TTG GEOCHEMICAL TRENDS CONTROLLED BY PLAGIOCLASE FRACTIONATION IN A MIDDLE-CRUSTAL MUSH

Archean crust is dominated by tonalite-trondhjemite-granodiorite (TTG) suites. Trace element concentrations and ratios of these rocks have traditionally been linked to the depth of their mafic source, with implications for geodynamic setting. Alternatively, variations in TTG compositions may be caused by fractional crystallization, with a less straightforward connection to depth of melting and tectonic regime. Many TTGs are preserved as the dominant component of grey gneiss at mid-crustal levels, and the post-emplacement behavior of TTG magma in this setting is poorly understood. Here, we present a petrological–geochemical model that accounts for the coexistence of TTGs with ‘low pressure’ and ‘high pressure’ geochemical signatures in a single middle crustal suite. We propose that TTG magmas accumulated at middle-crustal depth and cooled slowly, resulting in crystal mush complexes where regional deformation caused separation of early-crystallizing plagioclase from fractionated melt. Based on phase equilibrium modelling coupled with trace element modelling of a putative parental melt, we show that a suite of TTGs in the Superior Province represents variable proportions of accumulated plagioclase and fractionated melt. This work illustrates the potential for plagioclase—the dominant mineral in TTGs—to control whole-rock rare earth element trends, obscuring the connection between TTG composition, source depth, and geodynamic setting.