Isotopic and geochemical constraints on the evolution of the 1.93-1.79 Ga Svecofennian crust and mantle in Finland

Abstract

The central part of the Svecofennian domain in Finland is characterized by thick lithosphere with crustal thickness up to 60–65 km. Major collisional events at 1.91-1.90 Ga and 1.89 Ga and a thrusting event at 1.86-1.84 Ga, the latter due either to continuation of shortening or to a separate intracrustal collision, are considered the major causes of crustal thickening. The origin of the Svecofennian domain has been formerly attributed to mixing of depleted mantle melts with variable amounts of Archaean crustal component via subduction, but the new data and reinterpretation of older data imply that, besides juvenile crust, the Svecofennian domain also contains older Palaeoproterozoic crustal components. No Archaean component is found in the 1.93-1.91 Ga gneissic tonalites and related felsic volcanics ( ε Nd (t) + 1 – +4) which occur in the Savo Belt (SB), adjacent to the Archaean craton. The depleted mantle model ages vary from 1.94 Ga to 2.33 Ga with most values ≤2.0 Ga. The analysed syntectonic (post-thickening) granitoids (≤1.89 Ga) in the SB exhibit positive ε Nd (t) values ranging from +2.8 to +3.0 pointing to a juvenile source. The hypersthene granitoids are similar in age but show epsilon values around −1, which are only slightly lower than found in EM-derived mafic rocks. The isotopic and geochemical data for the Central Finland Granitoid Complex and Tampere Schist Belt indicate the occurrence of evolved thick crust (≥2.0 Ga) and associated lithospheric mantle already at 1.91 Ga. The southernmost part of the Southern Svecofennian in Finland also shows evolved-type crust (≥2.0 Ga). A tentative model for the development of crust and mantle is presented. It includes the occurrence of continental nucleus surrounded by more juvenile island arc accretions. Two successive collisional events thickened both the crust and the subcontinental lithospheric mantle, with the greatest relative amount of thickening occurring in the mobile zones. Convective removal of the lowermost parts of thickened lithosphere caused an upwelling of hot asthenosphere, increase in the geotherm, and melting of both the enriched subcontinental lithospheric mantle and the convective depleted asthenosphere. Mafic magmatism during the post-thickening stage (≤1.89 Ga) thus varied from dominantly enriched- to dominantly depleted-mantle melts and was associated with variable crustal contamination. The thickening of the lithosphere in the central part of the Svecofennian domain was not followed by large extension, suggesting that the thickness was already great 1.88-1.80 Ga ago. This, together with the thick high-velocity lower crust, could have provided the isostatic balance needed to stabilize the thick crust.