Jotunites from the Grenville Province, Quebec: petrological characteristics and implications for massif anorthosite petrogenesis

Abstract

We report new results on the geology, mineralogy and chemical compositions of jotunites (mainly pyroxene-monzodiorites) from the Morin, St. Urbain and Labrieville anorthosite complexes, all in the Grenville Province of Quebec. Jotunites occur between anorthosite and mangerite, as marginal units to anorthosite or mangerite plutons, or as fine-grained dikes in either rock type. Some occurrences are probably cumulates, but the dikes indicate the existence of jotunitic liquids. Jotunites consist predominantly of sodic plagioclase (∼An 25–An 45), clinopyroxene and orthopyroxene having intermediate X Mg (e.g., ∼En 60–En 40), along with significant ilmenite, magnetite and apatite; most samples contain small amounts of K-feldspar (locally mesoperthite) and quartz. It is noteworthy that plagioclase and pyroxene compositions in jotunites lie between those in associated anorthosite and mangerite. Jotunites from all areas span a similar range in SiO 2-content (40–55 wt.%) and are characterized by high levels of Fe 2O 3-T (⩽30 wt.%), TiO 2 (⩽6 wt.%) and P 2O 5 (⩽3.25 wt.%). All samples are alkalic, with K 2O (⩽3.6 wt.%) increasing in the sequence Morin → St. Urbain → Labrieville. They contain high Sr (up to 1400 ppm) and Ba (up to 4600 ppm) and concentrations correlate well with those in associated anorthosite in each case. Jotunites also are high in Ga (high Ga Al ), Zr, Zn, Y and REE, but low in Rb (high K Rb ), U, Th, Ni, Cr, Co, Sc and V (high Ti V . Jotunites can be linked spatially, mineralogically and chemically with associated anorthosite and mangerite, and we interpret them as transitional rocks in a largely comagmatic sequence from anorthosite to mangerite. We postulate a model for andesine- anorthosite petrogenesis involving early crystallization and accumulation of plagioclase from parental magmas of intermediate SiO 2-content—broadly “dioritic” in composition, resulting in Fe-, Ti- and P-enrichment in the remaining magma, leading to jotunite. High |(O 2) Table causes early crystallization of FeTi oxides, which suppresses Fe Mg enrichment in pyroxene. Fractionation of FeTi oxides, pyroxene and apatite ultimately results in Si- and K-enrichment in residual liquids, leading to quartz-mangerite. Significantly, liquids follow an initial trend of Si-depletion + Fe-enrichment followed by a reversal of this trend. This view contrasts strongly with current models that regard anorthosite, jotunite and mangerite as unrelated products of bimodal magmatism.