A Field Guide to Naming Migmatites and Their Textures, with Saskatchewan Examples

Transcription

1 A Field Guide to Naming Migmatites and Their Textures, with Saskatchewan Examples Ralf O. Maxeiner 1, Ken Ashton 1, Colin D. Card 1, Ryan M. Morelli 1 and Bernadette Knox 2 Information from this publication may be used if credit is given. It is recommended that reference to this publication be made in the following form: Maxeiner, R.O., Ashton, K., Card, C.D., Morelli, R.M. and Knox, B. (2017): A field guide to naming migmatites and their textures, with Saskatchewan examples; in Summary of Investigations 2017, Volume 2, Saskatchewan Geological Survey, Saskatchewan Ministry of the Economy, Miscellaneous Report , Paper A-2, 21p. Abstract Much of northern Saskatchewan has been affected by upper amphibolite- to granulite-facies metamorphism and consequently was subjected to partial melting processes, which led to the formation of migmatites. This paper is written as a guide to anyone working on Saskatchewan migmatites, in order to convey a uniform language to describe such rocks and their textures. A migmatite is a rock produced under high-grade metamorphic conditions by melting or partial melting of a pre-existing rock in the continental or oceanic crust, irrespective of proportion of melt. Crustal anatexis is generally accompanied by deformation, which can help facilitate other processes such as separation of melt from the solid phase and crystal fractionation. Paleosome is that part of a migmatite that was not affected by partial melting, and in which pre-existing features are commonly preserved. The neosome is the new material resulting from the partial melting process. It typically comprises two components: a light-coloured part (leucosome) that is dominantly quartzofeldspathic or feldspathic in composition, and a dark-coloured part (melanosome) that is enriched in ferromagnesian, aluminous and/or calcic minerals. Leucosome can be in situ, in-source or injected. Determination of a migmatite s correct protolith (precursor of a modified, metamorphic rock) should be attempted because this is crucial to unravelling lithotectonic history. In situations where this is impossible, a name containing mineralogical, compositional, and/or textural descriptive terminology can be used. One aspect that complicates determination of protoliths is the amount of neosome. In cases where a migmatite has not lost structural cohesion, which typically takes place at 26 to 60% neosome, it is considered a metatexite. A migmatite that has largely lost structural cohesion and is predominantly composed of neosome (generally >60%) with minor but variable amounts of melanosome and/or resisters is called a diatexite. Metatexitic and diatexitic rocks can be further subdivided based on a number of descriptive textural terms. Keywords: migmatite, rock classification, terminology, Saskatchewan, metamorphism, anatexis, partial melting, protolith, metatexite, diatexite 1. Introduction Much of the Canadian Shield currently exposed in northern Saskatchewan has been affected by partial melting resulting from upper amphibolite- to granulite-facies metamorphic conditions (Figure 1). Over the almost seven decades that Saskatchewan Geological Survey (SGS) geologists and affiliated researchers have systematically mapped these rocks, geologists worldwide have struggled to understand the geological processes involved in what we now understand as partial melting. In the early days, metamorphic rocks were commonly named based on their mineral make-up (e.g., hornblende-plagioclase gneiss, muscovite schist), and rocks apparently containing both supracrustal and plutonic components were thought to have been the product of a poorly understood process termed granitization. Much has changed since that time. Geologists are rarely content with mineral-based rock terms. This information continues to be conveyed in textural descriptions and/or map legends, but it has become preferable to name rocks based on their interpreted protolith wherever possible. A great deal of work has also been conducted on 1 Saskatchewan Ministry of the Economy, Saskatchewan Geological Survey, th Avenue, Regina, SK S4P 3Z8 2 Formerly Saskatchewan Geological Survey; currently Northwest Territories Geological Survey, Industry, Tourism and Investment, P.O. Box 1320, Yellowknife, NT X1A 2L9 Although the Saskatchewan Ministry of the Economy has exercised all reasonable care in the compilation, interpretation and production of this product, it is not possible to ensure total accuracy, and all persons who rely on the information contained herein do so at their own risk. The Saskatchewan Ministry of the Economy and the Government of Saskatchewan do not accept liability for any errors, omissions or inaccuracies that may be included in, or derived from, this product. Saskatchewan Geological Survey 1 Summary of Investigations 2017, Volume 2

2 the process we now understand as partial melting (e.g., Sawyer, 2008), but this evolution has left an abundance of terms defined in multiple ways by different authors (e.g., Milord et al., 2001; Brodie et al., 2007; Schmid et al., 2007; Wimmenauer and Bryhni, 2007). Poorly understood terms, such as diatexite, and terms that have become outdated, like mesosome, have resulted in a breakdown in communications: a geological author can never be confident that he/she is conveying the descriptive character of a rock/unit to the reader. Figure 1 Metamorphic map for the Canadian Shield of northern Saskatchewan; major plutonic complexes are not shown as they do not generally provide sufficient information to establish metamorphic facies. Saskatchewan Geological Survey 2 Summary of Investigations 2017, Volume 2

