中喜马拉雅日玛那地区退变榴辉岩岩石地球化学及年代学研究

喜马拉雅造山带是目前最年轻的仍处于碰撞阶段的陆陆碰撞造山带。高喜马拉雅结晶岩系内产出的（退变）榴辉岩记录了印度板块俯冲和折返的重要信息。本文对中喜马拉雅日玛那地区的（退变）榴辉岩进行了岩石学、全岩地球化学及锆石年代学研究，确定了其变质演化、原岩性质及构造背景。日玛那榴辉岩呈透镜状或岩墙状与花岗片麻岩接触，基于矿物组合识别出3个变质阶段：(1)峰期榴辉岩相：石榴子石+绿辉石+多硅白云母+金红石+石英；(2)高压麻粒岩相：以单斜辉石+斜长石后成合晶、黑云母+斜长石的后成合晶为代表；(3)角闪岩相：以基质中角闪石的大量出现为特征。锆石U-Pb定年给出两期年龄，峰期榴辉岩相锆石年龄为约15.5～14 Ma，具平坦的HREE分配模式，缺失Eu负异常，麻粒岩相的年龄约为12 Ma，锆石显著富集HREE，并呈现出Eu负异常的特征，指示石榴子石分解和斜长石生长。矿物温压计限定日玛那榴辉岩峰期变质条件为约730℃、1.9～2.1 GPa。榴辉岩记录了顺时针的变质p-T轨迹，从峰期榴辉岩相经近等温快速降压至麻粒岩相，最后降温降压至角闪岩相。另外，日玛那榴辉岩的原岩为亚碱性玄武岩并沿着拉斑玄武岩趋势演化，地...

Petrology, geochemistry and geochronology of granulitized eclogites from the Nyönno Ri region, central Himalaya

The Himalayan orogenic belt represents one of the youngest continent-continent collisional orogenic belts and the collision is still an ongoing process. Eclogites in the Greater Himalayan Crystalline complex contain important information about subduction and exhumation processes of the Indian plate. In this paper, we investigate the petrology, whole-rock geochemistry and zircon chronology of retrograde eclogites in the Nyönno Ri region, Central Himalaya. The objective of our study is elucidate the metamorphic evolution, nature of protolith and tectonic setting of these rocks. The eclogites are enclosed within granitic gneisses and occur as lens or bands. Three metamorphic stages are identified with different mineral assemblages:① peak eclogite-facies:garnet, omphacite, polysilicate muscovite, rutile, and quartz; ② high-pressure granulite-facies:symplectite of clinopyroxene and plagioclase, and symplectite of biotite and plagioclase; and ③ amphibolite-facies:amphibole in the matrix. Zircon grains of the peak eclogite-facies stage has an age of ~15.5~14 Ma and have chondrite-normalized REE patterns with flat HREE and without negative Eu anomalies, whereas zircon grains of the granulite-facies stage have an age of ~12 Ma and show significantly enriched HREE and negative Eu anomalies, indicative of garnet breakdown and plagioclase growth. From the mineral thermobarometry, the peak metamorphic conditions are estimated to be ~730℃ and 1.9~2.1 GPa. The eclogites have a clockwise metamorphic p-T path with the peak eclogite facies metamorphism followed by the near-isothermal rapid decompression, then the granulite-facies metamorphism. The final metamorphism was cooling and decompressing under the amphibolite-facies conditions. The eclogites in the Nyönno Ri region were formed from protoliths of alkaline basalts geochemically similar to mid-ocean ridge basalts (MORBs). Relict magmatic core of zircon grains has a protolith age of 890~850 Ma. In summary, we suggest that the eclogites in the Nyönno Ri region formed from Neoproterozoic MORBs that were subsequently subducted and metamorphosed under the eclogite-facies condition beneath the Eurasian continent at ~15.5 Ma, followed by exhumation.