Handling editor: Doug Robinson Abstract The Blåhø Nappe on the island of Fjørtoft, which represents an isolated portion of the Seve Nappe Complex in the Western Gneiss Region, Norway, has been suggested to have experienced two deep burial cycles during the Caledonian orogeny. However, evidence on this multiple burial process by the derivation of a pressure–temperature– time (P–T–t) path has never been given in the literature. In this study, the ‘diamondiferous’ kyanite–garnet gneiss from the Blåhø Nappe on Fjørtoft was revisited to determine if such a process was correct. Two types of garnet, porphyroblastic garnet1 and finegrained garnet2, were recognized in the gneiss. The core of garnet1 is poor in Ca and documents P–T conditions of 1.2–1.3 GPa at c. 880°C based on pseudosection modelling. The inner rims of garnet1 and the core of garnet2 are both richer in Ca, recording P–T conditions of 1.35–1.45 GPa and 770–820°C. Application of conventional geothermobarometry on the outer rim of garnet1 and the rim of garnet2 yielded retrograde P–T conditions of 0.75–0.90 GPa and 610–685°C. These estimates define an anticlockwise P–T path at pressures below 1.5 GPa. Accessory monazite was dated with the electron microscope. Relicts of detrital monazite in the gneiss point to Sveconorwegian and possibly also Cryogenian provenance for the detritus of the sedimentary protolith. Metamorphic monazite in the gneiss records a wide age range from 460 to 380 Ma, with a peak c. 435 Ma and a shoulder at 395 Ma. These data suggest that the original (Ediacaran?) Baltica margin sediment (gneiss protolith) was transported to the base of an overlying plate during the early Caledonian (preScandian) orogeny. A long residence time of the metasedimentary rock at this base caused its heating to 880°C and homogenization of the early garnet chemistry. The late Caledonian (Scandian) collision between Baltica and Laurentia led to further burial, during which the studied gneiss was close to the former surface of the downgoing continental plate and, thus, cooled. The reconstructed P–T–t path confirms the multiple burial history of the Blåhø Nappe but contradicts previous ideas of deep burial of the Fjørtoft gneiss to more than 100 km.

中文翻译：

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来自挪威西部片麻岩地区 Fjørtoft 岛的混合片麻岩在高压、高温条件下的逆时针 PTt 路径表明加里东造山运动期间的两次埋藏事件

处理编辑：Doug Robinson 摘要 Fjørtoft 岛上的 Blåhø Nappe 代表了挪威西部片麻岩地区 Seve Nappe 复合体的一个孤立部分，据推测在加里东造山运动期间经历了两次深埋循环。然而，文献中从未给出通过推导压力-温度-时间（P-T-t）路径来证明这种多重埋藏过程的证据。在这项研究中，我们重新审视了 Fjørtoft 上 Blåhø Nappe 的“含金刚石”蓝晶石-石榴石片麻岩，以确定这样的过程是否正确。在片麻岩中发现了两种类型的石榴石，斑纹石榴石 1 和细粒石榴石 2。garnet1 的核心在 Ca 中较差，并且在 c 处记录了 1.2-1.3 GPa 的 P-T 条件。880°C 基于假截面模型。石榴石 1 的内缘和石榴石 2 的核心都富含 Ca，记录的 P-T 条件为 1.35-1.45 GPa 和 770-820°C。在石榴石 1 的外缘和石榴石 2 的外缘应用常规地热气压测量法产生了 0.75-0.90 GPa 和 610-685°C 的逆行 P-T 条件。这些估计定义了压力低于 1.5 GPa 时的逆时针 P-T 路径。附件独居石用电子显微镜测年。片麻岩中的碎屑独居石残渣指向 Sveconorwegian 和可能还有 Cryogenian 的沉积原岩碎屑的出处。片麻岩中的变质独居石记录的年龄范围很广，从 460 到 380 Ma，峰值为 c。435 Ma 和 395 Ma 的肩峰。这些数据表明，原始（埃迪卡拉纪？）波罗的海边缘沉积物（片麻岩原岩）在加里东早期（前斯堪地亚）造山运动期间被输送到上覆板块的底部。变质沉积岩在该基地的长时间停留导致其加热至 880°C 并使早期石榴石化学均质化。波罗的海和劳伦西亚之间的晚加里东（斯堪的亚）碰撞导致了进一步的掩埋，在此期间所研究的片麻岩靠近下降的大陆板块的前表面，因此冷却。重建的 P-T-t 路径证实了 Blåhø Nappe 的多次埋藏历史，但与之前将 Fjørtoft 片麻岩深埋 100 公里以上的想法相矛盾。变质沉积岩在该基地的长时间停留导致其加热至 880°C 并使早期石榴石化学均质化。波罗的海和劳伦西亚之间的晚加里东（斯堪的亚）碰撞导致了进一步的掩埋，在此期间所研究的片麻岩靠近下降的大陆板块的前表面，因此冷却。重建的 P-T-t 路径证实了 Blåhø Nappe 的多次埋藏历史，但与之前将 Fjørtoft 片麻岩深埋 100 公里以上的想法相矛盾。变质沉积岩在该基地的长时间停留导致其加热至 880°C 并使早期石榴石化学均质化。波罗的海和劳伦西亚之间的晚加里东（斯堪的亚）碰撞导致了进一步的掩埋，在此期间所研究的片麻岩靠近下降的大陆板块的前表面，因此冷却。重建的 P-T-t 路径证实了 Blåhø Nappe 的多次埋藏历史，但与之前将 Fjørtoft 片麻岩深埋 100 公里以上的想法相矛盾。