Geological Society of London
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Contact metamorphism and partial melting of Dalradian pelites and semipelites in the southern sector of the Etive aureole

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posted on 2016-06-21, 12:08 authored by Giles T. R. Droop, Mohssen Moazzen

Contact-metamorphic mineral assemblages are described from Dalradian non-graphitic pelites and semipelites in the southern sector of the Etive thermal aureole, adjacent to the Quarry Diorite ring intrusion. Six metamorphic zones were mapped on the basis of incoming index minerals: I, Biotite; II, Cordierite; III, Andalusite+K-feldspar; IV, Corundum; V, Spinel; VI, Sillimanite. Garnet and orthopyroxene occur locally in high-grade quartz-bearing assemblages. Rocks of the Biotite and Cordierite zones retain their regional-metamorphic textures to a large extent, particularly S2 crenulation cleavage. Above the Andalusite+K-feldspar isograd the rocks are hornfelses, but relict features of S2 persist. Rocks of the upper Spinel and Sillimanite zones are agmatic/stromatic migmatites which preserve evidence of local partial melting in the form of S2-parallel leucosomes and thin fracture-filling vein leucosomes. All leucosomes are K-feldspar-rich. Vein leucosomes consist mainly of leucogranite with a metaluminous composition and, in the Sillimanite Zone, locally contain crystals of peritectic orthopyroxene and/or cordierite.

The compositions of coexisting minerals, determined by EPMA, were used to constrain metamorphic reactions and, with equilibrium thermodynamic calculations, to refine estimates of the P-T conditions of metamorphism. Isograd reactions can generally be modelled satisfactorily in the CNKFMASH system as an isobaric sequence at P=2.2 kbar. An exception is the spinel isograd which occurs at anomalously low T due to the partitioning of Zn into spinel. Estimated isograd temperatures for the And+Kfs, Crn, Spl, melt and Sil isograds are 610°C, 630°C, 660°C, 680°C and 705°C, respectively. Calculated dehydration equilibria yield low values of water activity in the Sillimanite Zone, confirming that partial melting in the innermost aureole occurred under fluid-absent conditions. Leucosome textures and structures suggest that the siliceous microlithons of the former S2 differentiated crenulation cleavage were the likely sites of melting and provided channels for the movement of melt into brittle fractures to form the vein leucosomes.