通化集安群石榴子石和黑云母微区化学成分特征及其对变质作用p—T—t轨

集巡群片麻岩中石榴子石和黑云母接触带两侧边缘广泛发育Ｆｅ－Ｍｇ交换反应所形成的扩散环带,但晶体内部各自保持变质峰期的成分,中温带石榴子石可保留生长环带。     

柴达木盆地北缘大柴旦和锡铁山榴辉岩中石榴子石环带对比及地质意义

大柴旦榴辉岩和锡铁山榴辉岩同为柴达木盆地北缘高压—超高压变质带的组成部分,但它们的岩相学特征、形成的PT条件及榴辉岩中石榴子石中保存的成分环带均具有明显的差异。大柴旦榴辉岩峰期变质的PT条件为T=620～730℃,P=2.3～2.8GPa,岩相学研究表明其经历了近等温降压或降温降压的后榴辉岩相历史,保存有进变质的矿物组合和石榴子石的生长环带及增生结构。相反,锡铁山榴辉岩的榴辉岩相的温度较高,岩相学研究显示其具有明显的后榴辉岩相的高温麻粒岩相的叠加,PT轨迹显示压力峰期早于温度峰期,具有略升温降压的后榴辉岩相变质历史,榴辉岩中石榴子石的成分环带表现为扩散环带,没有生长环带保留。大柴旦榴辉岩和锡铁山榴辉岩的不同的石榴子石环带特征反映了两者在形成过程中,特别是折返过程中不同的构造热历史。     

喜马拉雅造山带片麻岩中石榴子石的多期生长

石榴子石是高级变质作用的重要矿物之一,能够保留成分环带和不同变质阶段的矿物或流体包裹体,为示踪寄主岩石经历的变质演化历史提供重要信息。本文对采自雅拉香波片麻岩穹窿内高级变质岩中石榴子石进行了详细的包裹体成分、主量元素环带和微量元素特征的研究,揭示出石榴子石黑云母片麻岩至少记录了五期岩浆或变质热事件。第Ⅰ期石榴子石为来源于源区的岩浆型石榴子石。第Ⅱ期、第Ⅲ期和第Ⅳ期石榴子石为变质型石榴子石,但不同期次变质作用的温压条件和生长介质、矿物组合不同。第Ⅴ期石榴子石为转熔型石榴子石,是黑云母脱水熔融形成,记录了喜马拉雅造山过程早期加厚地壳条件下的深熔作用。喜马拉雅造山带变质岩中石榴子石具有复杂的成因机制和演化历史,在应用石榴子石进行变质作用研究时,需要仔细甄别,否则会得到错误的结论。     

拉萨地块吉朗榴辉岩的岩石学研究及其对古特提斯洋壳俯冲折返过程的限定

拉萨地块东部松多(超)高压榴辉岩记录了古特提斯洋俯冲及折返过程。松多榴辉岩带已发现松多、新达多、白朗和吉朗4个榴辉岩出露区,它们的峰期温压条件及变质p-T轨迹的研究对揭示拉萨地块古特提斯时期的俯冲及折返过程有重要意义。松多榴辉岩带东段吉朗榴辉岩的主要矿物为石榴子石、绿辉石、多硅白云母、角闪石、金红石、绿帘石、石英以及退变形成的后成合晶结构(透辉石+角闪石+斜长石)和少量的黑云母。石榴子石具有含丰富矿物包裹体的"脏"核和极少包裹体的"净"边,具有典型的进变质成分环带特征,从核部到边部镁铝榴石组分升高,锰铝榴石和钙铝榴石组分降低。石榴子石边部发育窄的角闪石+斜长石(An=28)组成的冠状体,表明石榴子石边部发生了后期角闪岩相退变质作用。通过变质相平衡模拟计算得到石榴子石以及多硅白云母记录的峰期温压条件为563℃、2. 4 GPa。结合岩相学特征,确定吉朗榴辉岩经历了4期变质演化阶段:(1)进变质阶段以石榴子石核部及其包裹体为代表性矿物组合;(2)峰期变质阶段矿物组合为石榴子石边部、绿辉石、多硅白云母、蓝闪石、硬柱石、金红石和石英;(3)早期退变质阶段以硬柱石分解产生绿帘石为特征;(4)晚期退变质阶段以绿辉石发育后成合晶和石榴子石生长冠状体为特征。认为吉朗榴辉岩为典型的低温高压榴辉岩,经历了顺时针p-T演化轨迹,折返过程为近等温降压过程。与松多带内其他(超)高压岩石相比,吉朗榴辉岩峰期温压条件较低,其围岩为变石英岩,区别于区内其他(超)高压榴辉岩的石榴子石白云母片岩及蛇纹岩围岩。推测吉朗榴辉岩来自于俯冲带浅部,由俯冲隧道中低密度沉积物裹挟折返。     

