Research ArticleRevaluation of “equilibrium” P-T paths from zoned garnet in light of quartz inclusion in garnet (QuiG) barometry
Introduction
Garnet has served as a central focus of metamorphic studies for the discernment of P–T paths because it has the capability of storing a record of its history through the crust through inclusion suites, chemical zoning, and age zoning (e.g. Ague and Carlson, 2013; Baxter et al., 2013; Baxter and Scherer, 2013; Caddick and Kohn, 2013). Although it is generally acknowledged that metamorphic rocks cannot react if in equilibrium and a finite amount of overstepping is required to drive metamorphic processes, it is generally assumed that the degree to which metamorphic rocks are out of equilibrium is sufficiently small that utilization of equilibrium constraints does not introduce a substantial error in the calculation of metamorphic P–T paths (e.g. Caddick and Kohn, 2013).
However, a number of studies have presented observations that challenge this assumption of a close approach to equilibrium throughout the duration of garnet growth. As early as 1969, Hollister (1969) demonstrated that overstepping of equilibrium reaction boundaries was required to explain textural relations of aluminosilicate minerals (staurolite, kyanite, andalusite and sillimanite) from the Kwoiek contact aureole in British Columbia. More recently, a pioneering study by Waters and Lovegrove (2002) provided unambiguous textural data documenting the sequence of porphyroblast growth in the contact aureole of the Bushveld complex and demonstrated that it was not the sequence that was predicted from equilibrium calculations, and drew the conclusion that considerable overstepping of the equilibrium phase boundary was required for porphyroblast nucleation. Pattison and Tinkham (2009) reach similar a similar conclusion for porphyroblast growth in the Nelson contact aureole by comparing the spacing of isograds mapped in the field with the spacing of isograds that would be predicted from equilibrium calculations and thermal modeling. Pattison and Spear (2018) presented compelling evidence that not only might garnet nucleation be overstepped during regional metamorphism, but also staurolite and aluminosilicates.
The introduction of a new tool into the petrologist's arsenal — inclusion barometry — has provided a technique for evaluating the extent of overstepping required for porphyroblast nucleation. In particular, quartz-in-garnet barometry, or QuiG, has been applied to rocks from collisional orogenic belts in New England (Spear et al., 2014; Wolfe and Spear, 2018, in review), the Cyclades subduction complex of Greece (Ashley et al., 2014; Castro and Spear, 2016), and to high pressure gneisses and eclogites (Alvaro et al., 2020; Gonzalez et al., 2019). In several of these studies, it was found that when the results of QuiG barometry were compared to the calculated P–T conditions of the garnet isograd based on equilibrium modeling, garnet was concluded to have nucleated not near the equilibrium isograd but only after considerable overstepping .
The extent to which the above results are true — that garnet only nucleates after considerable overstepping of the equilibrium phase boundary — raises a very important question as to the validity of previously published P–T paths that are based on the assumption of garnet having grown through a sequence of equilibrium states. If application of QuiG barometry reveals a P–T history for garnet growth that deviates from the P–T histories inferred from conventional chemical zoning analysis, then the nature and causes of this deviation urgently requires evaluation because of the numerous P–T paths that have been inferred from this latter approach. Inasmuch as metamorphic P–T paths, many of which have been constrained from garnet zoning studies, are a key constraint in the tectonic interpretation or orogenesis, evaluation of the accuracy of QuiG is of considerable importance.
The purpose of this paper is to summarize some of the recent results in which QuiG barometry has been applied to constrain metamorphic P–T conditions. In some of these studies, QuiG barometry reveals a different P–T history from that previously published whereas in others QuiG barometry reinforces published inferences about metamorphic P–T paths. Examples are also presented in which QuiG barometry provides new constraints on metamorphic histories in rocks for which traditional, equilibrium studies have seen only limited success.
Section snippets
Petrologic methods applied to garnet
Garnet has long enjoyed the status of an “uncommonly useful” mineral in petrologic studies (Baxter et al., 2013) and numerous approaches have been championed to deciphering P–T histories from garnet phase equilibria and compositional zoning (e.g. see Caddick and Kohn, 2013, for an excellent review). The earliest geothermometers utilized element partitioning between garnet and coexisting phases as a proxy for temperature of crystallization or equilibration (Kretz, 1959; Ramberg, 1952) and one of
The QuiG method
Inclusion barometry is based on the assumption that when a porphyroblast such as garnet overgrows another phase so that the phase becomes included in the host, the host-inclusion pair are initially in mechanical equilibrium such that the inclusion exactly fits inside of the hole in the host. Subsequent changes in P and T and eventual exhumation results in changes in the size of the host hole and inclusion so that at surface conditions the inclusion no longer fits perfectly inside the host and
Results
This section discusses the results of reevaluating P–T paths based on application of QuiG barometry to previously studied samples. Some examples reveal close consistencies between the originally published P–T path and the results of inclusion barometry whereas other examples show no consistency between the QuiG results and garnet zoning paths. Finally, two examples are discussed in which QuiG barometry places new constraints on the evolution of samples that were not previously accessible by
Discussion
The above examples demonstrate that in some cases QuiG barometry provides constraints on metamorphic P–T conditions that are consistent with previous estimates based on classical thermobarometry and thermodynamic calculations but in other cases there are major discrepancies. The question of whether QuiG, and other inclusion barometry, is providing an accurate assessment of the conditions of garnet nucleation and growth, is of considerable importance because many studies have utilized garnet
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.
Acknowledgments
The authors are greatly indebted to Jennifer Thomson for providing the sample of coticule for this study. This work was supported by NSF grants 1447468 and 1750674 to Spear and The Edward P. Hamilton Distinguished Professor of Science Education chair.
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