Repeated Caledonian burial and ultrafast cooling and exhumation of high‐pressure granulite facies rocks from the Blåhø Nappe on the island of Fjørtoft, Western Gneiss Region, Norway
Repeated Caledonian burial and ultrafast cooling and exhumation of high‐pressure granulite facies rocks from the Blåhø Nappe on the island of Fjørtoft, Western Gneiss Region, Norway
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DOI：
10.1111/jmg.12710
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发表时间：
2023-01
影响因子：
3.4
通讯作者：
Penglei Liu;H. Massonne
中科院分区：
文献类型：
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作者：
Penglei Liu;H. Massonne
位于挪威西部片麻岩地区 Nordøyane 超高压 (UHP) 域的异地 Blåhø Nappe 是研究大陆-大陆碰撞带地质过程的窗口，但有关其构造变质演化的许多方面仍然存在争议和难以捉摸。在本论文中，对来自 Fjørtoft 岛 Blåhø Nappe 的两块高压 (HP) 麻粒岩相岩石进行了一项综合研究，包括石榴石中的主量元素和微量元素分带、石榴石中的相平衡建模和阳离子扩散模拟。这些结果为了解大陆-大陆碰撞带中作用并最终形成碰撞造山带的复杂地球动力学过程提供了新的线索。包裹在石榴石中的多硅白云母、黑云母、角闪石、黝帘石-铜榴石和低 Zr 金红石可能证明所研究的岩石存在渐进榴辉岩相变质作用。此阶段的压力-温度 (P-T) 条件为 ~1.5-1.6 GPa 和 615-670°C。随后在 ~1.5 GPa 和 925 ± 50°C 的峰值 HP 麻粒岩相条件下进行广泛的再平衡。随后，岩石被冷却并重新埋藏至榴辉岩相条件为 ~1.8–1.9 GPa 和 805–825°C。随后进行最后阶段的减压和冷却，达到约 650–780°C 和 0.5–1.0 GPa 的角闪岩相条件。根据石榴石中阳离子扩散的模拟，估计该阶段的冷却和折返速率分别为 >400°C/Ma 和 >75 km/Ma，表明温度和压力超快下降。这里获得的逆时针 P-T 路径相对完整，并且与加里东造山运动期间的重复埋藏历史兼容，但不符合为 Blåhø Naappe 提出的超高压条件。我们的模型提出，后来形成 Blåhø Nappe 的岩石被埋藏在约 55 公里的较低地壳深度，相当于早期加里东造山运动期间地温梯度约为 13°C/km。随后将这些岩石加热至高压麻粒岩相条件可能是由板片断裂和热地幔上升流驱动的。斯堪的纳维亚大陆与大陆碰撞期间波罗的海的逆冲作用冷却并将布拉霍纳佩输送到更深的地方。获得的冷却和折返速率表明超快折返，可能是在折返通道中。
The allochthonous Blåhø Nappe in the Nordøyane ultra high pressure (UHP) domain, Western Gneiss Region in Norway, acts as a window to examine geological processes occurring in continent–continent collisional zones, but many aspects regarding its tectonometamorphic evolution remain debated and elusive. In this contribution, an integrated study including major‐ and trace‐element zoning in garnet, phase equilibrium modelling and the simulation of cation diffusion in garnet was conducted on two high‐pressure (HP) granulite facies rocks from the Blåhø Nappe on the island of Fjørtoft. The results shed new light on the complex geodynamic processes that act in continent–continent collisional zones and finally shape collisional orogens. Phengite, biotite, amphibole, zoisite‐allanite and low‐Zr rutile enclosed in garnet likely attest to a prograde eclogite facies metamorphism for the studied rocks. Pressure–temperature (P–T) conditions of ~1.5–1.6 GPa and 615–670°C were retrieved for this stage. An extensive re‐equilibration under peak HP granulite facies conditions of ~1.5 GPa and 925 ± 50°C followed. Subsequently, the rocks were cooled and reburied to eclogite facies conditions of ~1.8–1.9 GPa and 805–825°C. This was followed by a final stage of decompression and cooling to amphibolite facies conditions of ~650–780°C and 0.5–1.0 GPa. Cooling and exhumation rates of >400°C/Ma and >75 km/Ma, respectively, indicating an ultrafast temperature and pressure decrease are estimated for this stage from simulations of cation diffusion in garnet. The anticlockwise P–T path obtained here is relatively complete and compatible with a repeated burial history during the Caledonian orogeny but not with UHP conditions proposed for the Blåhø Nappe. Our model proposes that the rocks later forming the Blåhø Nappe were buried to lower crustal depths of approximately 55 km equating to a geothermal gradient of ~13°C/km during the early Caledonian orogeny. Subsequent heating of these rocks to HP granulite facies conditions was likely driven by slab break‐off and hot mantle upwelling. Baltica underthrusting during the Scandian continent–continent collision cooled and transported the Blåhø Nappe to greater depths. The obtained cooling and exhumation rates indicate ultrafast exhumation, presumably in an exhumation channel.