Modelling the thermal evolution of a collisional Precambrian orogen: High heat production migmatitic granites of southern Finland

Abstract

We present results of thermal and geochemical modelling of Paleoproterozoic southern Finland, which is characterized by formation of high heat production migmatitic granites, post-orogenic granitoids, and high temperature–low pressure metamorphism at 1.85–1.78 Ga during late Svecofennian time. Metamorphic grade is mostly amphibolite facies (4–5 kbar, 650–700 °C), but also granulite facies (6 kbar, up to 800 °C) occurs. Our results suggest that the thermal evolution of the area can be modelled as a process of rapid crustal thickening (up to 70 km) in a plate collision at ca. 1870–1860 Ma ago, followed by conductive heating of the crust, simultaneous exhumation and partial melting of middle crust, and emplacement of the melts in the upper crust as late orogenic high heat production granites at about 1825 Ma ago. The upper crust collapsed gravitationally at about 1820 Ma but the crust was still thick (57 km) when the main orogenic era terminated about 1800 Ma ago. In modelling the thermal evolution we applied crustal heat production values constrained by geochemical data, and a conservative estimate, i.e., relatively low value for mantle heat flow density. The modeled temperatures at 30–50 km depths in the thickened crustal stack exceed the dehydration melting temperatures of micas (at 40 km 750 °C for muscovite and 800 °C for biotite, respectively) 25–35 Ma after the collision and thickening of crust. The results explain the high temperature–low pressure metamorphism in southern Finland as an inevitable consequence of crustal thicknening leading to an increase of the total heat production of the crust. Crustal thicknening is the main heat source for melting and metamorphism of the orogeny. Advective and latent heat brought by the melts further increased the temperatures at the emplacement depth and contributed to the granulite facies metamorphism that is spatially related to the migmatitic granites. Geochemical models suggest that the high concentrations of U and Th in the migmatitic granites can be attributed to about 20 vol% partial melting of deeply buried sediments and volcanic rocks. About 36–71% of the original U, 18–54% of Th and 34–46% of K, respectively, in the source rock types entered the melt phase. No anomalous concentrations of these elements in the source rock are required. In addition to explaining the migmatitic late orogenic granite and post-orogenic granitoid genesis, the collision model implies that the middle-lower crust and the upper mantle continued warming until the Mesoproterozoic times, which suggests a causal link between the Svecofennian thermal evolution and the extensive bimodal rapakivi granite and anorthosite magmatism in the Fennoscandian Shield about 200–250 Ma later.