Computer Science
Scientific paper
Dec 1980
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980gecoa..44.2033v&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 44, Issue 12, pp.2033-2044
Computer Science
6
Scientific paper
The greenstone belt of Suomussaimi is composed of a lower volcanic sequence of komatiitic and tholeiitic composition, a middle sedimentary unit and a upper volcanic sequence that is mainly andesitic-rhyodacitic in composition. These series have been folded and then intruded by granitic to granodioritic plutons. The surrounding basement is mainly composed of migmatitic gneisses which have undergone a complex tectonometamorphic history. The Pb-Pb and Rb-Sr systematics reveal that 1. (1) All these rocks have been emplaced at about 2.65 b.y. and are therefore Archean in age; 2. (2) all the events in the basement and the belt lasted not more than the analytical uncertainties, that is 0.2 b.y.; 3. (3) all the rocks probably originated from source regions (the mantle and its derived basic rocks) characterized by similar isotopic compositions; the involvement of much older continental rocks was negligible; 4. (4) the basement gneisses have low calculated second stage values (1 7) and high calculated Th/U ratios (5 20); in contrast, the belt rocks (with the exception of komatiitic basalts) have second stage values from 6 to 18 and Th/U ratios from 2 to 5. These features can be interpreted as the result of U-depletion in a high grade metamorphism prior to the belt formation or due to the melting of underlying granulite facies crust; 5. (5) K-feldspars Pb-Pb systematics and biotites Rb-Sr ages show the influence of the 1.7 b.y. Svecokarelian thermal event; 6. (6) almost all the rocks have undergone a very strong recent U. depletion.
Blais S.
Capdevila Raymond
Jahn Bor-Ming
Tilton George R.
Vidal Ph.
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