Melt-producing versus melt-consuming reactions in pelitic xenoliths and migmatites

Lithos 116 (2010) 310–320

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Melt-producing versus melt-consuming reactions in pelitic xenoliths and migmatites Leo M. Kriegsman a,b, Antonio M. Álvarez-Valero c,a,⁎ a b c

NCB Naturalis, Darwinweg 2, 2333 CR, Leiden, The Netherlands Department of Geology, University of Turku, FIN-20014 Turun Yliopisto, Finland Instituto Andaluz de Ciencias de la Tierra (CSIC—University of Granada), Campus Fuentenueva, Faculty of Sciences, 18002 Granada, Spain

a r t i c l e

i n f o

Article history: Received 16 March 2009 Accepted 6 September 2009 Available online 24 September 2009 Keywords: Migmatites Xenoliths Melt production Melt consumption Textures

a b s t r a c t Melt-producing reactions have operated both in pelitic xenoliths and in pelitic migmatites, but meltconsuming reactions, related to in situ melt crystallization, are largely restricted to migmatites. Hence, xenolith reaction textures may provide clues on the relative roles of both reaction types in migmatites. A comparison of xenoliths and migmatites results in a catalogue of textural and chemical features, some of which are argued to be diagnostic. The results have implications for the identification of restite in igneous rocks and for the relative importance of prograde and retrograde processes in different domains of layered migmatites. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Anatectic migmatites commonly show both melt-producing and melt-consuming reactions (Cenki et al., 2002). The final textures, mineral modes and mineral chemistries are theoretically affected by four successive processes (Kriegsman, 2001a): (i) partial melting and small-scale differentiation into melt-rich domains and restitic domains; (ii) partial melt extraction; (iii) partial melt-consuming reactions between in situ crystallizing melt and the restite; and (iv) crystallization of remaining melt at the solidus, thereby releasing volatiles. The “retrograde” (melt-consuming) overprint, which in some cases is pervasive, hampers the recognition and interpretation of “prograde” (melt-producing) reactions in many ways. First of all, early reaction textures may be partly to completely obscured by late reactions involving minerals and melt, which may, for example, reproduce assemblages that had disappeared during partial melting (Waters, 2001). Secondly, the exact melt composition is generally modified and difficult to retrieve (e.g. Ashworth 1985; Powell and Downes, 1990; Ellis and Obata, 1992; Whitney and Irving, 1994), which prevents proper balancing of reactions and in some cases prevents the distinction between alternative textural interpretations. Thirdly, solid phases are also affected by retrograde reequilibration, causing major changes in chemical compositions in the case of elements with high diffusion rates (notably Fe–Mg exchange: e.g., Spear, 1991; also Na–K exchange: e.g., Thompson, 1974), that may in ⁎ Corresponding author. Instituto Andaluz de Ciencias de la Tierra (CSIC— University of Granada), Campus Fuentenueva, Faculty of Sciences, 18002 Granada, Spain. Tel.: +34 958246286; fax: +34 958243384. E-mail addresses: [email protected] (L.M. Kriegsman), [email protected] (A.M. Álvarez-Valero). 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.09.001

extreme cases show little resemblance to the compositions during or immediately after melt production. In contrast to migmatites, crustal xenoliths generally preserve original variations between individual reaction domains and can potentially characterize processes and reactions that operated during crustal melting (Grapes, 1986; Hanchar et al., 1994; Cesare et al., 1997; Álvarez-Valero et al., 2005, 2007). In addition, the chemical compositions of the melt and mineral phases are rarely modified by retrograde reequilibration and can thus help to constrain balanced melting reactions. What has not yet been done, however, is to compare a large range of textural phenomena in primary migmatites, i.e. migmatites that record a single event of partial melting, and xenoliths with the aim to distinguish melt-producing from melt-consuming reactions in migmatites and to constrain “prograde” and “retrograde” components. This is, however, an important avenue for potentially resolving the following issues in migmatites: – assessing the relative importance of melt-producing and meltconsuming reactions in melanosome domains of migmatites, – the correct interpretation of trace element (re)distributions across leucosome–melanosome interfaces (e.g. Kriegsman and Nyström, 2003), – the interpretation of age patterns in migmatites (e.g. Nyström and Kriegsman, 2003), – estimates of melt loss from migmatites. Under comparable conditions (e.g. similar P–T path, in terms of both shape and absolute values, similar starting material, similar heating rate), xenoliths may freeze in textures that are similar to the early stages of migmatisation. The remaining textures in migmatites

