Growth of non-typical garnet textures during amphibolite facies metamorphism: Dwalile Supracrustal Suite, Ancient Gneiss Complex, Eswatini

The Dwalile Supracrustal Suite of the Ancient Gneiss Complex (Eswatini) represents one of the oldest greenstone belts in the world with a crustal evolution history from Palaeo- to Mesoarchaean times. The main metapelitic rock type is dominated by garnet and staurolite porphyroblasts in a layered matrix of biotite, muscovite, quartz and retrograde sericite. Minor components are andalusite, chlorite and chloritoid with accessory ilmenite and monazite. The age for the amphibolite facies metamorphism is recorded by monazite at ca. 3.15 Ga.

The garnet-staurolite bearing metapelites have thus similar mineralogy and bulk rock compositions, but differ due to their unusual garnet microstructures. In some samples the garnet grains are distributed as thin layers consisting of elongated ribbons, with local resorption textures and peninsular features together with coarse recrystallised quartz. The euhedral garnet cores are only visible in compositional maps, which show a typical bell-shaped growth zoning.

EBSD maps of the crystallographic orientations of the garnet are created for four samples and two samples were imaged using high-resolution X-ray tomography for the visualization of the garnet morphology in order to test the relationship of the garnet porphyroblasts and consider the implications for the formation mechanism. For example, do the garnets share the same crystallographic orientation or is there evidence for deformation and/or rotation processes during growth. How are the garnets orientated in 3D space and are the ribbon textures build up by subgrains caused by deformation or are they separate grains formed by multiple nucleation events, possibly caused by fluid-rock interaction processes.