东昆仑早古生代变质演化:来自浪木日石榴斜长角闪岩的记录SCIEI北大核心CSCD

东昆仑造山带近年来被厘定为早古生代高压-超高压变质带。带内广泛出露早古生代的中-高级变质基性岩,这些岩石记录了不同的变质温压和多期的变质年龄,是反演和制约东昆仑早古生代变质演化的重要样品。本文选取东昆仑浪木日地区的石榴斜长角闪岩为研究对象,开展了变质岩石学及锆石年代学研究。石榴斜长角闪岩呈团块状出露在黑云二长片麻岩中,主要组成矿物为石榴子石、角闪石、斜长石、透辉石和石英,含少量黑云母、绿泥石、金红石、钛铁矿和榍石。石榴子石变斑晶的核部含有绿帘石、角闪石、斜长石、金红石和石英包裹体,其成分从核部到边部,锰铝榴石逐渐降低、钙铝榴石和Mg/(Mg+Fe;)比值逐渐升高,为进变质作用形成的环带。岩石中的矿物结构关系和成分特征显示其经历了进变质、峰期变质和退变质三个阶段的变质演化,变质温压分别为:T≈610℃和P≈6.5kbar、T≈700℃和P≈10.5kbar以及T≈650℃和P≈4.5kbar。这三阶段的变质作用构成顺时针的变质P-T轨迹,指示岩石经历进变质升温升压至峰期阶段,随后经历近等温降压的退变质阶段。同时该P-T轨迹特征表明岩石形成于俯冲-碰撞的构造背景。对石榴斜长角闪岩中的锆石进行SIMS U-Pb定年,得到492.8±5.1Ma的谐和年龄。锆石的形态特征与典型的变质锆石一致,其内包裹的石榴子石、角闪石和斜长石组合与岩石的峰期矿物组合一致。因此,锆石在峰期变质阶段结晶,所测年龄~493Ma为角闪岩相峰期变质年龄。本文研究的石榴斜长角闪岩与该区高压-超高压榴辉岩在野外产状、P-T轨迹和变质年龄等方面密切相关,暗示ca.490Ma是该区高压-超高压变质作用的一个重要时间节点。石榴斜长角闪岩和榴辉岩之间的变质差异,表明东昆仑早古生代经历了多阶段的变质作用,不同岩石记录了原特提斯洋俯冲-碰撞过程的不同阶段。本文获得的变质P-T轨迹和变质年龄可为进一步探究东昆仑早古生代高压-超高压变质作用提供限定。     

东疆哈尔里克变质带中石榴子石的特征

石榴子石为东疆哈尔里克变质带中的常见矿物。从外形上可分为两种 类型,一种是晶形发育较好的石榴子石,为断裂区域变质作用形成;另一种是破碎状、裂纹 发育的石榴子石,为早期热变质作用形成,呈残留状出现。二者在空间上共存。对于晶形较 好的石榴子石的化学成分研究表明,石榴子石发育生长环带,在晶体粒径与核部MnO的含量 之间存在着相关关系。其峰期MnO的含量反映了递增变质带变质温度变化的趋势。     

