Accessories after the facts: Constraining the timing, duration and conditions of high-temperature metamorphic processes

    Accessories after the facts: Constraining the timing, duration and conditions of high-temperature metamorphic processes Richard J.M. Taylor, Christopher L. Kirkland, Chris Clark PII: DOI: Reference:

S0024-4937(16)30275-4 doi:10.1016/j.lithos.2016.09.004 LITHOS 4065

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Received date: Revised date: Accepted date:

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Please cite this article as: Taylor, Richard J.M., Kirkland, Christopher L., Clark, Chris, Accessories after the facts: Constraining the timing, duration and conditions of hightemperature metamorphic processes, LITHOS (2016), doi:10.1016/j.lithos.2016.09.004

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Accessories after the facts: constraining the timing, duration and

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conditions of high-temperature metamorphic processes

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Richard J. M. Taylor*, Christopher L. Kirkland and Chris Clark

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The Institute for Geoscience Research (TIGeR), Western Australian School of Mines (WASM), Department of Applied Geology, Curtin University, GPO Box U1987, Perth WA 6845, Australia.

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Corresponding author: [email protected]

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ACCEPTED MANUSCRIPT ABSTRACT High-temperature metamorphic rocks are the result of numerous chemical and

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physical processes that occur during a potentially long-lived thermal evolution. These rocks chart the sequence of events during an orogenic episode including heating,

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cooling, exhumation and melt interaction, all of which may be interpreted through the elemental and isotopic characteristics of accessory minerals such as zircon, monazite and rutile. Developments in imaging and in situ chemical analysis have resulted in an

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increasing mount of information being extracted from these accessory phases. The

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refractory nature of these minerals, combined with both their use as geochronometers and tracers of metamorphic mineral reactions, has made them the focus of many studies of granulite-facies terrains. In such studies the primary aim is often to

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determine the timing and conditions of the peak of metamorphism, and high-

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temperature metasedimentary rocks may seem ideal for this purpose. For example pelites typically contain an abundance of accessory minerals in a variety of bulk

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compositions, are melt-bearing, and may have endured extreme conditions that

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facilitate diffusion and chemical equilibrium. However complexities arise due to the heterogeneous nature of these rocks on all scales, driven by both the composition of the protolith and metamorphic differentiation. In additional to lithological heterogeneity, the closure temperatures for both radiogenic isotopes and chemical thermometers vary between different accessory minerals. This apparent complexity can be useful as it permits a wide range of temperature and time (T–t) information to be recovered from a single rock sample. In this review we cover: 1) characteristic internal textures of accessory minerals in high temperature rocks; 2) the interpretation of zircon and monazite age data in relation to high temperature processes; 3) rare earth element partitioning; 4) trace element thermometry; 5) the incorporation of

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ACCEPTED MANUSCRIPT accessory mineral growth into thermodynamic modeling; and 6) Hf isotopic signature of zircon overgrowths and its use in linking zircon into major metamorphic phase

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development.

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Keywords: accessory minerals; granulites; petrochronology; thermometry; trace element partitioning; Hf isotopes.

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Highlights  Atlas of zircon and monazite textures in high temperature rocks.  Relationship between accessory mineral U–Pb ages and peak metamorphism.  Trace elements as a monitor of metamorphic mineral reactions and temperature.  Hf isotopic signature of zircon in metamorphic environments.

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1. Introduction

An enduring challenge in the earth sciences is to determine the rates and

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durations of orogenic processes. The application of in situ techniques to major and

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accessory minerals in regionally metamorphosed terrains has opened up the possibility of a detailed understanding of orogenic processes as temporally

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constrained minerals chart the evolution of this crust (e.g. Harley, 2008; Harley, 2016; Harley et al., 2007; Korhonen et al., 2013; Rubatto, 2002; Rubatto and Hermann, 2007b; Rubatto et al., 2001). Accessory mineral geochronology and thermometry are key tools in the arsenal of the metamorphic petrologist, and the application of these methods is now widespread in the interpretation of high-T terrains and terrain boundaries (e.g. Clark et al., 2009a; Clark et al., 2009b; Ewing et al., 2013; Korhonen et al., 2013; Korhonen et al., 2014; Pape et al., 2016; Yakymchuk et al., 2015). Highgrade metamorphic terrains provide seemingly ideal settings for the application of these techniques, with high temperatures facilitating diffusion of key elements (Cherniak et al., 2007; Cherniak and Watson, 2007). However the reality of applying 3

ACCEPTED MANUSCRIPT these analytical approaches to the granulite-facies and the even more extreme conditions of ultrahigh-temperature (UHT) crust is far more complex, with

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multifaceted interplay of mineral reactions, elemental and isotopic closure temperatures, and melt-bearing open system behavior (e.g. Harley et al., 2007;

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Yakymchuk and Brown, 2014b). This often requires careful interpretation of the meaning of ages and temperature estimates obtained on such rocks. Of particular importance are constraints on the metamorphic peak and the process that radiometric

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ages actually reflect.

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Here we aim to provide a resource of accessory mineral textures apparent in high-grade metamorphic rocks that have reached P–T conditions significant enough to experience partial melting, and what those textures may represent (Figure 1). We then

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look at examples of granulite and UHT granulite terrains that represent short-lived,

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protracted and multiple metamorphic events. An examination of modern analytical techniques applied to these terrains demonstrates the application of U–Pb in

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commonly occurring accessory phases such as zircon and monazite, combined with

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cation substitution thermometry in understanding P–T–t histories. In many cases some techniques may not provide a constraint on peak metamorphic conditions, but rather provide information on prograde and retrograde processes (Harley and Nandakumar, 2014; Johnson et al., 2015; Korhonen et al., 2013; Korhonen et al., 2014). Alongside our modern analytical techniques we also consider the pros and cons of integrating accessory phases into thermodynamic modeling of metamorphic rock samples in multivariant compositional space (pseudosections). Linking zircon and monazite to P–T pseudosections may be able to replicate simple metamorphic processes such a mineral growth during melt crystallisation, but may not be able to account for a multitude of other scenarios in which accessory minerals grow or are

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ACCEPTED MANUSCRIPT modified. Finally we also cover Lu–Hf in zircon, in particular what these isotopic measurements may mean in a rock system that contains, or has previously contained,

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phases such as garnet, which may result in variable fractionation of the isotopic systems held within zircon.

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Together these techniques provide powerful tools for investigating crustal metamorphism, and are becoming increasingly widespread as instrumentation improves and becomes cheaper and easier to operate. However, thoughtful collection

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and interpretation of the resulting datasets is still a prerequisite in order to unlock

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such data’s full potential.

2. Accessory mineral textures in high-grade rocks Zircon and monazite in high-grade metamorphic rocks often display a

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complex array of internal textures under a wide range of imaging techniques (e.g.

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Ayers et al., 1999; Corfu et al., 2003; Hanchar and Miller, 1993; Hanchar and Rudnick, 1995; Zhu and O'Nions, 1999). Whilst primary growth features of accessory

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minerals may be easily identified, the inherent modification that results from

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environments and processes such as high temperatures, melt interaction, and fluid ingress may lead to some degree of ambiguity in the interpretation of secondary features. The fact that the resulting accessory phase textures from even a wellconstrained rock suite may be variable, and often highly inconsistent within a single rock means that textural interpretation must be integrated with later acquired chemical datasets. In Figures 2 and 3 we present a compilation of zircon and monazite textures respectively and illustrate some of the consistencies and variations expected within high-grade metamorphic rocks to provide a basis for determination of what part of the P–T–t path such textures most likely represent. Localities and/or references for each image are founds in the figure captions.

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ACCEPTED MANUSCRIPT 2.1 Zircon textures The external morphology of zircon grains is commonly used to identify its

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origin, or provenance, in terms of the rock it formed in. The variety of morphologies has been described in specific detail in a number of regional studies and is not

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reiterated here, but rather the range of possible internal features indicative of zircon in high-grade metamorphic rocks is evaluated here in a comprehensive atlas. A variety of imaging techniques have been used to identify internal grain features throughout

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the history of in situ zircon geochronology. Whilst methods such as transmitted and

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reflected light, raman spectroscopy, and backscattered electron (BSE) images still hold some useful information, cathodoluminescence (CL) imaging using a Scanning Electron Microscope (SEM) system has become the tool of choice for identifying

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zircon mineral zones prior to further analysis with some of the pioneering work done

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over 20 years ago (e.g. Hanchar and Miller, 1993; Hanchar and Rudnick, 1995; Rubatto and Gebauer, 2000). These detailed images provide the opportunity to not

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only target zones within the grains, but also aid in determining the genesis of grains

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(Corfu et al., 2003; Hoskin and Schaltegger, 2003). Here we aim to provide a new atlas, complementary to that of previous workers, focusing on internal zircon features that are typical from high-grade metamorphic rocks (Figure 2). In its simplest terms metamorphic zircon represents two types, those that existed prior to metamorphism but have been modified, and those that have grown as a result of the metamorphic process. In terms of the resultant textures this basic scenario is then manifested as the result of the combined processes of dissolution, recrystallisation and neocrystallisation (Harley et al., 2007). An inherited zircon from an igneous source may retain much of its primary magmatic zoning with minimal recrystallisation, or may lose primary zoning through substantial recrystallisation,

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ACCEPTED MANUSCRIPT whilst retaining the original grain morphology (Hoskin and Black, 2000). The production of anatectic melts in the granulite facies may result in the

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consumption/dissolution of inherited zircon through saturation of such melts with hundreds of ppm Zr at high temperatures (Boehnke et al., 2013; Gervasoni et al.,

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2016; Watson and Harrison, 1983). This process will reduce the modal proportion of zircon in the metamorphic rock, resulting in a lack of inherited material, or corroded cores that may show resorption surfaces (Corfu et al., 2003). Neocrystallised zircon,

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often formed from the crystallisation of melt following peak temperatures (Roberts

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and Finger, 1997), may result in either the growth of new material on pre-existing, inherited zircon that forms a template for subsequent growth, or the formation of entirely new grains. It is possible that the local scale migration and crystallisation of

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small volumes of melt may also result in the growth of zircon during peak conditions

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(Harley, 2016; Harley and Nandakumar, 2014). Following melt crystallisation, or as a direct result of it, fluids present in the rock may alter the final texture of the zircon

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grain through either in situ recrystallisation or coupled dissolution–reprecipitation

