Nd isotope re-equilibration during high temperature metamorphism across an orogenic belt: Evidence from monazite and garnet
Nd isotope re-equilibration during high temperature metamorphism across an orogenic belt: Evidence from monazite and garnet
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造山带高温变质作用期间的 Nd 同位素再平衡：来自独居石和石榴石的证据
DOI：
10.1016/j.chemgeo.2020.119751
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发表时间：
2020-06
期刊：
影响因子：
3.9
通讯作者：
Hui Cao
中科院分区：
文献类型：
--
作者：
Da Wang;Christopher M. Fisher;Jeffrey D. Vervoort;Hui Cao
独居石是一种有效的U-Th-Pb地质年代学仪器，常用于确定造山过程的克里思。独居石作为一种轻稀土富集矿物，记录了构造活动中Sm-Nd同位素体系行为的重要信息。然而，独居石的Sm-Nd同位素研究受到的关注远远少于其作为U-Pb地质年代计的应用。通过对华北克拉通胶辽冀造山带部分熔融变质沉积岩中独居石的U-Pb年龄和Sm-Nd同位素组成的研究，为揭示高温变质和地壳熔融过程中的Sm-Nd同位素行为提供依据。我们利用几个互补的定年系统，在同一个样品，包括石榴石Lu-Hf和Sm-Nd，锆石U-Pb和独居石U-Pb年代学，以获得精确的时间约束的P-T演化的JLJB。石榴石Lu-Hf等时线年龄为1.97-1.94 Ga，代表石榴子石生长，与石榴子石变质锆石年龄1.96-1.90 Ga一致;石榴石Sm-Nd年龄与退变质锆石年龄一致，独居石U-Pb年龄定义了一个相对较窄的1.86-1.83 Ga年龄范围，代表退变冷却。缺乏较老的独居石颗粒相匹配的锆石或石榴石的日期表明，预先存在的独居石溶解成部分熔融。所有样品中的独居石，无论是作为石榴石包裹体还是在基质中出现，都在~1.85 Ga处产生均匀的Nd同位素组成（εNd~ −5），这与石榴石和块体岩石值一致。因此，在高温变质作用（~750 °C）期间，大块岩石和矿物之间的Nd同位素再平衡在整个造山带同步发生，其中涉及熔融，没有变质前同位素特征的证据。这表明在高级变质作用和熔融过程中Sm-Nd同位素系统的流动性，独居石的Nd同位素特征可能代表了最近的构造活动。
Monazite is a powerful U-Th-Pb geochronometer commonly used to determine the tempo of orogenic processes. As a light rare earth element (LREE)-enriched mineral, monazite also records key information for understanding the behavior of Sm-Nd isotope systematics during tectonothermal events. However, Sm-Nd isotope studies of monazite have received far less attention than its use as a U-Pb geochronometer. Here we investigate coupled U-Pb ages and Sm-Nd isotopic compositions of monazite from partially melted metasedimentary rocks in the Jiao-Liao-Ji orogenic belt (JLJB), North China craton, to provide insight into Sm-Nd isotope behavior during high-temperature metamorphism and crustal melting. We utilize several complementary dating systems in the same sample, including garnet Lu-Hf and Sm-Nd, zircon U-Pb, and monazite U-Pb geochronology, to gain precise time constraints on the P-T evolution of the JLJB. Garnet Lu-Hf isochron dates of 1.97–1.94 Ga are interpreted to represent prograde garnet growth, consistent with the prograde metamorphic zircon dates of 1.96–1.90 Ga; garnet Sm-Nd dates agree with retrograde metamorphic zircon dates, and all monazite U-Pb dates define a relatively narrow age range of 1.86–1.83 Ga, interpreted to represent retrograde cooling. The lack of older monazite grains matching zircon or garnet dates suggests the dissolution of pre-existing monazites into partial melt. Monazite in all samples, irrespective of occurring as garnet inclusions or in the matrix, yields homogenous Nd isotope compositions (εNd~ −5) at ~1.85 Ga, which are consistent with the garnet and bulk-rock values. Thus, Nd isotope re-equilibration between bulk-rock and minerals occurs synchronously across the orogenic belt during high-T metamorphism (~750 °C) where melting was involved, with no evidence of a pre-metamorphic isotope signature. This suggests mobility in the Sm-Nd isotope system during high-grade metamorphism and melting, where the Nd isotope signature of monazite likely represents the most recent tectonothermal event.
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影响因子：
3.8
作者：
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