{"title":"Constraining the P–T path of (U)HP rocks with reaction overstepping during subduction; example from the Western Gneiss Region (Norway)","authors":"Shah Wali Faryad, Josef Ježek, Jan Kulhánek","doi":"10.1111/jmg.12680","DOIUrl":null,"url":null,"abstract":"<p>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.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":null,"pages":null},"PeriodicalIF":3.5000,"publicationDate":"2022-06-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"5","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of Metamorphic Geology","FirstCategoryId":"89","ListUrlMain":"https://onlinelibrary.wiley.com/doi/10.1111/jmg.12680","RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"GEOLOGY","Score":null,"Total":0}
引用次数: 5
Abstract
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.
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.