3 This situation is similar to that of geologists trying to describe plutonic rocks using the cumbersome terminology of several decades ago. This of course led to a standardization of terms to describe plutonic rocks (Streckeisen, 1976), which was later expanded to include a variety of other rock types (Le Bas and Streckeisen, 1991). Thanks to these International Union of Geological Sciences (IUGS) classification schemes, there is worldwide agreement on, for example, what constitutes a tonalite or a granodiorite. The classification and naming of rocks is fundamental to our discipline. They are at the heart of generating maps, making correlations, conveying the geological history of an area and making predictions about mineral exploration potential. The Saskatchewan Geological Survey previously encountered the problem of inconsistent terminology when trying to describe clastic sedimentary rocks that passed along strike from a weakly metamorphosed state, in which primary structures and textures were visible, to a more intensely metamorphosed state, in which such early features had been obliterated. The result was an Open File Report (Gilboy, 1982), and a short paper (Maxeiner et al., 1999) in which a modified scheme for the standardized naming of metamorphosed clastic sedimentary rocks was presented for future use by the Saskatchewan Geological Survey. Based on the success of the standardized nomenclature for metamorphosed clastic sedimentary rocks, the current paper is intended to standardize the usage of terminology related to Saskatchewan rocks that have undergone partial melting (i.e., migmatites). There have been attempts at standardization including, for example, guidelines published by the British Geological Survey for naming migmatitic rocks as part of a document that deals with metamorphic rocks in general (Robertson, 1999). Several IUGS commissions (Brodie et al., 2007; Schmid et al., 2007; Wimmenauer and Bryhni, 2007) made recommendations regarding the definition of metamorphic rocks and migmatitic terms, but it appears that their proposals were never officially adopted. Most notably, Sawyer and coworkers have made a comprehensive attempt at creating an up-to-date language based on recent scientific research (Sawyer, 2008; Sawyer and Brown, 2008). Although these guides are excellent resources, geologists of the Saskatchewan Geological Survey are of the opinion that they fall short in terms of their usefulness for protolith mapping, as some of the recommended nomenclature uses terms like metatexite migmatite and stromatic metatexite (Milord et al., 2001; Brown, 2013). The South Australian Geological Survey has published their own User s guide to migmatites (Pawley et al., 2013, 2015), which is essentially a modified and shortened version of the publications put forward by Sawyer (2008) and Sawyer and Brown (2008). This publication is based on a series of in-house meetings at the Saskatchewan Geological Survey and on the comprehensive work of Sawyer (2008) and Sawyer and Brown (2008). Where applicable, definitions are adopted from these two publications, although in many cases they are slightly modified to suit our purposes. Therefore, it is important to note that this manuscript does not report on original scientific research, but, rather, constitutes a compendium of standardized terms and definitions to be used in the future by the Saskatchewan Geological Survey. In some cases, we have purposely not adopted terms used by Sawyer or other workers. In those cases, we have looked to the literature, gleaning and sometimes modifying the definitions from other publications. All of the terms defined have been illustrated using Saskatchewan examples. 2. Terminology Going forward, the Saskatchewan Geological Survey recommends that this guide be used by Survey geologists and clients as a reference for naming migmatitic rocks in the province. This publication is meant to be a hands-on guide, and assumes a basic understanding of metamorphic reactions and processes. Readers interested in the processes of migmatite formation and recent theories on granite formation can refer to Sawyer (2008, and references therein) as well as other journal publications (e.g., Milord et al., 2001; Weinberg and Mark, 2008; Brown, 2013; Weinberg et al., 2015). This guide refers to features of migmatites that can be observed from a scale of hand samples to outcrops. Microscale features are not taken into account in this classification, but we recognize that they do play an important part in determining the petrogenetic history of any rock, including migmatites. Saskatchewan Geological Survey 3 Summary of Investigations 2017, Volume 2

4 a) Protoliths Protolith is the term given to the precursor of a modified, metamorphic rock. Determination of the protolith of a migmatitic rock can be difficult because partially melted layers and/or zones have been compositionally changed. Parts of a migmatitic outcrop that have not partially melted (i.e., the paleosome) have escaped the melting process because they are at least slightly different in composition. Such relatively unaffected components of the migmatite can be extremely useful in helping to constrain the protolith of the partially melted rock. For example, the protolith of most sillimanite-garnet-biotite gneisses is generally considered to be an argillaceous sedimentary rock (e.g., mudstone, shale). However, rocks bearing that same mineral assemblage could also result from metasomatic processes associated with a shear zone developed in a granitoid rock, or the metamorphism of a volcanic rock previously affected by alteration associated with volcanogenic massive sulfide mineralization, to name two possible protoliths. In contrast, protolith determination for partially melted granitoid rocks can be relatively straightforward due to the large volume of compositionally homogeneous starting material. In such cases, the degree of partial melting (i.e., the component of the original rock that has melted) is commonly controlled by the availability of one of the reactants (e.g., water) more than by significant compositional variation. Determination of the correct protolith is crucial to unravelling lithotectonic history; therefore, an attempt should always be made. Nevertheless, there will be situations in which the protolith cannot be determined, in which case a descriptive name containing mineralogical, compositional, and/or textural terms is recommended (e.g., mafic calcsilicate gneiss). b) Anatexis Anatexis is the process of partial melting resulting from thermotectonic processes in the continental or oceanic crust. Ever since it was coined at the beginning of the 20th century, the term anatexis has created much controversy, and interesting accounts of debates between Sederholm (1907) and Holmquist (1916) can be found in Mehnert (1968). Most of these early debates focused on granitization and the question of whether or not granite has a magmatic origin (Sederholm, 1907; Holmquist, 1916; Mehnert, 1968; Ashworth, 1985). More specifically, researchers questioned whether nonmagmatic granite formed under wet or dry conditions. Sawyer (2008) presents a brief and insightful discussion of this debate and how it led to Mehnert s (1968) non-genetic descriptive terminology for migmatites. Today, research into the formation and evolution of granitic magmas includes study of the geochemical processes involved during crustal melt formation, the origin and depth of partial-melt formation, tectonic environment, degree of partial melting, wall-rock contamination, and crystal fractionation processes (e.g., Sawyer, 1998; Jung et al., 1999; Chappell and White, 2001; Frost et al., 2001). Therefore, we employ the following slightly modified version of Sawyer s (2008) definition of anatexis: Melting or partial melting of a pre-existing rock in the continental or oceanic crust, irrespective of proportion of melt; crustal anatexis is generally accompanied by deformation, which can help facilitate other processes such as separation of melt from the solid phase and crystal fractionation. The Glossary of Geology (Bates and Jackson, 1987) suggests that adjectives such as incipient, initial, intergranular, advanced, partial, differential, selective, and complete be added as modifiers to indicate the degree/nature of melting. c) Migmatite The term migmatite (Figure 2) is derived from the Greek word migma, which means mixture. In the early part of the 20th century it was applied to rocks comprising two or more components (Sederholm, 1907), irrespective of whether or not the components were genetically related. Sederholm described migmatite as a rock with one component being a schistose sediment or foliated eruptive, and the other component having formed by re-solution of the material like the first or by an injection from without. So strictly speaking, based on this early definition, any mixed two-component rock could be termed a migmatite. Mehnert (1968) stipulated that the term be reserved for metamorphic rocks that are composite on a megascopic scale, where one component is the country rock and the other is a rock of plutonic appearance. Saskatchewan Geological Survey 4 Summary of Investigations 2017, Volume 2

