THE IMPORTANCE OF LATE-OROGENIC AND POSTOROGENIC CRUSTAL GROWTH IN THE EARLY PROTEROZOIC - EVIDENCE FROM SM-ND ISOTOPIC STUDIES OF IGNEOUS ROCKS IN THE MAKKOVIK PROVINCE, CANADA

Sm-Nd isotopic studies suggest that large tracts of 1900-1700 Ma old c rust in Laurentia-Baltica is of 'juvenile' origin. This crust has gene rally been ascribed to arc magmatism, sustained over long periods, and most conceptual models for crustal growth emphasize this process. The late- to post-orogenic granitoid rocks that areally dominate many anc ient orogenic belts are commonly viewed as anatectic derivatives of th e earlier arc-type crust. However, in regions of short crustal residen ce, the time resolution of isotopic tracers, such as the Sm-Nd system, does not permit discrimination between this model and continued growt h of the crust during post-orogenic magmatism. The relative contributi ons of recycled crust and new, mantle-derived material in late- to pos t-orogenic magmas can only be assessed where they also transect much o lder crustal blocks. The Nd isotopic signatures of 1800-1720 Ma igneou s suites in the Makkovik Province define such a boundary between an Ar chean craton and a juvenile Proterozoic domain. In the juvenile domain , the Nd signatures of most igneous suites are equivocal (initial epsi lon(Nd) = 0 to + 2), and they could be anatectic derivatives of slight ly older orthogneisses; however, addition of new, mantle-derived mater ial is documented by 'A-type' granites with initial epsilon(Nd) up to + 4. In the cratonic domain, temporally and compositionally equivalent igneous suites mostly have initial epsilon(Nd) of - 7 to - 3, signifi cantly above local Archean basement, which has epsilon(Nd) of - 15 at 1800 Ma. Conservative calculations suggest that most of these suites c ontain more than 50% new, mantle-derived material. These results sugge st significant crustal growth via late-stage magmatism, and direct int eraction of mantle-derived magmas and lower crustal rocks. In contrast , 1650 Ma igneous suites lack clear systematic variation in epsilon(Nd ), and are interpreted as representing crustal growth via later, dista l, arc-type magmatism, with crust-mantle mixing mainly facilitated by sediment subduction. Crustal growth in this area thus involved signifi cant addition of new material through late- to post-orogenic magmatism . This process may partially explain the unusually high growth rates c alculated for some segments of Proterozoic crust, which approach or ex ceed modern global arc addition rates. This 'stage 3' crustal growth m ay have been proportionally more important in early Earth history due to higher ambient mantle temperatures.