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are expected to reflect the processes during subsequent restite–melt interaction. It has been postulated (Kriegsman, 2001a) that these processes mainly affect the melanosome domains of migmatites. Hence, the three hypotheses we intend to test through comparative petrology can be formulated as: – (1) xenoliths show only or dominantly melt-producing reactions, – (2) migmatites record both melt-producing and melt-consuming reactions, – (3) melt-consuming reactions have mainly operated in melanosome domains. Braun and Kriegsman (2001) compared textures in remelted xenoliths and migmatites, providing interesting details on a very particular case (remelted migmatites). The current paper differs from it by presenting a comparison of reaction textures in the far more common case of “primary migmatites”. We mainly, but not exclusively, use examples from the following high-grade provinces: – xenoliths from fossil volcanic suites within the Neogene Volcanic Province (NVP) of the Betic Cordillera (SE Spain), extensively studied by Zeck (1970), Cesare et al. (1997, 2003a,b, 2009), Cesare and Gómez-Pugnaire (2001), Álvarez-Valero et al. (2005, 2007), Álvarez-Valero and Kriegsman (2007, 2008), and Álvarez-Valero and Waters (2010), – granulite-facies migmatites from the Northampton Complex, so far only described in Kriegsman and Hensen (1998), – granulite-facies migmatites from southern Finland well-known since Sederholm (1913, 1934) and studied in detail by Hölttä et al. (1994), Kriegsman (2001a,b), Johannes et al. (2003), and Nyström and Kriegsman (2003), – granulite-facies migmatites from the Achankovil area in southern India (Cenki et al., 2002). We restrict our analysis to cases of water (vapour)-absent melting, because the alternative case of water (vapour)-present melting leaves little textural evidence (see next section; for an alternative view, see Sawyer, 2010-this issue). In addition, the influence of deviatory stress is not considered, except for the spatial distribution of reaction textures (Section 2.2). Stress strongly influences the shape, location and direction of melt pockets, with deviatory stress favouring elongate shapes, regular spacing and long axes commonly parallel with or perpendicular to the main shortening orientation (e.g., Van der Molen, 1985; Hand and Dirks, 1992; Sawyer, 1999). In addition, deviatory stress exerts a strong influence on melt escape thresholds (Vigneresse et al., 1996) and thus, indirectly, on the progress of restite–melt back reaction. Stress is beyond the scope of this paper, however, because it does not influence the reaction balance, or which type of reaction texture is developed. 2. Partial melting: theoretical considerations and approach 2.1. On reaction paths and textures There are two generic reaction types involving melt: water (vapour)-present melting and water (vapour)-absent dehydration melting. On any P–T path, the first melting occurs at the solidus, in suitable rock types, through a general equation A + V = M, where A are solid phases, V is water (vapour) and M is melt (see e.g., Thompson, 2001). An example in the KFMASH system is: Bt + Sil + Grt + Kfs + Qtz + V = M, (mineral abbreviations after Kretz, 1983), which requires that all five solid phases are in contact and start melting with vapour as the diffusion-controlling phase promoting contact and thermodynamic interaction between the phases. This is the reason why the term contact melting has been used. Water (vapour)-present melting quickly consumes the water present in the rock (e.g. Kriegsman, 2001a, and refs. therein), unless the system is open and a considerable influx of water occurs (e.g., Patiño Douce and

