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STRONTIUM IN FELDSPARS OF HIGH-K PROTEROZOIC IGNEOUS ROCKS OF THE ROBITAILLE SUITE, BUCKINGHAM, QUEBEC

Donald D. Hogarth and Michel J.L. Robin

Department of Earth Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada

E-mail address: dhogarth{at}uottawa.ca

Near Buckingham, Quebec, Sr-, K-rich rocks exposed in small, Mesoproterozoic, volcano-plutonic bodies of the Robitaille suite overlie or intrude the Grenville metasedimentary sequence and are cut by Neoproterozoic diabase dykes. They may represent near-surface equivalents of the Mont Laurier – Marmora alkaline plutons. The rocks includes latite and alkali-feldspar trachyte, lamprophyres (kersantite and minette), monzodiorite and alkali-feldspar syenite. Ultrapotassic compositions are common. The complexes, up to 2.7 km long and 1.0 km wide, are partly fringed with aureoles of Sr-rich, dolomitized calcitic marble. In 20 samples from three igneous complexes, Sr (up to 3960 ppm) is present in celestine (very rare), barite (rare), fluorapatite and calcite, but feldspars are the main hosts. K-feldspar without plagioclase occurs in samples with K₂O/Na₂O > 15 (weight ratio), and K-feldspar + plagioclase in samples with a lower ratio. Individual grains of plagioclase in the groundmass of lamprophyre are unzoned, but distinct ranges of An content are found in grains from restricted areas within a single polished thin section. The amount of Sr increases linearly with An content, from near zero at end-member albite to 1.0–2.6 wt% SrO at An₄₀. In one sample of latite, Sr peaked at An₃₀. Variation of Sr within a single sample is due to both differentiation (mainly by crystallization of sanidine) and equilibrium factors (such as temperature and An content). Clustering of point compositions on An – wt% diagrams for individual samples may reflect periods of volcanicity, separated by quiescent intervals, as differentiation evolved continuously in the parent magma-chamber. Barium is low, but can be correlated with Sr in one kersantite sample. The distribution of Sr between contiguous K-feldspar and plagioclase grains is governed by disequilibrium factors at low An content. For feldspar pairs, Sr preferentially enters more calcic plagioclase (An > ca. 10), but K-feldspar where plagioclase is more sodic (An < ca. 10).

Keywords: strontium, barium, lamprophyre, alkali-feldspar trachyte, ultrapotassic rock, plagioclase, sanidine, microcline, Robitaille suite, Buckingham, Quebec.