper suggests that the dikes of amphibolites intruding into the mica-schist were products of crust extension or lithospheric delamination and thinning triggered by thermal contraction during the post Caledonian orogen in the north Qinling orogenic belt. While layer-liked or blocked amphibolite involved in marbles might be the assemblages of basic extrusive rocks and carbonate cap of the oceanic rises, such as the OIBs and seamounts, most likely. Those oceanic rises were allochthonous in the Middle Permian.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2009年第9期2177-2194,共18页
Acta Petrologica Sinica
基金
国家自然科学基金项目(40872147和40472119)
国土资源部"百人计划"项目(2004)
中国地质调查局项目(1212010782003-13 和1212010633601)
国家重点基础研究发展规划项目(2002CB412608)
国家科技支撑计划课题(2006BAB01 A11)资助
