PETROLOGY OF THE MICA SCHISTS OF THE TANAELV BELT IN THE SOUTHERN TECTONIC FRAMING OF THE LAPLAND GRANULITE COMPLEX

The Tanaelv belt is a dynamometamorphic zone separating the regional o verthrust of Lapland belt granulites from the deformed rocks of the Ka relian craton (Belomorian Complex). Strongly deformed staurolite-two-m ica schists with garnet porphyroblasts are common in the belt. Two met amorphic zones were distinguished in the schists: the chlorite-staurol ite zone tectonically contacts gray gneisses and the kyanite-biotite z one that underlies garnet amphibolites of the Tanaelv belt. The bounda ry between them is defined by the prograde reaction Chl + St + Ms = Ky + Bt + Qtz + H2O. Rotated garnet porphyroblasts from the schists of t he chlorite-staurolite zone show a systematic increase in magnesium mo le fraction and a decrease in manganese from the center to margins of grains. This suggests that the garnet grew during prograde metamorphis m. Its peak occurred in the kyanite-biotite zone at 650 degrees C and similar to 8 kbar. Garnet porphyroblasts there have the snowball struc ture and are almost unzoned. An exception is a thin, 30-20 mu m, rim i n the garnet, where the magnesium mole fraction declines. The euhedral garnet porphyroblasts of the regressive stage are almost completely d evoid of mineral inclusions, and their magnesium mole fraction decreas es from the center to rims. Metamorphic parameters decreased to 530 de grees C and similar to 5 kbar. The overall form of the P-T path is tha t of a loop reflecting the process of crustal convection in the upper parts of the cratonized sequences of greenstone complexes owing to the emplacement into them of a giant granulite body from the lower crust (Perchuk et al., 1997a, 1997b).