The thermal and geodynamic evolution of the Lapland granulite belt: Implications for thermal structure of the lower crust during granulite-facies metamorphism

The tectono-thermal history of the Lapland Granulite belt is considered within the framework of a combined superplume-plate tectonic model on the basis of new geochronological and petrological data. The high-grade metamorphic events were linked with plume-related extensional episodes at ∼ 2.44 Ga (M0), ∼ 2.0–1.95 Ga (M1), ∼ 1.95–1.92 Ga (M2) and 1.92–1.91 Ga (M3). The last (M4) high-grade event at ∼ 1.89–1.87 Ga corresponds to the final collision. P–T parameters of metamorphism during M1 event ranged from 960 to 860 °C and from 14.0 to 10.3 kbar. They decreased from the lower to the upper part of the belt during the M2 event from 860 °C at 12.4 kbar and to 800 °C at 5.8 kbar, during the M3 event from 770 °C at 10.7 kbar to 640 °C at 4.8 kbar, and during the M4 event from 650 °C at 8.4 kbar to 550 °C at 4.5 kbar. Upward displacement of a hot tectonic slice during the M4 stage caused inverted metamorphic zoning in the autochthonous rocks and a clockwise P–T evolution within the mélange zone at the base of LGB 1. The internal thermal gradient within the lower crust during events M2 and M3 was 1.8–2.3 °C/km, and during the M4 event it fell to 4–5 °C/km. The agreement of thickness estimates of the granulite ensemble obtained from seismic and petrological data attests to the lithostatic nature of metamorphic pressures. The average thickness of the Lapland granulite slice was initially ∼ 21–23 km.