GARNET CHEMICAL ZONATION IN HIGH-PRESSURE MAFIC MIGMATITES IN THE NORTHERN KAPUSKASING STRUCTURAL ZONE, ONTARIO, CANADA

Locally preserved chemical zoning of garnet in lower-crustal Archean mafic migmatites offers new insight into the crustal evolution of the Kapuskasing Structural Zone (KSZ). The Groundhog River Block in the northern KSZ is dominated by granulite facies metabasaltic composition and metasedimentary rocks that display variable amounts of migmatization. Mafic migmatites preserve a wide range of metamorphic assemblages with a variation in modal mineralogy and fabric that define individual migmatite components: 1) melanosome composed predominantly of retrograde Amp and a peak-metamorphic assemblage of Grt + Cpx + Pl ± Opx ± Qz + Mgt ± Ilm, 2) leucosome composed of Pl + Qz ± Grt ± Cpx ± Opx + Ilm ± Mgt, and 3) mesosome composed of Cpx + Grt + Pl + Qz ± Amp ± Ilm ± Mgt. Detailed SEM compositional profiles were acquired across 16 garnet crystals in 4 samples representing the different migmatite components to evaluate chemical differences. Garnet crystals in the melanosome display some chemical zonation, specially a decrease in Xgr from core to rim (⁓0.20 to ⁓0.13), while there is no zoning in the garnet crystals in the mesosome except for the Xgr content which shows a minor increase from core to rim (⁓0.14 to ⁓0.18). The melt production reaction Amp + Pl + Qz à Grt + Cpx + Liq at high-pressures, likely in a chemically open system, generated chemical zonation in suprasolidus garnet in the melanosome that differs from the composition and preserved zoning in Xgr in some mesosome garnet.. Garnet embayments in the melanosome suggest some reabsorption of garnet that was unstable with the melt/matrix at lower pressures but higher temperatures. The different garnet crystals of each migmatite domain may preserve different parts of the metamorphic evolution in the northern KSZ. The contrasting Ca zoning in mesosome garnets and melanasome garnets may indicate that one is preserving prograde sub-solidus garnet growth zoning while the other is preserving suprasolidus growth zoning. Future phase equilibria modelling will test whether the decrease in Ca from core to rim represents a change in effective bulk composition during melt extraction or potential pressure decreases at high temperature. Resolving the conditions at which the garnets grew in the northern KSZ will advance our understanding the P-T evolution of the northern part of the KSZ and establish a basis for comparison with the P-T evolution of the southern KSZ.