Low-temperature metasomatic garnets in marine sediments

Abstract

The paragenesis of low-temperature metasomatic calcium-iron garnets or hydrogarnets in marine sediments is discussed. Oxygen isotope data indicate that the garnetiferous (micarb) chalk from DSDP site 251, southwest branch, Indian Ocean Ridge (gros ₅₂ and ₄₀ py ₆ alm _1.5 sp _0.5 ), formed at about 170 °C from circulating seawater hydrothermal solutions passing through the underlying altered basalts. Other possible examples of the hydrothermal metasomatic formation of garnets are found on bleached coprolites in the basal Bone Bed at the junction of the Rhaetic and the underlying Triassic Tea Green Marls at Larne, Antrim, in association with montmorillonite, analcime, magnetite, fluorite, and calcite, near to Tertiary basalts (and _89.4 gros _6.1 sp _3.1 alm _1.0 py _0.4 ) ; in recrystallized chalk and glauconitic sandstones, associated with zeolites and other minerals, near a dolerite dyke, at Portmuck, Antrim (andraditic); and in a calcareous sedimentary lens in bedded trachytic tuffs, cut by dolerite dykes, at Fuerteventura, Canary Islands (gros _94.5 and _5.0 py _0.5 ). An extensive search through DSDP basal sediments has revealed no further examples.