AGE AND DURATION OF SVECOFENNIAN PLUTONO-METAMORPHIC ACTIVITY IN THE LADOGA AREA, SOUTHEASTERN BALTIC SHIELD

Abstract

The U-Pb system was examined in minerals of the main rock types from the Ladoga area: early- and synorogenic enderbites, diorites, syn- and late-orogenic K-granites, the leucosomes of migmatites, and their host garnet-cprdierite and garnet-hypersthene gneisses. The peak of the granulite metamorphism was dated and the duration of each retrograde metamorphic stage was evaluated. The age of the Svecofennian plutono-metamorphic activity is constrained between 1.89-1.88 and 1.86-1.85 Ga. The geochemistry, morphology, and inner structure of the zircon and monazite of different genetic types are studied in detail to determine whether these minerals can be used as chronometers of each of the petrogenetic processes. The rocks crystallizing from melts (magmatic rocks and the leucosomes of migmatites) were determined to have similar U-Pb zircon and monazite ages. The Th/U ratios of monazite from the igneous rocks are generally higher than in this mineral of metamorphic and ultrametamorphic genesis. Age relations for two migmatite zones (sodic and potassic migmatites) in the Ladoga area are examined. The U-Pb system of monazite and zircon from migmatites and gneisses in the two migmatite zones suggest that they were produced simultaneously at 1871-1876 Ma.