Orogenic high-T granitoids of the central Grenville Province: characteristics and geodynamic implications

Conventional models of collisional orogens predict S-type granitoid emplacement during the orogenic climax, whereas high-T (also known as A-type) granitoids, commonly associated with mantle-derived magmas, are expected to form by late-orogenic extensional collapse. However, the Grenville Province lacks significant syn-orogenic S-type granites, while high-T granitoid bodies were intermittently emplaced in its hinterland throughout the duration of the Grenvillian Orogeny. These rocks are particularly abundant in central Grenville, where they intrude Mesoproterozoic Quebecia crust, in the granulite-facies mid and low-P segments of the orogenic hinterland and are spatially associated with ~1.16–1.13 Ga anorthosites. A review of a literature database shows that the high-T granitoids of the central Grenville Province were emplaced intermittently in the orogenic hinterland between ca. 1086 and 1007 Ma (with an apparent gap ~1042–1032 Ma) and are dominantly ferroan, metaluminous to weakly peraluminous, alkali-calcic, with strong negative HFSE anomalies. In addition, a petrological and geochemical investigation of samples from four granitoid bodies suggests crystallisation conditions at ~5–6 kbar and ~800–1000ºC and variable oxidising conditions. Within-plate transitional to arc-like geochemistry, oxidising ƒO₂ values, relatively evolved whole-rock εNd and zircon εHf with Paleo- to Mesoproterozoic TDM suggest significant inheritance from the Quebecia arc crust. The granitoids may have originated from the anhydrous, partial melting of the intermediate, granulitic mid- to lower crust. Irrespective of the magma sources, orogenic high-T magmatism over a span of ~80 Myr reflects substantial mantle heat flux. The high concentration of these granitoids and the ~1.16– 1.13 Ga anorthosites in Quebecia suggests that orogenic magmatism is heavily influenced by lithospheric structures inherited from the evolution of Quebecia in the pre-Grenvillian Mesoproterozoic Laurentian margin.