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processes, which may occur along grain boundaries or take advantage of internal fractures (e.g. Geisler et al., 2007; Rayner et al., 2005; Taylor et al., 2014). Many of the textures described above are seen in Figure 2. The variation of recrystallisation possible in inherited igneous zircon can be seen in Figure 2a-i. The majority of these grains still show an aspect ratio (1:2 – 1:4) expected of igneous zircon (Hoskin and Schaltegger, 2003), however the internal textures have been altered or masked to varying degrees. Grains b, e and h still show clear, CL bright, oscillatory zoning within the core of the grains though this may be overprinted by dark CL response at the grain tips and boundaries (grain b and e), and internal cuspate features and convolutions (grains e and h). Grain c shows a

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ACCEPTED MANUSCRIPT progressive loss of the original, CL bright internal features towards the bottom of the core, whilst grain d shows a total loss of internal features within a CL bright zone in

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the core of the original grain. Grains a and g show progressive recrystallisation of the original grain, faster along the c-axis, which has replaced the oscillatory and sector

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zoning with an almost featureless, CL dark texture. Grain f retains some remnants of the original zoning in the brighter CL regions, however this original grain has been altered, most likely along pre-existing fractures, which have since been annealed and

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recrystallized as featureless, CL dark regions which also progress in from the tip of

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the grain. Almost all these grains show a clear, final addition of zircon material only a few microns in thickness (larger in grain b), often but not always bright in CL response, which is typical for high-grade zircons. The recrystallisation of inherited

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material has a variable effect of U–Pb systematics, which should ultimately result in

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the expulsion of the pre-existing Pb, and resetting of the U–Pb system in a “fully” recrystallized grain. Whilst in a minority of grains a concordant, inherited age can be

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obtained, the reality of the U–Pb in grains such as these is discordant arrays and large

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uncertainties potentially as a result of patchy Pb distribution (Kusiak et al., 2013; Whitehouse et al., 2014), that forms as a result of migration of unsupported Pb through the grain to form microscale clumps often unrelated to crystal morphology or damage (Kusiak et al., 2013). Zircon grains j to q in Figure 2 are characteristic of granulite facies rocks that have undergone partial melting. These grains are multi-component with a distinct core, often with an igneous morphology, and metamorphic overgrowth, resulting in a more equant grain shape. Grain i shows a core region in the centre of the grain that shows no primary oscillatory zoning, and has been largely recrystallized to sector zoning with little variation in CL response. There is a clear boundary between this

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ACCEPTED MANUSCRIPT core region and the surrounding overgrowth, however the CL texture in both regions is very similar. Grains k, l, m, p and q show the clearest core–overgrowth texture,

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where the dissolution front in the inherited zircon is clearly marked by a bright CL response resorption line with irregular thickness (typically a few microns), that

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truncates the internal features of the core. This bright CL feature marks the boundary between the inherited zircon and the metamorphic overgrowth, which often show distinctly different textures. Grains n and o also have a clear core–overgrowth texture,

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however in this case there is no corrosion surface, just a distinct textural change (grain

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j and n) or a clear truncation of the internal features (grain o). In terms of core textures these grains also show a range in extent of recrystallisation. In grains j to o there is still clear oscillatory zoning, in many cases brighter in CL response than the

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overgrowths. In grain p this zoning has been overprinted by a component with sector

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zonation, though there is a remnant of oscillatory zoning present as a “ghost texture” (Hoskin and Black, 2000). Grain q shows a loss of primary features that has been

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replaced by a fine-scale network of vein like features. This may be related to

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deterioration of crystal structure at high temperatures (e.g. Harley and Kelly, 2007a), a similar process to that seen in the core of grain d. The overgrowths seen in textures such as these are prime targets for obtaining U–Pb ages relating to metamorphism. Grains r and s have distinct and complex internal features resulting from UHT metamorphism. The cores of these grains are inclusion-rich but the zircon itself is a featureless mass with very low CL response. It is unclear whether this lack of texture is due to metamictisation of high-U parts of the core, a degradation of crystallinity through other means (such as the partial reduction of the silicate structure to oxides at the micro-scale) at extreme temperatures, or a combination of these factors. Radiating cracks surrounding these zones are often associated with thermal expansion of

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ACCEPTED MANUSCRIPT metamict domains. Surrounding the damaged portion of the core there are clear sections showing the fine-scale oscillatory zoning that represents the growth of these

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grains in a crystallising igneous melt. The external features of these grains are variable, with grain r showing an additional overgrowth of coarse oscillatory, but in

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places slightly convoluted zoning, whereas grain s shows a progressive recrystallisation of the grain with a low CL response, featureless overprint. These differences are likely the result of the textural relationship of the zircon to locally

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crystallising anatectic melt at, or typically following, peak temperatures. Additional

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features in these grains can be seen as invasive, cuspate and lobate fronts of pervasive alteration, potentially along pre-existing, now annealed cracks. This feature may be the result of high temperature annealing or recrystallisation due to fluid alteration, and

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surrounding inclusions.

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is manifested in the metamict core by the appearance of new, crystalline zircon

Grains t–w show textures often described as ‘truly metamorphic’ zircon. The

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grains are luminescent, with similar CL responses to the overgrowths seen in the

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previous grains, and show equant morphologies. The internal textures of these grains are usually sector, or fir-tree zonation, in some cases showing some planar feature (grain t) with a similar CL response, or a late overgrowth (grain u) that shows coarse oscillatory zoning. In many cases the internal boundaries between sector zones are marked by a fine, bright CL zone that gives the internal textures a strong definition. This feature, also present in the overgrowths on grain m and very pronounced on grain n, appears to be a distinctive feature of sector zoning at high-grades of metamorphism. The internal features, combined with the external morphology, of grains v and w are classic “soccer ball” metamorphic zircon, which have previously been interpreted as super- and suprasolidus zircon growth at high temperature

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ACCEPTED MANUSCRIPT (Schaltegger et al., 1999; Vavra et al., 1999). Whilst grains such as these are often homogenous in appearance and often present the high likelihood of metamorphic U–

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Pb ages, the inefficiency of recrystallisation of zircon at high temperature means it is possible these grains can still contain pre-existing zircon with inherited Pb signatures.

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The final textures presented here in grains x and y are the result of pervasive fluid alteration, where post peak fluids have resulted in a coupled dissolution– reprecipitation process within the zircon grains (Geisler et al., 2007). These textures

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are very distinctive, with the remains of inherited zircon cores showing broad, lobate

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features truncating the original zoning and replacing the primary zoning with a largely featureless zircon that may retain the original grain shape. The extremely convoluted appearance of the zoning in the core of grain y is likely the result of these invasive

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features in the third dimension. The dissolution–reprecipitation process is variable in

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is efficiency of Pb-removal from the original grain, frequently resulting in erratically disturbed U–Pb ages (e.g. Taylor et al., 2014).

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2.2 Monazite textures

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Prior to in situ analysis monazite is also typically imaged using an SEM or electron probe to highlight internal features, with BSE imaging very common. A number of papers have described and used the internal features of monazite, particularly to identify secondary reaction features that can be ascribed to metamorphic events or fluid related processes. Basic classification of monazite into simple and complex grains (e.g. Triantafyllidis et al., 2010) is often inadequate, with a large proportion of BSE monazite textures summarized for metamorphic rocks in particular falling into the complicated category (e.g. Ayers et al., 1999; Catlos, 2013; Fitzsimons et al., 1997; Harley and Nandakumar, 2014; Kelly et al., 2012; Zhu and O'Nions, 1999). It is often hard to identify genuine overgrowths such as those seen in

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ACCEPTED MANUSCRIPT zircon, and in many cases the variation appears to be modification of the original monazite. However the descriptive terms core and rim are still often used in this

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circumstance. The complexity in monazite textures in BSE images is often attributed to Th zoning (Swain et al., 2005) as the large variety in Th content seen in many

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monazite grains should result in varying BSE response. However this is an oversimplification, and whilst it is likely that Th plays a role in some zoning patterns, there is clearly additional complexity. As a result, in order to successfully identify

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different age domains more detailed images are often taken prior to analysis using

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wavelength dispersive spectroscopy (WDS) on an electron microprobe analyser (EPMA). Typical elements mapped are Ce, Th, U, Y and may include other important elements such as other REE, Ca and Si. Apparent internal zones defined by Y are a

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common target for U–Pb dating, and in many cases have been shown to directly relate

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to age variation (Dahl et al., 2005; Mahan et al., 2006), however there are many cases where no such correlation exists (Hokada and Motoyoshi, 2006; Martin et al., 2007;

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Triantafyllidis et al., 2010) Of particular importance is the potential link between Y-

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bearing zones in both garnet and monazite that can link metamorphism to U–Pb ages (e.g. Catlos et al., 2001; Hacker et al., 2015; Mahan et al., 2006). However the exact nature of high or low Y in monazite in relation to the growth or destruction of garnet, or other high-Y phases such as allanite (Spear and Pyle, 2010), means this link may be enigmatic and require a case-by-case evaluation. The result of investigations into monazite using these X-ray maps is that the zoning does not necessarily match up between different elements in terms of style or extent, or match up with the BSE image (e.g. Ayers et al., 2006; Crowley et al., 2008; Harley and Nandakumar, 2014; Kelly et al., 2012; Shaw et al., 2002; Williams et al., 2007), and also that geochemically distinct zones may have no effect on resulting age domains. Monazite

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ACCEPTED MANUSCRIPT is therefore capable of displaying a bewildering array of possible textures that may, or may not, aid in identifying targets for in situ isotopic and trace element analysis, and

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are not always consistent even from a single, well characterized process. Despite the potential ambiguities it is still essential to investigate the complexity of monazite

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grains prior to analysis. Even complex internal textures still provide information on the growth process and its relative timing compared to other phases. Figure 3 is composed of an array of predominantly BSE images highlighting the different

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textures typically seen in high-grade metamorphic rocks, along with some additional

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techniques that can aid, or directly identify different age domains. Figure 3a–c shows the simplest texture generally seen in monazite from granulite facies rocks. A low BSE response grain is modified during metamorphism

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and displays a typically brighter BSE alteration zone. In grains a and b this is seen

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around the rim and varies in extent, whereas grain c shows no core-rim relationship. Grains d–f show an additional level of internal features. Broadly speaking the

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grains are dominated by two BSE zones, however within these zones there is a level

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of intricacy to the BSE response. Grain d shows a low BSE response core with a modified rim, both of which show patchy internal features, whilst grain f shows only minor additional detail in the rim. Grain e also shows some remnants of the dark BSE response material within a modified area of the grain. In many cases the outermost features of more complex monazite show very sharp boundaries with the remaining core, suggesting that these features, which may result in U–Pb resetting, are the result of dissolution-reprecipitation processes rather than diffusion (e.g. Seydoux-Guillaume et al., 2002). Grains g–k add a further amount of complexity with a broader range of potential BSE responses, and a larger variety of textural features. Grain g had three

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ACCEPTED MANUSCRIPT distinct BSE response zones, approximately concentric and increasing in response towards the rim of the grain. Some of these zones have internal features such as

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irregular or patchy zoning. Grains i and k also have multiple zones however the distribution of them is apparently random and not related to the general grain

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morphology. Grains h and j have an additional zone with a curved, oscillatory pattern of high BSE response that appears in an irregular fashion within the grain. Both of these grains also show an additional outer zone of lower BSE response with no

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internal features.

