Contact metamorphism associated with mafic intrusives is one of several mechanisms that has been invoked to produce extensive high‐temperature (HT) metamorphism and associated partial melting of the crust. Indisputable evidence for polymetamorphism in these settings can be difficult to decipher because both melt loss and retrogression (i.e. rehydration) can erase or obscure the records of earlier HT metamorphism by modifying HT mineral parageneses and compositions. Here, a combination of detailed field and petrographical observations, inverse mineral thermometry, and thermodynamic forward modelling is used to delineate the polymetamorphic history of migmatites from the Smith River Allochthon (SRA) in the central Appalachians. Bulk rock geochemical data suggest that some metapelitic samples lost a significant amount of melt during interpreted contact metamorphism with the Rich Acres gabbro, resulting in a residual bulk composition (<50 wt% SiO₂, ~30 wt% Al₂O₃). Garnet cores (Grt1) in SiO₂‐depleted samples are interpreted to grow during this HT contact metamorphism, with Fe‐Ti oxide thermometry on spinel inclusions in Grt1, cordierite–garnet thermometry, and thermodynamic forward modelling constraining peak P–T conditions during contact heating of the migmatites to ~800ºC and ∼0.5 GPa. This is associated with an inferred peak assemblage prior to melt loss of crd+kfs+pl+grt+bt+spl (mag+usp+hc)+ilm+sil+qtz+melt. Garnet in SiO₂‐depleted samples has a distinct high‐Ca rim (Grt2), which appears to record a younger metamorphic event. A combination of substantial melt loss and later rehydration appears to be a major control on the ability of SiO₂‐depleted samples to faithfully record evidence for this polymetamorphism. The tectonic implications of this younger metamorphic event are not entirely clear, but it appears to record renewed burial and heating of the SRA sometime after the Taconic orogeny, which may be related to either the neo‐Acadian or Alleghanian orogenies.

中文翻译：

---

高T变质岩中的多变质作用：以阿巴拉契亚中部为例

与镁铁质侵入体相关的接触变质作用是被调用来产生广泛的高温（HT）变质作用和相关的地壳部分熔融的几种机制之一。在这些情况下，多态性的无可争辩的证据可能很难破译，因为熔体损失和逆行（即再水化）都可以通过修饰HT矿物亚型和组成而消除或掩盖早期HT变质的记录。在这里，结合了详细的现场和岩相学观测，逆矿物测温和热力学正演模型，描绘了阿巴拉契亚中部史密斯河异位岩（SRA）的辉贵岩的多变质历史。₂，〜30 wt％Al ₂ O ₃）。SiO ₂贫化样品中的石榴石芯（Grt1）被解释为在这种HT接触变质过程中生长，通过对Grt1中尖晶石夹杂物的Fe-Ti氧化物测温，堇青石-石榴石测温和限制接触过程中峰值P–T条件的热力学正演模拟将蒙脱石加热到〜800ºC和〜0.5 GPa。这与在crd + kfs + pl + grt + bt + bt + spl（mag + usp + hc）+ ilm + sil + qtz + melt熔体损失之前推断的峰组装有关。SiO _2中的石榴石耗尽的样本具有独特的高Ca边缘（Grt2），似乎记录了一个年轻的变质事件。大量熔体损失和随后的再水化相结合似乎是对SiO ₂贫化样品忠实记录该多态性证据的能力的主要控制。这个年轻的变质事件的构造意义尚不完全清楚，但它似乎记录了在塔科尼克造山运动之后的某个时间，SRA的重新埋葬和加热，这可能与新阿卡迪期或阿勒格罕造山运动有关。