Revaluation of “equilibrium” P-T paths from zoned garnet in light of quartz inclusion in garnet (QuiG) barometry

Highlights

• QuiG barometry potentially provides an estimate of garnet growth conditions

• QuiG barometry in some localities is consistent with peak metamorphic estimates and in some localities not

• QuiG barometry suggests that some garnets grow nearly isothermally and isobarically

• QuiG barometry appears to preserve entrapment conditions through multiple metamorphic events

Abstract

Metamorphic P–T paths provide valuable constraints on the tectonics of collisional orogens and many such paths have been deduced from analysis of chemical zoning in garnet. Inclusion barometry (e.g. quartz-in-garnet or QuiG) provides complementary and sometimes contradictory information regarding the interpretation of garnet zoning and the P–T paths derived therefrom. For example, QuiG barometry on several generations of garnet from well-characterized samples from Fall Mountain, New Hampshire, is consistent with the P–T path inferred from other methods. However, QuiG barometry is inconsistent with P–T paths inferred from garnet zoning from the Orfordville Belt (Vermont), Townshend Dam (Vermont), the Connecticut Valley Trough (Vermont), and Sifnos (Greece). New data from the Perry Mountain Formation, southeastern New Hampshire, suggests that QuiG may preserve a record of polymetamorphic events. Specifically, QuiG barometry in coticule samples is interpreted to record a previously unrecognized early medium pressure (ca. 0.9 GPa) metamorphism and a later low pressure (ca. 0.3 GPa) metamorphism.

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.