Comment on “Petrology, geochemistry and Sm-Nd systematics of the Paleoproterozoic Itaguara retroeclogite from Sao Francisco/Congo Craton: One of the oldest records of the modern-style plate tectonics”
Graphical abstract
Introduction
P-T pseudosections are a forward modelling approach that shows every possible equilibrium assemblage that a rock composition may experience (Holland and Powell, 1998; Powell et al., 1998). This is incredibly useful, particularly when combined with inverse modelling because it is possible to use the P-T pseudosection to interpret the change in observed mineral assemblages or locate intersections of observed equilibrium mineral compositions (i.e. garnet), and define a P-T evolution, or P-T-t evolution if the timing of metamorphic mineral growth can be constrained (e.g. Mottram et al., 2015; Lanari and Engi, 2017; Williams et al., 2017). The method is not without complications– determining the effective equilibrium volume (or reactive bulk composition) of a specific sample can be difficult for a multitude of reasons (i.e. unknown H2O and Fe3+ content, melt loss, porphyroblast growth, local domain reactions, etc.; Rebay et al., 2010; Lanari and Engi, 2017; Tedeschi et al., 2017). This problem can be addressed by creating multiple diagrams investigating the effect of varying compositional components (T-X or P-X sections). This has been also successfully circumvented with thermodynamic calculations of equilibrium based on iterative thermodynamic models allowing the reactive bulk composition of a sample to also be modelled (in terms of fit) in relation to observed mineral assemblages, modes and compositions (Bingo-Antidote; Duesterhoeft and Lanari, 2020). P-T phase modelling relies upon, and is only as good as the sample petrography.
Conventional thermobarometry presumes equilibrium of two (or more) mineral compositions and determines what temperature or pressure these must have formed at. For this assumption to work, the textural relationships of the rock must be well known. However, metamorphic rocks experience a variety of P-T conditions on the path to peak conditions and also on the way back, and different minerals will be reactive to different parts of the P-T evolution. Unless every mineral is covered with compositional analyses and thousands of pressure and temperature estimates are calculated via a variety of thermobarometers (this can be done – see Lanari et al., 2013, Lanari et al., 2014), it is extremely unlikely that a selection of several points in different minerals represents equilibrium.
The objective of this comment is to re-evaluate the P-T conditions of the garnet amphibolite from Itaguara based on observable equilibrium relations and P-T phase diagram modelling. I show that the use of conventional thermobarometry and P-T modelling by Chaves and Porcher (2020) is based on flawed interpretation of the sample petrography and phase diagram (see Table 1). I have used their bulk and mineral compositions to reproduce P-T constraints that are more consistent with the observed mineral relationships and compositions. A detailed description of the methods used and additional diagrams referred to in the text are included in the supplementary material.
Section snippets
Petrography
Based on the images and descriptions presented by Chaves and Porcher (2020), it seems that the Itaguara mafic rocks have experienced two different P-T events. The earlier is responsible for the coarse-grained mineral assemblage including garnet, amphibole, ilmenite, quartz, biotite and relatively calcic plagioclase (XAb = 31–37; see Table 3 of Chaves and Porcher, 2020). Garnet (XAlm = 65–61; XGrs = 20–17; XPyr = 15–12; XSps = 5–4; Table 1 of Chaves and Porcher, 2020), amphibole, ilmenite,
P-T conditions
Additional modelling was done to explore the effect of compositional variation on the modelled P-T results using T-M and P-M sections. For the P-T evolution described above, the XPyr end-member was not included because the XPyr found in the sample plots above 20 kbar on the phase diagram (see supplementary Figs. 1 and 4). There are many reasons why this can happen, one is of course that the bulk rock composition is not appropriate (see below), another possibility is that the garnet selected
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Ian Buick, Hugo Moreira and Mahyra Tedeschi are acknowledged for discussion and comments that greatly improved the manuscript. The author acknowledges funding from Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) under the scheme Pós doc Nota 10.
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