發(fā)布時(shí)間：2018-04-03 08:49

  本文選題：麻粒巖　切入點(diǎn)：鋯石年代學(xué)　出處：《西北大學(xué)》2015年碩士論文

【摘要】：華北克拉通中部造山帶的中部西緣地區(qū),出露大量古元古代片麻狀花崗巖、變質(zhì)表殼巖及少量變質(zhì)基性巖脈,被認(rèn)為是一套經(jīng)不同程度變質(zhì)而成的雜巖而被稱之為“呂梁雜巖”。前人已對(duì)該區(qū)一些深變質(zhì)巖的變質(zhì)時(shí)代和期次有一定研究,但對(duì)其變質(zhì)作用和演化過(guò)程的研究還較薄弱,作為中部造山帶的重要組成部分和經(jīng)歷多期變質(zhì)作用改造的深變質(zhì)巖,其變質(zhì)年代及演化歷史的研究無(wú)疑將為華北克拉通早期構(gòu)造演化過(guò)程提供重要的依據(jù)。本文通過(guò)對(duì)呂梁雜巖東南部西榆皮地區(qū)的片麻巖及基性麻粒巖的野外特征、巖相學(xué)特征、礦物成分分析及同位素年代學(xué)等方面的研究,初步確定了片麻巖及基性麻粒巖所經(jīng)歷的變質(zhì)作用期次、峰期P-T條件及變質(zhì)P-T軌跡,進(jìn)而對(duì)該區(qū)的構(gòu)造演化做了初步探討。區(qū)內(nèi)主要發(fā)育石榴石夕線石黑云二長(zhǎng)片麻巖及基性麻粒巖組合,片麻巖中常伴有深熔長(zhǎng)英質(zhì)脈體,基性麻粒巖呈侵入脈體狀發(fā)育于石榴石夕線石黑云二長(zhǎng)片麻巖中。巖相學(xué)觀察顯示,石榴石夕線石黑云二長(zhǎng)片麻巖的主要礦物組合為：石榴子石+夕線石+黑云母+鉀長(zhǎng)石+斜長(zhǎng)石+石英；基性麻粒巖的主要礦物組合為：石榴子石+斜方輝石+單斜輝石+角閃石+斜長(zhǎng)石,均顯示出麻粒巖相的礦物組合特征,代表了呂梁地區(qū)一套深變質(zhì)巖石組合。石榴石夕線石黑云二長(zhǎng)片麻巖中的石榴子石富Fe、Mg,虧損Mn、Ca,且核部成分較均一,邊部成分變化較大,顯示出退變環(huán)帶特征；基質(zhì)黑云母富Mg,但包裹于石榴石中的黑云母與石榴子石邊緣的黑云母Mg、Fe成分差異很大,證明在退變質(zhì)過(guò)程中兩者發(fā)生了Fe-Mg交換；基性麻粒巖中的石榴子石富Fe、Ca,相對(duì)虧損Mg、Mn,同樣發(fā)育退變環(huán)帶；斜方輝石主要為富Mg、貧A1的紫蘇輝石；單斜輝石主要為富Ca,貧Fe、Mg的次透輝石。計(jì)算獲得了石榴石夕線石黑云二長(zhǎng)片麻巖的峰期溫壓條件為：T=～700℃,P=～6kbar,退變質(zhì)階段的溫壓條件為：T=～530℃,P=～2kbar;基性麻粒巖的峰期溫壓條件為：T=497℃,P=8.7kbar;由此獲得一順時(shí)針的變質(zhì)P-T演化軌跡。片麻巖中的深熔花崗巖脈的鋯石U-Pb年代學(xué)研究揭示,該深熔作用的時(shí)代為1981Ma,從而限定該片麻巖最大變質(zhì)年齡為1981 Ma�；月榱r中的鋯石定年,分別獲得1945±20Ma和1928±25Ma兩期年齡,1945Ma解釋為峰期變質(zhì)年齡,1928Ma則代表了麻粒巖抬升過(guò)程退變質(zhì)年齡。結(jié)合前人獲得的片麻巖中獨(dú)居石年代學(xué)結(jié)果,認(rèn)為西榆皮地區(qū)的峰期變質(zhì)年齡介于1947～1942Ma之間,退變質(zhì)可能發(fā)生在1928Ma及其后期的1919～1857Ma期間。因此認(rèn)為,呂梁地區(qū)同中部帶其它地區(qū)一樣,在1950Ma也經(jīng)歷了陸-陸碰撞陸殼增厚,并致使該區(qū)經(jīng)歷了麻粒巖相的變質(zhì)作用,此后在1930Ma陸殼抬升減薄,從而疊加了角閃巖相的退變質(zhì)作用,這一過(guò)程在呂梁西榆皮地區(qū)的片麻巖和基性麻粒巖中均有很好的記錄。
[Abstract]:In the central western margin of the central orogenic belt of the North China Craton, a large number of ancient Gu Yuan granitic granites, metamorphic epidermal rocks and a small amount of metamorphic basic vein are exposed.Lv Liang complex is considered to be a set of metamorphic complex rocks.The metamorphic ages and stages of some deep metamorphic rocks in this area have been studied, but the study on their metamorphism and evolution process is still weak. As an important part of the central orogenic belt and the deep metamorphic rocks undergoing multi-stage metamorphism transformation, the metamorphic rocks have been studied.The study of metamorphic age and evolution history will undoubtedly provide an important basis for the early tectonic evolution of the North China Craton.In this paper, the field characteristics, petrographic characteristics, mineral composition analysis and isotopic chronology of gneiss and basic granulite in Xiyupi area of southeastern Lv Liang complex are studied.The metamorphic stages, peak P-T conditions and metamorphic P-T loci of gneiss and basic granulite are preliminarily determined, and the tectonic evolution of the area is discussed.The gneiss and basic granulite assemblage are mainly developed in this area. The gneiss is often accompanied by deep melt feldspar vein, and the basic granulite is developed in the garnet siliite black cloud gneiss as an intrusive vein.The petrographic observation shows that the main mineral assemblages of garnet sillimanite biotite biotite feldspar plagioclase quartz are: pomegranate sillimanite biotite feldspar plagioclase quartz;The main mineral assemblages of basic granulites are pomegranate clinopyroxene clinopyroxene hornblende plagioclase which all show the mineral assemblage characteristics of granulite facies and represent a set of deep metamorphic rock assemblages in Lv Liang area.The pomegranate Shi Fu FeMgO in garnet silimanite black cloud gneiss is depleted in MnCa-Ca.The composition of the nuclear part is more uniform, and the composition of the edge part changes greatly, showing the characteristics of the degenerative ring zone.The matrix biotite is rich in Mg-rich, but the composition of biotite in garnet is very different from that in the edge of garnet, which indicates that Fe-Mg exchange occurred during retrograde metamorphism.In the basic granulite, the pomegranate Shi Fu Feancai, which is relatively deficient in MgPe mn, also developed a degenerative ring zone; the clinopyroxene is mainly Mg-rich and A1-poor perilla pyroxene, while the clinopyroxene is mainly Ca-rich and Fe-mg-poor subdiopyroxene.璁＄畻鑾峰緱浜?jiǎn)鐭虫Υ鐭冲绾跨煶榛戜簯浜岄暱鐗囬赫d博鐨勫嘲鏈熸俯鍘嬫潯浠朵負(fù):T=~700鈩，

本文編號(hào)：1704485