Visualising Garnets: Linking complex microstructures through a multi-modal approach to reveal metamorphic history

The 3.46- 3.1 Ga Dwalile Supracrustal Suite (DSS) of the Ancient Gneiss Complex in Eswatini constitutes one of the world’s oldest greenstone belts, recording a prolonged crustal evolution from the Palaeoarchaean to Mesoarchaean. Archaean metasediments are commonly poorly preserved, with matrix minerals frequently altered or no longer in equilibrium with garnet porphyroblasts due to superimposed metamorphic events. Consequently, garnet textures, when integrated with petrological observations and both major- and trace-element geochemistry, may provide valuable insights into the entire metamorphic history.

Garnet-staurolite schists of the DSS mainly differ in their garnet and staurolite modes and their unusual garnet microstructures. In some samples, the almandine garnets are distributed as thin boudinaged layers consisting of elongated ribbons, with local resorption textures and peninsular features surrounded by coarse recrystallised quartz. The euhedral garnet cores are only visible in compositional maps. Other schists consist of staurolite-mica rich layers intertwined with garnetite layers containing almandine garnet.

The complexity of these garnet grains cannot be adequately captured by spot analyses using techniques such as electron probe microanalysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Instead, the polyphase nature of the microstructures is investigated by a multi-scale, multi-modal imaging approach that integrates complementary techniques, including X-ray micro–computed tomography for three-dimensional structural information and electron backscattered diffraction, EMPA major element, and LA-ICP-MS trace element mapping.

The EBSD maps show distinct microsctructural differences between the samples. Many of the garnetite porphyroblasts are consisting of polycrystals with distinct crystal orientations, evidence for aggregation due to pervasive fluid influx which has accelerated garnet nucleation. Whereas, the garnet banding surrounding older euhedral cores often show the same preferred orientation as the cores themselves, but distinct differences in orientation occur between individual cores and between sections of the garnet banding. This may be the result of accelerated garnet growth due to channelled fluid flow during metamorphism.

The garnet growth is mainly associated with amphibolite-facies metamorphism recorded by monazite at ca. 3.16 Ga, at maximum pressures of ~4 kbar and temperatures of 510–540 °C. However, to better resolve the complexity of the microstructures, additional geochronology targeting distinct garnet generations and other mineral phases associated with fluid activity may be necessary.