Ti-free hogbomite in spinel- and sapphirine-bearing Mg-Al rock from the Palghat-Cauvery shear zone system, southern India

摘要

We report a new type of högbomite in a spinel- and sapphirine-bearing silica-deficient Mg-Al rock from Manavadi at the northern margin of the Madurai Block along the Palghat-Cauvery shear zone system, which defines the Archaean-Proterozoic collisional boundary in southern India. Högbomite occurs here as fine- to medium-grained euhedral to subhedral crystals enclosed within or along grain boundaries of greenish spinel. It is compositionally very unique due to the lack of TiO₂ (<0.2 wt.%) and enrichment in Al₂O₃ (66.4–70.8 wt.%) compared to other occurrences. The variety of högbomite described in this study, confirmed by laser Raman spectroscopy, has not been reported elsewhere previously. Högbomite in this study is subdivided into three groups based on the X_Mg (=Mg/(Fe_Total+Mg)) ratio; 0.54–0.57, 0.61–0.73 and 0.77–0.79. Compositional data suggest a linear element substitution of 3(Fe+Mg) = 2Al for the Ti-free högbomite, a trend that is distinct from previously reported Ti-bearing högbomites. Textures and mineral chemistry data suggest that the following reaction took place at retrograde stage (T högbomite + corundum + magnetite, implying that introduction of hydrous and oxidized fluid as well as low bulk-Ti content played an important role in the formation of Ti-free högbomite.