CAJ | 학술논문

近期在羌塘中西部红脊山地区一套二叠纪蛇绿岩中首次发现了埃达克质岩石，呈脉状侵入到蛇绿岩中。对该套岩石进行了详细的岩石学、锆石U-Pb年龄和地球化学研究，重点讨论岩石成因及地质意义。埃达克质岩石岩性为白云母花岗岩，岩石中锆石晶形较完整，具典型的岩浆生长环带，结合Th/U值（0.22~0.63），表明其为典型的岩浆锆石。锆石LA-ICP-MS定年结果为271.7Ma±2.3Ma，表明其形成时代为早二叠世。岩石具有较高的SiO2、Al2O3含量和较低的K2O/Na2O值，为弱过铝质岩石。稀土元素配分模式呈右倾的曲线，重稀土元素分馏中等，Eu异常变化较小；微量元素Y、Yb含量较低，Sr含量中等，在微量元素蛛网图上表现出Rb、U、K正异常和Nb、Ta、P、Ti负异常。这与俯冲洋壳熔融形成的埃达克质岩石特征相似。红脊山埃达克质岩石是俯冲的古特提斯洋在角闪岩相-榴辉岩相条件下部分熔融形成的，残留相主要为石榴子石+角闪石。较早产生的熔体与地幔橄榄岩充分发生交代作用，后产生的熔体与地幔橄榄岩交代作用较弱，向上运移过程中斜长石发生分离结晶。红脊山埃达克质岩石的发现，表明古特提斯洋在早二叠世开始俯冲消减，大洋进入消退阶段。
근기재강당중서부홍척산지구일투이첩기사록암중수차발현료애체극질암석，정맥상침입도사록암중。대해투암석진행료상세적암석학、고석U-Pb년령화지구화학연구，중점토론암석성인급지질의의。애체극질암석암성위백운모화강암，암석중고석정형교완정，구전형적암장생장배대，결합Th/U치（0.22~0.63），표명기위전형적암장고석。고석LA-ICP-MS정년결과위271.7Ma±2.3Ma，표명기형성시대위조이첩세。암석구유교고적SiO2、Al2O3함량화교저적K2O/Na2O치，위약과려질암석。희토원소배분모식정우경적곡선，중희토원소분류중등，Eu이상변화교소；미량원소Y、Yb함량교저，Sr함량중등，재미량원소주망도상표현출Rb、U、K정이상화Nb、Ta、P、Ti부이상。저여부충양각용융형성적애체극질암석특정상사。홍척산애체극질암석시부충적고특제사양재각섬암상-류휘암상조건하부분용융형성적，잔류상주요위석류자석+각섬석。교조산생적용체여지만감람암충분발생교대작용，후산생적용체여지만감람암교대작용교약，향상운이과정중사장석발생분리결정。홍척산애체극질암석적발현，표명고특제사양재조이첩세개시부충소감，대양진입소퇴계단。
The adakitic rock was recently found in Permian ophiolite complex of the Hongji Mountain within central and western Qiangtang Block. It intruded into the Permian ophiolite in the form of veinlets. The authors studied petrology, zircon LA-ICP-MS U-Pb age and whole-rock geochemistry of the adakitic rock, with the emphasis placed on its petrogenesis and geological implications. The adakitic rock is muscovite granite. Its high Th/U (0.22~0.63) and clear zoned zircons indicate magamatic origin. Zircon LA-ICP-MS age of muscovite granite is 271.7Ma ± 2.3Ma, indicating that the crystallization of adakitic rock took place in Early Permian. The adakitic rock is characterized by high SiO2 and Al2O3 and low K2O/Na2O, thus belonging to aluminous-peraluminous granite. The REE model is characterized by enrichment of light REE, moderate fractionations in HREE and small Eu anomalies. The values of Y, Yb are low and the content of Sr is moderate. The primitive mantle normalized trace element spider patterns show that they are enriched in Rb, U, K and have negative anomalies of Nb, Ta, Ti. Geochemical characteristics are similar to features of the adakite formed by partial melting of the subducted oceanic crust. The Hongji Mountain adakitic rock was most probably generated by partial melting of the subducted Paleo-Tethys oceanic crust under the amphibolite-eclogite facies condition, whereas the residual facies were hornblende and garnet. The earlier adakitic melt sufficiently replaced the mantle peridotite, whereas the later melt weakly replaced mantle peridotite. The plagioclase experienced fractional crystallization during the upward migration of the melt. The discovery of Hongji Mountain adakitic rock indicates that the Paleo-Tethys oceanic crust subducted in Early Permian and entered into the regression stage.