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Harris, 1998; Sawyer, 2010-this issue). Importantly, the only product phase at the solidus is melt and thus textural indications for this type of melting are not very strong. Instead, the arguments in favour rely heavily on geochemical analyses, including oxygen isotopes. By contrast, water (vapour)-absent melting derives the necessary water from the breakdown of micas or amphiboles and is therefore also called dehydration melting. Generally, these reactions produce an incongruent phase that was not present prior to melting and the appropriate process is therefore also called incongruent melting. A well-known example in the KFMASH system is: Bt + Sil + Kfs + Qtz = Grt + Crd + M. The relative importance of water (vapour)-present and -absent melting has been debated extensively in the literature (e.g., Patiño Douce and Harris, 1998; Clemens and Droop, 1998; Spear and Daniel, 2001; Kriegsman, 2001a). In addition to the above melt-producing reactions, Thompson (2001) also defined suprasolidus decompression dehydration reactions (SDDR) that consume melt + mica during decompression, producing an anhydrous assemblage and a vapour phase that is released from the system. In chemical systems dominated by univariant reactions, like many systems in experimental petrology, these reactions can be an important factor, but they may be less appropriate in more complex natural systems where shifting phase compositions lead to the dominance of mineral and melting reactions at higher variance (see, however, Section 3.3). The P–T path determines which particular melt-producing and melt-consuming reactions are encountered. Different paths give different assemblages, but some phenomena are expected to be diagnostic, for example: inclusion textures (but see discussion in Vernon et al., 2008), single and multiple reaction rims or coronas, intergrowths, etc. In general, one may expect the following features during melt-producing reactions: – leucosome present nearby, if the melt has not escaped from the system, – mica breakdown, e.g. skeletal (corroded) biotite, – growth of incongruent phases, – mica inclusions in incongruent phases are separated from other phases, – Plagioclase: Ab(-rich) breakdown, locally An(-rich) growth (see Álvarez-Valero and Kriegsman, 2010-this issue). Melt-consuming reactions, besides including the presence of nearby leucosome as a common point, are more likely to show the following features: – – – –

mica growth, breakdown of incongruent phases, coronas next to leucosome, impossibility to balance some reactions without involving a melt phase, – Plagioclase: An(-rich) breakdown, locally Ab(-rich) growth, – intergrowth of phases that had been consumed along the prograde path, – fine-grained intergrowths, notably involving micas. There are several complications to be expected before inspection of any thin sections: catalytical effects (Carmichael, 1969; see also Waters, 2001), where one partial equilibrium produces a phase that is consumed at the same rate by a second partial equilibrium. For example, a classical subsolidus reaction is the And–Ky transition that may make use of Ms as a reaction catalyser (Carmichael, 1969). In migmatites and xenoliths, similar complicated reactions may be expected that use biotite as a catalytic phase, giving rise to textures producing biotite and simultaneous textures consuming biotite. Reactants may be preserved by shielding in multivariant reactions if there are heterogenous rock domains (f > 1), which is a very common situation. In addition, there may be kinetic factors preventing nucleation.

