On oriented ilmenite needles in garnet porphyroblasts from deep crustal granulites: Implications for fluid evolution and cooling history

Abstract Garnet porphyroblasts from a litho-assemblage containing aluminous granulite and quartzofeldspathic gneisses of the Eastern Ghats granulite belt, India contain nanometer- to micrometer-thick ilmenite needles oriented crystallographically. Petrographic and chemical analyses reveal that garnet was formed by dehydration melting reaction(s) of titaniferous biotite in an oxidized condition. Elevated oxygen fugacity might have promoted enrichment of Ti-bearing andradite component of garnet porphyroblasts formed during pre- to peak metamorphic condition in appropriate bulk chemistry. During the post-peak cooling history, Ti-bearing components in garnet decomposed to rhombohedral oxide solid solution (ilmenite–hematite). Detailed transmission electron microscopic study of the host garnet and ilmenite solid solution indicates that though there is an overall parallelism of (011) plane of host garnet and (011) plane of ilmenite, structural coherence between the two phases was progressively lost during growth from thin to thick needles. Appropriate cooling rate from high-temperature peak metamorphic condition arguably promoted growth of ilmenite solid solution through reaction–exsolution process within garnet porphyroblasts.