View source for Fraochaidh prograde sequence, Ballachulish Igneous Complex - an excursion

=== Stop 3-7. Different types of veins in hornfels: onset of partial melting. NN 0235 5270, 440-460 m ===
'''''Directions: '''''From Stop 3-6, continue ESE and uphill in the direction of a large light-coloured boulder. After 150-200 m, you will find yourself on the edge of a ridge below which Coire Dubh opens out immediately to the east, and where the major crags swing round to take a more NNW-SSE, rather than WNW-ESE, trend. The exposures of Stop 3-7 occur for about 100 m leading up to this point.

'''''Description: '''''The hornfelses have a generally similar appearance and mineralogy to those of Stops 3-5 and 3-6, with local andalusite- and corundum-bearing layers, but show a greater variety of mm-scale vein types. The veins appear to be a little wider and stand up as ribs on exposure surfaces. Some are sub-vertical, discrete and relatively sharp-margined like those seen at lower grade, whereas a larger proportion show a variety of orientations, are less continuous, and have less sharp contacts with the enclosing hornfels, in places merging with the matrix. Some sub-vertical veins interconnect with more shallowly-dipping veins. Many of the veins are of a fine-grained, granular Kfs+Qtz-rich composition and resemble migmatitic leucosomes, contrasting with the thin Kfs-dominated mineralogy of the veins noted in Stops 3-4 to 36. In thin section, the more diffuse veins show a range of igneous-like textures similar to those observed in migmatites elsewhere in the aureole ('''Haile et al., 1991a'''). Taken together, these features are tentatively interpreted to be indicative of localized partial melting in the hornfelses. The temperature of these rocks was probably ca. 700 °C (see '''Introduction''').