Coexisting K-rich alkaline and shoshonitic magmatism of arc affinities in the Proterozoic: a reassessment of syenitic stocks in the southwestern Grenville Province

Abstract

Biotite-rich syenitic stocks in the Mont-Laurier area of the southwestern Grenville Province are shown to belong to the first recorded Proterozoic example of an ultrapotassic, K-rich alkaline and shoshonitic rock association with clear arc affinities. The plutons investigated were previously considered mostly syenitic, typical of nepheline syenite alkaline suites, slightly metamorphosed and late-tectonic with respect to the Grenville orogeny. We find that they postdate the regional metamorphism and comprise a felsic to ultramafic range of rock types belonging to two series: (1) a potassic-to-ultrapotassic, silica-undersaturated series of biotite-rich nepheline-bearing syenite, syenite, monzonite, diorite and pyroxenite, and (2) a shoshonitic, critically silicasaturated series of quartz syenite and amphibole-bearing syenite, with rare monzonite and diorite. The ubiquitous biotite, previously regarded as metamorphic, is reinterpreted as igneous and diagnostic of the potassic character. The shoshonitic and potassic series display the strong enrichment in Al, Ca, K and large-ion-lithophile elements relative to the high-field-strength elements (e.g. Ba/Nb≤722, La/YB∼45) and the low contents in Mg that are characteristic of arc-related magmas. The syenitic rocks consistently share the distinctive arc-related geochemical signature of their mafic counterparts. Syenites may thus represent a potential source of paleotectonic information for high grade terranes. Geochemical discriminants (NbN/TaN and HfN/TiN ratios) indicate that the shoshonitic and potassic series are unrelated by closedsystem fractionation processes. Rather, the chemical differences between the two series probably reflect differences in source characteristics and conditions of melting. Similar plutons occur throughout the Central Metasedimentary Belt of the southwestern Grenville Province. They define a 1089 to 1076 Ma, 450-km-long grenvillian potassic alkaline plutonic (PAP) province. The presence of this K-rich alkaline province indicates that the scarcity of K-rich rocks in the Precambrian could be only apparent and a consequence of misidentification of K-rich plutons in metamorphosed Precambrian terranes. These 1.1 Ga ultrapotassic to shoshonitic plutonic rocks are geochemically similar to shoshonites and leucitites of the Sunda arc. This similarity suggests that subduction-type enrichment processes were operating in the Proterozoic in ways similar to those of modern settings.