Abstract 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 initial garnet nucleation, 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 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 indicates 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 (eNd ~ −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.

中文翻译：

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造山带高温变质过程中钕同位素再平衡：来自独居石和石榴石的证据

摘要 独居石是一种功能强大的 U-Th-Pb 年代计，常用于确定造山过程的节奏。作为一种富含轻稀土元素 (LREE) 的矿物，独居石还记录了了解构造热事件期间 Sm-Nd 同位素系统行为的关键信息。然而，独居石的 Sm-Nd 同位素研究受到的关注远不如将其用作 U-Pb 测时仪。在这里，我们研究了华北克拉通胶辽冀造山带 (JLJB) 部分熔融变沉积岩中独居石的耦合 U-Pb 年龄和 Sm-Nd 同位素组成，以深入了解高温度变质作用和地壳熔化。我们在同一样本中使用了几种互补的测年系统，包括石榴石 Lu-Hf 和 Sm-Nd、锆石 U-Pb 和独居石 U-Pb 年代学，获得关于 JLJB 的 PT 演变的精确时间限制。1.97-1.94 Ga 的石榴石 Lu-Hf 等时线日期被解释为代表初始石榴石成核，与 1.96-1.90 Ga 的顺变质锆石日期一致；石榴石 Sm-Nd 日期与逆行变质锆石日期一致，并且所有独居石 U-Pb 日期都定义了 1.86-1.83 Ga 的狭窄年龄范围，解释为代表逆行冷却。缺少与锆石或石榴石日期匹配的较老的独居石颗粒表明先前存在的独居石溶解到部分熔体中。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，都会在 ~1.85 Ga 处产生均匀的 Nd 同位素组成 (eNd ~ -5)，这与石榴石和大块岩石的值一致。因此，在涉及熔融的高温变质作用（~750°C）期间，整个造山带同步发生大块岩石和矿物之间的 Nd 同位素重新平衡，没有任何变质前同位素特征的证据。这表明 Sm-Nd 同位素系统在高品位变质作用和熔融过程中具有流动性，其中独居石的 Nd 同位素特征可能代表了最近的构造热事件。