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Grains l–n have variable features that are the result of fluid assisted, coupled dissolution–reprecipitation mechanisms. These grains have been subject to post-peak, fluid alteration that has resulted in recrystallisation fronts with sharp boundaries.

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These features do not form a mantle around the grain edge, but form complex, lobate

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features that appear to overprint each other. Unlike the zircon grains these zones may have completely reset U–Pb systematics enabling the fluid alteration to be accurately

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dated and thus fluid–rock interaction constrained (Taylor et al., 2014).

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As a result of the complexity of monazite BSE images, and the possibility of highly variable major element compositions, alternative imaging methods have been extensively used to aid in age interpretation and analytical targeting. Images o–p in Figure 3 are X-ray maps produced using an electron microprobe. These maps show complex zoning directly from major and trace element compositions, in this case Th and U respectively. Figure 3q is an electron backscatter diffraction (EBSD) image of a complex monazite grain that has undergone plastic deformation during high-grade metamorphism (Erickson et al., 2015). In this case the EBSD image highlighted features seen in transmitted light, but not apparent using other imaging techniques, such as lattice distortion in zones with planar deformation features, and small sub-

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ACCEPTED MANUSCRIPT grains (dark blue) which are the only zones to yield concordant metamorphic ages. Most recently monazite dating via laser ablation split stream (LASS) (Hacker et al.,

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2015; Kylander-Clark et al., 2013) or quadrapole (e.g. Alagna et al., 2008; Kirkland et al., 2016; McFarlane and Luo, 2012; McFarlane and Frost, 2009) allows the

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collection of all elements and isotopic ratios of interest at the same time. This allows maps of complex grains to be produced (Figure 3 r–s) from multiple laser spots in a grid fashion that have values spatially interpolated, and the direct correlation of all

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major, trace and age information from the same sample volume for each spot. In this

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case the LASS chemical map (image r) directly correlates with the U–Pb obtained from each zone (image s) (Clark et al., Unpubl.). This technique allows the internal nature of the grain to be realized at the same time as the determination of quantitative

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information, however the exact nature of the boundaries between zones cannot be

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seen in the same way as a BSE image, and the shape and quality of the features shown depends on the number of data points and the method of interpolating between them.

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The nature of high-grade metamorphic mineral growth, consumption and

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recrystallisation clearly results in a wide range of accessory mineral textures of varying complexity. These textures result from the interplay of a number of processes including modification of protolith or inherited grains, reactions involving major or other accessory minerals, as well as the formation, flux, and crystallization of melt (Harley et al., 2007). Internal features of zircon and monazite must be identified by some means prior to in situ analysis, to identify both potential target zones and, just as importantly, highlight areas of fine scale complexity that should be avoided. 3. U–Pb geochronology in high-grade metamorphic environments It is often the case that granulite-facies terrains provide a continuum of concordant data that do not form a coherent statistical population. In some cases it is

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ACCEPTED MANUSCRIPT likely that the U–Pb age spread may represent the duration of high temperatures in a region, whilst in others there is partial resetting between separate thermal events, in

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either case a description of the full age range is important and may provide insight into the nature of the T–t path (Clark et al., 2011; Harley, 2016; Harley et al., 2007).

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Under high-grade metamorphic conditions new zircon typically forms in entirety from the breakdown of older components of the rock that liberate the necessary elements. An alternative process is that under metamorphic conditions trace elements in

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protolith crystals are purged during recrystallization and partitioned between the

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enriched recrystallization front and depleted recrystallized areas (e.g. Hoskin and Black, 2000). However, recrystallization is not always efficient, often leaving a ‘memory’ of the protolith trace element and isotopic composition. This may result in

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the measurement of ‘mixed’ U–Pb isotope ages. Nonetheless such complex U–Pb

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isotopic data can be interpreted and utilised to gain insights into the earlier history of the rock. Additional complementary chemical and isotopic information, which

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includes the REE, Ti, Hf isotopes and O isotopes can aid in confidently identifying

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the specific geochemical process that has affected the U–Pb isotopic system. In the following section the U–Pb isotopic systematics are considered for zircon and monazite in high-grade metamorphic environments. The portrayal of U–Pb data on a concordia diagram (either normal or inverse) and the interpretation of discordance in relation to U–Pb resetting is explained in detail in other reviews (e.g. Schoene, 2013) or specific articles relating to the examples below. In this section we aim to show how metamorphic zircon growth may form in relation to the evolution of a metamorphic terrain using specific examples. Following that we look at the formation of metamorphic monazite, and how to interpret its growth relative to that of zircon. 3.1 U–Pb Zircon in high-T environments

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ACCEPTED MANUSCRIPT Here we present case studies from regions where metamorphic processes have significantly influenced the U–Pb systematics. Under favourable circumstances the

the duration and thermal intensity of metamorphism.

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interpretation of complex U–Pb information can greatly assist in the understanding of

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In the simplest case a discordant array of protolith zircon may yield a lower intercept that constrains the time of radiogenic-Pb mobility. In many cases this period of radiogenic-Pb loss may coincide with new zircon growth, frequently ascribed to

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metamorphic zircon precipitation. Such data distribution on a concordia diagram may

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be taken to reflect a short episode of metamorphic overprinting affecting both preexisting zircon, liberating prerequisite elements for new zircon growth, and the precipitation of new “metamorphic” zircon.

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A well-constrained example of this process can be seen in zircons from the

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Munglinup Gneiss of the Northern Foreland in the Albany–Fraser Orogen. The Northern Foreland is the portion of the Yilgarn Craton reworked during the Albany–

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Fraser Orogeny, reflecting its position adjacent to this orogenic belt (Myers, 1990;

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Spaggiari, 2009). The Munglinup Gneiss is interpreted as a reworked part of the Yilgarn Craton based on similarities in lithologies, protolith ages and Lu–Hf isotopic signature (Kirkland et al., 2011a). U–Pb data from oscillatory-zoned zircon cores in a migmatitic granitic gneiss from this region, yields a discordia that intersects the concordia curve at 2709 ± 35 Ma and 1018 ± 21 Ma. The upper intercept date of 2709 ± 35 Ma is interpreted as the magmatic crystallization age of the granitic protolith, consistent with the age of other granitic rocks in the Yilgarn Craton (Kirkland et al., 2011). Homogeneous zircon overgrowths on these zircon cores yield an age of 1184 ± 6 Ma supporting new zircon generation during the regionally widespread Stage II Albany–Fraser Orogeny. The anomalously young lower intercept implies recent

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ACCEPTED MANUSCRIPT radiogenic-Pb loss affecting the discordant array. This result highlights the importance of targeting neocrystallised zircon identified by CL imaging, as partially

as both processes are most likely in metamict zircon grains.

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reset discordant zircon are the very grains most susceptible to more recent Pb mobility

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In the more complex case of continuous U–Pb age distributions along concordia two different approaches are apparent in the literature, aimed at extracting meaningful temporal information. Analyses may be interpreted with the aid of

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imaging and hence ages can be determined through grouping texturally similar

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populations. However, in some cases, certain growth or alteration processes occur multiple times over a metamorphic episode leading to similar textures forming at dissimilar times. Hence, there may be little unambiguous textural evidence to

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distinguish different age populations. Nonetheless, assuming that the observed data

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derive from zircon formed during a single magmatic event that has been subject to metamorphic overprinting, the oldest concordant datapoint could reflect the timing of

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that event assuming that radiogenic-Pb mobility had not compromised the protolith

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age. Again, textural evidence such as oscillatory zonation has frequently been argued as a means to distinguish pristine core domains carrying information from the earliest history of the crystal. An alternative approach that has been applied to polyphase zircon subject to potentially prolonged metamorphic processes again assumes that a single magmatic protolith age component is present and that this has been subject to later Pb-loss, resulting in a spread of ages. Under such a model exclusion of the youngest ages from the weighted average is performed until the scatter statistic (e.g. MSWD) reaches a value consistent with a single population, that is one in which the assigned analytical errors alone are able to account for the observed scatter within the subset of data. However, such a method is not based on independent petrographic

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ACCEPTED MANUSCRIPT information and cannot yield a meaningful value for prolonged metamorphic growth processes, though it may be feasible to place bounding limits on it. Additionally, in

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such an approach, extreme care is necessary to correctly interpret any coherent population in orthogneisses as it is feasible that even the oldest component does not

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reflect the magmatic protolith age; in that the oldest component may have been expunged from the zircon record. Both approaches, petrographic characterization or statistical modeling, may be applicable in individual case studies.

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The Musgrave Province, in central Australia, is located at the centre of

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Australia’s Proterozoic structural trends, reflecting the amalgamation of the North, West and South Australian Cratons. The province is dominated by felsic lithologies created and heavily metamorphosed, during a range of events in the late

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Mesoproterozoic, including the 1345–1293 Ma Mount West Orogeny, the 1220–1150

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Ma Musgrave Orogeny (Pitjantjatjara Supersuite) and the >1078–1026 Ma Giles Event. Considering the Musgrave Orogeny more specifically, this event was

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associated with a prolonged 100 Ma period (c.1220-1120 Ma) of UHT metamorphism

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and granite emplacement at temperatures of up to 1000°C (Kelsey et al., 2010; Smithies et al., 2011). Temperature constraints on the Musgrave Orogeny indicate that the mid crust (7 kbar) was at >1000°C with a geothermal gradient of >35-40°C km-1 (Walsh et al., 2015). SIMS zircon U–Pb geochronology of this event has yielded an essentially continuous record of granite magmatism ascribed to the Pitjantjatjara Supersuite from 1220–1150 Ma. In the early stages of the Musgrave Orogeny a switch from Yb-depleted to Yb-enriched granite compositions heralds a profound change from deep-crustal, garnet-present, melting to lower-pressure melting, related to delamination of the lower crust (Howard et al., 2015; Smithies et al., 2011).

