IMPLICATIONS OF ZONING IN GARNET FOR ITS GROWTH AND REEQUILIBRATION IN BLACK HILLS, SOUTH DAKOTA, METAPELITES

Garnets in metapelites in the Black Hills, South Dakota, were analyzed to determine the conditions of their growth and chemical reequilibration. The metapelites are the product of regional metamorphism beginning at ~1755 Ma due to the collision of Wyoming and Superior cratons and subsequent contact metamorphism by intrusion of the Harney Peak Granite. Garnet occurs in regional garnet-biotite zone and in contact-metamorphic zones that range from staurolite to sillimanite. Chemical zoning was determined by electron backscatter (BSE) imaging and x-ray mapping using an electron microprobe. Garnets display complex zoning patterns in Fe, Mn, Ca, Mg and Y. Garnets in the garnet-biotite zone have inclusions-rich centers, which are relatively dark in BSE images, and almost inclusions-free dodecahedral projections into the groundmass. Cores of the garnets contain high-Y and have fairly sharp boundaries. A high-Y annulus near rims marks growth of the dodecahedral projections. Concentrations of Ca are low in cores, but sharply increase near the rims. Mn decreases slightly from cores toward rims while Mg and Fe increase and are relatively high in the projections. However, the zoning of Mn, Mg, and Fe is more gradual than that of Ca and Y. BSE images of garnets in staurolite zone contain very bright cores, dark regions around the cores and relatively bright inclusion-free rims. Relatively high Ca in cores precisely coincides with the edges of dark regions in BSE images. High Y concentrations and the bright cores in BSE images overlap. Some garnets also have elevated Y near rims. Mn concentrations are higher in cores than rims, but the zoning does not correspond to the dark or bright regions of BSE images. Zoning of Fe and Mg is opposite to that of Mn and can be attributed to elevated temperatures in the aureole of the Harney Peak Granite. Relatively smooth concentrations profiles suggest that Fe, Mg, and Mn were partly modified by diffusion, in contrast to Ca and Y that show sharp breaks in their zoning patters. The high-Y cores in both the garnet-biotite zone and within the contact aureole suggest that the initial conditions for the crystallization of the garnets were similar throughout the Black Hills metapelites.