The thermal aureole of the Lochnagar Complex: mineral reactions and implications from thermal modelling

Abstract

Synopsis The Caledonian Lochnagar Complex in the NE Highlands of Scotland comprises a central granite and earlier marginal quartz diorites. Contact metamorphism of the Dalradian country rocks has caused a thermal overprint on upper amphibolite facies regional metamorphic assemblages. Distinctive assemblages in metapelites and amphibolites can be mapped across the aureole, showing that the thermal effects are greatest where the marginal quartz diorites occur. Mineral chemistry in the metapelites can be related to contact metamorphic reactions and to a change in bulk rock chemistry due to partial melting of the metasediments in the high-grade parts of the aureole. Amphibolites within the aureole show no evidence of partial melting; their mineral chemistry is controlled by contact metamorphic reactions. The form of the aureole is a function of the different phases of intrusion, and mathematical modelling has been applied to distinguish between different intrusive scenarios. It is likely that two phases of contact metamorphism, related to early quartz diorite and later granite intrusion, were necessary to produce the highest grades of metamorphism. Mismatch between the modelled aureole shape and that observed in some areas is probably due to enhanced local heat flow and the lateral extension of the intrusion below the present erosion level.