The aureole of the Traigh Bhàn na Sgùrra Sill, Isle of Mull: Reaction-driven microcracking during pyrometamorphism

Abstract

The intrusion of a Tertiary gabbroic sill of 6 m thickness, in which magma flow was locally turbulent, into garnet-grade pelitic gneiss and psammite at 600 bars at Traigh Bhàn na Sgurra on the Ross of Mull, Scotland, led to the development of a contact aureole of 3 m width. Observed reactions include muscovite breakdown, melting on quartz-feldspar grain boundaries, and isochemical breakdown of biotite and garnet. A simple, two-stage thermal model fitted to the profile of maximum temperature is consistent with a 5 month period of turbulent magma flow. Five generations of micro-cracks occur in the psammite. The oldest pre-dales contact metamorphism and is marked by sub-parallel fluid inclusion arrays. The next resulted from anisotropic thermal expansion caused by magma intrusion. Internally generated stresses related to an increase in volume associated with muscovite breakdown formed two further sets of cracks associated with release of H2O and melting. Melting on quartz-feldspar grain boundaries also resulted in crack formation. The final stage of cracking was the result of anisotropic thermal contraction. Despite the high crack density at the metamorphic peak, little or no melt segregation occurred, demonstrating that micro-cracking alone is not sufficient (at least on this time scale) for mell segregation in static anatectic environments.