Genesis of mafic migmatites from the Kapuskasing structural zone, Ontario, Canada

Abstract

Interlayered tonalitic and mafic gneisses from the Kapuskasing Structural Zone have been metamorphosed to upper-amphibolite/granulite facies. The mafic gneisses are heterogeneous, comprising diopside-garnet-plagioclasesphene- rich (Di-Grt-Pl-Spn) layers, hornblende-plagioclase-rich (Hbl-Pl) layers, and all gradations in between. Plagioclase-quartz-rich (Pl-Qtz) leucosomes occur in a variety of textures in the mafic gneiss, ranging in scale from ?m to dm. Leucosome is generally associated with the hornblende-rich layers and gives the rock a migmatitic appearance. A combination of outcrop textures, microscope textures, modal mineralogy, and mineral analyses suggest that hornblende and plagioclase reacted to form diopside, garnet, sphene and trondhjemitic leucosome. Mass balance analysis, using matrix-algebraic techniques, resulted in the following continuous migmatite-forming reaction consistent with textural, modal and chemical data: Hbl + 0.95 MgFe.1 + Ca-Pl + 0.16 NaSiCa_1A1_1 + 3.42 Qtz => => 1.41 Grt + 1.13 Di+ 0.19 Spn + 1.45 Pl-Qtz (leucosome) Textural, mineralogical and chemical evidence suggest that the plagioclasequartz leucosomes and films (micro-leucosomes) may represent crystallized trondhjemitic melt that formed according to the above dehydration-melting reaction. This is consistent with estimated P-T conditions of 740°C and 9 kbar (=27 km depth) for the KSZ migmatites.