Publication Type:

Journal Article


Canadian Mineralogist, Mineralogical Association of Canada, Volume 40, Number 5, p.1287-1311 (2002)




Basalt, cooling, Glass, silicates, textures


The Jurassic (201 Ma) quartz-normative continental tholeiitic basalts of the North Mountain Basalt (NMB) Formation of southern Nova Scotia, Canada, record evidence of pervasive silicate-liquid immiscibility. The basalts, up to 400 m thick, typically with phenocrysts of plagioclase (An50-70) and clinopyroxene (Wo40 En40Fs20), are subdivided into lower, middle, and upper units on the basis of the nature of the flows and petrographic features. Petrographic observations combined with image analysis indicate that the middle and upper units are characterized by an abundance of mesostasis material (i.e., quenched residual melt) that contains skeletal clinopyroxene, acicular plagioclase, skeletal Fe-Ti oxides, Fe-Ti-P. rich globules, and andesitic to rhyolitic glass. Raster analysis of the mesostasis tracks an in situ fractionation that culminated in formation of an interstitial felsic glass (i.e., 74 wt.% SiO2) that contains skeletal apatite and Fe-rich clinopyroxene. The composition of the skeletal clinopyroxene, ca. Wo20En15Fs65, acicular plagioclase (An35-55, &le2.5 wt.% FeO), Fe-Ti-P-rich globules, and intergranular glass of the mesostasis are consistent with formation from an interstitial melt that evolved in a disequilibrium environment promoted by rapid cooling of the lavas. The mineralogical and chemical features are consistent with the process of silicate-liquid immiscibility within the residual liquids of basaltic systems, in this case late-stage intergranular melts. The presence within the NMB of (1) Fe-rich clinopyroxene-bearing mafic pegmatite, (2) thin (i.e., 1-2 cm) seams of rhyolite associated with the mafic pegmatites, and (3) locally abundant segregation pipes of mixed mafic-felsic composition, is considered to reflect mobilization of the Fe-and silica-rich immiscible melts.


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