北秦岭西部榴闪岩的发现与变质作用过程:锆石年代学和Hf-O同位素制约

发布时间：2018-03-26 00:52

  本文选题：秦岭造山带　切入点：榴闪岩　出处：《岩石学报》2017年08期

【摘要】：秦岭造山带是由多期俯冲-碰撞造山和随后多阶段隆升成山形成的复合型造山带。近年来一个突破性进展是在北秦岭东部即陕西-河南交界地区以及桐柏地区的秦岭群高级变质岩中发现存在大量退变榴辉岩和高压麻粒岩,其中部分退变榴辉岩锆石包裹体中出现柯石英。这些发现证明北秦岭东部和桐柏地区的确在480~500Ma发生过深俯冲作用,随后造山带抬升形成退变榴辉岩。那么,北秦岭西部是否存在类似的(深)俯冲作用?本文首次在北秦岭西部宝鸡眉县地区发现了榴闪岩。该榴闪岩主要由石榴石和角闪石组成,含少量石英、透辉石和绿帘石。石榴石变斑晶都具有明显由退变质扩散作用形成的环带结构,绿帘石变斑晶具有与榴辉岩中绿帘石大致相同的XFe、CaO和Al2O3含量。同时,石榴石变斑晶核部还含有Amp+Cpx+Qtz包裹体,该包裹体矿物组合暗示其退变前可能是榴辉岩,而榴闪岩是在流体活动过程中退变质作用形成的。榴闪岩中锆石的离子探针SIMS定年获得很好的U-Pb谐和年龄(443.9±1.1Ma)和206Pb/238U的加权平均年龄(443.2±2.5Ma)。锆石的低Th和U含量和非常高Th/U比值(0.47~1.66)说明其并非是在"干"体系下形成的进变质锆石,应该是在流体强烈活动下形成的或是进变质锆石被退变质过程完全重置。因此,443Ma应该是榴闪岩的形成年龄即记录退变质作用发生的时间。这些锆石具有亏损的Hf同位素和类似地幔的氧同位素组成。因遭受退变质作用的强烈影响,其εHf(t)值和δ18O值有较宽的分布范围(εHf(t)=2.4~14.7;δ18O=4.25‰~6.39‰),其亏损地幔模式年龄为0.48~0.95Ga。所以,锆石的Hf-O同位素特征指示榴闪岩的原岩主要来自新元古代时期的亏损地幔来源的玄武质岩石。结合北秦岭东部地区榴辉岩发生退变质作用时代间隔可以大致判断北秦岭西部的榴闪岩的峰期变质时代可能为470~480Ma。由此发现,北秦岭东西部早古生代俯冲-碰撞事件时限上存在差异,即北秦岭早古生代俯冲-碰撞作用沿商南-宝鸡一线可能是自东向西发生的。
[Abstract]:The Qinling orogenic belt is a composite orogenic belt formed by multiple stages of subduction-collision orogeny and subsequent multi-stage uplift into a mountain. In recent years, a breakthrough has been made in the eastern part of the north Qinling mountain, that is, the Shannxi-Henan border area and the Tongbai area. A large number of retrograde eclogites and high-pressure granulites were found in the high-level metamorphic rocks of the Qinling Group. Coesite is found in zircon inclusions of some retrograde eclogites. These findings prove that deep subduction occurred in 480~500Ma in the eastern part of the North Qinling Mountains and Tongbai area, and then the orogenic belt uplifted to form retrograde eclogite. Is there a similar (deep) subduction in the western part of the North Qinling Mountains? In this paper, the eclogite was first found in Baojimei County, west of North Qinling Mountains. The eclogite is mainly composed of garnet and hornblende, and contains a small amount of quartz. Both diopside and verdant. The garnet changed into porphyry with the ring structure formed by retrograde metamorphism and diffusion, and the porphyry of green curtain has the same content of XFeO and Al2O3 as that of the eclogite in the eclogite, and the content of XFeO and Al2O3 is about the same as that in the eclogite. There are also Amp Cpx Qtz inclusions in the core of garnet speckle. The mineral assemblage suggests that the inclusion may be eclogite before its degeneration. The eclogite was formed by retrograde metamorphism during fluid activity. The zircon ion probe SIMS dating in the eclogite obtained a good U-Pb harmonic age of 443.9 卤1.1 MaA and a weighted average age of 443.2 卤2.5MaA of 206Pb/238U. The low Th and U contents of zircon were very high. The Th/U ratio of 0.47 ~ 1.66) shows that it is not a progressive zircon formed in "dry" system. It should be formed under strong fluid activity or completely reset during retrograde metamorphic process. Therefore, 443 Ma is supposed to be the age of formation of the eclogite, that is, the time when retrograde metamorphism occurred. These zircons have depleted HF. Isotopes and oxygen isotopic compositions similar to the mantle. Strongly influenced by retrograde metamorphism, The values of 蔚 HfT and 未 18O have a relatively wide distribution range (蔚 Hfft = 2.4 ~ 14.7; 未 18O=4.25 鈥，

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