| Abstrakt: |
Detailed petrographic, microstructural, and chemical analysis of specimens collected at Garnet Hill, central Paulding County, Georgia, provide insights on the conditions that led to growth of large (> 1 cm) garnet porphyroblasts during Appalachian metamorphism. Relationships between fabrics of minerals included in garnet porphyroblasts and matrix minerals indicate that lower amphibolite facies metamorphism overlaps deformation and continued after deformation ceased. Garnet cores overprint an early foliation fabric that curves into parallelism, potentially due to porphyroblast rotation, with the matrix fabric; garnet cores are wrapped by fabrics of minerals included in the garnet mantle. Shielded by the garnet, inclusion assemblages also show key differences from the matrix mineralogy. Chloritoid and K-rich white mica are preserved in the garnet mantle; chlorite and Na-rich white mica are found exclusively in the matrix. Preliminary exchange thermometry for garnet core, mantle, and rim give the following approximate temperatures for the growth of garnet along a prograde path: nucleation at 400 °C, growth of mantle at 460 °C and growth of skeletal rim at 510 - 550 °C. Continuing work will determine whether the sharp transition in garnet chemistry and inclusion fabrics found at the core-mantle boundary in the porphyroblasts represents a brief hiatus or a profound time break in garnet growth representing two distinct orogenic pulses. [ABSTRACT FROM AUTHOR] |