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ACCEPTED MANUSCRIPT During the Musgrave Orogeny five magmatic zircon age probability peaks are recognized (Figure 4) and over the same period homogenous zircon overgrowths

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developed on pre-existing zircon from both magmatic and sedimentary protoliths. It has been pointed out that that the probability age peak in metamorphic zircon ages

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occurs after a lag of about 5–10 Ma from the magmatic probability age peak (Howard et al., 2015) (Figure 4). This may reflect cooling of the country-rock after granite emplacement to sub-UHT conditions where anatectic zircon may crystallise (Howard

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et al., 2015).

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3.2 The relationship between zircon and monazite U–Pb ages at high-T Due to relatively high U contents compared to other accessory minerals the U–Pb ages obtained from monazite can be very precise, however the interpretation of

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monazite ages can be anything from simple to cryptic, particularly when the spread of

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ages is large (e.g. Horton et al., 2016; Kirkland et al., 2016). Whilst monazite often returns very similar ages to zircon due to mineral and melt reactions at high

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temperature (e.g. Hermann and Rubatto, 2003), there are examples of granulites

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where metamorphic monazite U–Pb ages post date metamorphic zircon (e.g. Clark et al., 2014; Harley and Nandakumar, 2014). The simplistic notion that monazite will record younger ages than zircon is inadequate when dealing with high temperature metamorphism, and while some estimates of the Pb closure temperature are conflicting, the more recent experimental constraints place it in excess of 900ºC (Cherniak et al., 2004). The variable major element composition, and therefore physical and chemical stability, of monazite means that there may be multiple generations of monazite in a metamorphic rock reflecting different processes (Spear and Pyle, 2002; Williams et al., 2007). Even in simple scenarios variations between zircon and monazite ages in metamorphic rocks are poorly understood, and may

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ACCEPTED MANUSCRIPT reflect variations in composition of the host rock, or mineral reactions that result in accessory mineral growth. Where high temperatures exist for a prolonged period of

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time it becomes possible to investigate the nature of some of these variations as periods of monazite and zircon growth can be temporally distinguished. A natural

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example of this can be found in southern India’s Southern Granulite Terrane (SGT), which experienced temperatures of 900ºC for approximately 100 Myr during the amalgamation of Gondwana (Collins et al., 2014 and references therein). There have

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been a large number of geochronological studies on the SGT due to its position in the

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middle of the Gondwana supercontinent, between the Congo and Dharwar Cratons (Collins and Pisarevsky, 2005). The SGT makes an excellent natural laboratory for the study of monazite as almost every lithology contains an abundant amount of this

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accessory mineral. Several recent studies looking at each of the major tectonic units

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(Madurai Block, Achankovil Zone, Trivandrum Block, Nagercoil Block) of the SGT have focused on the high temperature metamorphism as recorded by zircon and

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monazite (Blereau et al., 2016; Clark et al., 2015; Harley and Nandakumar, 2014;

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Johnson et al., 2015; Taylor et al., 2014; Taylor et al., 2015a) providing insight into the relative ages recorded by the two geochronometers. Figure 5 shows relative probability distributions for zircon and monazite ages from the southern part of the SGT, south of the Madurai Block based on the recent SHRIMP geochronology studies above. These data identify some of the likely scenarios for the timing of monazite growth during a long-lived metamorphic event. Figure 5a shows zircon and monazite age peaks from a typical metapelite (grt-sill-crd gneiss) in the Trivandrum Block of the SGT (Taylor et al., unpubl.). This dataset is obtained from the same locality as Harley and Nandakumar (2014) who carried out a detailed in situ study of the structurally youngest leucosome sheet. The solid black line shows zircon data in the

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ACCEPTED MANUSCRIPT metapelite, with recrystallization of pre-existing grains during high temperature metamorphism resulting in a slow increase of metamorphic ages, followed by the

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majority of metamorphic zircon forming during melt crystallization (535–520 Ma) after the metamorphic peak (Roberts and Finger, 1997). The solid grey line shows

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zircon data from within a granite sheet hosted by the metapelite, which as expected simply shows zircon forming post peak. Monazite from this locality however shows a distinctly different distribution with two distinct peaks prior to and following the

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inferred peak of regional metamorphism. This matched the monazite data seen in a

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metapelite from the Nagercoil Block (Johnson et al., 2015) where the prograde monazite population was interpreted to have formed during melt production following the breakdown of apatite, while post-peak monazite results from the recrystallization

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of preexisting grains during melt crystallisation. This scenario, of prograde and

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retrograde monazite growth, potentially explains some of the complexity seen in metamorphic terrains where multiple monazite age peaks could have been interpreted

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to result from multiple heating events, even where zircon data suggests a single, long-

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lived event. An inherent limiting factor in the growth of prograde monazite may be the supply of Th, however many other accessory minerals, such as xenotime and allanite, that break down on the prograde path may contribute to the Th budgetrequired. It is in fact notable that in the SGT examples early monazite is particularly low in Th and Th/U ratio compared to that seen later during melt crystallization and fluid flow (Johnson et al., 2015; Taylor et al., 2014). Figure 5b shows another dataset from a metasedimentary rock (grt-bt gneiss) from the Trivandrum Block, however in this case there has been a modification of the accessory phase U–Pb systematics by a late stage fluid event c. 495 Ma (Taylor et al., 2014). The zircon data show a similar trend to the previous example with melt

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ACCEPTED MANUSCRIPT crystallization dominating the age signal, with maybe some younger ages attributed to a response to the fluid ingress. The monazite, in contrast, shows a distinctly younger

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age peak as a result of more complete resetting of the U–Pb systematics due to dissolution–reprecipitation reactions during fluid flux, including a distinct group at c.

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495 Ma with particularly high Th/U values (Taylor et al., 2014). An interesting aspect to this data is that the 535 Ma recrystallized monazite peak seen in metasedimentary rocks through the Trivandrum Block (Figure 5a) is lost during fluid event, whilst the

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older, pre-peak, monazite signal is still present. This highlights the previously

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unrecognised possibility that this early monazite is more stable in the presence of such fluid, whilst domains that have already been recrystallized are subsequently more susceptible to dissolution–reprecipitation reactions.

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Figure 5c shows a larger dataset of zircon and monazite compiled the entire

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SGT. As with the site specific examples the majority of zircon data forms during melt crystallization, with 540–525 Ma ages dominating in the Trivandrum and Nagercoil

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Block (Johnson et al., 2015; Taylor et al., 2014) similar to the range seen in other

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recent studies (Harley and Nandakumar, 2014), and younger 515 Ma ages present in the Achankovil Zone (Taylor et al., 2015a). The apparent hiatus in zircon ages following the recrystallization peak at c. 580 Ma is likely due to zircon consumption by melt as it approaches peak conditions. The monazite spectra shows prograde monazite formed during melting as the dominant age signal in all tectonic units. Subordinate age peaks are present as a result of post-peak modification during melt crystallisation at c. 535, and fluid ingress at 510–495 Ma. Importantly, although not surprisingly, there are no specific age peaks related to the recently interpreted thermal peak of metamorphism for the SGT (e.g. Johnson et al., 2015). This full spectra of ages for a single metamorphic event, accentuated by the protracted nature of high-

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ACCEPTED MANUSCRIPT temperatures in the SGT, highlights the essential requirement of imaging of accessory phases prior to analysis and understanding the processes that result in the internal

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features of the grains. It must be noted that even with these explanations of the origin of the distinct age peaks, the nature of a terrain being held above the solidus for long

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periods could result in large ranges even for single textures. For example, zircon overgrowths related to melt crystallization may result for a number of open–closed system changes, including melt migration, stagnation, and wall-rock interaction as

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described in Harley and Nandakumar (2014).

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Accessory mineral U–Pb data from within granulites do not form single populations as they do not result from a single process. The spread of ages in some high-grade rocks might even be resolvable into multiple, process related peaks if the

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duration of high temperatures is protracted, and the analytical resolution sufficiently

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precise. This complexity can be used to interrogate metamorphic processes by detailed observations of accessory mineral textures in relation to their petrographic

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context. To better characterize terrain scale regional metamorphism it is advantageous

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to analyse accessory minerals from a variety of bulk compositions, and across a wide geographical area. 4. Linking accessory mineral growth to the mineral assemblage evolution through REE geochemistry Increasingly trace and in particular rare earth element (REE) information is collected in addition to U–Pb ages in order to place the temporal information in a mineralogical or process context. Zircon in particular, due to its propensity to incorporate heavy rare earth elements (HREE) and slow diffusion (Cherniak et al., 1997), can be linked to garnet growth (Rubatto, 2002). Monazite often has a more enigmatic trace element signature, in part due to the REE transitioning from major structural components

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ACCEPTED MANUSCRIPT (LREE) to trace elements (HREE), and variable major element composition (Catlos, 2013).