5 Figure 2 Typical examples of migmatitic rocks from an upper amphibolite facies metamorphic terrane in northern Saskatchewan. A) A stromatic migmatitic psammopelite displaying about 65% grey, fine-grained paleosome (P) and 35% neosome, the latter consisting of white, medium-grained granitic leucosome (L) in semi-continuous layers up to about 10 cm thick, bounded by millimetre-scale, biotite-rich melanosome (M). B) A partially melted granodiorite in which a gneissic grey paleosome (P) makes up about 40% of the rock. The neosome consists of a pink, coarse-grained granitic leucosome (L) and the bounding dark biotite-rich melanosome (M). For rocks such as this, in which the leucosome is compositionally and texturally similar to the paleosome, assigning proportions to each component can be difficult and tenuous. In keeping with most other modern definitions (Sawyer, 2008; Sawyer and Brown, 2008; Pawley et al., 2013), the SGS has chosen to use the term migmatite for rocks with two or more components, found in upper amphibolite- to granulite-facies metamorphic areas, that contain pre-existing material (paleosome) and a new component derived by partial melting (neosome). If possible a distinction is made between in situ migmatite, in which both components (paleosome and neosome) are petrogenetically related, and injection migmatite, in which the neosome is injected into an unrelated rock. Although the term migmatite will be used for all two-or-more-component systems derived by partial melting, regardless of the quantity of each component, it is important to record the various proportions to fully describe the rock and the map unit. Therefore, component proportions should be listed in unit descriptions in publications and on map legends (e.g., Unit contains up to 20% white to light pinkish grey granitic leucosome and up to 5% melanosome, rich in garnet and biotite. ). Although migmatite can be used as a rock name, much like the term gneiss, the SGS prefers to use it as a descriptive term (also see Robertson, 1999) and to instead name rocks based on their lithological precursors. For example, the rock name migmatitic granite is preferable to granitic migmatite. In the first case, the rock is interpreted as a granite that has been partially melted. In the second case, the partially melted rock is seen as having Saskatchewan Geological Survey 5 Summary of Investigations 2017, Volume 2

