Formation of jasper and andradite during low-temperature hydrothermal seafloor metamorphism, Ongeluk Formation, South Africa

Abstract.

Andradite and andradite–grossular solid solutions have repeatedly been described as minor constituents of weakly metamorphosed ocean-floor metabasites and associated calcareous or siliceous sediments. The conditions of garnet formation, however, have remained rather elusive. Here, wedescribe the discovery of andradite as a minor but widespread constituent of chert and jasper closely associated with only very weakly metamorphosed basaltic andesites of the Early Paleoproterozoic Ongeluk Formation, Transvaal Supergroup, South Africa.The andradite is most commonly fine-grained and forms skeletal crystals that poikilotopically enclose and replace quartz and hematite. Macroscopically coarse-grained andradite is scarce and was recognized at one locality only. Up to 3-mm largeandradite grains occur closely associated with quartz in short veins that crosscut thin beds of andradite-bearing jasper. The cogenetic association of quartz and andradite was studied in detail to constrain the conditions of garnet formation. Our results indicate that andradite crystallized at temperatures well below 200 °C in the sub-seafloor environment in siliceous sediments of hydrothermal origin that provided an ample source of SiO₂ and Fe₂O₃. Andradite formed when hydrothermally modified seawater introduced Ca that was leached when seawater interacted with plagioclase and clinopyroxene in the surrounding volcanic pile. Other important preconditions for garnet formation were the lack of mixing between the hydrothermal fluid and cold seawater and low fCO2. It appears reasonable to extend the findings of this study to other recorded occurrences of andradite in modern and ancient ocean-floor metabasites.