The igneous complex of Ballachulish is a composite calc-alkaline pluton of Caledonian age (412 ± 28 Ma), emplaced in Dalradian metasediments at a pressure of 3 ± 0–5 kb (c. 10 km depth). The 4 by 7 km intrusion is composed of a zoned monzodiorite-quartz diorite envelope with a distinct flowand deformation-foliation, surrounding a younger core of porphyritic granite. Two-pyroxene thermometry, Fe-Ti oxide thermobarometry, and stability relationships of ternary feldspars, biotite, and amphiboles are used to calibrate the 3 kb isobaric crystallization sequence with respect to the following parameters: the fractionation stage of the host rocks, the water content of the magmas, phase compositions, and oxygen fugacity. Plagioclase, augite, and oxides generally yielded submagmatic temperatures due to the extensive recrystallization and re-equilibration of these phases in the 900–l550°C subsolidus range. The ‘dry’ monzodiorites apparently contained less than 1 wt. % initial magmatic water, and remained H2O-deficient and vapor-absent throughout their entire crystallization range. In contrast, 2.5–3 wt.% initial H2O is estimated for the more fractionated quartz diorites and the younger granites. The main crystallization interval for Opx–Cpx–Plg primocrysts in the diorites covers c. 1100–950°C. Late-magmatic biotite and alkali feldspar join the paragenetic sequence below 980°860°C, at fO2 near NNO. A solidus temperature of c. 900°C is inferred for this ‘dry’ system, in which amphiboles are entirely subsolidus. At the present level of emplacement, crystallization intervals of ˜ 1050–690°C and˜ 900–680°C are suggested for the quartz diorites and the granites, which probably terminated crystallization in the presence of a hydrous fluid.
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Weiss et al. (1989) studied this question.