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Understanding the petrogenesis of Li-rich magmas: a case study of the Li-granites of Cornwall, UK

Matthew Morris , Owen Weller, Caroline Soderman and Marie Edmonds

Abstract

Lithium-enriched granites are of increasing interest globally due to the explosion in demand for lithium (Li), primarily due to its importance within Li-ion battery systems. However, many questions remain about the co-genesis of Li-rich and Li-poor magmas, such as in the Cornubian batholith (Cornwall, UK). Previous models have required multi-stage melting [1] and/or unique source lithologies [2] to explain this co-occurrence. This study utilises advanced phase equilibria modelling to produce a model which bypasses these previous requirements, using the c.290-270 Ma granites of the Cornubian batholith as a case study. Our modelling broadly agrees with previous studies, which showed that the granites (G1-G5) were sourced by melting of a greywacke at pressures of c.3-6 kbar [2]. However, to explain the co-genesis of both Li-poor (G1-G4) and Li-rich (G5) granites within a single melting event, we present a new model invoking both fractional melting and the co-presence of 'standard' and F-rich (Fluorine-rich) biotites within areas of the source greywacke. G1-G4 melt extraction occurs after the breakdown of 'standard' biotite, with G5 melts subsequently extracted after the breakdown of the F-rich biotites. These F-rich biotites are stable to higher temperatures [3] and have a much greater biotite-melt DLi [4], which allows for the retention of Li during G1-G4 melting, followed by the release of this Li after breakdown, during G5 melting. References: [1] Koopmans et al. (2023) Geology 52:7-11; [2] Simons et al. (2016) Lithos 260:76-94; [3] Brigatti and Guggenheim (2002) Rev Mineral Geochem 46:1-97; [4] Beard et al. (2025) VMSG-MDSG 2025