ANDRADITE-UVAROVITE SOLID SOLUTION FROM HYDROTHERMALLY ALTERED PODIFORM CHROMITITE, RUTLAND OPHIOLITE, ANDAMAN, INDIA

Abstract

Compositions of chromium-bearing garnet with little grossular content belong to the uvarovite–andradite solid-solution series. They are extremely rare in nature. Here, we report the occurrence of chromian andradite with up to 50 mol.% uvarovite component from hydrothermally altered podiform chromitite of the Rutland ophiolite, Andaman Island, India. The mantle tectonites of this ophiolite are characterized by a broad spectrum of peridotitic rocks ranging from depleted harzburgite to clinopyroxene-bearing harzburgite that hosts impersistently developed podiform chromite. Chromian garnet is concentrated at the margins of serpentine-rich veins that dissect the chromitite pods. The mineral assemblages are suggestive of low-temperature hydrothermal alteration of the host peridotite. The formation of chlorite and tremolite is simultaneous, and linked to the process of magnesiochromite oxidation. Garnet formed by reaction between tremolite and chromian spinel, which, during its growth, engulfed the tremolite needles.