6 the overall bulk composition of a granite but its protolith is unclear (i.e., it may have been derived from a rhyolite, felsic tuff, arkose, etc.). d) Components of a Migmatite Paleosome is defined as that part of a migmatite that was not affected by partial melting, and in which features older than the partial melting (i.e., bedding, layering, foliation, folding) have been preserved (Figure 2). The neosome is the new material resulting from the partial melting process. It typically also comprises two components: a lightcoloured part (leucosome) that is dominantly quartzofeldspathic or feldspathic in composition (e.g., generally enriched in silica, sodium and potassium), and a darker-coloured part (melanosome) that is enriched in ferromagnesian, aluminous and/or calcic minerals (e.g., biotite, garnet, cordierite, orthopyroxene and hornblende). Some light-coloured, aluminous minerals (e.g., sillimanite, cordierite) can also be found concentrated in the melanosome. The leucosome is the part of the rock that melted, whereas the melanosome represents the mineralogically reconfigured, solid-state residual part of the rock from which the leucosome was derived. Both are part of the neosome and result from the partial melting process. Anatexis of naturally occurring rocks is very complex and is greatly influenced by protolith composition and the temperature of melting (Winkler, 1979; Yardley, 1989; Sawyer and Brown, 2008). It is beyond the scope of this paper to discuss the science of partial melting experiments, but to better illustrate the products of anatexis, it is useful to consider a partial melting reaction, such as the minimum melt reaction: Muscovite + quartz + biotite + H2O = sillimanite + melt (equation 1; e.g., Thompson, 1982) Without a further increase in temperature, the melting reaction above will stop in natural settings once all the water has been consumed. However, further melting can occur through the breakdown of hydrous phases, in which case K- feldspar is produced in addition to melt and sillimanite: Muscovite + quartz = K-feldspar + sillimanite + melt (equation 2; Storre, 1972) Therefore, typical leucosomes produced from either of the reactions above are generally leucogranitic to trondhjemitic 3 or granodioritic to tonalitic, depending on the starting material (Winkler, 1979). The leucosome will most commonly be enriched in alkali (i.e., K, Na) and to a lesser extent calc-alkali elements (i.e., resulting plagioclase is generally more Na-rich than Ca-rich), but will also contain appreciable amounts of Al and Si. A certain amount of Al and Si will move into the melt phase and combine with the alkali elements to form feldspars upon crystallization of the melt (leucosome). Other Al will be used to make solid-state aluminous minerals in the reaction products (i.e., the sillimanite in reactions 1 and 2). Unrequired Al will remain at the site of melting to produce sillimanite, or combine with relatively enriched Fe and Mg to form concentrations of aluminous ferromagnesian minerals such as biotite, garnet or cordierite. The part of the original rock that preferentially releases the dominantly quartzofeldspathic melt component (leucosome) becomes relatively enriched in other major elements (e.g., generally Fe, Mg, Ca). As a result, solid-state reactions lead to concentrations of minerals like biotite, garnet, cordierite, sillimanite and orthopyroxene (or hornblende and clinopyroxene in a more calcic system) with little feldspar due to the lack of essential alkali elements. This concentration of ferromagnesian, aluminous and/or calcic minerals, developed in response to a partial melting reaction, represents the melanosome (e.g., Figures 2, 3). Thus, the leucosome, melanosome and the parent rock are petrogenetically related to each other. Some workers prefer the term residuum to melanosome (e.g., Sawyer, 2008; Pawley et al., 2015), partly as a way to include some lighter-coloured aluminous residual phases such as sillimanite in the solid residual fraction. Sawyer (2008) also noted that in some cases residuum can be dominated by light-coloured minerals such as quartz and feldspar (e.g., the partial melting of granites); however, we prefer to include all residual mineral phases (i.e., light and dark coloured) resulting from melt extraction under the term melanosome. 3 A trondhjemite is a leucocratic, sodium-rich tonalite. Saskatchewan Geological Survey 6 Summary of Investigations 2017, Volume 2

7 For rocks that were compositionally layered and foliated prior to partial melting, the resulting neosome generally develops parallel to the layering and/or tectonic foliation. Therefore, melanosome occurs as thin layers of generally dark material enriched in one or more ferromagnesian and/or aluminous minerals that mantle coplanar layers of leucosome (Figure 2). However, neosome need not develop as layers coplanar to a pre-existing foliation. It can be crosscutting or irregular in form. Types of Leucosome Since the leucosome produced by partial melt reactions is in a liquid state, it can be mobile, resulting in its segregation from the melanosome and paleosome. Three types of leucosome are differentiated based on whether it has remained at the site of melting (in situ), migrated a small distance (in-source) or moved a large distance (injected) from the site of melting (Figures 3, 4). Figure 3 Examples of in situ leucosome. A) Typical example of in situ, medium-grained, quartzofeldspathic leucosome (L) and nearubiquitous, spatially associated melanosome (M) derived from psammopelite. P denotes paleosome (component of psammopelite that did not partially melt). B) Close-up of A) showing spatial relationship of leucosome (L), melanosome (M) rich in biotite, garnet and sillimanite, and paleosome (P). C) Close-up of neosome within partially melted psammopelite. Note the close spatial relationship of white, medium-grained leucosome (L) to melanosome (M) dominated by clots of beige sillimanite. Also note the paleosome (P), which, by definition, was not affected by the partial melting. The long diagonal black line crosscutting the outcrop (S) is a shear zone that postdated the partial melting event. Saskatchewan Geological Survey 7 Summary of Investigations 2017, Volume 2

8 In situ leucosome (Figures 3, 4 and 5) is defined as being located at the original site of partial melting, and can be recognized by the adjacent presence of melanosome. If the leucosome has migrated away from the residual solidphase melanosome into neighbouring layers, but can easily be traced back to its source (e.g., a leucosome generated in a pelitic layer having migrated into neighbouring psammitic layers), it is referred to as being in-source (Figures 4, 5). In-source leucosome can be discordant, with sharp boundaries, or layer-parallel, with less well-defined boundaries and generally no adjacent melanosome. If the leucosome has moved into neighbouring rocks and can no longer be traced back to its source, it is referred to as injected (Figures 4, 6), as long as it is still in the region that has been affected by the partial melting event. For cases in which the source of the leucosome is not known, it is assumed to be injected (Figure 6). We recommend that these terms be used as adjectival modifiers to stipulate the location of the leucosome relative to its site of derivation (e.g., in situ leucosome versus injected leucosome). Figure 4 Schematic sketch maps of two outcrops (intervening areas in grey not exposed) showing several map units. A) A unit of foliated granite has produced in situ (Lu) and in-source (Lr) leucogranitic leucosome; the resulting rock is a migmatitic granite. On a neighbouring outcrop, granitic leucosome has invaded units of tonalite and quartz diorite. As it is not possible to trace this leucosome back to its source, this rock can be described in general terms as an injection migmatite and more specifically as a migmatitic tonalite-quartz diorite with injected granitic leucosome (Lj). B) In the same area of upper amphibolite facies rocks, a layered unit of pelite, psammopelite and psammite shows the same relationships; pelitic and psammopelitic layers display in situ leucosome (Lu) and melanosome (M). The psammitic layer has not partially melted, but because the leucosome is traceable back to its source, it is referred to as in-source (Lr). In a neighbouring outcrop of layered psammite, quartzite and mafic volcanic rock, granitic leucosome is not traceable back to its source and is therefore considered injected leucosome (Lj). Saskatchewan Geological Survey 8 Summary of Investigations 2017, Volume 2

