Fractionation of alkali feldspar: on making high Ga/Al granite

The classification of granites is marked by inconsistencies. For example, the widespread classification of granites into A-type was initially based on associations with extension-related tectonic environments while the I- and S-type classifications were based on inferred source rocks using major-element chemistry and mineralogy. Here, we revisit the significance of one geochemical signature that has been widely used to classify granites. Specifically, elevated Ga/Al has long been used as an empirical signature of A-type granites, but the petrogenetic processes responsible for Ga/Al enrichment in granitic melts remain poorly quantified. Because the term ‘A-type’ originally combines compositional, tectonic and petrogenetic implications, whether Ga/Al systematics reflect tectonic environment or reflect petrogenetic processes requires an understanding of the processes that control Ga/Al fractionation. Here, we present a comprehensive evaluation of Ga/Al exchange coefficients among major rock-forming minerals from experimental and natural mineral-melt pairs that inform the petrologic processes of generating high Ga/Al magmas. We find that segregation of alkali feldspar is the primary driver for high Ga/Al ratios in granitic melts. We demonstrate that significant increases in Ga/Al ratios occur predominantly when alkali feldspar segregates efficiently from the melt. Elevated Ga/Al ratios are not unique to specific tectonic settings or source rocks; rather, they reflect a distinct petrogenetic history of alkali feldspar saturation and subsequent segregation. Thus, Ga/Al ratios should be utilized as a tool to understand magmatic differentiation processes (specifically, the efficiency of alkali feldspar segregation) rather than as an indicator of tectonic environment.