拉萨地块东南缘晚白垩世高温变质作用及其地质意义:来自石英出溶金红石的证据和锆石U-Pb定年

Late Cretaceous high-temperature metamorphism in the southeastern Lhasa terrane: Evidence from rutile exsolutions in quartz and zircon LA-ICP-MS dating

文中对拉萨地块东南缘林芝杂岩中的含榴斜长角闪岩进行了详细的岩相学研究和锆石U-Pb定年。岩相学观察表明,含榴斜长角闪岩经历了峰期麻粒岩相变质和角闪岩相退变质作用。峰期麻粒岩相矿物组合为石榴子石+高Ti角闪石+紫苏辉石+斜长石+石英+金红石,其中的石榴子石、石英和角闪石中含有大量金红石出溶体,说明这些矿物的初始成分具有高Ti含量。角闪岩相退变质矿物组合为低Ti角闪石+斜长石+斜黝帘石+石英+金红石。利用Ti在石英中的含量(TitaniQ)温度计计算得到峰期麻粒岩相变质温度为803～924℃,后期角闪岩相退变质温度为555～732℃。样品中的锆石具有明显的核边结构,核部为典型岩浆型锆石,具有高的Th/U值,强烈富集HREE,明显的正Ce异常和负Eu异常,206Pb/238U年龄为(89.3±0.6)Ma,代表含榴斜长角闪岩原岩结晶年龄。锆石边部呈无环带结构,同核部相比,具有低的Th/U值,低的M-HREE和弱的Eu负异常,为变质峰期生长的锆石,206Pb/238U年龄为(81.1±0.8)Ma,代表麻粒岩相峰期变质年龄。我们认为约81Ma高温麻粒岩相变质作用可能与洋中脊俯冲造成的板片窗导致的软流圈上涌有关。

In this study,we present detailed petrographic study and LA-ICP-MS zircon U-Pb dating for a garnet-bearing amphibolite from the Nyingchi Complex in the southeastern Lhasa terrane.Petrographic study indicates that the garnet-bearing amphibolite had undergone two stages of peak granulite-facies and retrograde amphibole-facies metamorphism.The peak mineral assemblage is characterized by Grt＋high-Ti Amp ＋ Hy＋Pl＋Qtz＋Rt.There are a large number of rutile exsolutions in the garnet,quartz and amphiboles,suggesting that the primary composition of these minerals have high Ti contents.The retrograde amphibole-facies assemblage is characterized by low-Ti Amp＋Pl＋Czo＋Qtz＋Rt.Based on the Ti-in-quartz（TitaniQ） thermometers,the peak and retrograde metamorphic temperatures are 803-924 ℃ and 555-732 ℃,respectively.The zircons in the garnet-bearing amphibolite show core-rim structure in CL images.The cores are characterized by high Th/U ratios,enrichment in HREE,a positive Ce anomaly and a marked negative Eu anomaly,suggesting that they are typical of magmatic zircon.They yielded a weighted 206Pb/238U mean age of 89.3±0.6 Ma,which is interpreted as the crystallization age of the protolith of garnet-bearing amphibolite.By contrast,the rims exhibit no zoning.They have low Th/U ratios and M-HREE contents,and show weak or no Eu anomaly,indicating that they formed in the peak metamorphic stage.The 206Pb/238U age of 81.1±0.8 Ma obtained on these rims is interpreted as the peak metamorphic age.Combined with regional studies,we suggest that the about 81 Ma high-temperature metamorphism resulted from the upwelling of asthenosphere through the slab window opened as a result of ridge subduction.