Emplacement mechanism and thermobarometry of the Sveconorwegian Bohus granite, SW Sweden

Abstract

The 920 Ma, post-kinematic Sveconorwegian (Grenvillian) Bohus granite of south-western Sweden was principally emplaced along a gently easterly dipping weakness zone at mesozonal brittle crustal depth, in an extensional stress regime. The fusion of the protolith gneisses was facilitated by a Sveconorwegian post-orogenic uplift which accelerated the dehydration melting of biotite. The crystalrich magma surges crystallised predominantly as biotite monzogranites, locally rich in secondary muscovite. Biotite + muscovite assemblage became stable in the late magmatic stage, principally in the pegmatitic-aplitic facies. Amphibolite facies para and orthogneiss xenoliths are very frequent in the northern part of the granite massif, where they are interpreted to represent remnants of a giant, brecciated roof pendant. Contact metamorphic transformation of large paragneiss xenoliths, at a specific locality, Tjärnö island, enables garnet-biotite geothermometry and garnet-plagioclase geobarometry studies of physical emplacement conditions during intrusion of the granite. Garnet-bearing Bohus granite samples from four additional localities were also used for the geothermobarometric calculations. Element partitioning in garnet-biotite indicate that the temperature of intrusion was at least 715[ddot]C and the final crystallisation (solidus) temperature 670–680[ddot]C. Garnets in the granite and the granofelsic xenolith are low-calcium almandine-spessartines, in contrast to the generally calcium-rich almandinetype garnets of the metamorphic rocks of SW Sweden. Pressure is determined with the independant approaches of quartz + muscovite + biotite + plagioclase + garnet and garnet-only Gd/Dy geobarometry, supported by evaluation of emplacement features and mineral assemblages. Pressure is found to have been c. 4 kbar or slightly higher during the final crystallisation of the Bohus granite, corresponding to depths of c. 15 km.