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2.2. Spatial distribution of melt-producing and melt-consuming reactions In Kriegsman's (2001a) migmatisation model, melt-producing reactions have affected all migmatite domains, whereas melt-consuming reactions are confined to the melanosome, and melt crystallization has occurred mainly in the leucosome. Based on discussions with colleagues over the last few years and comments on earlier papers, the model requires some modifications: – an original layering may predetermine meso- and melanosome, – there may be a melt production gradient, with melanosome having produced more melt than mesosome, – high biotite concentrations can be explained by different mechanisms operating in tandem: (i) volume loss by melt extraction, leaving behind a biotite-rich residue; (ii) significant changes in biotite composition, notably increase in Ti content and concomitant decrease in H (or water) content (Cesare et al., 2007). 3. Catalogue of reaction textures and chemical features 3.1. Reaction textures in xenoliths A literature scan of 13 articles from three areas (10 papers on the Neogene Volcanic Province in SE Spain; two studies on Monte Amiata, Italy: Van Bergen, 1980, 1983; and one paper on the Pannonian basin: Harangi and Downes, 2001; see Table 1 for references), revealed a considerable variety of reaction textures in pelitic crustal xenoliths. In addition, we re-examined thin sections from Zeck (1970) and those in our private collection. Table 2 summarises more general published observations and refers to figures in the current paper and in the literature. Some key observations are: – the presence of layers or pods of glass, either as isolated domains or as interstitial melt in a layered matrix rich in fibrolite (“mix”: Cesare et al., 1997) (Fig. 1A), – preservation of microfolds in such layers (Fig. 1B), possibly resulting from melting of an earlier crenulated, mica-rich assemblage, – breakdown of an early generation of Bt, resulting in skeletal, corroded morphologies (Fig. 1A; see also figure 3a of ÁlvarezValero and Kriegsman, 2010-this issue), – disappearance of the early assemblage Bt + Sil + Qtz + Pl (Fig. 1A,B), – growth of porphyroblasts, generally Grt or Crd, with single and multiple inclusions of Bt ± Pl, Sil ± Pl (Fig. 1C), and Kfs, with Bt or Qtz inclusions, – at higher T, porphyroblasts may include Spl (with Sil inclusions) and Opx (with Bt or Qtz inclusions), – pseudomorphs of a single phase or multiple phases after an earlier porphyroblast (e.g. Crd ± Pl after Grt; Sil after And; see, e.g. figures 2 and 3 of Álvarez-Valero and Kriegsman, 2008),

– intergrowths of porphyroblasts sharing similar inclusions: Crd–Kfs intergrowth (Fig. 1D); Crd–Grt intergrowth (see, e.g. figure 2 of Álvarez-Valero et al., 2007; figure 3 of Álvarez-Valero and Kriegsman, 2008), – coarse coronas of one “anhydrous” phase (including fluid-bearing Crd) around another anhydrous phase are common: Crd rimming Grt (Fig. 1E), – plagioclase with so-called fingerprint textures, in which finegrained breakdown products after plagioclase give the strongly altered feldspar an isotropic appearance. Such textures are common in anatectic xenoliths and are regarded to result from melting of plagioclase (Wyllie, 1961; Preston et al., 1999). They have been reported in remelted migmatite xenoliths (Braun and Kriegsman, 2001) and also occur in primary xenoliths (Van Bergen, 1980) – coarse- to fine-grained Bt–Pl intergrowths (see figures 2a and 3a of Álvarez-Valero and Kriegsman, 2010-this issue) – breakdown of intermediate Pl and precipitation of An-richer Pl in the same thin section (Álvarez-Valero et al., 2007; Álvarez-Valero and Kriegsman, 2010-this issue) – Replacement of garnet by a symplectitic intergrowth of Bt + Kfs + Opx ± Crd (figure 3 in Van Bergen, 1983) – Rare coarse prismatic Sil with minor Bt transecting incongruent phases (Álvarez-Valero et al., 2005) (Fig. 1F). In addition, melt inclusions are a common feature, but they are present in all phases and can, therefore, not be used to distinguish reactants and products. One possible explanation is high-T recrystallization, but it may also result when (a component of) biotite acts as a catalyser (Carmichael, 1969; Waters, 2001) and is both a reactant and a product of melting reactions. In such cases, reactions may show imbalance at small scale, but balance at larger scale (Carmichael, 1969). In either case, the environment is melt-rich and biotite-rich, resulting in fairly high diffusion rates. 3.2. Reaction textures in migmatites The textural variety is more significant in migmatites and our literature scan showed repetition after about fifteen articles from fourteen basement areas (see Table 1 for references). In addition, we re-examined thin sections from our private collection. Table 2 summarises more general published observations and refers to figures in the current paper and in the literature. Some key observations are: – breakdown of an early generation of micas (Ms, Bt), reacting with other solids and acquiring corroded morphologies (Fig. 2A), – Folded and recrystallized Fib and Sil layers, with biotite, possibly resulting from muscovite dehydration melting (Fig. 2B),

