Regional metamorphism is characterized by complicated mineral-mineral reactions and fluid-mineral interactions, which may promote elemental migration and ore genesis. A robust tracer of metamorphic reactions and fluid activities is a key for identifying material sources and quantifying mass flow along metamorphic pathways. Here, through examinations of not only bulk-rock nitrogen (N) isotopic variation along six prograde metamorphic zones but more importantly inter-mineral comparison of isotopic equilibration/disequilibration in the classic Barrovian metamorphic sequence in the Mica Creek area in the Canadian Cordillera, we show that N isotopes are a sensitive tracer for probing metamorphic devolatilization, mineral reaction and external fluid infiltration at various spatial scales from prograde to retrograde stages. In detail, our results show that, despite a relatively large variation in bulk-rock N/Al molar ratio, low-grade rocks (slates and phyllites) display a narrow δN range, which suggests relatively homogeneous N isotope compositions in their protoliths. Biotite-zone schists show reduced N/Al ratios with elevated δN values, which is consistent with devolatilization of N-depleted NH from rocks during prograde metamorphism at T < 500 °C. In contrast, samples from high-grade rocks (kyanite zone and sillimanite–K-feldspar zone) do not follow the metamorphic devolatilization trend but decrease in both N/Al ratios and δN values, indicating overprinting by a relatively large-scale flow (over at least tens of kilometers) of a N-depleted external fluid during the high-grade stage. The fluid was likely derived from the granitic magmas that intruded into these rocks during or slightly post peak metamorphism. The δN values of minerals in kyanite and sillimanite–K-feldspar zones displays negative correlations between biotite and both muscovite and plagioclase, with high δN values close to isotope equilibrium but low δN values at isotope disequilibrium with muscovite and plagioclase. These negative correlations can be best explained by a kinetic isotopic effect associated with metamorphic reactions that consume muscovite and plagioclase to produce biotite. This suggests that plagioclase may be more involved in the metamorphic reactions in pelitic rocks than previously assumed. The associated kinetic isotopic effect resulted in a ΔN > 5 ‰, much larger than the theoretically predicted equilibrium N isotope fractionation factor (ΔN < 0.5 ‰ at T > 350 °C) but close to the large N isotope fractionation (ΔN ≈ 8 ‰) recently observed in field samples. Our data also imply that plagioclase is susceptible to retrograde alteration. Overall, combined N signatures of bulk rocks and coexisting minerals can provide multiple lines of constraints on mineral reactions and fluid activities from prograde to retrograde metamorphism.

中文翻译：

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氮同位素作为区域变质作用中流体活动和矿物反应的有力示踪剂


区域变质作用具有复杂的矿物-矿物反应和流体-矿物相互作用的特点，可能促进元素迁移和成矿。变质反应和流体活动的强大示踪剂是识别物质来源和量化沿变质路径的质量流量的关键。在这里，不仅通过检查沿着六个顺变质带的块岩氮（N）同位素变化，而且更重要的是通过加拿大科迪勒拉山脉云母溪地区经典巴罗夫变质序列中同位素平衡/不平衡的矿物间比较，我们结果表明，N 同位素是一种敏感的示踪剂，可用于探测从顺行到逆行阶段的不同空间尺度的变质脱挥发分、矿物反应和外部流体渗透。具体而言，我们的结果表明，尽管块岩 N/Al 摩尔比变化相对较大，但低品位岩石（板岩和千枚岩）显示出较窄的 δN 范围，这表明其原岩中的 N 同位素组成相对均匀。黑云母带片岩表现出 N/Al 比降低，δN 值升高，这与 T < 500 °C 进行变质作用过程中岩石中贫氮 NH 的脱挥发分一致。相比之下，高品位岩石（蓝晶石带和硅线石-钾长石带）的样品不遵循变质脱挥发分趋势，但 N/Al 比和 δN 值均下降，表明相对较大规模的流动（超过在高品位阶段，至少有数十公里）的贫氮外部流体。这种流体很可能来自于变质作用高峰期间或稍晚时期侵入这些岩石的花岗岩岩浆。 蓝晶石和硅线石-钾长石带矿物的 δN 值在黑云母与白云母和斜长石之间表现出负相关性，在接近同位素平衡时 δN 值较高，而在与白云母和斜长石同位素不平衡时 δN 值较低。这些负相关性可以通过与消耗白云母和斜长石产生黑云母的变质反应相关的动力学同位素效应来最好地解释。这表明斜长石可能比以前假设的更多地参与泥质岩的变质反应。相关的动力学同位素效应导致 ΔN > 5 ‰，远大于理论预测的平衡 N 同位素分馏因子（T > 350 °C 时 ΔN < 0.5 ‰），但接近大 N 同位素分馏（ΔN ≈ 8 ‰）最近在野外样本中观察到。我们的数据还表明斜长石容易发生逆行改变。总体而言，大块岩石和共存矿物的组合氮特征可以为从顺变质到逆变质的矿物反应和流体活动提供多种限制。