安徽安庆铜铁矿床矽卡岩岩相学和矿物学特征及意义

安庆铜铁矿床系长江中下游铁铜成矿带中一典型矽卡岩型矿床.矿体产于月山岩体与下三叠统南陵湖组碳酸盐岩之间的接触带,外接触带矽卡岩中透辉石富集,以铁铜矿化为主;内接触带矽卡岩中石榴子石富集,以铜矿化为主.岩相学研究表明研究区含铜矽卡岩演化经历了矽卡岩期和热液蚀变期,其中,矽卡岩期包括早期矽卡岩阶段、磁铁矿阶段和晚期矽卡岩阶段;热液蚀变期包括早期热液交代阶段、石英-硫化物阶段和石英-碳酸盐阶段.大规模的黄铜矿化发生于石英-硫化物阶段.矿物学研究表明,石榴子石均为钙铁榴石,早期矽卡岩阶段的粒状石榴子石发育韵律环带,其FeO和Al2O3含量表现为振荡变化.与粒状石榴子石相比,晚期矽卡岩阶段脉状石榴子石的And组分更高.早期矽卡岩阶段的粒状辉石为透辉石,具有环带结构,由核部到边部MgO含量减少,FeO含量增加;晚期矽卡岩阶段的脉状辉石为钙铁辉石.空间上,从外接触带到内接触带,辉石的MgO含量与石榴子石的Al2O3含量分别减少,而FeO含量均分别增加.岩相学、矿物学,结合已有地球化学研究成果综合表明,安庆铜铁矿矽卡岩为岩浆热液接触交代成因,Mg来自碳酸盐岩,由外带向内带迁移;Fe来自岩浆热液,由内带向外带迁移,由于磁铁矿阶段的温压条件改变,Fe在外接触带以磁铁矿的形式沉淀富集.     

松潘—甘孜造山带中丹巴地区变质岩演化及其地质意义

丹巴地区位于松潘—甘孜造山带中部。区域动热变质作用主要发生于印支晚期—喜马拉雅早期,与深层滑脱-逆冲作用有关。根据特征矿物组合．丹巴地区变质岩可划分出六个变质带：绢云母—绿泥石带、黑云母带、石榴子石带、十字石带、蓝晶石带和夕线石带。石榴子石生长环带代表每段时间矿物晶体边部的平衡,可用于推测石榴子石生长时的P－t轨迹。利用石榴子石—黑云母温度计和石榴子石—斜长石—Al2SiO5-石英压力计得到石榴子石的参考温度、压力,根据环带定量计算得到的变质作用p-t轨迹为顺时外形式,与其所处的构造背景为大陆碰撞造山带是一致的。     

南秦岭增生杂岩带马道地区变泥质岩的变质作用

摘要：南秦岭构造带出露于勉略断裂和虞关—留坝断裂之间,是一条复杂的增生杂岩带,也是秦岭造山带的重要组成部分。增生杂岩带内马道地区发育一套由黑云母片麻岩、片岩组成的变泥质岩,内部包含有石英岩、大理岩及超基性岩等岩块,构成了典型的“block in matrix”结构。选取了含石榴子石黑云母片麻岩样品进行详细的岩石学研究。结果显示,北部变质岩样品中的石榴子石具有弱退变质成分环带,利用岩石矿物组合中的石榴子石-黑云母温度计、石榴子石-黑云母-斜长石-石英组合温度-压力计,估算峰期压力为078~079 GPa,温度为705~707 ℃,退变质时期压力为064~076 GPa,温度为602~650 ℃,揭示出岩石峰期高角闪岩相变质后,经历降温减压过程。南部岩石样品中含有特征的十字石+蓝晶石组合,样品中的石榴子石具有进变质成分环带,其峰期压力为049~057 GPa,温度为553~562 ℃,相当于低角闪岩相。通过与其他典型增生杂岩带变质岩的剥露机制对比,认为马道变泥质岩的变质作用演化与南秦岭地区碰撞作用有关,而其剥露过程则主要受到双重逆冲构造控制。     