Table 1 Non-exhaustive catalogue of equilibrium and disequilibrium (reaction) textures in migmatites and xenoliths from the literature. Texture or chemical feature

Common interpretation

Pelitic xenoliths

Pelitic migmatites

Isolated Bt inclusions in porphyroblasts (Grt, Crd, Spl) Isolated Sil inclusions in porphyroblasts (Grt, Crd, Spl) Bt separated from Sil in porphyroblasts Crd coronas around Grt Bt intergrown with Sil in matrix Corroded Bt present Skeletal Bt present Corroded Pl present Crystallized melt present Glass present Bt–Pl or Bt–Qtz intergrowths

Bt reactant in products Bt, Sil reactant in products Sil reactant in products High-diffusion textures Low-diffusion textures Incomplete breakdown Incomplete breakdown Incomplete breakdown Melting Melting Reaction products

NVP papers NVP papers NVP papers NVP papers NVP papers NVP papers NVP papers NVP papers devitrified NVP papers NVP papers, B83

G96, G98, J99, NK03, S08, WI94 J99, K99, NK03, S08 J99, NK03, S08 C02, P97, W04 C08, P97 GI82 W01 W01 G08, S99 – NK03, P97, W94, W01, TP

NVP papers are all those that involve Cesare, Alvarez-Valero or Zeck in the reference list. B83 = Van Bergen (1983); C02 = Cenki et al. (2002); C08 = Cruciani et al. (2008); G08 = Grew et al. (2008); G96 = Greenfield et al. (1996); G98 = Greenfield et al. (1998); GI82 = Gil Ibarguchi and Martínez (1982); J99 = Janák et al. (1999); K99 = Kalt et al. (1999); NK03 = Nyström and Kriegsman (2003); P97 = Palmeri (1997); S08 = Stípská et al. (2008); S99 = Sawyer (1999); TP = this paper; W01 = Waters (2001); W04 = Wheeler et al. (2004); WI94 = Whitney and Irving (1994).

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Table 2 Textures, structures and chemical features in pelitic xenoliths and migmatites. Feature

Figures/literature examples

Interpretation

Pelitic xenoliths

Pelitic migmatites

Presence of (former) glass Glass/melt inclusions in porphyroblasts Subdivision leuco-, meso-, melanosome Solid inclusions in porphyroblasts Coronas Symplectites Mica breakdown Equilibrium textures Mica growth Melt–solid interfingering Complex phase required in reaction balance Structures suggesting volume loss Partly dissolved zircon Growth of euhedral zircon Anhedral porphyroblasts Skeletal biotite Fingerprint plagioclase Growth of Ti-bearing oxides Breakdown of Ti-bearing oxides Porphyroblast intergrowths (Crd–Kfs, Crd–Grt) Patches with multiple porphyroblasts Bulk enrichment of Al, Fe, Mg, Ti Bulk depletion of Si, Na, K Mn decreasing from Grt core to rim Mn increasing from Grt core to rim Y, Zr and HREE increasing from Grt core to rim Y, Zr and HREE decreasing from Grt core to rim Pl: Ab(-rich) dissolution, An(rich) precipitation

1A,C,E; 2F 1C,E,F 2C,D,E 1C,D,F; 2C,D,F,H 1C,E; 2F,K 2G,L; AKti/2E and 3B 1A,B,E; 2A,C 2E,I 2G,H,J,K,L; AKti/2E and 3B,C 1A,C 1A,B,C,E K01/1 O99b O99b; NK03/9 1C,E; 2C,F,G,H 1A; 2A 1A; W61; P99; BK01 1D,E; 2C,F AKti/3B,D,E 1D; 2C,D 1C,E; 2H SG78; C97; GS03 SG78; C97; GS03 YR02/10ab; AV05/5,6 YR02/10ab; NK03/6-8; AV07/4 NK03/6,7; PS99/12e NK03/6,7; PS99/12e AKti/2,3

