Compositional diversity of TTGs controlled by heterogeneous accumulation of accessory minerals

Abstract

The compositional diversity of Archean tonalite-trondhjemite-granodiorite (TTG) rocks is generally attributed to partial melting of mafic rocks at various depths or magmatic differentiation driven by the accumulation of plagioclase and/or amphibole. Our investigation of the approximately 130 km² Paleoarchean Stolzburg pluton in the Barberton area of South Africa reveals a substantial compositional variability that cannot be explained by those processes. The variable Sr/Y and La/Yb ratios can correspond to high-, medium-, and low-pressure types based on pressure-dependent classification criteria, thus conflicting with partial melting simulations and ruling out the model of variable melting depths for such a small, single pluton. Instead, an integrated study incorporating petrographic analysis, mineral mapping, and thermodynamic simulation suggests that the heterogeneous accumulation of accessory minerals during crystal-melt separation in the magma chamber is the leading cause of the compositional variation of the ca. 3.45 Ga TTG pluton. This study highlights the critical role of accessory minerals in intrinsic magmatic differentiation within individual TTG plutons and advise caution when using bulk-rock compositions of TTGs to infer melt physochemical conditions and tectonic settings.