Inhaltszusammenfassung

Mineralogical and mineral chemical evidence for prograde metamorphism is rarely preserved in rocks that have reached ultrahigh-temperature (UHT) conditions (> 900 degrees C) because high diffusion and reaction rates erase evidence for earlier assemblages. The UHT, high-pressure (HP) metasedimentary rocks of the Leverburgh belt of South Harris, Scotland, are unusual in that evidence for the prograde history is preserved, despite having reached temperatures of similar to 955 degrees C or more. ...Mineralogical and mineral chemical evidence for prograde metamorphism is rarely preserved in rocks that have reached ultrahigh-temperature (UHT) conditions (> 900 degrees C) because high diffusion and reaction rates erase evidence for earlier assemblages. The UHT, high-pressure (HP) metasedimentary rocks of the Leverburgh belt of South Harris, Scotland, are unusual in that evidence for the prograde history is preserved, despite having reached temperatures of similar to 955 degrees C or more. Two lithologies from the belt are investigated here and quantitatively modelled in the system NaO-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O: a garnet-kyanite-K-feldspar-quartz gneiss (X-Mg = 37, A/AFM = 0.41), and an orthopyroxene-garnet-kyanite-K-feldspar quartzite (X-Mg = 89 A/AFM = 0.68). The garnet-kyanite gneiss contains garnet porphyroblasts that grew on the prograde path, and captured inclusion assemblages of biotite, sillimanite, plagioclase and quartz ( 84, consistent with its very rare occurrence in nature. The preservation of unusual high P-T mineral assemblages and chemical disequilibrium features in these UHT HP rocks is attributed to a rapid tectonometamorphic cycle involving arc subduction and terminating in exhumation. » weiterlesen» einklappen

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Klassifikation

DFG Fachgebiet:
3.42 - Geologie und Paläontologie

DDC Sachgruppe:
Geowissenschaften