东喜马拉雅构造结泥质高压麻粒岩的锆石和独居石定年与地质意义

东喜马拉雅构造结的南迦巴瓦杂岩含有广泛分布的高压麻粒岩，但由于以前获得了许多不同的年龄，对这些麻粒岩的变质与深熔时代、持续时间和成因存在不同认识。本文对泥质高压麻粒岩（蓝晶石榴黑云片岩）中的锆石和独居石进行了系统的内部结构、U-（Th）-Pb定年和微量元素分析，以求揭示这些岩石是否具有相同的演化过程。所研究的6个蓝晶石榴黑云片岩由石榴石、蓝晶石、黑云母、石英、钾长石、斜长石、夕线石、白云母、石墨和副矿物金红石、钛铁矿、锆石和独居石组成，峰期矿物组合是石榴石+蓝晶石+斜长石+钾长石+黑云母+石英+金红石。6个样品中的锆石均由继承碎屑核+变质（深熔）幔+变质（深熔）边组成。其中3个样品中的锆石幔和边较宽，均可进行原位定年，幔部给出了类似的较老年龄范围（39.6~31.6Ma、40.8~32.0Ma和38.1~31.3Ma），而边部给出了类似的较年轻年龄范围（26.8~17.3Ma、28.3~18.6Ma和28.4~18.8Ma）。另外3个样品的锆石幔部较窄，不能进行分析，其边部给出了与前3个样品锆石边部类似的年轻年龄范围（22.0~17.0Ma、20.9~16.9Ma和22.2~16.6Ma）。一个片岩样品中的独居石给出了与其锆石幔部+边部年龄类似的较宽年龄范围（38.1~17.5Ma），而另外3个样品中的独居石获得了与其锆石边部年龄相似的年轻年龄范围（26.0~18.8Ma、22.3~16.9Ma和26.4~19.4Ma）。随着年龄的减小，锆石和独居石的Th/U比值增大，Eu/Eu^*减小，独居石的HREE和Y含量减小。基于这些分析结果，笔者认为所研究的6个片岩记录了相同的、从~41Ma持续到~17Ma的进变质与深熔过程。但是，由于某些样品中的锆石和独居石在早期变质和深熔过程中形成的结晶域（锆石幔部）很窄，无法定年，导致不同的样品获得了不同的年龄范围。结合现有研究成果，笔者推测南迦巴瓦杂岩中的高压麻粒岩经历了相似的长期进变质与深熔过程。

Zircon and monazite dating of pelitic high-pressure granulite in the Eastern Himalayan Syntaxis and geological significance

The Namche Barwa complex in the Eastern Himalayan Syntaxis contains widely distributed high-pressure granulites. However, due to many different ages previously obtained, there are different understandings on the metamorphic and anatectic age, duration and genesis of these granulites. In this paper, zircon and monazite in high-pressure pelitic granulite (kyanite garnet biotite schist) samples are systematically analyzed by internal structure, U-(Th)-Pb isotope and trace element, in order to reveal whether these rocks have the same evolution process. The six schists studied are composed of garnet, kyanite, biotite, K-feldspar, plagioclase, quartz, sillimanite, muscovite, graphite and accessory minerals rutile, ilmenite, zircon and monazite. The peak mineral assemblages are garnet+kyanite+plagioclase+K-feldspar+biotite+quartz+rutile. The zircons in the six samples are composed of inherited detrital core+metamorphic (anatectic) mantle+metamorphic (anatectic) rim. The zircon mantle and rim in three samples are sufficiently wide and can be dated in situ. The zircon mantles give a similar older age range of 39.6~31.6Ma, 40.8~32.0Ma and 38.1~31.3Ma, respectively, while the rims give similar younger age ranges of 26.8~17.3Ma, 28.3~18.6Ma and 28.4~18.8Ma, respectively. The zircon mantles of the other three samples are narrow and cannot be analyzed, while the zircon rims give young age ranges of 22.0~17.0Ma, 20.9~16.9Ma and 22.2~16.6Ma, respectively. Monazite in one schist sample gives a wide age range of 38.1~17.5Ma, which is similar to that obtained by zircon mantle+rim from the same sample. Monazite in the other three samples obtains young age ranges of 26.0~18.8Ma, 22.3~16.9Ma and 26.4~19.4Ma, respectively, which are similar to those given by the zircon rims from the same samples. With the decrease of age, Th/U ratio of zircon and monazite increases, Eu/Eu^* of zircon and monazite decreases, HREE and Y of monazite decrease. Based on these analytical results, we suggest that the six schists recorded the same and long-lasting prograde metamorphic and anatectic process from ca.41Ma to ca.17Ma. However, the domains of zircon and monazite in some samples formed during early metamorphism and anatexis are too narrow to date via the in-situ analysis method adopted by this study. Combined with the existing research results, we speculate that the high-pressure granulites in the Namche Barwa complex have experienced similar prolonged prograde metamorphism and anatexis.