High-Temperature Metamorphism and the Role of Magmatic Heat Sources at the Rogaland Anorthosite Complex in Southwestern Norway

Abstract

The Rogaland complex covers ∼1000 km2 in southwestern Norway and consists mainly of anorthosite massifs and the layered Bjerkreim–Sokndal lopolith (BSL). These rocks intrude charnockitic migmatites containing intercalated marbles and garnetiferous migmatites. High-temperature mineral isograds (pigeonite, osumilite and orthopyroxene) in the metamorphic basement are subparallel to and increase in grade towards the intrusive complex. P–T estimates from the country rocks show a roughly linear increase in temperature towards the BSL consistent with the distribution of isograds. The peak P–T conditions at 20 and 2·5 km from the contact at ∼5 kbar range from 700 to >1000°C. Field relations and age determinations link the high-T metamorphism and the magmatism. The two-dimensional thermal modelling indicates that heat from a single magmatic cooling unit is not sufficient to produce the array of isograds and the peak metamorphic temperatures. Two magmatic episodes separated by ∼3 Myr, however, can account for the high-temperature metamorphism. In this model, the emplacement and crystallization of the anorthosite produces a regional thermal gradient (from 750 to 600°C). After a brief hiatus a second, smaller body (BSL) provides an additional thermal input that results in an array of high-temperature isograds and country-rock temperatures >1000°C.