CHRONOLOGY OF MULTIPHASE EMPLACEMENT OF THE SALMI-RAPAKIVI GRANITE-ANORTHOSITE COMPLEX, BALTIC SHIELD - IMPLICATIONS FOR MAGMATIC EVOLUTION

The U-Pb dating of 18 samples, representing the principal rock types o f the 4000 km(2) Salmi anorthosite-rapakivi granite complex and its sa tellite Uljalegi pluton, southeastern Baltic (Fennoscandian) Shield, r eveals that six temporally distinct episodes of igneous activity occur red in a timespan of 17 million years. From oldest to youngest they ar e: (1) gabbronorite and monzonite at 1546.7 Ma; (2) syenogranite at 15 43.4 Ma; (3) early wiborgite and pyterlite at 1540.6-1537.9 Ma; (4) bi otite granite and more evolved granite at 1538.4-1535 Ma; (5) late pyt erlite at 1535.2 Ma; (6) olivine gabbro and biotite-amphibole granite at 1530 Ma. The resolvable intervals between magmatic episodes are 3.5 -5.0 million years. Early wiborgite and pyterlite (3, above) and bioti te granite (4, above) probably crystallized from multiple magma intrus ions. Age differences of 3.4 +/- 1.5 million years between zircon and baddeleyite in olivine gabbro (6, above) are probably a result of xeno crystic origin of baddeleyite extracted from an earlier mafic phase of the Salmi complex. The ages and chemical features of early and late z ircon populations, together with our modeling of magma crystallization and zircon growth, show that the duration of magma crystallization an d Pb-diffusion in zircon was short lived and insignificant compared to the precision of dating of about +/- 1-2 million years. Hence, the ra nge of U-Pb ages for each of the major rock types may approximate the emplacement intervals of their respective magmas. Average rate of magm a emplacement was about 0.01 km(3)/year for the most voluminous phase of early biotite-amphibole rapakivi granite, and about 0.0024 km(3)/ye ar for the Salmi complex as a whole. Compositional changes of the Salm i magmas over time are in agreement with the model of magmatism relate d to lithospheric extension.