Nd Model Age Mapping of the Central Grenville Province in the Lac St. Jean Region, Quebec

Abstract

<p> Detailed Neodymium ( Nd ) and Lead ( Pb ) isotopic mapping was performed on gray gneisses from the Lac St. Jean region of Quebec, in the Central Grenville Province. The Nd model ages determined were based upon the Depleted Mantle Model of DePaolo ( 1981c ). The use of this model is justified by a Sm-Nd whole-rock isochron as well as common Pb-Pb whole-rock dating. </p> <p> The field area was divided up into three sections: Western, Central, and Eastern. The boundaries of these divisions corresponded to abrupt changes in Nd model ages. The Western section was located in the Parautochthonous Belt of Rivers et al., ( 1989 ) and almost exclusively consisted of tonalitic gray gneisses with Archean Nd model ages. Whole-rock Pb isotopic analyses produced a Transposed Palaeoisochron, implying that the rocks of the Western section were significantly affected by the Grenville event approximately 1.1 Ga ago. Major and trace element analyses indicate that the Western section represents an arc and may be the exhumed basement of the Abitibi Volcanic Belt to the north, consistent with previous work by Ciesielski ( 1992 ).</p> <p> In contrast to the Western section, the Central section which is separated from the Western section by the Allochthon Boundary Thrust of Rivers et al., ( 1989 ) has a very wide range of Nd model ages from 1.65 Ga to 2.35 Ga. The temporal and spatial range of Nd model ages, coupled with major and trace element analyses, indicates that the Central section represents an Andean style ensialic continental margin arc. The diapirs that reworked these rocks contain varying amounts of Archean material implied by the Early-Proterozoic Nd model ages of some samples, particularly in regions close to the Allochthon Boundary Thrust. Whole-rock Pb isotopic analyses produce a Transposed Palaeoisochron indicating significant U depletion caused by the Grenville orogeny.</p> <p> The Eastern section is composed almost exclusively of gneisses with Nd model ages of ca. 1.5 Ga. The boundary between the Central and Eastern sections is based solely on lithological differences and an isotopic age break form the variable Nd model ages of the Central section, to the consistent model ages of the Eastern section. A Sm-Nd isochron for all samples with ca. 1.5 Ga Nd model ages produces an age of 1.50 +/- 0.07 Ga, in close agreement with previous work ( Dickin and Higgins, 1992 ). The consistency of model ages ( > 100,000 Km^2 when combined with data from Dickin and Higgins, 1992 ) coupled with major and trace element analyses, indicates the presence of an island arc which formed at ca. 1.5 Ga and was sutured on to the Laurentian craton within 100 Ma of its formation.</p> <p> Whole-rock Pb isotope analyses indicate that the Eastern section was not affected by the Grenville event as much as the Central or Western sections were.</p> <p> Pb-Pb zircon dating for samples from the Central and Eastern sections failed to place a tight constraint between crystallization ages and Nd model ages because of excessive Pb loss.</p>