Melting Incongruent melting Stagnant melt transfer Reactants in products High-diffusion textures Low-diffusion textures Prograde textures Peak T textures Retrograde textures Incongruent melting Former melt presence Melt loss Dissolution during melting Precipitation during melt crystallization Incomplete breakdown Incomplete breakdown Fast melting Bt breakdown by melt-producing reaction Bt growth by melt-consuming reaction Reaction products Reaction products Melt loss Melt loss Grt growth by melt-producing reaction Grt breakdown by melt-consuming reaction Grt growth by melt-producing reaction Grt breakdown by melt-consuming reaction Melt-producing reaction

(Devitrified) glass (Devitrified) glass Noa Yes Yes Rare Yes Yes Minor Yes Yes No No data found No data found Yes Yes Yes Yes AK09 (rare) Yes Yes Yes Yes Yes Outer rim No data found No data found Yes

Crystallized melt Crystallized melt Yes Yes Yes Rare Yes Yes Yes Yes Yes Yes (field) Yesc Yesc Yes Rarely No Yes C02 Yes Yes Yesd Yesd Yes Outer rim Yes Outer rim Yes

X/y means Fig. y in paper X. AKti = Álvarez-Valero and Kriegsman (2010-this issue); AV05 = Álvarez-Valero et al. (2005); AV07 = Álvarez-Valero et al. (2007); BK01 = Braun and Kriegsman (2001); C97 = Cesare et al. (1997); GS03 = Guernina and Sawyer (2003); K01 = Kriegsman (2001b); NK03 = Nyström and Kriegsman (2003); O99 = Oliver et al. (1999); P99 = Preston et al. (1999); PS99 = Pyle and Spear (1999); SG78 = Sighinolfi and Gorgoni (1978); W61 = Wyllie (1961); YR02 = Yang and Rivers (2002). a Only in remelted migmatites (e.g., Eifel: Braun and Kriegsman, 2001). b Decreasing from core to inner rim, increasing again to corroded outer rim. c Complex patterns: anhedral zircon in mesosome, anhedral core overgrown by euhedral in melanosome; euhedral zircon in leucosome. d In migmatite melanosome and in granulite bulk. e Increasing from core to inner rim, decreasing again to corroded outer rim.

– disappearance of the early assemblages Ms + Qtz + Pl (muscovite dehydration melting: e.g. Spear et al. 1999), and Bt + Sil + Qtz + Pl (biotite dehydration melting; Fig. 2C), – growth of porphyroblasts, commonly Grt or Crd, with single and multiple inclusions of Bt ± Pl, Sil ± Pl (Fig. 2C,D), and more rarely Bt + Qtz, – other observed porphyroblasts, at more advanced melting stages and higher T, include Spl with Sil inclusion, Opx with Bt or Qtz inclusions, and Kfs with Bt or Qtz inclusions (Fig. 2E), – coronas between porphyroblasts and leucosome (Fig. 2F), interpreted in terms of a reactions between restitic phases and crystallizing melt, – intergrowths of micas ± feldspars ± quartz at the edges of corroded porphyroblasts (Fig. 2G,H; also figures 3 and 4 of Cenki et al., 2002), – growth of porphyroblasts near or within the leucosome, with large, rounded Qtz inclusions, locally with a graphic texture (Fig. 2I), – regrowth of assemblages that had disappeared earlier in a migmatite's history, locally cross-cutting the preexisting fabric: e.g., reappearance of the early assemblages Ms + Qtz + Pl (Nyman et al., 1995; Kohn et al., 1997) or Bt + Sil + Qtz (Kriegsman and Hensen, 1998, Fig. 2J), – growth of late Bt intergrown with aluminosilicate, replacing earlier incongruent phases (Fig. 2K); often low-Ti biotite, in contrast to texturally earlier (brown) high-Ti biotite – blocky intergrowth of Ti-poor Bt (often green) at the interface of Kfs and incongruent Fe–Mg phases (Fig. 2L). 3.3. Textural comparison: discussion A comparison of reaction textures in the two rock types (Figs. 1 and 2) shows both similarities and marked differences (Tables 1–3).

