Abstract Garnet is a prominent recorder of metamorphic processes and plays a pivotal role in deciphering the pressure-temperature evolution of metamorphic rocks. However, the potential of regional metamorphic garnet for laser ablation U–Pb dating remains unexplored, as variable and low U ( We applied LA-ICP-MS U–Pb dating to garnet and zircon, monazite, and xenotime from The Straits Schist, western Connecticut, USA, to constrain the timing of garnet growth within the overall Acadian-age metamorphic history recorded by this unit. We assessed the effects of U-rich inclusions on garnet U–Pb data using high-resolution microbeam sampling, grain size analysis, and calculations assessing how inclusion size and abundance affect U concentrations measured in garnet. When analysing garnet, our results demonstrate that by scrutinizing the time-resolved U-signal, U-rich inclusions can be identified and effectively removed during data evaluation. If U-rich inclusions are unaccounted for, ‘garnet’ analyses have elevated U contents compared to pure garnet and in a Tera-Wasserburg diagram, data points are shifted to the left, right, or along the garnet regression line, for inclusions older, younger or of the same age as the garnet, respectively. Garnet from two metapelitic samples yielded lower intercept ages of 409 ± 10 Ma and 406 ± 15 Ma, and a combined U–Pb age of 406.1 ± 8.3 Ma. Metamorphic zircon yielded age clusters at 446.2 ± 4.2 Ma, 413.3 ± 3.0 Ma, and 382.9 ± 2.8 Ma, monazite ages range between 390 and 355 Ma, and xenotime yielded 370.1 ± 5.6 Ma. The ∼446 Ma age peak is attributed to detrital zircon formed during the waning stages of the Taconian orogeny. Early Acadian metamorphism is recorded by garnet and ∼413 Ma zircon, whereas monazite and zircon ages of 390 to 383 Ma, coincide with peak metamorphic conditions. Early Carboniferous monazite and xenotime ages could reflect the incursion of fluids into The Straits Schist during Neo-Acadian metamorphism, coeval plutonism in New England, or breakdown of garnet during regional exhumation.

中文翻译：

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探索区域变质石榴石的激光烧蚀 U-Pb 测年——美国康涅狄格州海峡片岩

摘要 石榴石是变质过程的重要记录者，在破译变质岩的压力-温度演化方面起着举足轻重的作用。然而，区域变质石榴石用于激光烧蚀 U-Pb 测年的潜力仍未探索，因为可变和低 U（我们将 LA-ICP-MS U-Pb 测年应用于来自西部海峡片岩的石榴石和锆石、独居石和磷钇矿美国康涅狄格州，为了在该装置记录的整个阿卡迪亚时代变质历史中限制石榴石生长的时间。我们使用高分辨率微束采样、晶粒尺寸分析评估了富含 U 的包裹体对石榴石 U-Pb 数据的影响，和计算评估包裹体大小和丰度如何影响石榴石中测量的 U 浓度。在分析石榴石时，我们的结果表明，通过仔细检查时间分辨 U 信号，可以在数据评估期间识别并有效去除富含 U 的内含物。如果没有考虑富含 U 的内含物，与纯石榴石相比，“石榴石”分析会提高 U 含量，并且在 Tera-Wasserburg 图中，数据点向左、向右或沿石榴石回归线移动，对于更古老的内含物，分别比石榴石更年轻或同龄。来自两个变泥岩样品的石榴石产生了较低的截距年龄 409 ± 10 Ma 和 406 ± 15 Ma，以及 406.1 ± 8.3 Ma 的联合 U-Pb 年龄。变质锆石产生的年龄簇分别为 446.2±4.2 Ma、413.3±3.0 Ma 和 382.9±2.8 Ma，独居石年龄范围在 390-355 Ma，磷钇矿产生的年龄范围为 370.1±5.6 Ma。约 446 Ma 年龄峰值归因于 Taconian 造山运动减弱阶段形成的碎屑锆石。石榴石和~413 Ma 锆石记录了早期阿卡迪亚变质作用，而独居石和锆石年龄为 390 至 383 Ma，与峰值变质条件一致。早期石炭纪独居石和磷钇矿年龄可以反映流体在新阿卡迪亚变质作用、新英格兰的同期深成岩作用或区域挖掘过程中石榴石分解期间侵入海峡片岩。