Computer Science
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994e%26psl.125...71k&link_type=abstract
Earth and Planetary Science Letters (ISSN 0012-821X), vol. 125, no. 1-4, p. 71-88
Computer Science
14
Earth Crust, Earth Mantle, Geochemistry, Geochronology, Geomorphology, Igneous Rocks, Neodymium Isotopes, Orography, Samarium Isotopes, Volcanology, Earth (Planet), Magma, Mountains, Planetary Evolution
Scientific paper
Sm-Nd isotopic studies suggest that large tracts of 1900-1700 Ma old crust in Laurentia-Baltica is of 'juvenile' origin. This crust has generally 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 ancient orogenic belts are commonly viewed as anatectic derivatives of the earlier arc-type crust. However, in regions of short crustal residence, the time resolution of isotopic tracers, such as the Sm-Nd system, does not permit discrimination between this model and continued growth of the crust during post-orogenic magmatism. The relative contributions of recycled crust and new, mantle-derived material in late- to post-orogenic magmas can only be assessed where they also transect much older crustal blocks. The Nd isotopic signatures of 1800-1720 Ma igneous suites in the Makkovik Province define such a boundary between an Archean craton and a juvenile Proterozoic domain. In the juvenile domain, the Nd signatures of most igneous suites are equivocal (initial epsilon(sub Nd) = 0 to +2), and they could be anatectic derivatives of slightly older orthogneisses; however, addition of new, mantle-derived material is documented by 'A-type' granites with initial epsilon(sub Nd) up to +4. In the cratonic domain, temporally and compositionally equivalent igneous suites mostly have initial epsilon(sub Nd) of -7 to -3, significantly above local Archean basement, which has epsilon(sub Nd) of -15 at 1800 Ma. Conservative calculations suggest that most of these suites contain more than 50% new, mantle-derived material. These results suggest significant crustal growth via late-stage magmatism, and direct interaction of mantle-derived magmas and lower crustal rocks. In contrast, 1650 Ma igneous suites lack clear systematic variation in epsilon(sub Nd), and are interpreted as representing crustal growth via later, distal, arc-type magmatism, with crust-mantle mixing mainly facilitated by sediment subduction. Crustal growth in this area thus involved significant addition of new material through late- to post-orogenic magmatism. This process may partially explain the unusually high growth rates calculated for some segments of Proterozoic crust, which approach or exceed modern global arc addition rates. This 'stage 3' crustal growth may have been proportionally more important in early Earth history due to higher ambient mantle temperatures.
Fryer Brian J.
Kerr Andrew
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