High-field-strength-element precipitation during the magmatic-hydrothermal transition in lithium pegmatites of SE Ireland

摘要

Trace-element imaging of white mica and columbite group minerals (CGM) reveals that the magmatic-hydrothermal transition during the formation of late-orogenic anatectic (Barros and Menuge 2016) lithium-caesium-tantalum-enriched (LCT) pegmatites of Lein-ster, SE Ireland, can be described by a three-stage process (Kaeter et al. 2018). The first stage is characterised by crystallisation from a water-undersaturated silicic melt. During the second stage, crystallisation from coexisting silicic melt and aqueous melt, as well as auto-metaso-matic overprint of early magmatic units, are observed. The third stage is marked by final crystallisation from melt, followed by hydrothermal alteration of earlier units and precipitation of hydrothermal minerals. Vola-tiles exsolve into a fluid phase, possibly at some point during the second stage, and the resulting overpressure causes hydraulic fracturing, creating pathways and facilitating further metasomatism of earlier lithologies.