TEXTURAL TYPES, MINERAL CHEMISTRY, TEMPERATURES AND OXYGEN FUGACITIES OF IRON-TITANIUM OXIDES FROM THE CENTRAL INLIER, JAMAICA

Individual grain of detrital iron-titanium oxides were studied in order to determine provenance, utilising their complex compositions and textural types. Fe-Ti oxides were examined from sediments and possible Cretaceous bedrock sources in central Jamaica. The minerals include titanomagnetite, ilmenite, titanohematite, altered ilmenite/leucoxene(?), pseudobrookite, rutile and hematite. Accessory sphene and chlorapatite are commonly associated with the Fe-Ti oxides. The Fe-Ti oxides occur in a number of textural types, which include sandwich, trellis, composite, homogeneous, secondary alteration, curvi-linear and lensoid. The sandwich texture includes both single lamella and multi-lamellae. The composite texture includes both internal and external inclusions. Exsolved phases vary from titanomagnetite to rutile, while the host phase is generally titanomagnetite, hematite and or titanohematite. The MnO content of the Ti-rich phases varies from 0.86 to 23 wt. %; TiO₂ varies from 27.16 to 97.86 wt. %. Estimated temperatures for the coexisting phases of the different textural types range from 272 to 1333 ^oC and estimated oxygen fugacities range from log(fO₂) -42.09 to -6.34, plotting in the field for FMQ buffer. This field is consistent with Fe-bearing silicates and consistent with the compositions of calcalkaline magmas (evidence supported by source rocks). Higher temperatures and oxygen fugacities within the field indicate magmatic processes, while the lower temperatures suggest post-magmatic processes (e.g., metasomatic/hydrothermal alteration, or weathering).

The textural types and mineral compositions reflect various oxidation stages (C1-C7/R1-R7) of either the original titanomagnetite/ilmenite or the primary magmatic conditions. Variations in the oxidation assemblage dependent on (1) the composition of the original crystallizing magma, (2) the composition of the primary oxides and the extent of decomposition, (3) the values of temperature and oxygen fugacity at the time the primary Fe-Ti oxides started to crystallize, and (4) cooling history. The study demonstrates that the determination of provenance of detrital Fe-Ti oxides must take into account the complex textural evolution of these minerals.