9 Figure 5 Examples of in situ and in-source leucosome. A) Coarse in-source leucosome (Lr) developed in the axial plane of folds in partially melted granodiorite containing layer-parallel, in situ leucosome (Lu). B) Crosscutting in-source leucosome (Lr) developed broadly parallel to axial planes in partially melted, garnetiferous granitic rock that also contains layer-parallel in situ leucosome (Lu). Some workers suggest using terms such as vein or dyke for the leucosome component of migmatitic rocks (e.g., Sawyer, 2008; Pawley et al., 2013), and although we do not disagree with this approach, we would emphasize that these terms need to be used according to common definitions: e.g., a dyke is a tabular igneous intrusion that cuts across the bedding or foliation plane of the country rock (Bates and Jackson, 1987); a vein is a mineral-filled fracture or fault, commonly related to epigenetic hydrothermal processes (ibid). Saskatchewan Geological Survey 9 Summary of Investigations 2017, Volume 2

10 Figure 6 Typical examples of injected leucosome. A) Pale pink, medium-grained leucosome (Lj2) from migmatitic granodioritic orthogneiss (Gdg) at top of photo has been injected into meta-basite (M) at bottom. Note that there is an earlier folded and dismembered leucosome in the meta-basite (Lj1?) that is probably coeval with the earlier layer-parallel to chaotic leucosome in the orthogneiss (Lj1). B) Granodioritic leucosome (Lj1) has been injected into psammite (Ps). Note that the entire 10 m scale outcrop (not shown) is made up of this psammite and that it is not possible to tell where the leucosome was generated. Subsequent to isoclinal folding, a second generation of leucosome (Lj2) was injected. Peritectic Minerals Since the reactants of equations (1) and (2) contain little or no Fe or Mg, the resulting products are felsic and the melt has an essentially granitoid composition. Granitoid melt-forming reactions for more realistic, naturally occurring rock compositions at higher metamorphic grades involve ferromagnesian reactants such as biotite and, because of the low solubility of Fe and Mg in the melt phase, also result in ferromagnesian phases in the products: Biotite + plagioclase + quartz = orthopyroxene + garnet + K-feldspar + melt (equation 3; Gardien et al., 1995) Such ferromagnesian phases are generally not part of the resulting melt (e.g., Sawyer and Brown, 2008), but are rather produced simultaneously in the solid state and are termed peritectic phases (Figure 7). Saskatchewan Geological Survey 10 Summary of Investigations 2017, Volume 2

11 Figure 7 Examples of peritectic phases and their concentration in migmatitic plutonic rocks. A) Solid-phase garnet formed as peritectic phase during a melting reaction. Note that the garnet (G) is generally mantled by leucosome (Lu). B) Migmatitic granodiorite comprising grey fine- to medium-grained paleosome (P), white medium-grained leucosome (Lu, Lr) and hornblende (H). Note that separation of white in-source leucosome (Lr) produces anomalous concentrations of peritectic hornblende (Hp) (i.e., hornblendite pods). C) A mafic rock partially melted to form a plagioclase-rich leucosome (Pl) and solid-phase orthopyroxene (O) that has partially retrogressed to amphiboles. Mafic rocks undergoing hornblende dehydration melting produce clinopyroxene-plagioclase-orthopyroxene-liquid bearing peritectic assemblages (e.g., Rapp and Watson, 1995; Pattison, 2003, and references therein). Saskatchewan Geological Survey 11 Summary of Investigations 2017, Volume 2

12 Such peritectic mineral phases formed during melt-producing reactions (equations 2 and 3; Figures 3 and 7) are part of the neosome. They are distinct from the melanosome, which consists of solid-phase material left behind during development of the leucosome, and rather constitute a solid phase formed with, and as part of, the leucosomal melt. However, since peritectic minerals form in the solid state and the melt component is liquid, contemporaneous deformation can lead to their partial or near-complete segregation. In such cases, the melt component can migrate from the site of anatexis, leaving up to metre-scale pods dominated by the peritectic phase (e.g., garnetite, hornblendite; Figure 7). This process is particularly common during the partial melting of granodioritic rocks, which commonly produces peritectic hornblende by the breakdown of biotite (e.g., equation 4; Figure 7). Biotite + plagioclase + quartz = hornblende + An-rich plagioclase + titanite + melt (rich in K-feldspar + An-poor plagioclase + H2O + quartz) ± magnetite (equation 4; Gardien et al., 2000, and references therein) 4 The resulting pods of concentrated peritectic hornblende can be near-monomineralic (Figure 7). Typical peritectic mineral pods are commonly mantled by, or otherwise spatially associated with, the variably segregated leucosome and can in some cases be difficult to distinguish from melanosome. e) Restite The term restite has historically been used for that component of a migmatite that represents the modified remnants of a metamorphic rock that are left behind after substantial amounts of the mobile components have been removed (Bates and Jackson, 1987; Wimmenauer and Bryhni, 2007). The difference between it and melanosome has to do with scale. Whereas melanosome is used when dealing with migmatites at the outcrop scale, restite refers more to entire rock units that have been substantially modified by the extraction of very large volumes of partial melt. Thus, it is a term mainly used in the description of granulite-facies metamorphic terranes and in discussions of granite genesis (e.g., Chappell et al., 1987; Braun et al., 1996; Maas et al., 1997; Droop et al., 2003; Dorais and Spencer, 2014). As such, the term restite will not be used in our standardized terminology for the description of outcrop-scale migmatites. f) Resisters Resisters are rock types that, due to their extreme bulk compositions (e.g., mafic, ultramafic or quartz-rich rocks), have not partially melted under the same metamorphic P T conditions that produced migmatites in adjacent rocks (Figure 8). Mafic rocks, quartzites and calc-silicate rocks (Figures 4A, 4B) are good examples of resisters because their unusual compositions result in melting temperatures that are beyond the common range of metamorphic conditions. Note that resisters may still contain injected leucosome derived from nearby partially melted rocks. Intermediate and mafic rocks (e.g., andesites, quartz diorite) begin melting at higher temperatures than their felsic counterparts, so it is not uncommon in a compositionally layered outcrop to find intermediate and mafic resisters coexisting with migmatites (Figures 4, 8). Having said this, at sufficiently high metamorphic grades and especially in the presence of water, intermediate and mafic rocks can begin to partially melt and develop a leucosome (Figure 7C). In summary, resisters represent those compositional components of a migmatitic outcrop that were not affected by partial melting and in which structures (foliation, folding, and layering) older than the partial melting might be preserved (Figure 8). The characteristic that distinguishes them from other paleosome is their extremely high melting temperature, which is a function of their composition. 4 Magnetite and titanite form due to excess Fe and Ti from the reactant biotite, which cannot all be accommodated by the peritectic hornblende. Saskatchewan Geological Survey 12 Summary of Investigations 2017, Volume 2

