EPISODIC EARLY PROTEROZOIC GRANITOID PLUTONISM IN THE MAKKOVIK PROVINCE, LABRADOR - U-PB GEOCHRONOLOGICAL DATA AND GEOLOGICAL IMPLICATIONS

Episodic granitoid plutonism in the Early Proterozoic Makkovik Province of the easternmost Canadian Shield is revealed by high-precision U - Pb zircon studies of a range of intrusive rock types. The oldest granites yield ages of 1893 +/- 2 and 1891 +/- 5 Ma and document a previously unrecognized event that may correlate with an early migmatization of reworked Archean basement. These dates also constrain early structural reworking of the basement, the earliest deformational event grouped as part of the Makkovikian orogeny. Four samples have essentially identical zircon ages of 1801 +/- 2, 1802 +/- 2, 1802(-7)+13 , and 1825 - 1799 Ma, and a fifth is slightly older, at 1837(-4)+6 Ma. These ages suggest correlation with local volcanic sequences, dated in part at 1807 +/- 4 Ma. The plutonic suites include both syn- and posttectonic granitoid assemblages and define the main magmatic pulse associated with the Makkovikian orogeny and constrain its final deformational episode. Distinctive, fluorine-enriched "A-type" granites yield an age of 1719 +/- 3 Ma and represent a previously unrecognized late postorogenic to anorogenic magmatism of regional significance. Two layered, gabbro-diorite-monzonite-syenite suites yield identical zircon ages of 1649 +/- 1 and 1649 +/- 3 Ma. A regionally extensive granodioritic unit gives an age of 1647 +/- 2 Ma, and a high-level alaskitic granite is dated imprecisely at 1640 - 1650 Ma. These plutonic suites correlate with volcanic rocks previously dated at 1649 +/- 1 Ma. These data show that the plutonic evolution of the Makkovik Province is significantly more complex than previously supposed. The clustering of ages suggests episodic, rather than continuous, magmatism. The different age groupings can, to some extent, be correlated with compositional associations defined by major- and trace-element geochemistry. The new data also raise questions about the regional configuration of Early and Middle Proterozoic orogenic belts in Labrador. Previous correlations between the Makkovik Province, die Ketilidian Mobile Belt of Greenland, and the Svecofennian and Trans-Scandinavian belts of Sweden are supported and expanded by these new results.