THE IMPORTANCE OF CONSIDERING CARBONIC FLUID COMPONENTS AND CARBONATE MINERALS WHEN MODELING THE INITIAL GROWTH OF GARNET IN METAPELITES

Metamorphic P-T assemblage diagrams for metapelites and contours of garnet composition often suggest that the initial growth of garnet has overstepped the theoretical initial garnet-in reaction in terms of temperature. Overstepping by as much as 50°C has been reported. The production of garnet is usually assumed to be related to a continuous destruction of chlorite. This assumption is problematic for growth of spessartine-rich garnet because it would require the presence of a very Mn-rich chlorite. In metamorphosed black shales, garnet cores often have close to 50% spessartine component but that proportion substantially decreases with further growth of garnet, which suggests that most Mn is acquired by garnet right away as it nucleates. By the onset of sillimanite isograd, garnet may have only 15% spessartine component. In sillimanite-grade conditions, evidence that the protolith of the schist was a black shale may be lost because most organic matter, including graphite, would likely have been consumed by reaction with H₂O.

Within the Proterozoic terrane of the Black Hills, South Dakota, spessartine-almandine garnet nucleated on small crystals of rhodochrosite-siderite solution within chlorite-bearing, graphitic schists. Quartz veins in the chlorite-zone schist contain fluid inclusions dominated by CH₄, CO₂, and N₂ but at higher grades are mostly a combination of CO₂-H₂O. The fluid species are derived from organic components within the sedimentary protoliths. Garnet formed by the reaction

MnFe(CO₃) + Ms + Chl + Qz → Grt(alm,sps) + Bt + H₂O + CO₂.

A themodynamic model that includes data for rhodochrosite-siderite and assumes variable C/H ratio in fluid shows the reaction occurring at c. 450°C and C/H = 0.07 to 0.2 when P = 3 to 4 kbar and H/O ratio in the fluid = 2. The temperature is consistent with conventional garnet-biotite thermometry and the reaction does not appear to have been overstepped. Because the Black Hills terrane typifies collisional metamorphic terranes elsewhere, carbonate minerals and variable C/H ratios in fluids should be considered in mineral assemblage models for schists metamorphosed during continental collisions.