VARIATION OF CHLORITE CHEMISTRY IN METAMORPHIC ROCKS OF THE BLACK HILLS, SOUTH DAKOTA

Petrographic and microprobe data obtained from chlorite are used to show relationships of chlorite textures and chemistry to variations in metamorphic grade and bulk composition in metamorphic rocks from the Black Hills, South Dakota.

Chlorite bearing pelite samples collected from biotite through staurolite zones in the Black Hills, South Dakota are the basis of this study. Previous studies on pelites in the Black Hills from the biotite through staurolite zones indicate a temperature range of 350 to 600 degrees Celsius and a pressure range of 2-4 kb. The samples used in this studies are found in assemblages ranging from: chlorite + muscovite + quartz + biotite (biotite zone) to chlorite + garnet + biotite + muscovite + quartz + staurolite ± plagioclase (staurolite zone).

Chlorite occurs in several textural habits ranging from: fine-grained matrix, inclusions in garnet, reaction textures with biotite, to porphyroblasts which crosscut foliation. Most textural evidence indicates late metamorphic growth during the emplacement of Harney Peak Granite. Variation diagrams are used to show the compositional and metamorphic grade influences on chlorite chemistry. Variations include a tendency of increasing tetrahedral aluminum with increasing metamorphic grade, and a decrease of octahedral iron and manganese. The manganese content of Black Hills chlorite is significantly higher than that found in other metamorphic terrains of a similar petrogenetic setting.