Prograde and retrograde högbomites in sapphirine + quartz bearing Mg-Al rock from the Palghat-Cauvery Suture Zone, southern India

Abstract

We report rare hogbomite in a sapphirine + quartz bearing Mg-Al rock from the Palghat-Cauvery Suture Zone (PCSZ), the trace of the Cambrian Gondwana suture in southern India. Fine-grained (∼ 0.8 mm) hogbomite occurs mantled by sapphirine and spinel with or without rutile, and the entire assemblage is included within poikiloblastic garnet. The microstructure suggests the progress of a possible prograde reaction; Grt + Hog → Spr + Spl + Rt + H2O. As the garnet hosting hogbomite also includes equilibrium sapphirine + quartz, the hogbomite probably underwent T > 1000 °C peak metamorphism, in which case this the first report of this mineral formed under ultrahigh-temperature conditions. On the other hand, coarse-grained (∼ 12 mm) hogbomite, which is slightly richer in Fe + Mg (6.7-6.9 pfu) and poor in Al (14.5-14.7 pfu) than the former type (6.4-6.6 and 14.7-15.0 pfu, respectively), occurs in the matrix around spinel and/or sapphirine. The texture suggests retrograde formation of the latter hogbomite.