The geochemistry of mafic gneisses from the Renzy terrane, western Grenville Province, Quebec: Implications for the geodynamic setting of the early Mesoproterozoic Laurentian margin

Abstract

This study presents new geochemical data on mafic gneisses from the Renzy terrane (RT), located in the Grenvillian Parautochthonous Belt of western Quebec. This region is of particular interest since it contains one of the rare Ni–Cu–Co massive sulfide deposits ever mined in the Grenville Province. RT mafic gneisses have been divided into 3 groups based mainly on major element compositions, trace element profiles normalized to the primitive mantle, and incompatible trace element ratios. The behaviour of high field strength elements (HFSE) (Ta, Zr, Hf, Ti, Th) and large ion lithophile elements (LILE) (Ba, Rb) are particularly useful for discriminating RT mafic gneisses. Major and trace element compositions of group 1 mafic/ultramafic gneisses are very similar to those of the RT ultramafic sheets; the mafic gneisses are therefore interpreted as melagabbroic sills cogenetic with these sheets. The trace element composition of group 1 mafic gneisses is typical of magmas formed from a previously metasomatized depleted mantle source. These rocks have high Th/Ta ratios and show strong to moderate negative anomalies in Ti, Zr, and Hf on multi-element plots normalized to the primitive mantle. Mafic gneisses of group 2 show geochemical features transitional between extensional and compressional settings. These rocks generally have low concentrations of HFSE and high concentrations of LILE, typical features of mafic rocks formed in a compressional environment; however, some of them are Fe–Ti rich, with minor or no negative anomalies in HFSE and low to moderate concentrations of LILE on multi-element diagrams normalized to primitive mantle, features more typical of an extensional environment. Mafic gneisses of group 3 have a geochemical signature typical of extensional environments or of ocean island basalt (OIB). These gneisses have no negative anomalies in HFSE and have low concentrations of LILE. RT mafic gneisses are similar in composition to mafic granulites in the Bondy gneiss complex in the Central Metasedimentary Belt and to Shawanaga Domain amphibolites from the Central Gneiss Belt, two paleoarc/back-arc environments in the Grenville Province. Based on the contrasting geochemical signatures found in the RT mafic gneisses and the similarities with known arc/back-arc settings in the Grenville Province, we propose that the RT was formed by arc/back-arc magmatic activity most likely associated with a period of arc magmatism along the Proterozoic Laurentian margin. OIB-like mafic gneisses found in the RT are similar to those from the Kipawa region, formed by intraplate continental magmatism in the Parautochthonous Belt of the Grenville Province.