13 Figure 8 Typical examples of resisters. A) Migmatitic outcrop containing partially melted psammopelite (Psp; Lu denotes in situ leucosome) grading into psammite (Ps) that has not melted. Note boudinaged calc-silicate layer (Rc) in centre of photo. Both the psammite and the calc-silicate layers are resisters and part of the paleosome. B) Outcrop of largely melted pelite (Pp) with resisters of amphibolite (Ra) and psammite (Rp). g) Metatexite and Diatexite Early attempts to describe and name migmatitic rocks led to the term metatexite, which was originally used for twocomponent rocks in which the younger, more felsic, mobile component was believed to be of plutonic parentage (Scheumann, 1936; Mehnert, 1968). In this sense, metatexite was essentially synonymous with migmatite as defined in this paper. However, because of the emphasis on being a mixed-component rock, use of the term metatexite was restricted to rocks showing evidence of low to moderate degrees of partial melting, involving only the low- to moderate-temperature melting components of a rock. Therefore, it was used only for rocks in which the melted (leucosome) and unmelted (paleosome) components could be easily distinguished (Mehnert, 1968). For more complete or near-complete melting, where melted and unmelted components could not be easily distinguished (Figures 9, 10), the term diatexite was used (Gürich, 1905; Mehnert, 1968). Saskatchewan Geological Survey 13 Summary of Investigations 2017, Volume 2

14 Figure 9 Examples of diatexitic rocks. A) This rock is considered to be a closed-system diatexite; based on composition and mineralogy, the protolith is interpreted to have been a pelite and the entire rock has melted so that the colour index and bulk composition of the final product is similar to that of the protolith. B) Close-up view of A) showing the neosome of the diatexitic pelite, containing garnet (G), sillimanite (S) and cordierite (C), along with quartz and feldspar. (L = leucosome.) One problem with this definition is that the degree of partial melting can be very difficult to determine. Consequently, the main discriminating factor between metatexite and diatexite (Pattison and Harte, 1988; Sawyer, 2008) has been changed from the degree of partial melting to the proportion of neosome present. This new criterion is not only much easier to assess, but also addresses another aspect implied in the term diatexite. Metatexites retain their basic structural integrity, preserving primary layering, pre-existing structural elements, and/or structures developed during partial melting (Pawley et al., 2013). The term diatexite is used when this pre-existing structural cohesion is largely lost (Figure 9A). Any remaining paleosome retains its early structures, but otherwise the pre-existing structural framework is lost due to the large volume of neosome (Figure 9A), which may be massive or exhibit new structures formed during or subsequent to the melting event. Sawyer (2008) suggested that this transition from a migmatite that retains its structural cohesion (metatexite) to one that does not (diatexite) occurs over a range of melt fraction between 26% and 60%. Based on the relative ease of determining structural cohesion and the proportion of melt neosome, the SGS has adopted this basis for the distinction of metatexite and diatexite. Saskatchewan Geological Survey 14 Summary of Investigations 2017, Volume 2

