Volcanic rocks of Mississippian age occur on the Magdalen Islands as cap rocks and within collapse breccias above salt diapirs that have formed the islands. They consist of coarse volcaniclastic deposits and basaltic flows, intruded by minor mafic dykes and plugs. Petrologic studies of the basaltic rocks show that they are extensively altered. Original plagioclase, clinopyroxene, olivine, and interstitial glass are partially to entirely replaced by mixtures of chlorite, sericite, smectite, sphene, carbonate, epidote, albite, potassium feldspar, and iron oxides, and the samples display a relatively wide range in chemical compositions. Especially mobile were K, Na, and Ca, and most samples are classified as potash spilites (poenites). Using standard discriminant diagrams for mafic igneous rocks, it can be seen that the basalts appear to range from continental tholeiitic to continental alkalic. However, relict clinopyroxene compositions and the presence of kaersutitic amphibole and titaniferous biotite in some samples imply that the suite may originally have been more alkalic than tholeiitic.

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