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Department of Geology, University College of Wales, Aberystwyth, Dyfed SY23 3DB, UK
A number of previous studies of the possible causes of Archaean high-grade metamorphism have suggested that the heat source was large bodies of (intermediate) magma emplaced within pre-existing crust. Geochronological data for the Lewisian indicate that there was probably a gap of c. 200 Ma between the cessation of significant igneous activity and the peak of high-grade metamorphism. This is sufficiently long to allow conductive dissipation of magmatic heat and to render a model of the metamorphism relying upon heat transported in by intrusions unsatisfactory. The quantity of deep-derived CO2 that would be required to make it the primary cause of the metamorphism is larger than petrological considerations allow. Alternative models, based on enhanced basal heat-flow in a static crust or heat derived from the decay of radionuclides resident within the crust coupled with a more normal mantle heat-flow and tectonic crustal-thickening have been erected, and reveal that the P-T conditions and age data recorded by the Scourian granulites can be satisfied by such models.
Thermal models of this nature, and a consideration of the available petrological and geochronological data, suggest a very slow cooling rate which would allow considerable intercrystalline diffusion to occur. This could give rise to the observed zoning profiles and other forms of re-equilibration found within the granulite-facies rocks of the Lewisian complex.
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  References Geological Society, London, Memoirs, 2002; 26: 73 - 76. [PDF]
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