Trace-element and Nd isotopic variations in Early Proterozoic dyke swarms emplaced in the vicinity of the Kapuskasing structural zone: enriched mantle or assimilation and fractional crystallization (AFC) process?

Abstract

Several Early Proterozoic Hearst–Matachewan (2.454 Ga), Kapuskasing (2.14 Ga), and Preissac (2.04 Ga) dykes were emplaced within the Archean crust surrounding the Kapuskasing structural zone (KSZ). The dykes are composed of moderately to highly fractionated tholeiitic basalts (Mg number = 24–55) that exhibit trace-element characteristics similar to those of intraplate basaltic magmas or ocean–island basalts (e.g., Zr/Nb = 6–21, Zr/Y = 2–5, high TiO2 = 0.9–3.2 wt.%, and (Fe2O3)t = 12.4–18.7 wt.%). Their initial Nd isotopic compositions display a range of depleted [Formula: see text] to enriched [Formula: see text] values that are negatively correlated with the degree of light rare-earth element enrichment. We evaluate two models for the origin of these dykes: (i) The basaltic parental magmas were derived from two distinct sources, an isotopically depleted asthenospheric mantle (εNd = +4 and La/Sm = 2.7) and an isotopically enriched lithospheric(?) mantle (εNd = −4 to−8 and La/Sm = 5.1). The magmas subsequently underwent mixing and fractionation during ascent in the mantle or the lower crust. (ii) The parental magmas originated from a homogeneous Nd isotopically depleted asthenospheric mantle but later assimilated a substantial amount of Archean crustal material upon fractionation and ascent in the lower crust. Results derived for the latter model preclude any participation of the exposed crustal rocks in the KSZ, and the assimilation and fractional crystallization (AFC) model remains a viable hypothesis only if the parental magmas assimilated an older and perhaps more isotopically enriched crust than that represented in the KSZ.

Continental tholeiitic mafic rocks of the Paleoproterozoic Hurwitz Group, Central Hearne sub-domain, Nunavut: insight into the evolution of the Hearne sub-continental lithosphere

“…An extended trace-element plot (Fig. 9B) demonstrates that the elements Nb, Ta, P, and Ti are depleted relative to other elements for all of the samples, a feature characteristic of magmas generated at volcanic arcs (Pearce 1982), but also observed in many continental tholeiites worldwide (Weaver and Tarney 1981;Thompson et al 1983Thompson et al , 1984Nelson et al 1990;Boily and Ludden 1991;Cadman et al 1993;Zhao and McCulloch 1993) that have been interpreted to have been derived, at least in part, from a sub-continental lithospheric mantle source. If the primary magma(s) interacted with continental crust during ascent, then the observed elemental variations imply that the assimilant had a remarkably consistent composition.…”

“…Floyd 1989;Kerr et al 1996;Neal et al 1997;White et al 1999;Hauff et al 2000;Kerr et al 2000;Tejada et al 2002) or as tholeiitic to alkalic dykes and pillowed to massive subaerial flows in continental flood basalt provinces (cf. Weaver and Tarney 1981;Menzies et al 1984; Thompson et al 1983Thompson et al , 1984Waters et al 1990;Boily and Ludden 1991;Cadman et al 1993;Baker et al 1996; Baragar et al 1996;Condie 1997). Identification of ancient, Precambrian examples of flood basalts is difficult because of their commonly deformed and metamorphosed nature and, moreover, because they have undergone substantial erosion.…”

“…Petrochemical studies of continental within-plate basaltic rocks, comprising both mafic dike swarms (e.g., Weaver and Tarney 1981;Boily and Ludden 1991;Cadman et al 1993Cadman et al , 1997Baragar et al 1996) and plateau basalt sequences (e.g., Menzies et al 1984; Thompson et al 1983: Gibson et al 1996Kent et al 1997;Sharma 1997;Chandrasekharam et al 1999;Baker et al 2000) have discussed the roles of numerous distinct sources in the petrogenesis of these controversial rocks. Possible sources include asthenospheric mantle, sub-continental lithospheric mantle (SCLM), or plumelike mantle (OIB, ocean-island basalt), and the petrogenesis may involve fractional crystallization-assimilation of crustal material.…”

“…On the basis of their variable geochemical and isotopic characteristics Thompson et al (1984) outlined a number of major geochemical types of continental within-plate suites and emphasized that they were characterized by distinct extended trace-element patterns reflecting one or more possible sources. Many researchers now consider that two or more distinct mantle sources (asthenospheric, plume-like or OIB, and SCLM) give rise to continental intraplate basalts (Weaver and Tarney 1981;Dostal et al 1989;Paces and Bell 1989;Leat et al 1989;Waters et al 1990;Turner and Hawkesworth 1995;Condie 1997), but controversy over the importance of assimilation -fractional crystallization (AFC) processes in the crust persist (Boily and Ludden 1991;Dostal et al 1989;Baker et al 1996Baker et al , 2000.…”

Evolution of Palaeoproterozoic mafic intrusions located within the thermal aureole of the Sudbury Igneous Complex, Canada: Isotopic, geochronological and geochemical evidence

“…There are a number of Palaeoproterozoic mafic dyke swarms in the map area which are not shown, both in the Superior and Southern Province. These are, however, relatively unhelpful from the point of view of geochemical comparison, since their compositions, where geochemical data are available, are typically highly variable and probably very strongly influenced by local host-rock contamination (e.g., Boily and Ludden, 1990;Nelson et al, 1990), although it has been suggested more recently that elements of primary plume signature may be preserved in Hf/Zr ratios in the Matachewan dyke swarm (Phinney and Halls, 2001). While this is less true of the Nipissing Diabases and Huronian volcanics, they are also strongly affected by combined crustal assimilation-fractional crystallisation processes, such that isotopically and compositionally primitive compositions are extremely rare in the former case (e.g., Lightfoot et al, 1993) and unknown in the latter (e.g., Jolly et al, 1992).…”