The 1.79 Ga Särkilahti leucogranite – a horizontal magma layer below granulite-grade migmatites in SE Finland

Abstract

The heterogeneous, peraluminous Särkilahti garnet-cordierite leucogranite occupies ~ 75 km2 area in Svecofennian granulite-grade terrain of SE Finland. Country rocks are metasedimentary garnet-cordierite-biotite migmatites with some orthopyroxenegarnet paragneisses and orthopyroxene metatonalites. The leucogranite represents the exhumed upper portion of an extensive horizontal magma layer in the crust. Regional scale heat flow (initiated by energy input from the mantle) caused the formation of this magma layer and it also gave rise to granulite-grade migmatization (formation of in-situ leucosomes) in the above country rocks by heat conduction. When the magma layer slowly rose, country rocks at its upper limit were melted and partly merged in the magma. Within the magma layer, lighter granitic melt ascended as a consequence of the residue sinking. Late-stage crystallization products of the layer resulted in cross-cutting granite dykes in the above migmatites. U–Pb ages on monazite for the Särkilahti leucogranite samples are 1.79–1.78 Ga. The age slightly postdates the culmination of the regional metamorphism (1.84–1.80 Ga). As the leucogranite (magma layer) represents the lowest horizontal rock unit, it has – during crustal exhumation – crystallized slightly after the peak of the regional metamorphism.

Keywords

• leucogranite
• migmatite
• partial melting
• metamorphism
• magma emplacement
• molten crust
• u–pb geochronology
• finland