Isotopic and geochemical evidence for differentiation and crustal contamination from granitoids of the Berens river subprovince, Superior province, Canada

Abstract

Nd isotope and geochemical data are presented for biotite tonalite, hornblende tonalite, hornblende granodiorite, biotite granite and peraluminous granite from the Archean Berens River subprovince of the Superior province in northwestern Ontario, Canada. The older tonalitic suites were synchronous with greenstone belt volcanism and were followed by granodioritic and granitic intrusions. The suites vary from 57 to 74% SiO 2, are light rare-earth element (REE) enriched, with variable Eu and Nb anomalies and heavy REE depletion among the tonalites. The Nd isotope compositions for the granitoids vary from depleted values of ɛ Nd = +1.5 (tonalites) to enriched values of −1.5 (peraluminous granite). The range of isotopic composition supports a model where the granitoids were emplaced into, contaminated by, and locally melted an older pre-existing Archean crust (ca. 3.0 Ga). The Nd isotope compositions of the granites and the tonalites decrease towards the north (increased crustal contamination) away from the greenstone belts that form the southern margin. This implies a thickening of the crust towards the north and is consistent with an Andean-style convergent margin in which oceanic crust subducted northwards beneath an older Archean crustal plate. Crustal contamination increased heavy REE contents in the tonalites and decreased the Sr/Y ratios for all granitoids. Failure to account for this crustal contamination can lead to errors in estimating the degree of partial melting based on the Sr/Y ratio. Compositional differences between the biotite tonalites and hornblende tonalites likely reflect different degrees of partial melting of a garnet-bearing amphibolite source.