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

Abstract

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.

The authors' abstract, as published at the source. International Geology Review, 2026 · DOI ↗