Other
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59...97c&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 1, pp.97-105
Other
2
Scientific paper
Samples from six localities in the late Archean Mulcahy layered intrusion of the Wabigoon greenstone belt have been analysed for their U and Pb concentrations and their lead isotopic composition in order to estimate the initial isotopic composition of the intrusion. Leaching experiments have been performed on plagioclase fractions and seven out of nine residues have yielded very similar calculated initial compositions ( ; ). These seven plagioclas residues also define a U-Pb isochron corresponding to an age of 2764 ± 20 Ma from which initial 206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios of 13.33 and 14.43, respectively, are obtained, indicating that the Mulcahy intrusion had a homogeneous initial lead isotopic composition. Plagioclase and whole-rock fractions from a titanomagnetite-rich layer near the top of the middle zone of the intrusion has yielded a Pb-Pb age of 2722 ± 4 Ma, which is only slightly younger than the reported U---Pb zircon age of 2733 ± 1 Ma (MORRISON et al., 1985) for the same sample. Plagioclase, clinopyroxene, orthopyroxene, and magnetite mineral separates from the five other samples, however, yield older internal Pb---Pb ages, ranging between 2.76 and 2.80 Ga, and all of the data lie on a single line in the Pb---Pb isochron diagram, corresponding to an age of 2787 ± 14 Ma. This is believed to represent the crystallization age of the bulk of the Mulcahy intrusion. The 2733 Ma event may be the age of a small influx of magma intruded late after the bulk of the intrusion had crystallized or of a sill intruded within the middle zone and possibly associated to the adjacent 2732 Ma Atikwa batholith. When compared to available initial lead isotopic composition of various rocks from other greenstone belts of the Superior Province, the Mulcahy intrusion has one of the least radiogenic initial lead isotopic compositions, identical to that of komatiite lava flows from the Abitibi belt. These rocks are believed to represent melts of a late Archean depleted mantle component, which was homogeneous at that time, at the scale of greenstone belts of the Superior Province. Syn- to late-tectonic rocks in the Wawa, Wabigoon, and Abitibi belts have homogeneous and more radiogenic 207 Pb/ 204 Pb initial ratios than the depleted mantle values, indicating the melting of an enriched component of the mantle at that time, whereas post-tectonic plutons chiefly resulted from the melting of crustal precursors in a thickened lithosphere in response to the juxtaposition of greenstone belts and metasedimentary-plutonic-gneiss belts.
Carignan Jean
Gariepy Clément
Machado Nuno
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