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4.1 REE in zircon - Empirical observations The seminal paper by Rubatto (2002) marked a pivotal point for the use of

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zircon as a geochronometer in metamorphic rocks. In this paper SHRIMP U–Pb geochronology of zircon was directly correlated with LA–ICP–MS trace element analyses. A combination of geochronological and textural evidence was used to

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demonstrate that metamorphic overgrowths formed at granulite facies conditions had

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REE patterns reflecting growth in the presence of garnet. This was the first time that partition coefficients for the REE between zircon and garnet had been calculated, and opened up a new line of evidence for the interpretation of metamorphic ages. Over

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the next several years this method took hold and became a key tool in metamorphic

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geochronology (Buick et al., 2006; Harley and Kelly, 2007a; Harley et al., 2007; Harley et al., 2001; Hermann and Rubatto, 2003; Hokada and Harley, 2004; Kelly and

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Harley, 2005; Whitehouse and Platt, 2003), linking accessory mineral growth with

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major metamorphic minerals and therefore quantifying specific points within a P–T–t evolution. These studies used careful textural observations to determine that zircon growth occurred whilst garnet was stable in the rock, and cover a variety of lithologies and P–T estimates. The result of these studies was the publication of a variety of apparent equilibrium partition coefficients between these two minerals (D zrc/grt) that covers more than an order of magnitude for the HREE (Figure 6a). The study by Rubatto (2002) investigated DREE(zircon–garnet) relationships in well constrained, high-grade terrains at both granulite (Reynolds Range; 750–800ºC, 4.5–5 kbar) and eclogite (Sesia-Lanzo; 550–620ºC, 15–18 kbar) facies assemblages. Both residue (GP7) and leucosome (GL7) in the granulites facies sample show steep

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ACCEPTED MANUSCRIPT zircon REE patterns, strongly favouring the HREE, similar to the inherited cores, whilst the garnets have flat patterns through the M–HREE. The resultant DM– slopes for the restite and leucosome are steep with DGd of 0.94

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HREE(zircon–garnet)

and 1.6 respectively whilst DYb is 8.6 and 17 respectively. The study of Hermann and

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Rubatto (2003) showed a number of metamorphic zircon overgrowths on inherited cores in a granulite facies (850–700ºC, ~10 kbar) metapelite from Val Malenco, northern Italy. This study favoured the correlation of the oldest (~280 Ma), most

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HREE-rich metamorphic zircon with the garnet in the rock, resulting in steep

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DREE(zircon–garnet) patterns with DDy of 1.3 and DYb of 8.2 (Fig 6a). Later zircon overgrowths had approximately an order of magnitude lower HREE but this was interpreted as being due to growth after garnet. This paper was the first to make the

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additional correlation of monazite REE partitioning into the interpretation of ages,

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calculating DREE(zircon–monazite) and DREE(monazite–garnet) values for the samples. Buick et al. (2006) also favoured steep D(zircon–garnet) patterns from the

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M–HREE. In this study c. 2.02 Ga metamorphic zircon overgrowths from the

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Limpopo Belt, S. Africa (800ºC, 8–10 kbar) with a range of REE distributions were evaluated against a zoned garnet. Garnet rim compositions were taken to represent chemical equilibrium with the metamorphic zircon and returned DGd values ~2 and DLu values ~9-10 (Fig 6a). In contrast to the steep DREE(zircon–garnet) patterns described above several studies showed patterns with D values of approximately 1 through the M–HREE, regardless of temperature. The granulites of Whitehouse and Platt (2003) formed at 750–800ºC, ~4 kbar, contained zoned garnet and preserved a polyphase zircon history. As before the rims of the garnet were interpreted as being in equilibrium with homogenous, c. 21 Ma metamorphic zircon rims, DGd ~0.7 and DLu ~0.9 (Fig 6a).

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ACCEPTED MANUSCRIPT Correlation with garnet cores returned DHREE values strongly in favour of garnet by almost two orders of magnitude. This study identified the possibility that many factors

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may be at play regarding the zircon–garnet partition coefficients, such as temperature, pressure and garnet composition. Flat DM–HREE(zircon–garnet) profiles with values

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close to unity are also apparent in several studies of ultrahigh temperature (UHT) granulites in the Napier Complex, east Antarctica formed at up to 1100ºC, ~11 kbar (Harley et al., 2001; Hokada and Harley, 2004; Kelly and Harley, 2005). Detailed

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observations of zircon overgrowths and garnet in feldspathic leucosomes (Hokada and

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Harley, 2004) were used to identify textural, and accordingly chemical, equilibrium. A suite of orthogneiss and paragneiss samples in Kelly and Harley (2005) were analysed to investigate REE variations in zircon populations and identified the ~2.5

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Ga populations during melt-present conditions to show zircon–garnet equilibrium.

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Steep DM–HREE favouring zircon at Lu were only recorded for zircon grains which had been interpreted as magmatic. The Napier Complex samples all show DM– values in the range 0.6–1.1 (Fig 6a).

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HREE(zircon–garnet)

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4.2 REE in zircon – Experimental constraints In order to establish the parameters that control this disparity in DREE(zircon–

garnet) datasets a pioneering set of high P–T experiments were performed by Rubatto and Hermann (2007a). These piston cylinder experiments grew zircon and garnet within hydrous granitic melts doped with P, Y, REE, Zr, Hf, Th and U at 20 kbar across the temperature range 800–1000°C, and provided the first experimentally derived zircon-garnet partition coefficients for the REE. The results of these experiments (Figure 6b) provide a seemingly satisfactory solution to the observed variation in empirical estimates of equilibrium REE partitioning between zircon and garnet. On a DREE(zircon–garnet) plot the experimental data shows some consistent

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ACCEPTED MANUSCRIPT trends, such as a positive slope through the M–HREE showing preferential incorporation of HREE into zircon relative to the MREE. The calculated partition

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coefficients for the high temperature experiments give D values close to unity for the M–HREE (1000ºC - DGd = 0.81; DYb = 1.2), with a clear trend towards DHREE

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favouring zircon as temperature decreases (800ºC - DGd = 1; DYb = 8) (Figure 6b). This observed trend potentially means that the variation seen in the natural examples in the previous section can be entirely attributed to the peak temperature that those

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rocks experienced, with the lower temperature experiments matching those studies

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with steep partitioning plots (Buick et al., 2006; Hermann and Rubatto, 2003; Rubatto, 2002), and the high temperature experiments matching the UHT examples (e.g. Hokada and Harley, 2004; Kelly and Harley, 2005). However, there is a

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mismatch with the study of (Whitehouse and Platt, 2003) in that the zircon and garnet

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in this study are estimated to have equilibrated at relatively low temperatures, ~750ºC, and yet show DM–HREE(zircon–garnet) values close to unity, therefore matching the

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high temperature experiments. An additional complication is noted in that the

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variation in DREE also correlates with garnet major element chemistry in these experiments with 1000ºC garnet containing 3.05 wt% CaO and 11.26 wt% MgO, and 850ºC garnet containing 7.86 wt% Ca and 6.49 wt% MgO. This means there is some uncertainty as to whether the primary control over the DREE variations is temperature or garnet chemistry. However, as both these parameters may be correlated in a natural system (e.g. grossular garnet in high P terrains) the covariance between temperature, chemistry and DREE may be logical necessity of different rock systems. More recently a set of experiments was performed investigating DREE(zircon– garnet) values in systems more appropriate to high temperature metamorphism in the mid to lower crust (Taylor et al., 2015b). These Ca-absent experiments comprised

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ACCEPTED MANUSCRIPT granitic starting materials representing anatectic melts at 900–1000ºC and 7 kbar in internally heated gas pressure vessels (IHPV). The data from these experiments

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(Figure 6b) show a coherent set of zircon/garnet D values for the M–HREE at these temperatures with values close to unity. In more detail the shape of the partitioning

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plot (Figure 6b) shows a concave up pattern with the LREE strongly incorporated into zircon, the MREE slightly favouring garnet, and the HREE with values close to 1. There is an indication that the exact shape of the D value plot has a slight dependence

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on temperature, with the highest temperature experiments still favouring garnet for

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the HREE, however it is not truly resolvable within the analytical errors. These experiments are an excellent match for the empirical studies in both the simple UHT leucosomes (Hokada and Harley, 2004) and other studies in UHT terrains (e.g. Kelly

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and Harley, 2005). This pattern of DM–HREE(zircon–garnet) values close to unity

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appears to be consistent in high grade rocks e.g. south India (Clark et al., 2015; Johnson et al., 2015; Taylor et al., 2014; Taylor et al., 2015a), particularly where the

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lithology is relatively simple, e.g. felsic gneisses and garnet bearing granite sheets.

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The results of the 7 kbar Taylor et al. (2015b) study closely match that of the highest temperature experiments in the 20 kbar Rubatto and Hermann (2007a) study, suggesting that rocks that have equilibrated at greater than 900ºC should show DM– HREE(zircon–garnet)

values close to unity in low-Ca systems, and therefore these

values are the most appropriate for high temperature, mid to lower crustal, felsic rocks. 4.3 Trace elements in in monazite Linking trace elements in monazite to the formation or breakdown of major metamorphic phases is often cryptic. Distinct age populations in monazite are often identified by zonation in both major (e.g. Th) and trace (e.g. Y) elements and in many

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ACCEPTED MANUSCRIPT cases these zones clearly distinguish domains separated by many millions of years (Ayers et al., 1999; Gibson et al., 2004; Holder et al., 2015; Mahan et al., 2006;

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Santosh et al., 2006). As with zircon the appearance of garnet, an efficient sink for the HREE may result in populations of monazite with correspondingly low HREE

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contents. However unlike with zircon, where there is high quality experimental data to compare to, there is little quantitative information on the REE signature for monazite in equilibrium with garnet. Therefore to really identify a “garnet signature”

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in monazite it is most useful if there are multiple monazite populations, some of

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which are relatively depleted in HREE compared to others. However it has also been observed in some granulite facies rocks that there is an inverse relationship between the ThO2 + UO2 content and the total HREE in monazite (Krenn and Finger, 2010),

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signatures in monazite.

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and therefore there may be additional substitution mechanisms relating to the REE

Zonation in the Y content of monazite is often a key discriminator for

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temporally distinct growth zones, and also provides the possibility of linking

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monazite growth/recrystallisation events to other key minerals such as zircon and garnet (Rubatto et al., 2006). Compositional differences in Y within individual monazite grains may track mineral reactions involving other Y-bearing phases during prograde (e.g. allanite and xenotime breakdown) or retrograde (e.g. garnet breakdown) processes. Whilst in many cases the Y zoning in monazite has clearly identified different age populations (e.g. Krenn et al., 2009; Martins et al., 2009; Shaw et al., 2002), it is equally likely that the appearance of distinct chemical zones may not reflect a clear age progression from core to rim, or even any age progression at all, resulting in misleading, apparent geological complexity in the investigated

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ACCEPTED MANUSCRIPT samples (e.g. Hinchey et al., 2007; Kelly et al., 2012; Martin et al., 2007; Spear and Pyle, 2010; Triantafyllidis et al., 2010).

