俯冲过程中反应越级对(U)HP岩石P-T路径的约束西片麻岩地区(挪威)的例子

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2022-06-10 DOI:10.1111/jmg.12680
Shah Wali Faryad, Josef Ježek, Jan Kulhánek
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引用次数: 5

摘要

本文介绍了北峡湾西片麻岩区高压-超高压(U)HP)变质岩中石榴石的岩石学观测和扩散模拟结果。含蓝晶石云母岩中的石榴石保存了与前加里东期麻粒岩相和加里东期榴辉岩相变质事件有关的两代石榴石生长。在云母片岩中形成透镜体的基性榴辉岩,在角闪岩相条件下仅含有部分再结晶的榴辉岩相组合。云母岩和赋矿榴辉岩中的加里东期石榴石在550℃/2.0 GPa附近或以上均显示出反应越界和成核。根据榴辉岩相事件的拟剖面模型计算,最大压力和温度分别为~2.6 GPa和650℃。在云母岩中,前加里东期和加里东期石榴石的界面显示出强烈的成分梯度或可能的成分跳跃。这种梯度的保存以及加里东期榴辉岩石榴石的丘状成分剖面表明,岩石在其峰值温度条件下没有松弛或有短时间的松弛,以及通过冷却将其挖掘出来。或许,通过等温减压加热或挖掘岩石可以很容易地改变石榴石剖面上的这种成分不规则性。考虑到从最大深度和峰值温度到地表的温度和运输距离变化与石榴石模型计算的时间(3.5 Ma)成正比,得出HP岩石的冷却速率为~187°C/Ma,垂直方向的挖掘速率为~2.5 cm/年。
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Constraining the P–T path of (U)HP rocks with reaction overstepping during subduction; example from the Western Gneiss Region (Norway)

This paper presents the results of petrological observations and diffusion modelling on garnet from high-pressure to ultrahigh-pressure ((U)HP) metamorphic rocks of the Western Gneiss Region in the Nordfjord. Garnet from kyanite-bearing micaschist preserves two generations of garnet growth that are related to the Pre-Caledonian granulite facies and Caledonian eclogite facies metamorphic events. Mafic eclogite, forming lenses in the micaschist, contains only eclogite facies assemblages with partial recrystallization under amphibolite facies conditions. Caledonian garnet in both the micaschist and hosting eclogite indicates reaction overstepping and nucleation near or above 550°C/2.0 GPa. Maximum pressure and temperature, calculated using pseudosection modelling for the eclogite facies event, were ~2.6 GPa and 650°C. The interface between the Pre-Caledonian and Caledonian garnet in the micaschist shows a strong compositional gradient or possibly a compositional jump. The preservation of such a gradient together with the hummocky-shaped composition profiles in the Caledonian garnet from the eclogite indicates either no relaxation or a short-time of relaxation of the rocks at their peak temperature conditions, as well as their exhumation by cooling. Possibly, heating or exhuming of the rocks by isothermal decompression could have easily modified such compositional irregularities along the garnet profiles. A cooling rate of ~187°C/Ma and exhumation rate in the vertical direction of ~2.5 cm/year for the HP rocks were obtained by considering that the temperature and transport distance changes from their maximum depth and peak temperature to the surface were proportional to the time (3.5 Ma) calculated by modelling for the garnet.

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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
期刊最新文献
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