本文選題：秦嶺造山帶　切入點：榴閃巖　出處：《巖石學(xué)報》2017年08期

【摘要】：秦嶺造山帶是由多期俯沖-碰撞造山和隨后多階段隆升成山形成的復(fù)合型造山帶。近年來一個突破性進展是在北秦嶺東部即陜西-河南交界地區(qū)以及桐柏地區(qū)的秦嶺群高級變質(zhì)巖中發(fā)現(xiàn)存在大量退變榴輝巖和高壓麻粒巖,其中部分退變榴輝巖鋯石包裹體中出現(xiàn)柯石英。這些發(fā)現(xiàn)證明北秦嶺東部和桐柏地區(qū)的確在480~500Ma發(fā)生過深俯沖作用,隨后造山帶抬升形成退變榴輝巖。那么,北秦嶺西部是否存在類似的(深)俯沖作用?本文首次在北秦嶺西部寶雞眉縣地區(qū)發(fā)現(xiàn)了榴閃巖。該榴閃巖主要由石榴石和角閃石組成,含少量石英、透輝石和綠簾石。石榴石變斑晶都具有明顯由退變質(zhì)擴散作用形成的環(huán)帶結(jié)構(gòu),綠簾石變斑晶具有與榴輝巖中綠簾石大致相同的XFe、CaO和Al2O3含量。同時,石榴石變斑晶核部還含有Amp+Cpx+Qtz包裹體,該包裹體礦物組合暗示其退變前可能是榴輝巖,而榴閃巖是在流體活動過程中退變質(zhì)作用形成的。榴閃巖中鋯石的離子探針SIMS定年獲得很好的U-Pb諧和年齡(443.9±1.1Ma)和206Pb/238U的加權(quán)平均年齡(443.2±2.5Ma)。鋯石的低Th和U含量和非常高Th/U比值(0.47~1.66)說明其并非是在"干"體系下形成的進變質(zhì)鋯石,應(yīng)該是在流體強烈活動下形成的或是進變質(zhì)鋯石被退變質(zhì)過程完全重置。因此,443Ma應(yīng)該是榴閃巖的形成年齡即記錄退變質(zhì)作用發(fā)生的時間。這些鋯石具有虧損的Hf同位素和類似地幔的氧同位素組成。因遭受退變質(zhì)作用的強烈影響,其εHf(t)值和δ18O值有較寬的分布范圍(εHf(t)=2.4~14.7;δ18O=4.25‰~6.39‰),其虧損地幔模式年齡為0.48~0.95Ga。所以,鋯石的Hf-O同位素特征指示榴閃巖的原巖主要來自新元古代時期的虧損地幔來源的玄武質(zhì)巖石。結(jié)合北秦嶺東部地區(qū)榴輝巖發(fā)生退變質(zhì)作用時代間隔可以大致判斷北秦嶺西部的榴閃巖的峰期變質(zhì)時代可能為470~480Ma。由此發(fā)現(xiàn),北秦嶺東西部早古生代俯沖-碰撞事件時限上存在差異,即北秦嶺早古生代俯沖-碰撞作用沿商南-寶雞一線可能是自東向西發(fā)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 鹵1.1 MaA and a weighted average age of 443.2 鹵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，

本文編號：1665618