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The widespread use of LA-ICPMS techniques means that trace element information can be acquired rapidly for almost all minerals. The understanding that

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U–Pb information in metamorphic rocks relates to a multitude of potential processes means that trace elements such as the REE are critical because it enables a link to the bulk silicate mineral evolution of the rock. Whilst few experimental datasets exist to

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directly link accessory–major mineral growth such as zircon–garnet REE

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distributions, simple observations such as trace element rich/poor zones related to the growth or destruction of key sinks can be enough to tie U–Pb ages to specific

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processes.

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5. 4+ thermometry applications and complications Over the past decade a number of accessory mineral trace element thermometers have

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been calibrated with the aim of directly linking temperature to time. The two most

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widely utilized accessory mineral thermometers applied to granulite facies rocks are the Ti-in-zircon thermometer (Ferry and Watson, 2007; Hofmann et al., 2013; Watson and Harrison, 2005; Watson et al., 2006) and the Zr-in-rutile thermometer (Ferry and Watson, 2007; Tomkins et al., 2007; Watson et al., 2006; Zack et al., 2004). These thermometers are particularly relevant to high-T geological processes due to the limited diffusivity of the 4+ cations within their lattices as constrained by the experimental diffusion studies of Cherniak and Watson (2007) and Cherniak et al. (2007). Kelsey and Hand (2015) provide an extensive review of the application these thermometers with particular reference to UHT rocks and readers are pointed to this study for more specific details of the limitations of the application of these and other

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ACCEPTED MANUSCRIPT thermometers to high-grade rocks. In this section we will focus on some of the advantages and limitations of these two thermometers with a particular emphasis on

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the link between the timing of accessory mineral growth and how the temperature recorded by these minerals may be linked to a specific time during the evolution of a

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high-grade metamorphic rock. Whilst there are a number of additional 4+ cation thermometers such as Ti-in-quartz (Thomas et al., 2010; Wark and Watson, 2006) and Zr-in-titanite (Hayden et al., 2008), for this review we will concentrate on the high

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temperature, accessory mineral thermometers Ti-in-zircon and Zr-in-rutile.

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5.1 Ti-in-zircon

The first, and arguably the most essential, step in the application of the Ti-inzircon thermometer is establishing the saturation of the rock in the trace elements of

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interest. Saturation is demonstrated petrologically by the presence of rutile and quartz

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for the Ti-in-zircon thermometer. Studies that report Ti-in-zircon temperatures without having established the saturating criteria should be viewed with considerable

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caution. In high-grade rocks the temperatures reported using the Ti-in-zircon

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thermometer are quite commonly cooler than the peak temperatures inferred by mineral equilibria modeling and corresponding Zr-in-rutile thermometry (e.g. Baldwin et al., 2007; Ewing et al., 2012; Kohn et al., 2015; Korhonen et al., 2014). Rather than these sub-peak temperatures being related to the diffusion of Ti from the zircon lattice at high-T Korhonen et al. (2014) have related this phenomenon to the post peak growth of zircon as the rock cools toward the solidus (e.g. Baldwin et al., 2007). Therefore the bulk of Ti-in-zircon temperatures in high-grade rocks when coupled with the U–Pb ages constrain the timing of melt crystallization rather than the timing of peak metamorphism. 5.2 Zr-in-rutile

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ACCEPTED MANUSCRIPT Kelsey and Hand (2015) recommend that the Zr-in-rutile thermometer should be preferentially used when trying to constrain the temperature of high-grade rocks

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due to its stability at temperatures in excess of 900ºC (e.g. Kooijman et al., 2012), resistance to subsequent modification during retrogression (e.g. Ewing et al., 2012)

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coarse grain size, ease of integration with petrologic forward modeling (Kelsey and Hand, 2015). However, there are a number of factors that must be taken in to consideration when applying this thermometer and trying to link it to a specific time

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during the evolution of the rock. The first and most significant limitation is that the

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diffusivity of Pb in rutile is significantly faster than Zr in high-grade rocks and it is very unlikely that the U–Pb age retrieved will be related to the timing of growth and the temperature recorded rutile. For example, Korhonen et al. (2014) describe an UHT

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event in the Eastern Ghats at ~950 Ma, an age that has been constrained by zircon U–

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Pb geochronology coupled with REE data from zircon and garnet. However despite the rutile occurring within the 950 Ma UHT mineral assemblage with a Zr content

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recording <1000ºC at this time, it has a U–Pb age of 450 Ma, some 500 Myr younger

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than the inferred growth (Taylor et al., 2012). This decoupling of U–Pb age and temperature means that there is limited use in extracting useful coupled T–t information purely from rutile in high-grade rocks. Complications may arise due to the inability of rutile to retain high concentrations of Zr following peak T, resulting in the exsolution of Zr-bearing phases. Reintegraion of the Zr of such phases can result in good estimate peak T (e.g. Pape et al., 2016), however if such phases are precipitated around the rim of the rutile this may be impossible. A further complexity that must be taken in to consideration when applying the Zr-in-rutile thermometer has recently been illustrated in a study by Taylor‐Jones and Powell (2015). Their study suggests that Zr-loss from rutile due to

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ACCEPTED MANUSCRIPT diffusion processes may be controlled by the availability of Si and Zr rather than solely by the ability of Zr to move from the lattice at the rate determined by the

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diffusion experiments of Cherniak et al. (2007). In their model if rutile is isolated from zircon during cooling the chances of recording a high-T event are maximized, as

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Zr is unable to leave the rutile lattice. Whereas if rutile is in chemical communication with zircon, Zr diffusion will continue down to the lower temperatures determined by experimental data from Cherniak et al. (2007). To overcome the potential for different

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rutile grains to record different temperatures in the same rock multiple analyses of

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rutile grains from a number of petrographic settings should be undertaken. Korhonen et al. (2014) and (Walsh et al., 2015) present examples of such analysis where multiple grains in a number of petrographic settings constrain peak temperatures

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recorded by rocks that experienced UHT conditions. The recorded temperatures must

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be used with careful observation for each case. For example, Harley (2016) describes a Zr-in-rutile dataset for the classic UHT Napier Complex in which, despite the rutile

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forming in the UHT mineral assemblage, appears to record only later amphibolite

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facies conditions. In comparison the corresponding Ti-in-zircon data show UHT temperatures, seemingly the opposite of the expectations of Kelsey and Hand (2015). 6. Thermodynamic modeling of accessory mineral growth The integration of the growth and breakdown of accessory minerals with thermodynamic models is in its infancy. There have been a handful of studies that have started down the path of linking the chemistry and age of phases into constrained P–T space and the results to date present at best a framework within which to interpret the results of U–Pb geochronological datasets. The first study that attempted to integrate zircon and monazite into the suprasolidus evolution of metasedimentary rocks was that of Kelsey et al. (2008). This study took the zircon and monazite

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ACCEPTED MANUSCRIPT dissolution experiments of Watson and Harrison (1983) and Rapp and Watson (1986) recast them in a form that was able to be integrated with the melt models used in

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Thermocalc (Powell et al., 1998). This study found that zircon and monazite growth should occur just before the solidus and provided an explanation for the occurrence of

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inheritance in zircon and lack of inheritance in monazite that is unrelated to closure temperature.

Kelsey and Powell (2011) took the simple approach of the Kelsey et al. (2008)

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study a step further by developing a series of activity composition models for Zr

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bearing phases (garnet, rutile, and melt) based on empirical datasets. By integrating Zr in to more than one phase the growth and breakdown of zircon could then be modeled more completely rather than just relying on melt as the controlling factor.

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The integration of monazite with thermodynamic forward models is

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considerably more difficult than zircon owing to its more complex chemistry and the fact that it is a phosphate rather than a silicate. One of the contradictions from the

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original modeling of Kelsey et al. (2008) was that a melt loss event 20°C above the

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solidus should lead to all of the monazite forming components escaping with the melt. The implication being that the residual rock is devoid of the material required to grow monazite during cooling, clearly not the case observed in the majority of metapelitic granulites. Stepanov et al. (2012) undertook a number of further experiments that demonstrated that the dissolution parameters for monazite used by Kelsey et al. (2008) overestimated its solubility at high-temperature. The data of Stepanov et al. (2012) has subsequently been integrated in to a refined of version the monazite and zircon dissolution models by (Yakymchuk and Brown, 2014a). The new models have enabled the construction of P–T pseudosections that more thoroughly model the behavior of monazite in melt-bearing rocks.

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ACCEPTED MANUSCRIPT The numerous circumstances in which accessory minerals grow or modify at high makes the incorporation in to thermodynamic models complicated. It remains to

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be seen whether robust thermodynamic frameworks will ever be able to account for features such as the prograde growth of zircon and monazite (e.g. Blereau et al., 2016;

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Johnson et al., 2015) or the sub-solidus and fluid mediated features observed in the majority of granulite facies rocks (e.g. Taylor et al., 2014). However, work in this area continues with studies such as Kohn et al. (2015) who incorporated Zr in to a

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number of minerals that occur in mafic rocks and used this to semi-quantitatively

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investigate the behavior of sub-solidus zircon along a variety of P–T paths in HP/UHP rocks.

The combination of U–Pb and trace element data with P–T information such

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as pseudosections, combined with mineral modes and accessory mineral

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thermometers should be used together to better constrain peak metamorphic conditions, as well as the location of mineral reactions on P–T–t space to more fully

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understand the metamorphic evolution of the terrain of interest. Whilst the

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combination of multiple techniques is clearly important, there is still the likelihood of discrepancies due to the caveats of each technique. Therefore it is essential to carefully evaluate each dataset, distill the most appropriate information for interpretation, whilst still presenting all the information available and the likely reasons for any complexity. 7. Lu-Hf systematics in metamorphic zircon 7.1 The Lu-Hf record as a monitor of metamorphic process The Hf isotopic system in zircon exhibits remarkable stability managing to retain its isotopic signature through events that manage to reset the U–Pb system (Choi et al., 2006; Guitreau and Blichert-Toft, 2014). Metamorphic zircon as preserved as either

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ACCEPTED MANUSCRIPT individual grains and/or domains within a single crystal can result from quite different processes. Zircon related to metamorphism may result from recrystallization

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(pseudomorphic alteration) of primary (magmatic) zircon grains (e.g. Geisler et al., 2007; Rubatto and Hermann, 2007b) or from new zircon growth. The latter process

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likely is dominated by precipitation from a liquid phase (e.g. Rasmussen, 2005; Rubatto and Hermann, 2007b) although near solid-state mineral reactions have also been proposed (e.g. Möller et al., 2003). As discussed by Zeh et al. (2010) it is first

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necessary to identify whether zircon is newly grown, or is the result of

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recrystallization of preexisting grains for a correct geological interpretation of the zircon Lu–Hf isotope data. Metamorphic zircon that formed by a recrystallization process will likely preserve its primary Hf isotopic composition even if the original

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U–Th–Pb information and zoning is obliterated, whereas metamorphic zircon that

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precipitates from a fluid or melt can have a distinct Hf isotopic composition because of incorporation of isotopically dissimilar Hf from external Hf reservoirs with

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differing Lu/Hf ratio (Gerdes and Zeh, 2009; Lenting et al., 2010; Patchett, 1983).