苏鲁造山带威海地区石榴角闪岩中“环状”石榴子石的成因

发育特征结构的石榴子石是研究俯冲带矿物演化和元素迁移的理想对象。本文首次报道苏鲁造山带东北端威海地区出露的含"环状"石榴子石的石榴角闪岩,并对其开展了详细的岩相学、矿物化学、锆石SHRIMP U-Pb年代学和变质作用研究。X射线主元素扫面显示,"环状"石榴子石经历了3个阶段生长:最高Ca部分的成核阶段、相对低Ca部分的扩展阶段和最低Ca部分的快速塑形阶段,分别对应M_1、M_2和M_3阶段变质作用,相应的矿物组合为石榴子石高Ca环核部及其矿物包裹体(如角闪石+富Na斜长石+金红石+绿帘石+磷灰石)、石榴子石低Ca环内边及其后成合晶矿物(如角闪石+富Ca斜长石边部+钛铁矿±金红石±磷灰石)和最低Ca石榴子石环外边±角闪石±钛铁矿,未发现超高压变质矿物或假象。传统地质温压计估算出M_1和M_2阶段的变质P-T条件分别为620～740℃/6.8～10.4kbar和705～775℃/5.3～7.1kbar,而M_3阶段可能发生于温度稍高和压力稍低的P-T范围。其中M_1至M_2阶段,发生的变质反应主要为角闪石+绿帘石+富Na斜长石→石榴子石+角闪石+富Ca斜长石,随着反应物绿帘石、斜长石和角闪石被消耗,石榴子石Ca逐渐降低,Mg有所升高或变化不明显; M_2至M_3阶段,变质反应物绿帘石和斜长石几乎被消耗完全,主要消耗角闪石中的Mg、Fe和重稀土元素(HREE),使其形成"钟形"稀土配分模式,而使M_3阶段生长的石榴子石环边极度贫Ca、富Mg和Fe,且HREE含量升高。变质锆石U-Pb定年结果为232.9±2.2Ma,代表该样品受到三叠纪陆-陆俯冲碰撞过程的强烈响应。此外,M3阶段是形成石榴子石"环形"结构的重要阶段,可能受变质温度、粒间流体和俯冲带外部流体渗透作用共同影响,加快元素在石榴子石中的迁入和迁出,且外部流体可为石榴子石快速生长提供充足的Mg和HREE。综上所述,"环状"石榴子石记录了相对完整的生长过程,整体形成于角闪岩相变质条件,为非超高压变质产物。     

赣中周潭群石榴石、斜长石和黑云母微区化学成分特征及其地球动力学意义

周潭群变质岩中石榴石、斜长石和黑云母微区化学成分变化明显，石榴石变斑晶具典型的生长环带，由晶体中心向两侧边缘XMg、XFe值以光滑曲线递增，XCu、XMn值以光滑曲线递减，反映其增温过程；晶体最边缘的化学成分反映变质峰期的温度条件。通过石榴石变斑晶生长环带剖面分析，应用Grt-Bi温度计和GASP压力计，确定本区变质作用PT轨迹为顺时针形式，发生于大陆碰撞造山带环境。     

The microarea composition of garnet and biotite from the Ji’an Group in Tonghua district and its significance as an indicator of metamorphicP-T-t path

The garnet (Grt) and biotite (Bt) from gneisses of the Ji’an Group are characterized by diffusion zoning at the rim, but equilibrium composition of metamorphic peak is usually remained in extensive interior area. Garnet with growth zoning is also found in the kyanite zone. In the light of microarea compositional variation of Grt and Bt, the temperature and pressure at the progressive, peak and post-peak metamorphic stages are determined by correctly using Grt-Bt thermometer and GASP barometer. On this basis, a counterclockwiseP-T-t path can be constructed, which reflects the closing process of an ensialic rift belt in this region during the Early Proterozoic. The project was financially supported by the State Educational Commission Ph. D. Station Foundation (No. 96018702).     

赣中周潭群变质岩中石榴石的演化史及其地质意义

周潭群变质岩系中石榴石、十字石和黑云母微区化学成分变化明显,石榴石变斑晶具典型的生长环带,由晶体中心向两侧边缘XMg,XFe值递增,XCa,XMn值递减,反映增温过程,晶体最边缘的化学成分反映变质峰期的温度条件。变质岩系经历了角闪岩相变质作用,划分为3个变质阶段。各阶段石榴石的分布、形态、矿物组合、化学成分及其成因条件存在一定差异。通过成因矿物学的系统研究,确定本区变质岩系石榴石的结晶生长演化史及变晶变形序列。     