15 Figure 10 Metatexitic to diatexitic migmatites. A) Metatexite comprising a volumetrically minor irregular mass of leucosome (L) injected into amphibolite (A). Note that the structural integrity of the amphibolite is preserved. B) Diatexite dominated by leucosome (L) injected into amphibolite (A). Note that the amphibolite has been variably dismembered to form inclusions in the leucosome. C) Schollen (German for raft ) diatexite in which only minor quartzite (Q) and amphibolite (A) inclusions (resisters) exhibiting an early foliation are left in the dominant near-massive leucosomal component; large mappable bodies of such rocks could be referred to as anatectic granites. D) Another anatectic granite containing schlieren (German for swirls or streaks ) and pods of dismembered sillimanite-biotite melanosome (M). Diatexites can form in situ (e.g., Figures 9A and 9B) but, given the large proportion of melt, they can also become mobile, moving through older rock units in much the same way as igneous rocks, but distinguished from the latter by their irregular form and minor components of associated melanosomal remnants and/or inclusions of resisters (Figure 10). Where large abundances of mobile leucosome amass, they can become mappable bodies (Figures 10C, 10D). In such cases, the protolith of the diatexite is generally not known. In fact, such large volumes of leucosome may amass from more than one protolith. Given the stated goal and practise of the SGS to produce interpreted protolith maps, it is generally preferable to use metatexite and diatexite as adjectives to reflect the proportion of neosome and degree of structural cohesion. However, given the unknown parentage of some diatexites, a rock name such as anatectic granitoid may be preferable. Note that this does not preclude describing the rock as diatexitic. h) Migmatitic Textures Containing generally subordinate proportions of neosome, metatexites can take on a variety of appearances. For example, the metatexites illustrated in Figures 11A and 11B are termed stromatic based on the layered nature of their paleosome, leucosome and/or melanosome. Based on their compositions and mineral assemblages, which are typical of upper amphibolite facies metamorphic conditions, both are examples of stromatic migmatitic psammopelites (Figures 11A, 11B). Saskatchewan Geological Survey 15 Summary of Investigations 2017, Volume 2

16 Figure 11 Typical examples of metatexitic textures. A) Stromatic psammopelite containing about 15% granodioritic in-source and in situ leucosome. B) Stromatic psammopelite with strongly dismembered lit-par-lit leucosome accounting for about 30% of the rock. C) Patch (or fleck) metatexitic intermediate volcanic rock. D) Patch metatexitic gabbro containing melt blebs cored by peritectic hornblende. Stromatic metatexites are also commonly referred to as having lit-par-lit (French for bed-by-bed ) textures, alluding to the fact that the leucosome layers are parallel to each other and to the pre-existing structure of the paleosome (Figure 11B). A patch or fleck metatexite (Figures 11C, 11D) is a migmatite that typically develops at the initiation of partial melting in relatively non-layered, homogeneous rocks (e.g., mafic rocks, psammites), with leucosome nucleating in isolated areas of a rock. Textures in diatexitic rocks are generally related to the presence of melanosome and/or resisters. A psammitic to pelitic sequence of sedimentary rocks that has undergone near-complete partial melting may be recognized by a homogeneous garnet-biotite rich granodioritic neosome, containing resisters of quartz-feldspar rich psammite (Figure 12A). Such resisters commonly occur as rafts or schollen (Figure 10C). They range in scale up to a metre, commonly display an older foliation, and can be randomly oriented. Agmatitic textures are generated when prevailing neosome contains inclusions of resisters, most commonly amphibolites derived from mafic volcanic rocks or mafic dykes (Figure 12B). In some diatexites, the neosome retains diffuse relicts of pre-existing structures and/or paleosome; such migmatites are referred to as having nebulitic textures (Figure 12C). Leucosome oriented in two or more preferred directions can join together to form a stockwork or net-textured metatexite (Figure 12D). Saskatchewan Geological Survey 16 Summary of Investigations 2017, Volume 2

17 Figure 12 Examples of migmatitic textures. A) Diatexitic psammopelite containing decimetre-scale psammitic raft (Ps) in garnet-biotite rich granodioritic neosome (N). B) Outcrop of intermediate volcanic rock (I), which is metatexitic with mediumgrained dioritic leucosome (Lu). Amphibolitic layers (A), originally representing mafic volcanic layers or mafic dykes, are injected by tonalitic leucosome (T) of unknown origin, causing that part of the outcrop to lose structural cohesion and take on an agmatitic appearance. C) A diatexitic leucogranodiorite displaying a nebulitic texture. Note that a tight fold is still discernible in the lower left corner of the photo. D) Net-textured migmatitic tonalite displaying in situ and in-source granitic leucosome that together form a network. 3. Glossary of Terms and Concise Definitions (modified from Sawyer, 2008 and Sawyer and Brown, 2008) Agmatite, agmatitic A textural term to describe a breccia-like migmatitic rock in which the breccia matrix is composed of injected leucosome. Generally an agmatitic rock will have a larger proportion of paleosome disrupted by lesser amounts of in-source or injected leucosome. See also Schollen. Anatectic granitoid An open-system partial melt/leucosome that has migrated away from its source, forming intrusive sheets and plutons up to the scale of kilometres. Anatexis Melting or partial melting of a pre-existing rock in the continental or oceanic crust, irrespective of proportion of melt. Diatexite A migmatite that has largely lost structural cohesion and that is predominantly composed of neosome (generally >60%); it may contain minor amounts of melanosome as well as resisters. Diatexite can occur at the Saskatchewan Geological Survey 17 Summary of Investigations 2017, Volume 2