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Hence, the 176Hf/177Hf and Lu/Hf ratio of zircon crystals that grew at different stages of metamorphism during attendant, mineral destruction and growth processes, can potentially be used as tracers of zircon-forming metamorphic reactions in high-grade rocks (Sláma et al., 2007) We provide three case studies where the Hf isotopic signature has provided critical additional information on the metamorphic history. Two of the case studies demonstrate that under high-grade conditions the isotopic response in zircon is dependent not only on the associated metamorphic mineral growth but also on the stability of pre-metamorphic zircon. The last case study in this section reveals the

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ACCEPTED MANUSCRIPT important link that can be drawn between the metamorphic grade of melt source and its isotopic signature.

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All case studies come from the Albany-Fraser-Orogen (AFO). The AFO is part of the West Australian Craton and is dominated by Paleoproterozoic and

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Mesoproterozoic rocks that reworked, up to granulite facies, the southern Archean, Yilgarn Craton from at least 1810 Ma through to 1140 Ma. The orogen is constructed from a number of large scale lithotectonic blocks (Tropicana, Biranup, Fraser, and

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Nornalup) that contain rocks with variable protolith ages and geological histories that

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are interpreted to reflect moderate to heavy modification of pre-existing Archean Yilgarn Craton crust (Kirkland et al., 2014; Occhipinti et al., 2014; Spaggiari et al., 2011; Spaggiari et al., 2014).

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7.2 Hf isotopic signature of high-grade metamorphic zircon associated with

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metastable garnet.

The following example of metastable garnet and zircon growth comes from

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the Biranup Zone which comprises largely mid-crustal rocks, including orthogneiss

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and metagabbro with ages of c. 1810 to 1625 Ma (Spaggiari et al., 2011). Zircon overgrowths were developed widely during Stage II (c. 1200 Ma) metamorphism in the Biranup Zone.

Three Biranup Zone samples, whose age and isotopic composition have been previously discussed (Kirkland et al., 2011b), help to elucidate the growth of metamorphic zircon along with garnet. Zircon rims in two garnet present samples, a mylonitic siliclastic schist and a monzogranitic gneiss are homogeneous and dark in CL images and surround corroded magmatic cores (Kirkland et al., 2011b), which suggests breakdown and recrystallization of original zircon, presumably in the presence of a metamorphic fluid. However, zircon rims in a garnet absent

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ACCEPTED MANUSCRIPT granodioritic gneiss are different, with moderate to high CL response and broad zoning.

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The Hf isotope system also allows assessment of whether zircon rim growth was isochemical (i.e. no net gain or loss of components) on an intergrain scale. Within the

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three metamorphic zircon samples the rims (or discrete metamorphic zircon grains) have εHf(t) values up to +11 ε units greater than the magmatic domains when calculated at a common time. This indicates that the metamorphic zircon was

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associated with a metamorphic medium that had greater radiogenic Hf. While the

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metamorphic domains contain more radiogenic Hf than the original igneous domains, their Lu/Hf ratios are generally lower (with a lower Lu content), relative to the

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igneous domains. Lu, a heavy-rare-earth element (HREE), is partitioned into garnet much more strongly than Hf, a high-field-strength element (Chen et al., 2010; Zhang

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et al., 2010). Garnet crystallization or recrystallization during high-grade metamorphism can significantly deplete the associated metamorphic medium in

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HREE to produce a metamorphic fluid or melt with a low Lu/Hf ratio. If garnet co-

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precipitates with zircon, the metamorphic zircon will acquire a lower 176Lu/177Hf ratio than the igneous domains due to the preferential partitioning of Lu into garnet (Hoskin and Schaltegger, 2003). However, breakdown of garnet will lead to a metamorphic fluid or melt with a high

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Hf/177Hf ratio as a result of time-integrated

high Lu/Hf ratios in the garnet (Zheng et al., 2005). The higher εHf(t) values for the metamorphic rims compared to the igneous cores in the Biranup example indicate that this zircon overgrowth precipitated from metamorphic fluids elevated in but deficient in

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Hf/177Hf

Lu/177Hf. The generation of a radiogenic reservoir in garnet is

naturally a function of both the

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Lu/177Hf composition and time. Whilst the amount

of time from garnet formation to the end of a metamorphic episode may be relatively

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ACCEPTED MANUSCRIPT short on crustal evolution timescales, the extreme generate radiogenic

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Lu/177Hf ratio may be enough to

Hf/177Hf ratios that are significantly different. This suggests

that formed during metastable breakdown of garnet.

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that in our case study metamorphic zircon growth may be influenced by a Hf reservoir

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In contrast the third sample contains zircon rims and discrete grains with high CL emission and coarse oscillatory zoning, but importantly does not contain garnet (Figure 7). These zircon rims and grains yielded a 207Pb*/206Pb* date of 1154 ± 25 Ma

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which reflects metamorphism and in situ anatexis of the host rock at this time (Bodorkos and Wingate, 2008). Hf analyses of these metamorphic zircons yield

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Hf

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/177Hf values which are less radiogenic than the metamorphic zircons in the other two (garnet-bearing) samples. The Hf isotopes of these grains are consistent with zircon

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growth within a small-volume melt as opposed to growth from a metamorphic fluid

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during recrystallization of garnet. In situ melting is indicated by widespread leucosome development in the garnet-bearing rocks at this time (Bodorkos and

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Hf/177Hf can be variable due to the continuous loss or discontinuous

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zircon

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Wingate, 2008; Spaggiari, 2009). These studies demonstrate that the effect on the

breakdown of garnet as this may change how garnet retains, banks up, or releases the REE. Nonetheless, despite these complexities the possibility of a garnet effect cannot be ignored when interpreting metamorphic zircon Hf isotope ratios. 7.3 Hf isotopic signature in granulite facies zircon associated with zircon breakdown. Another important example of zircon Hf isotope systematics in granulite facies zircon comes from the Tropicana Zone, in the north eastern part of the AFO. Archean rocks of the Tropicana Zone include the Tropicana Gneiss which hosts the Tropicana gold deposit, and the Hercules Gneiss, which hosts disseminated and veinrelated gold, both gneisses are interpreted to have affected by granulite facies

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ACCEPTED MANUSCRIPT metamorphism (Kirkland et al., 2015). The Hercules gneiss represents a single suite of sanukitoid magmas, with affinity to the Yilgarn Craton. Kirkland et al. (2015)

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recently reported the Hf isotopic character of zircon crystals within this region. Four zircon samples yielded pre-metamorphic as well as granulite-facies zircon and the Hf

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isotopic signature of these pairs indicated that the discrete granulite facies material had εHf values up to 6 εHf units less radiogenic than the magmatic domains when calculated to a common reference time. The granulite-facies zircons also indicated

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generally lower Lu/Hf ratios. Hence, a newly released source of unradiogenic Hf was

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required to account for the evolved signature in the granulite zircon. Preferential destruction and release of the unradiogenic Hf budget in inherited material could

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supply the unradiogenic Hf required to account for the metamorphic zircon’s Hf signature, perhaps augmented by breakdown of primary ferromagnesian minerals

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under garnet-present conditions.

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Plots of εHf versus calculated apparent density of zircons (Figure 8c) were used to show positive correlations between pre-metamorphic magmatic zircon density

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and εHf, where zircon with lower density was less radiogenic. Zircon densities are calculated as a function of age, U content, and Th content (Murakami et al., 1991). Hence, it was suggested that the preserved radiogenic vestigial cores were, due to their lower U and Th content, less likely than unradiogenic crustal inheritance in the rock to be mobilized during granulite facies metamorphism. This situation may be a more general response of the Hf isotopic system, in which zircon grown in a more mafic melt, with lower U and Th, is less likely to contribute to the metamorphic Hf reservoir than its felsic counterpart (Figure 8a-b). The implication of this is that information on metamorphic zircon breakdown reactions and subsequent regrowth can be gleaned from Hf isotopes.

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ACCEPTED MANUSCRIPT 7.4 Decoupling of Nd and Hf systems – implications for metamorphic grade In this final section we investigate the effect of garnet on the isotopic

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systematics of the source of granitic melts. Such melts are often the subject of zircon geochronology studies. The combination of the Nd–Hf isotopic systems is particularly

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useful in tracing the metamorphic grade of a melt source, because of the similar geochemical affinity of the two isotope systems under most geological processes except where garnet is involved in the source. There is a strong tendency for the Nd-

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and Hf-isotopic compositions of all rocks to fall along a uniform array. This array has

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been interpreted as evidence of an efficient mixing process in the crust, and an indication that none of the minerals that account for substantial fractionation of Sm/Nd and Lu/Hf (e.g. garnet) achieves extensive Hf–Nd decoupling on a crustal

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scale within the terrestrial system. The coherent trend between Sm/Nd and Lu/Hf,

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where εHf = 1.36 × ε Nd + 2.95, is referred to as the terrestrial array. Nonetheless, Nd–Hf decoupling has been observed in the continental mantle (e.g. Aulbach et al.,

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2004; Bedini et al., 2004; Bizimis et al., 2003; Choi et al., 2006; Salters and Zindler,

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1995; Schmidberger et al., 2002). One of the mechanisms proposed to account for this decoupling is melting in the presence of garnet (Schmidberger et al., 2002). This should not significantly alter Sm/Nd ratios but will fractionate Lu from Hf, producing Lu-enriched residual crust. Since 176Lu decays to

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Hf, remelting of this residual Lu-

enriched crust under conditions where garnet is no longer stable will result in melts with more radiogenic Hf isotopic compositions. Johnson and Beard (1993) use the term ΔεHf to define the Hf isotope deviations from the principal axis of dispersion on the terrestrial array, and calculate this as the difference between the measured ɛHf value (which can be estimated from the average zircon Hf value) and that predicted based on the whole rock ɛNd.