石榴石钇(Y)元素电子探针分析及应用——以佛子岭石榴云母片岩为例SCIEI北大核心CSCD

石榴石是最重要的造岩矿物之一,通常能够保留早期的矿物结构和物质并记录较为晚期的变形和变质反应。石榴石钇(Y)元素环带特征丰富、复杂,不同的环带特征通常暗示不同的形成环境或经历了不同的变质事件,是变质演化历史研究的重要媒介之一。以往的研究中,多以LA-ICP-MS作为石榴石Y元素的主要分析手段,EPMA主要用于主量元素的分析。但是,LA-ICP-MS的束斑尺寸(44μm)和基底效应较EPMA(0~5μm)大,当石榴石颗粒小、包体和裂隙发育或成分环带以微区尺寸内存在较大变化时,大束斑更容易覆盖某些特殊信息。通过对石榴石Y元素测试参数的调试和标样验证,最终确定峰位测试时长和背景测试时长分别为140s和70s,并进行了PHA谱峰干扰剥离,降低检测限至54×10^(-6)。本文将通过对比佛子岭石榴云母片岩(LD025)4颗石榴石的EPMA主、微量原位分析(Ca、Mg、Mn、Fe、Y、Al、Si、Cr、Ti、Na)和LA-ICP-MS石榴石Y元素分析结果,论证EPMA分析Y元素的可行性。石榴石X-ray Mapping和主量成分剖面揭示该4颗石榴石均为生长环带,Mn呈钟形分带,Y与X Sps呈强烈正相关性,与X Grs、X Alm、X Prp相关性不清晰。EPMA和LA-ICP-MS分析结果显示Y含量曲线在核部和幔部具有良好的一致性,Grt1~Grt3中Y均表现出自核部(500×10^(-6)~1200×10^(-6))向幔部(200×10^(-6)~500×10^(-6))逐渐降低,极边部Y含量低(20×10^(-6)~200×10^(-6))且变化复杂;Grt4中Y含量差异相对较小(180×10^(-6)~450×10^(-6)),仅在边部出现不同程度的升降。因EPMA对于Y元素含量较低(<200×10^(-6))时灵敏度不够或者LA-ICP-MS束斑尺寸大容易掩盖边部窄带成分真实变化等原因,二者在边部Y元素差异较大。分别对EPMA和LA-ICP-MS的分析结果应用Grt-Xtm温度计和Grt单矿物压力计获得的变质PT结果显示Grt1~Grt3(核-幔-边)和Grt4(核-边)均记录较为完整、统一的温压演化过程。M1→M2→M3的变质温压变化分别为T=530~544℃、P=0.78~0.82GPa→T=577~616℃、P=0.89~0.98GPa→T=631~661℃、P=1.01~1.07GPa,表现为顺时针演化型式,M1至M3反映的是一个“暖俯冲”过程。根据温度评价结果,Grt1~Grt3(1.2~1.4mm,自形程度高)形成时间应早于Grt4(0.8mm,自形程度低)。由此可知,大颗粒的石榴石Y元素含量及变化特征通常更容易揭示相对完整的变质演化历史。本次研究为变泥质岩演化历史、变质温压评价等研究提供了不同视角和思路,结合EPMA主量(矿物成分、X-ray mapping、BSE分析)和微量元素(Y等)分析能够更加精准、全面地解读地质信息。     

Petrology and metamorphism of khondalites from the Jining complex,North China craton

The basement of the North China craton (NCC) can be divided into eastern and western blocks separating the Trans-North China orogen on the basis of petrologic associations, structures, metamorphic processes, and isotopic ages. Aluminous gneiss khondalites occur in the western block, and record a clockwise metamorphic P–T history characterized by nearly isothermal decompression following peak metamorphism at ca. 1.3 GPa and 825°C. Four metamorphic stages are recognized based on mineral assemblages. The early prograde metamorphic assemblage contains Ky+Bt+Ms+Grt+Pl+Qtz. The peak metamorphic mineral assemblage is characterized by Grt+Sil+Bt+Kfs+Pl+Qtz and the formation of cordierite after garnet, leading to a retrograde assemblage of Grt+Sil+Crd+Pl+Kfs+Qtz. Garnet retrogrades to biotite and the formation of pervasive matrix muscovite define a final metamorphic stage, inferred at ca. < 0.6 GPa and 700°C. Quantified metamorphic stages and a related clockwise P–T path derived from pseudosection analysis in the KMASH system suggest collision of the north Yinshan block with the South Ordos block at 1.92 Ga, before final suturing of the entire NCC basement.     

