CAJ | 학술논문

西藏泽当蛇绿岩位于雅鲁藏布江缝合带东段，由地幔橄榄岩、辉长岩、玄武岩等组成，地幔橄榄岩中发育有少量辉石岩、辉长岩、异剥钙榴岩、斜长花岗岩和角闪辉长岩。本文研究的角闪辉长岩侵入于泽当地幔橄榄岩的辉石岩中，主要由角闪石、葡萄石+绿纤石和少量Fe-Ti氧化物组成。岩石中角闪石呈自形，伟晶结构，褐色，均一的干涉色，与交代成因的角闪石相比具有较高的Al2O3(7.0%~11.0%)、TiO2(1.0%~2.5%)和Na2O (1.0%~1.9%)含量，指示其为岩浆成因；Fe-Ti氧化物以磁铁矿为主(TiO2=1.2%~13.2%；Fe2O3=40.6%~61.4%；FeO=31.1%~43.6%)，与钛铁矿(TiO2=36.2%~50.8%；Fe2O3=2.5%~24.6%；FeO=33.0%~43.7%)经常呈矿物对共生或出溶钛铁矿，磁铁矿—钛铁矿计算矿物平衡氧逸度(-logfo2)为12.4~22.8，约为NNO+1，比正常MORB氧化程度高，指示岩浆结晶时具有较高的水逸度。其锆石εHf(t)值为11.0~19.8，显示亏损地幔源区特征。岩石地球化学分析显示其具有高的Al2O3(20.4%~22.4%)、TiO2(1.0%~1.5%)，低的MgO(4.1%~5.5%)含量类似高铝玄武岩的特征。泽当角闪辉长岩可能由亏损地幔部分熔融形成的含水岩浆经过演化(分离结晶作用等)形成的低镁高铝类似高铝玄武岩的岩浆结晶形成。然而样品严重亏损轻稀土(LREE)，与常见于岛弧区的高铝玄武岩不同，可能为蛇纹石化的洋壳或地幔橄榄岩在俯冲早期脱水使得地幔楔发生部分熔融导致。对泽当蛇绿岩中侵入岩及地幔橄榄岩进行更详细的调查研究将对揭示泽当蛇绿岩的形成环境构造背景提供更多的线索。
서장택당사록암위우아로장포강봉합대동단，유지만감람암、휘장암、현무암등조성，지만감람암중발육유소량휘석암、휘장암、이박개류암、사장화강암화각섬휘장암。본문연구적각섬휘장암침입우택당지만감람암적휘석암중，주요유각섬석、포도석+록섬석화소량Fe-Ti양화물조성。암석중각섬석정자형，위정결구，갈색，균일적간섭색，여교대성인적각섬석상비구유교고적Al2O3(7.0%~11.0%)、TiO2(1.0%~2.5%)화Na2O (1.0%~1.9%)함량，지시기위암장성인；Fe-Ti양화물이자철광위주(TiO2=1.2%~13.2%；Fe2O3=40.6%~61.4%；FeO=31.1%~43.6%)，여태철광(TiO2=36.2%~50.8%；Fe2O3=2.5%~24.6%；FeO=33.0%~43.7%)경상정광물대공생혹출용태철광，자철광—태철광계산광물평형양일도(-logfo2)위12.4~22.8，약위NNO+1，비정상MORB양화정도고，지시암장결정시구유교고적수일도。기고석εHf(t)치위11.0~19.8，현시우손지만원구특정。암석지구화학분석현시기구유고적Al2O3(20.4%~22.4%)、TiO2(1.0%~1.5%)，저적MgO(4.1%~5.5%)함량유사고려현무암적특정。택당각섬휘장암가능유우손지만부분용융형성적함수암장경과연화(분리결정작용등)형성적저미고려유사고려현무암적암장결정형성。연이양품엄중우손경희토(LREE)，여상견우도호구적고려현무암불동，가능위사문석화적양각혹지만감람암재부충조기탈수사득지만설발생부분용융도치。대택당사록암중침입암급지만감람암진행경상세적조사연구장대게시택당사록암적형성배경구조배경제공경다적선색。
Zetang ophiolite is mainly composed of peridotite, gabbro and basalt. Hornblende gabbro, which was intruded into the peridotite, comprises amphibole, prehnite+pumpellyite and minor Fe-Ti oxides. The amphibole in hornblende gabbro is brown euhedral with homogeneous interference color and has higher Al2O3 (7.0%-11.0%), TiO2 (1.0%-2.5%), as well as Na2O (1.0%-1.9%) values than the amphibole of metasomatic genesis, implying that the amphibole is of primary magmatic origin. The Fe-Ti oxides consist of magnetite (TiO2=1.2%-13.2%; Fe2O3=40.6%-61.4%; FeO=31.1%-43.6%) and ilmenite (TiO2=36.2%-50.8%;Fe2O3=2.5%-24.6%; FeO=33.0%-43.7%). The ilmenites always exhibit trellis and sandwich textures or are closely spaced (composite type) in the magnetite host. The oxygen fugacities (expressed as-logfo2) of the Fe-Ti oxides calculated by ilmenite-magnetite geothermobarometry are 12.4-22.8 (approximately NNO+1), indicating that the presence of high water activities during crystallization. The positive εHf(t) values (+11.0 to +19.8) of the sample suggest that it came from depleted mantle (DM). The hornblende gabbro is characterized by high Al2O3 (20.4%-22.4%) and TiO2 (1.0%-1.5%), low MgO (4.1%-5.5%) values, similar to features of high-alumina basalt (HAB). The hornblende gabbro might have been formed by evolved mantle-derived hydrous magma rich in volatile, Ti and Al2O3. However, the sample shows depleted light rare earth element (LREE) patterns, which is different from features of HAB in the subduction zone. The rocks were probably formed by partial melting of mantle wedge induced by dehydration of serpentinized subducted oceanic lithosphere.