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ACCEPTED MANUSCRIPT (Smithies et al., 2015) recently used trends in the co-variation of Nd- and Hfisotopic compositions to provide insight into the evolution and metamorphic

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environment of some Biranup Zone magmas within the AFO (Figure 9). Biranup Zone magmas show a strong exponential correlation between La/Sm versus ΔεHf,

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indicating that more decoupled samples, with a more radiogenic εHf value than predicted from their whole rock εNd, had typically lower La/Sm, La/Yb ,and La/Nb

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ratios, with the most decoupled granites having the least fractionated REE profiles. Such negative correlations were considered to reflect the presence of garnet in the

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melt source and indicate the strongly heavy REE enriched nature of this mineral. Certain periods of magmatism within the Biranup Zone stood out as being strongly

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decoupled in terms of their Nd and Hf isotopes. One clear example of such was the c. 1700 Ma Bobbie Point granite suite of A-type magmatism requiring strongly

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enhanced geotherms (Smithies et al., 2015). High HREE concentrations and ΔɛHf

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values in these melts suggest melting of a crustal source that previously resided at a deeper level in the garnet stability field (Kirkland et al., 2011b), leading to an

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interpretation that c. 1700 Ma magmatism might also reflect a period of significant crustal thinning (Smithies et al., 2015). Hafnium isotopic signatures within zircon are frequently used to reveal

information on crustal evolution. However, Hf isotopes within metamorphic zircon can provide an important means to link zircon growth to the major metamorphic phases. For example, the high Lu content within garnet results in a reservoir that may strongly influence the subsequently formed zircon. For example the continuing stability of garnet may result in zircon Hf signature significantly different to zircon grown during garnet breakdown, assuming there is time to evolve a radiogenic Hf reservoir within garnet. This disparity between garnet-present and garnet-absent

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ACCEPTED MANUSCRIPT reservoirs may also manifest as deviations from the Hf–Nd terrestrial array, and may serve to indicate depth of melt generation.

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8. Concluding remarks This review covers some of the most important techniques using accessory

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phases as monitors of high temperature metamorphic processes. These methods range from the observation of internal grain features, to the linking of isotopic and chemical systems of the rock as it moves through P–T space over time. The ability to collect

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isotopic and chemical information from within the same analytical volume, e.g. by

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LA-ICPMS (quadrapole or split stream) or electron probe is of extreme importance when dealing with grain scale heterogeneity. However it must be taken into account that recrystallisation, along with variable rates of diffusion for different chemical

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systems can result in disparate processes being represented even when taking a single

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analytical volume.

There are some important topics that will play a key role in the future of high-T

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metamorphic research. For example there is a current paucity of experimental data

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making direct chemical links between accessory minerals and major silicate minerals. Whilst zircon–garnet REE relationships are arguably the best characterized and most used, even the variation between these datasets is not fully understood in terms of either P–T or bulk composition, and the ability to link monazite chemistry to metamorphic silicate minerals has no current experimental backing. Linking accessory mineral growth into thermodynamic modeling is also in its very early stages, currently such models only link mineral growth to elemental saturation in a crystallising melt. The ability to take into account dissolution and regrowth of minor phases, armoring of grains within silicate mineral hosts, and the effects of fluid alteration within thermodynamic models will greatly add to our understanding of

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ACCEPTED MANUSCRIPT high–T processes. In this review we have only touched upon a few accessory minerals directly relating to the study of granulites. The study of other isotopic and chemical

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systems that are either emerging or in their infancy, such as Lu–Hf in rutile, and Sm– Nd in monazite and titanite, as well as Zr-in-titanite thermometry will open up further

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avenues for interpreting additional processes occurring in a wider range of orogenic settings.

Almost 10 years later we are still at a stage where as Harley and Kelly (2007b)

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inferred, our ability to collect geochemical data has outpaced understanding of the

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multitude of geological processes, particularly in melt-bearing metamorphic rocks. However as highlighted in this review, great strides have been made in the last decade with regard to the ability to interpret P–T–t information, e.g. experimental partitioning

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constraints and 4+ cation thermometry. As the ability to acquire petrochronologic data

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increases, so does the requirement of new and innovative ways to both statistically interrogate and visualize these data, in order to extract the maximum amount of

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knowledge from the rock record.

Acknowledgements CC and RT acknowledge support from Curtin University Strategic Research

Funding. Funding for analyses and fieldwork was provided through Australian Research Council Discovery and DECRA projects DP0879330 and DE1201030 and Australia-India Strategic Research Fund project #ST030046 and the Geological Survey of Western Australia.

Figure Captions Figure 1

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Stylised representation of a P–T–t path for a metamorphic rock that has experienced partial melting. Also shown are the associated zircon and monazite textures along with expected garnet behaviour. a) Inherited zircon may show oscillatory zoning highlighting an igneous origin. Rounded grain edges are accentuated by long transport distances between source and sink. Inherited monazite is less common, and Pb-loss may result in spurious inherited ages. Garnet forms on prograde path, may “armour” early accessory mineral compositions b) Attainment of high temperatures may result in recrystallized zircon rims, and prograde monazite growth. c) Melt present at peak temperatures consumes zircon, and may result in monazite modification. Peritectic garnet forms as part of melting-producing reactions. d) Crystallisation of melt on retrograde path results in metamorphic zircon, as either new grains or overgrowths. Monazite textures may increase in complexity. Garnet breaks down during decompression. e) Sub-solidus fluid flux may further alter both zircon and monazite textures.

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Figure 2 Atlas of granulite facies zircon textures. CL images showing inherited, recrystallized, and new metamorphic zircon features. Dark circles are the analytical pits resulting from LA-ICPMS analysis. Scale bars are approximately 25μm. Textures described in detail in Section 2.1. Localities for different grains are as follows. India. Achankovil Zone – grains b, k, l, m, p, q, t, u (Taylor et al., 2015a): Trivandrum Block – grains c, g, h, x, y (Taylor et al., 2014) grain i (Clark and Taylor, Unpubl.): Nagercoil Block – grain j (Johnson et al., 2015): Eastern Ghats Belt – Grains a, o, r, s (Korhonen et al., 2015): Antarctica. Enderby Land – grains d, e, v, w (Clark and Taylor, Unpubl.): Windmill Islands – grain f (Morrissey et al., Unpubl.): Bunger Hills – (Tucker et al., Unpubl.).

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Figure 3 Atlas of granulite facies monazite textures. BSE, EPMA, EBSD and LASS images showing inherited, recrystallized, and new metamorphic monazite features. Dark circles are the analytical pits resulting from LA-ICPMS analysis. Scale bars are approximately 15μm. Textures described in detail in Section 2.2. India. Madurai Block – grains r, s (Clark, Unpubl.): Achankovil Zone – c, d, g, h, I, j, k (Taylor et al., 2015a): Trivandrum Block – grains l, m, n (Taylor et al., 2014), grains o, p (Taylor, Unpubl.): Nagercoil Block – grains a, b, e, f (Johnson et al., 2015): Sandamata Complex, Rajasthan – grain q (Buick et al., 2010; Erickson et al., 2015). Figure 4 Relative probability distribution for igneous and metamorphic grains during longlived high temperatures in the Musgrave Province, central Australia. Relative probability peak heights consistent within each grain type. Peaks show a consistent time lag of 5-10 Myr from each magmatic age to the corresponding metamorphic peak, as a result of cooling following each granite emplacement event. CL images show typical zircon textures for the Musgrave Province samples interpreted as magmatic (elongate with finescale oscillatory zoning) and metamorphic (equant with predominantly sector zoning. Figure 5 Relative probability diagrams from zircon and monazite in a long lived hightemperature orogen - Southern Granulite Terrain, southern India. Relative probability

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peak heights consistent within each grain type. a) Metapelite examples show a build up of zircon ages during the prograde path as inherited zircon undergoes U–Pb resetting, followed by a large probability peak on the retrograde path as new zircon grows during melt crystallization. Monazite shows both prograde and retrograde growth/modification. b) Example from a fluid altered lithology. Retrograde probability peaks are shifted towards a younger fluid related resetting age, particularly for monazite. Prograde monazite ages are still retained. c) Compilation for multiple lithologies in the SGT including metasediments, metaigneous and fluid alterated rocks. No probability peak is directly related to the leak temperature in the region.

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Figure 6 a) Empirical examples of zircon/garnet partitioning profiles for the REE. Apparent textural equilibrium between zircon and garnet highlights a disparity as to the chemical equilibrium signature. b) Experimentally derived zircon/garnet partition coefficients for the REE. Disparities between experiments may reflect pressure, temperature or chemical controls. DM–HREE(zircon/garnet) values close to unity appear to reflect the likely equilibrium signature in high temperature, low-Ca lithologies.

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Figure 7 The result of zircon recrystallization with metastable garnet on zircon Lu–Hf systematics in the Albany–Fraser Orogen. The garnet-bearing samples show recrystallized zircon with a more radiogenic (juvenile) Hf signature than the garnetabsent sample (a), and a correspondingly low Lu/Hf ratio (b). This is the expected behaviour of zircon in a rock where garnet has sequestered the HREE.

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Figure 8 a) Cartoon showing the variable behaviour of zircon from different bulk compositions during metamorphism. High Th+U, Hf-unradiogenic zircon from a felsic source breaks down more readily, forming rims on low Th+U, Hf-radiogenic zircon from mafic sources. b) The result is the opposite trend from that seen in figure 7, metamorphic overgrowths formed in the presence of garnet appear less radiogenic than the inherited igneous cores. Figure 9 Hf and Nd isotopic signatures in the Albany–Fraser Orogen as a result of zircon formation in different source regions. Early plutons such as the Bobbie Point Granite have a deeper source, as shown by strongly positive ΔεHf values. This reflects decoupled Nd and Hf signatures after garnet has sequestered Lu. Later granites from shallower sources have ΔεHf values close to zero, indicating a garnet-absent source region.

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HIGHLIGHTS Atlas of zircon and monazite textures in high temperature rocks. Relationship between accessory mineral U–Pb ages and peak metamorphism. Trace elements as a monitor of metamorphic mineral reactions and temperature. Hf isotopic signature of metamorphic zircon.

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