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

Abstract

SynopsisContact-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 S 2 crenulation cleavage. Above the Andalusite+K-feldspar isograd the rocks are hornfelses, but relict features of S 2 persist. Rocks of the upper Spinel and Sillimanite zones are agmatic/stromatic migmatites which preserve evidence of local partial melting in the form of S 2 -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 S 2 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.

Variations in fluid activity across the Etive thermal aureole, Scotland: evidence from cordierite volatile contents

“…The emplacement temperature of the Quarry Diorite was estimated to be around 1000 °C (Moazzen & Droop, 2005). The ensuing contact metamorphism produced an extensive, 2‐km‐wide, thermal aureole which was delineated on the basis of mapping metamorphic index minerals (Droop & Moazzen, 2007). The effects of contact metamorphism are particularly evident within the predominantly pelitic Leven Schist and pelitic units of the Islay Quartzite to the south of the complex in the vicinity of Loch Awe.…”

“…The most abundant rocks in the southern aureole are pelitic and semi‐pelitic hornfelses, which preserve systematic changes in the mineral assemblages from Qtz + Ms ± Bt ± Chl (abbreviations from Kretz, 1983) up to a variety of high‐grade assemblages involving, progressively, cordierite (zone II), andalusite + K‐feldspar (zone III), corundum (zone IV), spinel (zone V) and sillimanite (zone VI) (Moazzen et al. , 2001; Droop & Moazzen, 2007). Cordierite is present and modally abundant in all assemblages upgrade of zone I.…”

“…Muscovite persisting to higher grade in such rocks underwent the dehydration reaction: which was responsible for the corundum isograd. The appearance of spinel in zone V suggests the KFMASH model univariant reaction: The sillimanite isograd is marked by the onset of direct replacement of andalusite by sillimanite: No fibrolitic sillimanite was detected in any of the samples studied by Droop & Moazzen (2007). Vernon (1979) has argued that fluid presence is necessary for the production of fibrolite.…”

“…Coordinates: UK National Grid sheet NN. (c) The southern sector of the Etive thermal aureole in the vicinity of Lochawe: metamorphic zones and isograds from Droop & Moazzen (2007); formation boundaries from British Geological Survey (Scotland) Sheet 45(E)/Dalmally/. Coordinates: UK National Grid sheet NN.…”

Conflicting P–T paths within the Central Zone of the Limpopo Belt: A consequence of different thermobarometric methods?

“…Conversely, the pseudosection approach can be applied to a wide range of rocks, irrespective of the variance. Consider the following mineral assemblage developed in a pelitic rock (MM184) from the Etive thermal aureole, Scotland (Droop and Moazzen, 2007;Rigby et al, 2008d): and + cd + ksp + bi. How can conventional thermobarometry determine the P-T conditions of equilibration for this assemblage?…”