Genetic Mechanism of Mineral Inclusions in Zircons from the Khondalite Series, Southeastern Inner Mongolia

The early Precambrian khondalite series is widely distributed in the Jining-Zhuozi-Fengzhen-Liangcheng area, southeastern Inner Mongolia. The khondalite series mainly consists of sillimanite garnet potash feldspar (or two-feldspar) gneiss and garnet biotite plagioclase gneiss. These gneissic rocks have commonly experienced granulite-facies metamorphism. In zircons separated from sillimanite garnet potash feldspar gneisses, many mineral inclusions, including Sil, Grt, Ky, Kfs, Qtz and Ap, have been identified by the Laser Raman spectroscopy. Generally, prograde metamorphic mineral inclusion assemblages such as Ky + Kfs + Qtz + Ap and Ky + Grt + Kfs + Qtz are preserved in the core of zircon, while peak granulite-facies metamorphic minerals including Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap are identified in the mantle and rim of the same zircon. However, in some zircons are only preserved the peak metamorphic minerals such as Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap from core to ri     

Constraining the conditions of Barrovian metamorphism in Sikkim,India: P–T–t paths of garnet crystallization in the Lesser Himalayan Belt

Garnet crystallization in metapelites from the Barrovian garnet and staurolite zones of the Lesser Himalayan Belt in Sikkim is modelled utilizing Gibbs free energy minimization, multi‐component diffusion theory and a simple nucleation and growth algorithm. The predicted mineral assemblages and garnet‐growth zoning match observations remarkably well for relatively tight, clockwise metamorphic P–T paths that are characterized by prograde gradients of ～30 °C kbar^?1 for garnet‐zone rocks and ～20 °C kbar^?1 for rocks from the staurolite zone. Estimates for peak metamorphic temperature increase up‐structure toward the Main Central Thrust. According to our calculations, garnet stopped growing at peak pressures, and protracted heating after peak pressure was absent or insignificant. Almost identical P–T paths for the samples studied and the metamorphic continuity of the Lesser Himalayan Belt support thermo‐mechanical models that favour tectonic inversion of a coherent package of Barrovian metamorphic rocks. Time‐scales associated with the metamorphism were too short for chemical diffusion to substantially modify garnet‐growth zoning in rocks from the garnet and staurolite zones. In general, the pressure of initial garnet growth decreases, and the temperature required for initial garnet growth was reached earlier, for rocks buried closer toward the MCT. Deviations from this overall trend can be explained by variations in bulk‐rock chemistry.     

Growth zoning of garnet porphyroblasts: Grain boundary and microtopographic controls

Chemical zoning in the outer few 10s of microns of garnet porphyroblasts has been investigated to assess the scale of chemical equilibrium with matrix minerals in a pelitic schist. Garnet porphyroblasts from the Late Proterozoic amphibolite facies regional metamorphic mica schists from Glen Roy in the Scottish Highlands contain typical prograde growth zoning patterns. Edge compositions have been measured via a combination of analysis of traverses across the planar edges of porphyroblast surfaces coupled to X-ray mapping of small areas within polished thin sections at the immediate edge of the porphyroblasts. These approaches reveal local variation in garnet composition, especially of grossular (Ca) and almandine (Fe) components, with a range at the edge from <7 mol.% grs to >16 mol.% grs, across distances of less than 50 µm. This small-scale patchy compositional zoning is as much variation as the core–rim compositional zoning across the whole of a 3 mm porphyroblast. Ca and Fe heterogeneity occurs on a scale suggesting a combination of inefficient diffusive exchange across grain boundaries during prograde growth and the evolving microtopography of the porphyroblast surface control garnet composition. The latter creates haloes of compositional zoning adjacent to some inclusions, which typically extend from the inclusion towards the porphyroblast edge during further growth. The lack of a consistent equilibrium composition at the garnet edge is also apparent in the internal zoning of the porphyroblast and so processes occurring during entrapment of some mineral inclusions have a profound influence on the overall chemical zoning. Garnet compositions and associated zoning patterns are widely used by petrologists to reconstruct P–T–t paths for crustal rocks. The evidence of extremely localized (10–50 µm scale) equilibrium during growth further undermines these approaches.     

Polymetamorphism in high‐T metamorphic rocks: An example from the central Appalachians

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.