Multistage formation of the titanite-centered ocellar texture in magma mixing zones: New insights from field, petrographic and mineralogic evidence in the Paleoproterozoic Mineiro belt, Brazil

Abstract

The titanite-centered ocellar texture is among the rarest and most intriguing textures associated with magma hybridization zones in plutonic environments. In order to contribute to the discussion regarding its formation process and relation to mixing, we investigated detailed field, petrographic, and mineralogical aspects of two occurrences of ocellar hybrid rocks associated with the Paleoproterozoic Macuco de Minas metagranitoid, Mineiro belt, southern São Francisco Craton. The studied rocks display oriented elliptic leucocratic aggregates composed of quartz, oligoclase (An15-20), microcline, apatite, and zircon, with an up-to centimetric poikilitic or euhedral diamond-shaped titanite crystals in their center (ocelli). The ocelli are often partially connected and immersed in a mesocratic tonalitic matrix composed of quartz, oligoclase (An15-20), Fe-biotite, titanite, allanite, fluorapatite, and zircon. The titanite crystals in the ocelli are enriched in Ti, LREE, Y, Th, and Zr and depleted in Al and F compared to the ones from the matrix, indicating that these compose two distinct groups of titanite. We propose that the development of the titanite-centered ocellar texture is a multi-stage interaction between mafic and felsic end-members, in which a biotite-rich tonalitic hybrid mush is injected by a fractionated leucogranitic melt. The leucogranitic melt migrates through veins that follow the preferred orientation of crystals in the hybrid mush and becomes enriched in Ti by the destabilization of biotite, resulting in the crystallization of the euhedral diamond-shaped titanite crystals. The veins are then disrupted, turning into elliptic ocelli in response to the pressure applied by the denser hybrid mush.