Digital Mapping and Three-Dimensional Model Building of the Ben Nevis Igneous Complex, Southwest Highlands, Scotland: New Insights into the Emplacement and Preservation of Postorogenic Magmatism

• Published in: The Journal of geology 2017-11, Vol.125 (6), p.607-636
• Main Authors: Muir, R. J., Vaughan, A. P. M.
• Format: Article
• Language: English
• Subjects: Calderas; Carapace; Cenozoic; Devonian; Digital mapping; Dikes; Embankments; Geologic depressions; Geology; Granite; Highlands; Igneous rocks; Magma; Mapping; Modelling; Petrogenesis; Plutons; Precambrian; Preservation; Rocks; Sedimentary facies; Silurian; Three dimensional models; Tuff; Volcanic activity; Volcanic rocks; Volcanism
• ISSN: 0022-1376; 1537-5269
• DOI: 10.1086/693858

Caledonian postorogenic magmatic processes are not well understood. New digital mapping and three-dimensional modeling of the end-Caledonian (late Silurian to Early Devonian) Ben Nevis Igneous Complex in the SW Highlands of Scotland provide quantitative estimates of magmatic rock volumes for the first time and argue against local sources for the volcanic rocks or caldera interpretations for structural evolution. Reexamination of the Ben Nevis Intrusive Ring Tuff, key evidence for caldera collapse, shows it to be a restricted marginal facies of the trondhjemitic Inner Granite, and there is no evidence for a ring fault, previously argued as evidence for cauldron subsidence. The volcanic rocks at the core of the complex appear to form a 1.3-km³ roof pendant in the Inner Granite, and paleoflow evidence suggests that they had a distal volcanic source to the NW. Compositional comparisons between the Inner Granite and the monzonitic Outer Granite indicate that they are unlikely to have had a common petrogenesis or be coeval. The plutonic rocks form a concentric, composite, 6.4 × 8.5-km tabular body, elliptical in plan, with a total volume in the range 44–105 km³. Large areas of late Precambrian Dalradian Supergroup metasedimentary country rocks form parts of a steeply dipping carapace and roof pendants at the edge of the complex. Comparison with calc-alkaline volcanic rocks and plutons of the comparable Cenozoic San Juan Volcanic Complex in Colorado suggests that the plutonic rocks may have grown as laccoliths during regional caldera volcanism. Structural data suggest that the laccoliths inflated toward the SE, fed by NE-SW dikes in the core of the tightly folded Appin Syncline. This reevaluation of a classic area of world geology sheds light on a lost volcanic landscape that once covered much of the SW Highlands of Scotland.