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MINERAL REACTIONS AT BOUNDARIES BETWEEN AMPHIBOLITE AND MARBLE IN THE SOUTHERN GRENVILLE PROVINCE, QUEBEC, CANADA

Ralph Kretz

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

In a high-grade metamorphic terrane north of Ottawa, in the southern Grenville Province of Quebec, numerous reaction-zones from 4 to 20 mm wide have formed at boundaries between amphibolite and marble. Rock compositions are used to identify elements that were added to or removed from two of these zones, and electron-microprobe data on minerals are used to construct reaction equations, which provide additional information on gains and losses. Near the village of Otter Lake, a K-feldspar-bearing amphibolite, composed of aluminous calcic amphibole [Mg/(Mg + Fe²⁺) = 0.66] and K-feldspar [Na/(Na + K) = 0.07] with minor titanite, reacted with calcite–quartz marble to produce a 6-mm zone of calcic pyroxene [Mg/(Mg + Fe²⁺) = 0.68] and scapolite [Ca/(Ca + Na + K) = 0.71; Cl:S:CO₂ = 0.1:0.1:0.8]. Sodium, Cl, and S were introduced, and Fe, K, Rb, Ba, F, H₂O and CO₂ were removed. Near Gracefield, amphibolite composed of pargasitic amphibole [Mg/(Mg + Fe²⁺) = 0.50], plagioclase [Ca/(Na + Ca) = 0.20] and minor ilmenite reacted with calcite–dolomite marble to produce a 10-mm subzone of mainly amphibole [Mg/(Mg + Fe) = 0.83, increasing toward marble to 0.87] and scapolite [Ca/(Na + Ca) = 0.37], with minor plagioclase [Ca/(Na + Ca) = 0.27] and calcite, and a 7-mm subzone (adjacent to marble) composed almost entirely of biotite [Mg/(Mg + Fe) = 0.87, increasing toward marble, to 0.92]. Here, K, P, Zn, Rb, Ba, Cl, and H₂O were introduced, and Na, Sr, and CO₂ were removed. Estimates of gains and losses per cm³ of reaction zone (both occurrences) fall in the range of 9000 to 2000 µmol for H₂O and CO₂, 2000 to 600 µmol for Na, K, and Fe, and <200 µmol for trace elements. In the Otter Lake occurrence, approximately 3 µmol of each of Si, Al, Fe, and Mg passed through the reaction zone from amphibolite to marble, relative to 12 µmol of pyroxene produced, whereas 5 µmol of Ca moved in the opposite direction. In reactions of the kind here described, the rate of migration of atoms that were added to the reaction sites, and the rate of migration of atoms through the reaction zones, have possibly both determined the overall rate of crystallization.

Keywords: amphibolite, marble, reaction zone, calcic amphibole, calcic pyroxene, scapolite, biotite, Grenville Province, Quebec.

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