Pulsating Evolution of Metamorphism in Granulite Terrains: Kolvitsa Meta-anorthosite Massif, Kolvitsa Belt, Northeast Baltic Shield

Abstract

From detailed petrographic and microprobe study and application of an internally consistent system of mineralogical thermometers and barometers (the TPF program), the pressure-temperature-time path of metamorphism of the Kolvitsa meta-anorthosites (metagabbro-anorthosites) (Kolvitsa belt of the Lapland-Kola orogen, Baltic shield) was estimated. The massif formed at a depth of more than 45 km. Crystallization of anorthosites (gabbro-anorthosites) occurred in the Paleoproterozoic (2450-2462 Ma) at a temperature of 1200°C. sif preserve evidence of several metamorphic events as relics of different (mostly kinetic, decomposition-crystallization) mineral reactions. The subsequent metamorphic history of the massif, spanning more than 600 Ma, evolved in two main stages, early Paleoproterozoic rifting (extensional conditions) and Svecofennian collision (compressional conditions), comprising five dated metamorphic (tectonothermal) events. Temperatures obtained are: for M1, 990°C-primary metamorphism soon after 2450-2462 Ma (time of magmatic crystallization of the massif); for M2, 870-915°C at 2423-2433 Ma; for M3, 750-830°C at 2374-2394 Ma; for M4, 670-710°C a; for M5, 570-600°C at 1827-1833 Ma. The rocks were affected by sub-isothermal decompression from a depth of ∼45 km to 40 km (P from 12.37 down to 11.2 kbar) during M2, from 40 km to 28.5-31 km during M3 (P from 11.2 down to 8-8.6 kbar), from 28.5 km to ∼23 km during M4 (P from 8-8.6 down to 6.5 kbar). Early Paleoproterozoic tectonothermal activation of the region (lasting from 55 to 75 Ma) was characterized by high rates of exhumation, 0.2-0.3 mm per year, and cooling, 3.3-4.5°C/m.y. During the Svecofennian stage, these rates were considerably lower, 0.045 mm per year and ca. 1°C/m.y., respectively. Both periods of regional development, despite differences in geodynamic conditions and exhumation mechanisms, are characterized by similar P-T paths with alternating periods of subisothermal decompression (activation) and subisobaric cooling (relaxation). Each decompression period was accompanied by intense tectonic deformation, with increase in rock permeability and acceleration of thermal, fluid, and mineral-forming processes.