Emplacement and formation of archean anorthosites in the lower crust via hydrous magma underplating and differentiation: Evidence from a new pressure estimator

Understanding the emplacement depth of Archean anorthosites is crucial for deciphering the mechanisms governing Earth's continental crust formation. Traditionally, Archean anorthosites were thought to crystallize in shallow, low-pressure environments, favoring high anorthite plagioclase formation. However, recent research, including hydrous experimental studies and investigations of Phanerozoic arc root complexes, suggests that anorthosites may have formed in high-pressure environments within the lower crust. Our study contributes to this understanding by systematically comparing amphibole and plagioclase datasets from Phanerozoic arc root complexes with independent geothermobarometric results. These results validate the use of early amphibole and late plagioclase crystallization as a robust proxy for high-pressure conditions. Building upon these insights, we conduct a comprehensive re-examination of the emplacement and formation of the Archean Fiskenæsset Anorthosite Complex (FAC). Our multifaceted approach combines field investigations, petrographic analysis, and geochemical methods to unravel the processes involved in underplating hydrous magma and its subsequent differentiation. The pressure estimated for the FAC from a new approach based on amphibole-plagioclase crystallization sequences and supported by independent geothermobarometers suggests an emplacement depth of approximately 25–28 km, placing it in the lower crust. The formation of extremely calcic plagioclase (An 94–99) in Suite B gabbros likely requires crystallization from hydrous basalts with anomalously high CaO/Na₂O values (>8) because normal-ratio basalts lack sufficient calcium enrichment. This detailed study advances our understanding of the formation mechanisms of Archean anorthosites and associated rocks, including gabbronorites and hornblendites, highlighting their important role in shaping early continental crust.