TEXTURAL AND GEOCHEMICAL ANALYSIS OF GARNET PORPHYROBLASTS IN THE CASTNER MABLE, WEST TEXAS, USA

The Mesoproterozoic Castner Marble contains four texturally distinct populations of garnet porphyroblasts within relatively small volumes of rock (<10 cm³). Textures include: elongate crystals in hornfels at hornfels-marble compositional boundaries; subhedral to euhedral oscillatory zoned garnet growing from the marble-hornfels boundary into marble; subhedral equant grains in marble; and, elongate garnet in marble with oscillatory zonation parallel to compositional layering. Garnet compositions are dominated by the andradite end-member with a lesser grossular component, which are inversely correlated in zoned crystals. Chondrite normalized REE patterns show a positive LREE slope, a flat MREE pattern, and a flat to negative HREE slope with higher temperature samples typically having higher total REE abundances. Equant, unzoned garnets show little variation in composition, while brighter areas in oscillatory or patchy zoned grains have higher andradite and LREE contents. Euhedral oscillatory zoned garnet in marble layers show increasing HREE content from core to rim, and reflect zones of high nucleation density along the hornfels-marble boundary. In contrast, elongate oscillatory zoned crystals may reflect compositional boundaries with low nucleation density, and individual grains grow by delivery of nutrients to crystal faces oriented sub-perpendicular to compositional layering. Both textures are surround by coronas of coarse grained recrystallized calcite suggesting some nutrients were delivered from rock volumes immediately adjacent to nucleation zones. It is proposed that nucleation density is a reflection of effective reaction volume, with low nucleation density representing crystallization of garnet in a large volume in which advection controlled element transport. Higher nucleation densities may have been promoted in smaller effective reaction rock-volumes in response to slower rates of element transport. The Castner Marble represents a geochemical system with effective reaction volumes that vary in size between a few mm³ and several cm³, which operate immediately adjacent to one another. The magnitude of fluid availability controls the scale of nucleation density and crystallization in the heterogeneous matrix.