Atoll garnet: insights from LA-ICP-MS trace element mapping

Atoll garnets are uncommon features that have been recognized in contrasting metamorphic environments worldwide, but their origin remains largely debated. Several models have been proposed to explain their formation, including preferential dissolution of garnet cores by fluid infiltration, polymetamorphism, and the coalescence of subgrains. We report atoll-shaped garnets in an amphibolite facies schist from the Palaeoproterozoic New Quebec Orogen, Canada, and investigate their textural and chemical zoning through petrography, electron probe microanalysis (EPMA) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) maps. Textural evidence indicates a subhedral poikiloblastic core, an inclusion ring composed of matrix minerals, and a euhedral rim. Major element distribution maps show flat zoning, whereas trace elements show concentric growth zoning. Such characteristics are consistent with rapid, post-kinematic growth involving Rayleigh fractionation of trace elements and coeval with accessory phase breakdown. Our observations rule out the preferential dissolution, polymetamorphism and coalescence models, and support that the formation of atoll garnet in these rocks is best explained by a kinetic control and rapid growth. Our study concludes that the term “atoll” is more descriptive than genetic, and that the physio-chemical mechanisms leading to its formation should be assessed on a case-by-case basis using complementary tools, primarily including trace element mapping.