Geothermobarometry of igneous and metamorphic rocks around the Åna-Sira anorthosite massif: Implications for the depth of emplacement of the South Norwegian anorthosites

Orthopyroxenes and clinopyroxenes in metabasites in contact with the massif-type Åna-Sira anorthosite yield the following equilibrium temperatures using various geothermometers: 840°C (Wells 1977); 800°C (Wood & Banna 1973), 650°C (Lindsley & Andersen 1983). Gamet-cordierite-plagioclase-sillimanitequartz assemblages in neighbouring metapelites give values around 600°C and 3 ± 0.5 kb by means of several geothermobarometers. Armoured relics of symplectitic quartz + spinel in gamet suggest that equilibrium was attained during decreasing temperature. Fayalite + quartz and Ca-poor + Ca-rich clinopyroxene assemblages in charnockitic rocks of late tectonic intrusions yield 800-850°C (Lindsley & Andersen 1983) and 6-7.5 kb for the upper part of the Bjerkreim-Sokndal lopolith and a minimum pressure of 5-5.5 kb for the Hidra massif (Boblen & Boettcher 1981). When interpreted in terms of the regional geological evolution, the conditions measured reflect two successive stages of a PT path characterizing the post-magmatic evolution. It is inferred that the emplacement of the Rogaland massiftype anorthosites took place at greater depths than these massifs, i.e. not less than 22-28 km near the Bjerkreim-Sokndal massif and not less than 18-20 km near the Hidra body.