18 mesoscopic to megascopic scale, and can be leucocratic (colour index <5% including light-coloured minerals such as sillimanite, etc.), mesocratic (colour index 5 to 25%) or melanocratic (colour index >25%). Dyke A tabular igneous intrusion that cuts across the bedding or foliation of the country rock (Bates and Jackson, 1987, p.184). This is not a strictly migmatitic term but can be used for crosscutting bodies of injected leucosome. Fleck migmatite German for patch or spot ; see Patch migmatite. Injected leucosome Leucosome that has traveled away from the rock where it was generated into neighbouring rocks. Injection migmatite A migmatite in which the leucosome is injected into paleosome; the term can also be used for a rock that has undergone melting but whose leucosome is volumetrically dominated by injected material from elsewhere. In situ leucosome Leucosome that is located at the site of melting, which can generally be recognized by the adjacent presence of melanosome. In-source leucosome Leucosome that has mobilized away from its melanosome into neighbouring layers within the same source region (e.g., leucosome generated in a pelitic layer having migrated into neighbouring psammitic layers) and which can be physically traced back to its source. In-source leucosome can be discordant to the regional foliation or layer-parallel and have sharp to less well-defined boundaries. Leucosome The light-coloured quartz-feldspar rich part of the neosome in a migmatite that is derived by partial melting of a rock. Leucosome may be located at the place where it is generated (in situ), or have migrated away a short distance (in-source) or large distance (injected). Megascopic Features observable at property to regional map scale, i.e., on the scale of hundreds of metres to kilometres. Melanosome That part of the neosome that represents the solid residual fraction after the melt fraction (i.e., leucosome) has been extracted. It is typically composed of minerals rich in Fe, Mg and/or Al, which are generally dark coloured (e.g., biotite, garnet, cordierite, orthopyroxene and hornblende), but can also include light-coloured minerals (sillimanite, cordierite). Mesoscopic Observable at outcrop scale, i.e., on the scale of metres to tens of metres. Metatexite A migmatite that has not lost structural cohesion, typically containing between 26 and 60% neosome at the mesoscopic scale. Microscopic Observable at the scale of a thin section, i.e., micro- to millimetre scale. Migmatite A two-or-more-component rock found in upper amphibolite- to granulite-facies metamorphic terranes that contains pre-existing material (paleosome) and a new component derived by partial melting (neosome). Nebulitic A textural term to describe a migmatitic rock that has diffuse relicts of pre-existing structures and/or paleosome; the neosome in these rocks is difficult to distinguish from the paleosome. Neosome The newly formed material resulting from partial melting, consisting of a melt fraction (leucosome) and a modified residue from the zone of melting (melanosome); the leucosome can migrate away from the site of partial melting, leaving behind the melanosome. Net-textured A textural term to describe a migmatitic rock in which the distribution of the neosome is in the form of a network. The network can be regular or irregular in nature, but will always contain more than one preferred direction of neosome, which intersect each other. Very commonly the leucosome in these migmatites will be somewhat mobilized (in-source or injected). Outcrop scale See Mesoscopic. Saskatchewan Geological Survey 18 Summary of Investigations 2017, Volume 2

19 Paleosome That part of a migmatitic outcrop that did not undergo partial melting and in which structures older than the partial melting (e.g., foliation, folding, layering) are generally preserved. Patch migmatite A textural term to describe a migmatite characterized by unconnected, round to irregularly shaped blebs of partial melt, generally not accounting for more than 5 to 10% of an outcrop. It typically forms in homogeneous, poorly layered rocks (e.g., psammites, granitoids); synonym: fleck migmatite. Peritectic mineral In the context of anatexis, a solid-state mineral formed by a melt-producing reaction. Protolith The non-metamorphosed, precursor rock from which the migmatite formed. For example, the protolith of a sillimanite-garnet-biotite gneiss is, in most cases, an argillaceous sedimentary rock (e.g., mudstone, shale). Raft A textural term to describe an inclusion in a migmatitic rock in which the proportion of leucosome greatly outweighs that of the paleosome or melanosome. Commonly these will be diatexitic rocks in which the paleosome will be made up of disrupted resisters (i.e., rafts); synonym: schollen. Residuum Some workers use residuum as a broader term for melanosome that includes non-melanocratic minerals such as sillimanite; based on our definition of melanosome, this term is redundant and therefore not needed. Resister A rock that is especially resistant to partial melting in a given metamorphic pressure temperature regime due its extreme composition (e.g., mafic rocks, quartzites and calc-silicates). Restite On a regional scale, rocks and rock units that have been left behind after substantial amounts of partial melt have been extracted. Scale Migmatite textures are most obvious at the mesoscopic scale. Although results of migmatitic processes are also visible at the microscopic and macroscopic scale, they are much more difficult to recognize, describe and reconcile in the field. Schlieren A textural term; German for streaks or swirls. Schollen A textural term; German for raft; synonym: raft. See also Agmatite. Stromatic A textural term to describe a layered migmatitic rock. Layering can be on the scale of centimetres to decimetres, and is produced by alternating layers and lenses and other segregations of leucosome, melanosome and paleosome. Vein A thin, sheet-like igneous intrusion into a fissure; or an epigenetic mineral filling of a fault or other fracture in a host rock, in tabular or sheet-like form, commonly with associated replacement of the host rock (Bates and Jackson, 1987, p.720). The term is most commonly used for material precipitated from fluids. We do not recommend use of this term when describing leucosome in migmatites. 4. Summary Given the range of rock compositions capable of partially melting under metamorphic conditions, together with the complexity of the partial melting process, it is not surprising that the description of migmatites is complicated and confusing. It is hoped that this brief attempt at clarification will reduce at least some of the confusion and standardize the way migmatite terminology is used in future publications of the Saskatchewan Geological Survey. 5. Acknowledgments We would like to thank Charles Normand, Sean Bosman and Jason Berenyi for joining the authors in many lively discussions during a number of in-house migmatite workshops. Though at times spirited, these sessions ultimately helped in making this a better and more comprehensive paper. We would also like to thank Dr. Dave Pattison Saskatchewan Geological Survey 19 Summary of Investigations 2017, Volume 2