The Aureole of the Traigh Bhan na Sgurra Sill, Isle of Mull: Reaction-Driven Micro-cracking During Pyrometamorphism

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The intrusion of a Tertiary gabbroic sill of 6 m thickness, in which itself is dependent on the ease with which partial melts can segregate from their source and this process remains magma flow was locally turbulent, into garnet-grade pelitic gneiss an important subject of research. Complete underand psammite at 600 bars at Traigh Bhan na Sgurra on the Ross standing of the segregation process is dependent on of Mull, Scotland, led to the development of a contact aureole of constraining both the distribution of melt in the source 3 m width. Observed reactions include muscovite breakdown, melting region and the mechanisms by which it can move at low on quartz–feldspar grain boundaries, and isochemical breakdown volume percent (<10%). of biotite and garnet. A simple, two-stage thermal model fitted to Experimental determination of equilibrium melt geothe profile of maximum temperature is consistent with a 5 month metries under hydrostatic conditions in crustal materials period of turbulent magma flow. Five generations of micro-cracks demonstrates that melt-filled pores should form an inoccur in the psammite. The oldest pre-dates contact metamorphism terconnected network (e.g. Jurewicz & Watson, 1985; and is marked by sub-parallel fluid inclusion arrays. The next Wolf & Wyllie, 1991; Laporte, 1994). Such a network resulted from anisotropic thermal expansion caused by magma should, in principle, permit the segregation of all but a intrusion. Internally generated stresses related to an increase in very small proportion of melt from the source region volume associated with muscovite breakdown formed two further (McKenzie, 1984). However, the high viscosity of crustal sets of cracks associated with release of H2O and melting. Melting melts is believed to require deformation in a deviatoric on quartz–feldspar grain boundaries also resulted in crack formation. stress field to ‘squeeze’ out the melts before they solidify The final stage of cracking was the result of anisotropic thermal (e.g. Sawyer, 1991; Brown, 1994; Rutter & Neumann, contraction. Despite the high crack density at the metamorphic peak, 1995). Recently it has been pointed out that it is probable little or no melt segregation occurred, demonstrating that microthat pore topologies controlled entirely by textural equicracking alone is not sufficient (at least on this time scale) for melt librium will only very rarely be attained in fluid-producing segregation in static anatectic environments. environments. This is because the rate of reaction and fluid production is generally greater than that of textural adjustment driven by minimization of surface energies