The Harvey volcanic suite, New Brunswick; II, Postmagmatic adjustments in the mineralogy and bulk composition of a high-fluorine rhyolite

The Harvey volcanic suite, New Brunswick, of early Carboniferous age, consists of three units: 1) York Mills (base): volcanogenic sediments, minor ash-flow and laminated rhyolite; 2) Cherry Hill: ash-flow and ash-fall tuffs, quartz-feldspar porphyry, and 3) Harvey Mountain, rhyolitic lava flows. Quartz paramorphs after tridymite are present in the latter, and may indicate a high T of emplacement and devitrification (870-900 degrees C). Analyses of the feldspars indicate that, in most cases, two feldspars coexist and represent a secondary low T (<300 degrees C) assemblage. Intermediate microcline is only very locally developed. The chemical compositions of the rocks suggest highly evolved rhyolites: high Si (>76% SiO (sub 2) ), low Fe (<2% Fe (sub 2) O (sub 3) ), Mg (<0.05% MgO), Ti (<0.15% TiO (sub 2) ), and Ca (<2% CaO), high Rb, Nb, Y, Th, and low Ba, Sr and Zr. Comparison with the composition of glass trapped in quartz shows a net dilution by SiO (sub 2) added after devitrification. The mineralogy, chemical composition, peraluminous character and F-enrichment of the Harvey rocks reflect the characteristics of the F-rich magma. The Harvey rocks were probably derived by partial melting of continental crust in a rifting (extensional) environment, in response to the influx of mantle-derived basic magmas.