Hogbomite in migmatitic paragneiss in the Betroka shear belt, Vohidava area, Precambrian of southern Madagascar

The Betroka sinistral shear belt is a major geotectonic unit in the Precamb rian of southern Madagascar. It consists of migmatitic paragneiss commonly interlayered with phlogopite-bearing diopsidite, phlogopite-humite-diopside -spinel marble, sillimanite-garnet quartzite and syn-tectonic S-type leucog ranite. Hogbomite occurs sporadically in the migmatitic paragneiss in patch es of magnetite with hercynite, and at the border of magnetite where it is in contact with hercynite, rare ilmenite, rutile and cordierite, which cont ains a network of chlorite, pyrophyllite and rare corundum/diaspore. X-Mg = Mg/(Mg + Fe) decreases as follows: Crd > Bt > Chi > Hog > He The textural relations suggest the following hogbomite-participating reactions: Ti-bearing hercynite double right arrow hercynite + hogbomite (intergrown/e xsolution lamellae) ilmenite + cordierite double right arrow hercynite + hogbomite + rutile + c hlorite/pyrophyllite hogbomite double right arrow hercynite + ilmenite + corundum The chemical composition of hogbomite varies substantially from grain to gr ain in individual samples and from sample to sample, this variation being h ighly dependent on the associated minerals. There is a weak zoning from cor e to rim in individual grains intergrown with hercynite and also in grains at the margin of hercynite, but this zoning is overprinted by zones formed at grain rims depending on the surrounding phases. In contact with hercynit e, hogbomite has FeO (total Fe as FeO) 27.1-28.5 wt.%, and MgO 4.5-5.8 wt.% , and in contact with magnetite FeO 24.9-26.5 wt.%, and MgO 6.0-8.5 wt.% an d the core contents are within these values. TiO2 7.5-4.0 wt.% and Al2O3 62 .0-59.0 wt.% show zonations with increase from core to rim. Estimated P-T c onditions are 6.0 +/- 1.0 kbar and 700 +/- 100 degrees C reached during a p eak metamorphic stage of the Pan-African orogeny. However, the presence or diaspore with exsolved hercynite-magnetite indicates extreme retrograde met amorphism in the decompressional central part of this shear belt of souther n Madagascar.