Comment on: ‘Spatially controlled Fe and Si isotope variations: an alternative view on the formation of the Torres del Paine pluton’ by Gajos et al. (Contrib Mineral Petrol 171:93, 2016)

Abstract

The Torres del Paine pluton is one of the best examples of an incrementally constructed shallow crustal intrusion. Contrib Gajos et al. (Contrib Mineral Petrol 171:93, 2016) show that relatively large differences in δ56Fe and δ30Si exist in the mafic to granitic rocks of the Torres del Paine pluton. They propose that the data is spatially distributed from the bottom of the intrusion to its roof. They propose that thermally induced diffusion or source magma variations are the most likely processes to explain the data. While we do believe that non-traditional stable isotopes indeed contribute significantly to the understanding of igneous and hydrothermal processes, we suggest that the interpretation of the Fe and Si isotopic data by Gajos et al. (2016) is not consistent with field relations. The observed field relations and geochronology of the Torres del Paine are adequately explained by our original interpretation. In this comment, we will show that (a) the preferred interpretation of Gajos et al. (2016) that thermal migration zone refining can explain Fe and Si isotopic data from the Torres del Paine is incorrect, based on simple field observations, (b) that ignoring the known geological context as published on maps and cross sections will lead to erroneous interpretations of how granitic magmas are assembled, and (c) the correlation of Sr, Si and Fe isotopes as shown by Gajos et al. (2016) can be readily interpreted as a combination of fractionation and eventual source magma variation and/or late stage fluid exsolution.