DEFINITION OF AN ARCHEAN-PROTEROZOIC CRUSTAL SUTURE BY ISOTOPIC STUDIES OF BASEMENT INTERSECTIONS FROM OFFSHORE WELLS IN THE SOUTHERN LABRADOR SEA

Eight hydrocarbon exploration wells in the southern Labrador Sea penet rated Precambrian basement rocks, adjacent to the course of a Lithopro be marine seismic reflection survey. The rock types are mostly Early P roterozoic (1870-1800 Ma) calc-alkaline plutonic rocks or their metamo rphic derivatives, but one example is a Middle Proterozoic (1270 Ma) a norogenic intrusion. These samples of buried basement document a trans ition from juvenile Proterozoic crust in the southeast (shown by epsil on(Nd) from + 0.8 to + 2.2) to ancient Archean crust in the northwest (shown by epsilon(Nd) from - 2.7 to - 7.0). However, initial Sr isotop e ratios do not show such a clear pattern. The transition in epsilon(N d) is similar to results of onland studies in Labrador and formerly ad jacent south Greenland, and delimits the hidden edge of the North Atla ntic Archean craton within the Early Proterozoic Makkovikian-Ketilidia n Orogen. These results from close to the seismic profile confirm that regional reflectivity contrasts on either side of the isotopic bounda ry record fundamental differences between local Archean and Proterozoi c crust. Southeast-dipping reflectors that broadly correspond with the boundary zone probably represent a collisional suture zone, along whi ch juvenile terranes were probably placed structurally above the Arche an craton.