Oxide enrichment by syntectonic melt-rock interaction

Abstract

Processes that enrich rocks in oxides, such as ilmenite, are controversial. Current models include magmatic accumulation, crystallisation of veins from immiscible liquids and syntectonic differentiation. In this contribution, we investigate examples of oxide enrichment in both the oceanic and continental crust. The oceanic samples are of oxide gabbros (with up to 45 vol% oxides) from the Atlantis Bank oceanic core complex, Southwest Indian Ridge. The continental sample is from the Cattle Water Pass shear zone (with up to 20 vol% oxides) associated with the intracontinental Alice Springs Orogeny, central Australia. We argue for the occurrence of an open chemical system, with melt rock reactions as a key process involved in oxide enrichment in melt-fluxed shear zones. Our detailed microstructural characterisation reveals that oxides replace silicates and form interstitial grains, grain boundary films and low dihedral angles between silicates often making up an interconnected skeletal texture. Quantitative orientation data reveals that the oxides: 1) have limited internal deformation, 2) form clusters of grains that are connected in 3D, 3) have crystal faces matching the orientation of the grain boundary of nearby newly crystallised diopside (oceanic sample) and 4) form part of the foliation defining assemblage with biotite (continental sample). This evidence suggests the oxides crystallised in the presence of melt and formed during melt-rock interaction. Syntectonic melt migration is known to result in low strain microstructures in shear zones, as the strain is accommodated by the melt that existed in the deforming rock. This produces a high strain rock with silicate and oxide minerals that show limited internal deformation. Microchemical data shows major element variability in silicates and ilmenite at the thin section scale, supporting an open chemical system with local variability in both oceanic and continental settings. It further argues that syntectonic melt migration is important in oxide enrichment. Mineral chemistry data implies that the oceanic tectonic setting involved melt-rock interaction with fractionated gabbroic melt while the continental setting involved peraluminous granite melt driving mineral replacement and enrichment of oxides. We propose that deformation assisted reactive porous flow of near liquidus melt through rocks in any tectonic setting may result in melt-rock interaction induced crystallisation of oxides in preference to silicates and that with high time-integrated melt flux, the accumulation of oxides can be significant.