雅鲁藏布蛇绿岩中的变质底板:快速俯冲起始与折返

Metamorphic soles from the Yarlung-Tsangbo ophiolites:Rapid subduction initiation and exhumation

蛇绿岩中的变质底板通常是俯冲起始的关键记录,并且对探究蛇绿岩的形成机制至关重要。最近在我国西藏南部雅鲁藏布蛇绿岩中陆续发现了高温变质底板。本文对雅鲁藏布蛇绿岩白朗剖面采集到的变质底板样品(基性麻粒岩)进行了系统的岩石学,副矿物U-Pb年代学以及模拟研究,包含相耦合的相平衡、石榴石扩散以及热动力学数值模型。岩石学观察表明,基性麻粒岩记录了四期变质阶段。在新鲜的麻粒岩样品中,石榴石核部保留了早期进变质阶段的矿物包裹体,尽管石榴石的核部成分在峰期发生了再平衡。基质矿物组合连同石榴石的核部成分代表了岩石的峰期变质阶段。退变质阶段由石榴石边部成分以及后成合晶矿物组合所记录。绝大多数的变质底板样品遭受了后期亚绿片岩相交代作用。相平衡模拟表明岩石经历了顺时针变质轨迹,并记录了约1.6~1.7GPa和829~863℃的峰期条件以及约0.9~1.1GPa和823~923℃的近等温减压退变质条件。利用相关系对交代样品中亚绿片岩相的矿物组合进行了温压估计,得到了306~324℃/<0.3GPa的条件。榍石的U-Pb年代学以及温度计算结果表明变质底板的进变质年龄为132.4±4.8Ma和133.9±5.9Ma。金红石记录了126.8±6.2Ma和132.3±3.1Ma的后期冷却年龄,与榍石年龄在不确定度内重叠。先前以及本文年代学数据表明变质底板从原岩形成到进变质并随后折返的时间尺度非常短。石榴石扩散和热动力学模型计算也与年代学结果相吻合,整个变质过程模拟的持续时间仅为2.6~2.8Myr,并且折返发生非常迅速,速率为6.7cm/yr。结合变质底板以及上覆蛇绿岩所记录的数据,本文认为雅鲁藏布蛇绿岩的形成机制可能十分复杂,并不是单一的洋中脊环境或俯冲带相关环境,并强调一些俯冲起始前形成的洋中脊玄武岩作为变质底板参与了深俯冲循环并很快导致俯冲相关的岩石形成,这些俯冲相关岩石与那些未俯冲的洋中脊岩石一起形成了现今的雅鲁藏布蛇绿岩。

Metamorphic soles within ophiolites are typically considered as the primary evidence of subduction initiation and are crucial for understanding the formation mechanism of ophiolites.Recently,high-temperature metamorphic soles have been discovered in the Yarlung-Tsangbo ophiolites from southern Tibet.In this study,metamorphic sole samples(mafic garnet-bearing granulite)collected from the Bairang section of the Yarlung-Tsangbo ophiolites were systematically analyzed using petrology,U-Pb geochronology of accessory minerals,and coupled modeling techniques,including phase equilibrium,garnet diffusion,and thermo-mechanical models.Petrological observations reveal that the mafic granulite records four stages of metamorphism.In the fresh granulite samples,the mineral inclusions in the garnet core represent the early metamorphic residues,despite the composition of the garnet core being re-equilibrated at the peak stage.The matrix mineral assemblages and the garnet core compositions represent the peak metamorphic stage,while the retrograde metamorphic stage is recorded by garnet rims and symplectites.Most of the samples underwent late metasomatism at sub-greenschist facies.Phase equilibrium modeling indicates that the rock experienced a clockwise P-T path,recording P-T conditions of ca.1.6~1.7GPa and 829~863℃at the peak stage,and subsequent near-isothermal decompression conditions of ca.0.9~1.1GPa and 823~923℃.The temperature and pressure of 306~324℃/<0.3GPa of the sub-greenschist assemblages were estimated using phase relations.U-Pb geochronology and temperature calculation results of titanite indicate that the age at the prograde stage of the metamorphic sole is 132.4±4.8Ma and 133.9±5.9Ma.Rutile recorded late cooling ages of 126.8±6.2Ma and 132.3±3.1Ma,which overlap with the titanite age within uncertainty.The geochronological data suggests that the time scale of metamorphic soles from protolith formation to subduction and exhumation is very short.The garnet diffusion and thermo-mechanical model calculations are also consistent with the geochronological results.The modeling of the entire metamorphic process is only 2.6~2.8Myr,and exhumation occurs at a very fast rate of 6.7cm/yr.The results of this study,in conjunction with previous data on the overlying ophiolites,suggest that the formation of the Yarlung-Tsangbo ophiolites is extremely complex and may involve a combination of mid-ocean ridge and subduction-related environments.We highlight that some initial mid-ocean ridge basalts are involved in a cycle of deep subduction,generation of subduction-related melts,and the final formation of supra-subduction-zone rocks.These subduction-related rocks,along with the mid-ocean ridge basalts that were not subducted,form the present Yarlung-Tsangbo ophiolites.