METAMORPHIC EVOLUTION OF GARNET-BEARING GRANULITES IN THE WESTERN LADOGA AREA

New materials on the western Ladoga area provide evidence for a revisi on of preexisting concepts of the prograde metamorphic zonation, sugge st its retrograde nature, and, taking into account its temporal relati onships with plutonic rocks, make it possible to recognize the princip al stages in the evolution of the complex. There are only very scarce data on the prograde preculmination metamorphic stage. The stage (1.88 -1.87 Ga) included the origin of a norite-enderbite plutonic complex, metamorphism, and migmatization under granulite-facies conditions. The peak metamorphic conditions, calculated using the cotes of zoned garn et grains in assemblage with cordierite and hypersthene, attained 880- 945 degrees C and 5.6-6.5 kbar at a geothermal gradient of approximate ly 40 degrees C/km. The early retrograde stage (1.87-1.86 Ga) comprise d the emplacement of a gabbronorite-tonalite intrusive complex, high-t emperature retrograde metamorphism, and migmatization, which continued under these conditions. The metamorphic parameters, determined by the compositions of garnet and cordierite occurring in physical contact, compose a continuous Linear trend, which terminates at 600 degrees C a nd 4 kbar. This trend suggests a slow uplift at a nearly constant geot hermal gradient of about 40 degrees C/km. The late retrograde stage wa s marked by the intrusion of late-synorogenic granite and a further de crease in the metamorphic parameters, which were estimated at 500-460 degrees C and 1.0-1.4 kbar by contacting garnet-biotite pairs. The P-T path of this stage implies a rapid uplift and a simultaneous rapid de crease in the gradient to 25, possibly 10-15 degrees C/km. The evoluti on of the complex was generally controlled by a continuous but uneven crustal growth at a nearly constant and relatively high heat flow.