Formation mechanism of the Vaasa Batholith in the Fennoscandian shield: Petrographic and geochemical constraints

Abstract

The ~1.88 Ga Vaasa Batholith in western Finland is a large granitoid area (8 000 km2) with gradational contacts against the garnet- and cordierite-bearing diatexite and metatexite migmatites of the Pohjanmaa Belt. It is located in the middle part of the Fennoscandian Shield and was formed during the Svecofennian orogeny (1.9–1.8 Ga). The Batholith area consists of porphyritic and even-grained granodiorite with smaller proportions of granite and pyroxene granitoid. The various phases in the Vaasa Batholith mainly grade into each other and usually contain relicts of Ca concretions and schists, biotite and garnet, locally resembling uniform diatexites. All the phases are peraluminous in composition. The Mg# of biotite in the granodiorites and pyroxene tonalites is high (50–60), in contrast to lower values in the granite (20–35). Orthopyroxene in the tonalite has an elevated Mg# value and Al2O3 concentration. In Harker diagrams, bulk composition of the tonalites, granodiorites and granites of the Vaasa Batholith evolves along the same path, but the metatexites are beyond or cutting the path. The textural, mineralogical and chemical continuity between the granodiorites of the Vaasa Batholith and the adjacent diatexites indicate that the Vaasa Batholith represents a magma layer formed by extensive in situ melting of the crust. The diatexites are mainly from the inhomogeneous magma, although they texturally grade to the metatexites. The Vaasa Batholith and adjacent diatexites are separated from the metatexites of the Pohjanmaa Belt by ‘magma interface’ (MI). The metatexites formed in a conduction heat flow zone above the MI. The Vaasa Batholith is an exceptional type example of the voluminous and diverse granitoids in Finland.

Keywords

• granites
• granodiorites
• tonalite
• migmatites
• diatexite
• anatexis
• geochemistry
• magmas
• Paleoproterozoic
• Vaasa
• Finland