The Ballachulish Igneous Complex, Scotland: Petrography, Mineral Chemistry, and Order of Crystallization in the Monzodiorite-Quartz Diorite Suite and in the Granite |
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| Authors: | WEISS STEFAN; TROLL GEORG |
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| Institution: | Mineralogisch-Petrographisches Institut der Universit?t Theresienstr. 41, D-8000 M?nchen 2, W. Germany |
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| Abstract: | The igneous complex of Ballachulish is a composite calc-alkalinepluton of Caledonian age (412 ? 28 Ma), emplaced in Dalradianmetasediments at a pressure of 3 ? 0–5 kb (c. 10 km depth).The 4 by 7 km intrusion is composed of a zoned monzodiorite-quartzdiorite envelope with a distinct flowand deformation-foliation,surrounding a younger core of porphyritic granite. Two-pyroxene thermometry, Fe-Ti oxide thermobarometry, and stabilityrelationships of ternary feldspars, biotite, and amphibolesare used to calibrate the 3 kb isobaric crystallization sequencewith respect to the following parameters: the fractionationstage of the host rocks, the water content of the magmas, phasecompositions, and oxygen fugacity. Plagioclase, augite, andoxides generally yielded submagmatic temperatures due to theextensive recrystallization and re-equilibration of these phasesin the 900–l550?C subsolidus range. The ‘dry’monzodiorites apparently contained less than 1 wt. % initialmagmatic water, and remained H2O-deficient and vapor-absentthroughout their entire crystallization range. In contrast,2.5–3 wt.% initial H2O is estimated for the more fractionatedquartz diorites and the younger granites. The main crystallizationinterval for Opx–Cpx–Plg primocrysts in the dioritescovers c. 1100–950?C. Late-magmatic biotite and alkalifeldspar join the paragenetic sequence below 980?860?C, at fO2near NNO. A solidus temperature of c. 900?C is inferred forthis ‘dry’ system, in which amphiboles are entirelysubsolidus. At the present level of emplacement, crystallizationintervals of {small tilde} 1050–690?C and{small tilde}900–680?C are suggested for the quartz diorites and thegranites, which probably terminated crystallization in the presenceof a hydrous fluid. |
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