Petrography and petrochemistry of scapolite in the Grenville of Southern Ontario

Abstract

<p> The aureole surrounding the Loon Lake pluton, notable for the presence of scapolite, is situated in an upper amphibolite facies terrane in the Grenville Province of southern Ontario. Four distinct scapolite-bearing parageneses are found to surround the pluton in more or less concentric zones. Furthest from the pluton a clinopyroxene-scapolite gneiss grades into Ca-amphibole gneiss, Ca-amphibole-biotite gneiss and biotite gneiss. Scapolite is also found in the marbles surrounding the aureole, as well as in all gneisses of the aureole. As the mineralogy of the gneisses change from clinopyroxene through biotite, the textures grade from granoblastic polygonal to a granoblastic polygonal texture modified by abundant acicular and platy minerals. </p> <p> As the pluton is approached, scapolite often assumes a ragged fine-grained nature with alteration to sericite. The whole rock chemistry is characteristic of that derived from a mixture of intermediate volcanics (latite) and carbonate-evaporites. This sequence was undoubtedly deposited in a shallow marine environment. Subsequent metamorphism to upper amphibolite facies produced a band of clinopyroxene gneiss. As the Loon Lake pluton was emplaced increasing temperature and volatiles in an aqueous solution caused the clinopyroxene-scapolite gneiss to undergo retrograde metamorphism and formed the Ca-amphibole, Ca-amphibole-biotite and biotite gneisses. </p> <p> Studies of mineral chemistry indicate that the minerals coexist in equilibrium and that they are related, to some degree, to whole rock composition. As the retrograde metamorphsim occurred elements within the minerals redistributed themselves according to the nature of the new phases produced. Finally, the partitioning between phases followed the ideal binary solution model. Chlorine is found to be partitioned between scapolite, Ca-amphibole and biotite, and all three phases contain chlorine, the relative amounts present being controlled by the crystal structure of the minerals. The relation of Lin and Burley (1973a) between lattice parameter and scapolite composition is confirmed. </p>