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Oxidation-induced postmagmatic modifications of primary ilmenite, NYG-related aplite dyke, Tibchi complex, Kalato, Nigeria

Department of Earth and Planetary Sciences, 3450 University Street, McGill University, Montreal, Quebec H3A 2A7, Canada

^* E-mail: emmanuel{at}eps.mcgill.ca

We describe an ilmenite-bearing aplitic syenite dyke in the roof zone of the Tibchi granite, exposed at Kalato, in the Tibchi ring-complex, northern Nigeria. Inclusions of ferrocolumbite, rutile and ixiolite in the ilmenite are inferred to have been trapped at the magmatic stage. The main mafic mineral is annite. Compositionally, the ilmenite, rutile and ferrocolumbite have near-end-member compositions. A positive correlation between Sc and Ta/(Ta+Nb) indicates that Sc behaved incompatibly as ferrocolumbite grew. Such entrapped accessory minerals may well have formed by local saturation at the ilmenite-melt and annite-melt interface. During and after their crystallization, the melt reached saturation in H₂O and degassed. A second generation of ilmenite enriched in Mn and Zn replaced the primary ilmenite along fractures and grain margins. As f_O2 began to increase, composite blebs and rinds of ‘ferropseudobrookite’, rutile and hematite began to develop by oxidation-induced exsolution in the primary ilmenite. Incorporation of Nb, Ta, Sc and Si in the ‘ferropseudobrookite’ may well have stabilized it at Kalato. Ultimately, it is transformed to hematite + rutile. The IMA-sanctioned view that the solid solution between pseudobrookite and Ti₃O₅ is complete, and thus that ‘ferropseudobrookite’, as an intermediate member of the series, does not merit species status, needs to be re-evaluated.

KEYWORDS: ilmenite, ferrocolumbite, ‘ferropseudobrookite’, aplite dyke, syenite, oxygen fugacity, ring complex, Tibchi, northern Nigeria

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