Basement tracing using Mid-Proterozoic anorthosites straddling a palaeoterrane boundary, Ontario, Canada

Abstract

Abstract The St. Charles and Mercer anorthosites intrude Mesoproterozoic gneisses and granitoids in the northwestern Grenville Province of Ontario. The two intrusions are hosted by the West Bay Batholith, but lie on opposite sides of a neodymium model-age break as identified by Dickin [Can. J. Earth Sci. 37 (2000) 165]. The St. Charles and Mercer anorthosites give imprecise but essentially identical ages of ca. 1245 Ma, assuming Grenvillian lower intercepts. The high errors are largely consequences of very shallow discordia. While there are some differences in the extent of deformation (such that metamorphic facies vary from greenschist to amphibolite in both, but with more pervasive gneissosity in the Mercer Anorthosite), the age dates and setting suggest that the two intrusions have experienced essentially identical geological histories. The petrography and geochemistry of the two intrusions is dominated by the effects of plagioclase accumulation, modified by varying amounts of intersitial pyroxene and trapped liquid. The samples show chondrite-normalised REE profiles with modest LREE-enrichment and pronounced positive Eu anomalies, and high field strength element (HFSE)-depletion (Nb, Zr, Hf). The Mercer Anorthosite displays higher trace element abundances, on average, smaller europium anomalies, and more LREE-enrichment, with (La/Yb) N from 6 to 9 for the Mercer and around 2–4 for St. Charles, consistent with differences in cumulate–liquid proportions. The Sm–Nd isotopic system shows a similar distinction, whereby the Mercer samples give very consistent signatures with e Nd 1240 values of −2.1 to −2.6, while those for St. Charles are −3.9 to −6.5. The isotopic distributions are interpreted in terms of assimilation of two distinct lower crustal rock types during ascent through the crust. The St. Charles Anorthosite ascended through relatively old radiogenic Archaean basement which is comparatively incompatible-element poor, and the Mercer Anorthosite ascended through basement comprising a mixture of Archaean and Proterozoic material, with more incompatible-enriched and more isotopically juvenile signatures. This interpretation suggests that the apparent ‘suture’ between Archaean foreland and an accreted Palaeoproterozoic arc represents a significant deep-crustal lineament, in accord with previous interpretation. This study supports the efficacy of the application of anorthositic rocks as sensitive samplers of lower crust, attributable to the fact that anorthosites are mineralogically and geochemically relatively simple systems (being largely monomineralic) and that the amount of crustal assimilation is small, and therefore does not overwhelm the host signature.