The first striking observation is that textural variability is much larger in migmatites, which fits the hypothesis that migmatites record both melt-producing and melt-consuming reactions. We do not wish to imply that all migmatites are complex, nor that all of them record melt-consuming reactions. Some granulite-facies migmatites, for example, show a marked scarcity of both melt-producing and meltconsuming reaction textures. 3.3.1. Common features Features that are clearly similar in xenoliths and migmatites, in the presence of glass (xenoliths) or leucosome (migmatites), are: the breakdown of an early generation of biotite (Figs. 1A and 2A); the disappearance of early mica-rich assemblages like Bt + Sil + Qtz + Pl (Figs. 1C and 2C,D); the presence of single porphyroblasts and intergrown multiple porphyroblasts (Figs. 1C and 2C,D); coarse coronas of one “anhydrous” phase (here meant to include fluid-bearing Crd) around another anhydrous phase. Where these features are accompanied by mica breakdown (Figs. 1E and 2C,D), we postulate that they are due to melt-producing processes. Where they are accompanied by mica growth (Fig. 2F,G,H), we interpret them in terms of melt-consuming processes. Where neither mica breakdown nor growth is observed, proper mass balance using realistic melt compositions may often provide the answer (e.g. Cenki et al., 2002). 3.3.2. Main differences As a clear distinction, texturally early microfolds with sharp hinges, reminiscent of folds in mica-rich layers, have been observed in xenoliths (Fig. 1B), but are rare in migmatites (Fig. 2B). However, sillimanite needles mimicking a crenulated fabric have been observed as microfolded fibrolite inside garnets in metapelitic granulites (e.g., in Sri Lanka: Raase et al., 1994). A possible explanation is that

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Fig. 1. Microscopic, plane-polarized light views (except D) of key reaction textures in xenoliths: (A) melt-Fib layers (“mix”) with skeletal, corroded Bt that is partly broken down; late Spl-felsic haloes have overgrown the mix and result from the reaction Bt+ Sil + Pl= Spl + Crd + M; (B) as (A), with pre-anatectic microfolds; (C) breakdown of early assemblage Bt + Sil + Qtz +Pl to a Grt-bearing assemblage that further reacts to form Spl + Crd +M; (D) Crd–Kfs intergrowth resulting from the breakdown of the almost completely consumed Bt+ Sil + Qtz (crossed-nichols view); (E) Corroded Grt +Pl+ Bt reacting to form Crd + melt; (F) coarse prismatic Sil ± Bt transecting incongruent phases (e.g., local Grt), but overprinted by Spl–Kfs patches.

such folds are preserved in low-strain domains (xenoliths shielded from deformation by magma; similar shielding within garnet grains), but are strongly overprinted in high-strain domains (stromatic migmatites). In summary, Bt–Sil foliations may result from various processes: (i) subsolidus reactions, particularly during deformation or mimicking an earlier foliation; (ii) muscovite dehydration melting via Ms + Qtz = Sil + M (± Kfs), with Bt present as inert phase, both in xenoliths and migmatites; and (iii) restite–melt back reaction via Grt (or Crd) + M = Bt + Sil + Qtz in migmatites. A combination of textural analysis,

chemistry and reaction balancing should be sufficient to distinguish these scenarios. Coarse intergrowths of garnet and lobate quartz, locally with graphic textures (Fig. 2I) are common in leucosomes and at the leucosome–melanosome interface in migmatites, but have not been recorded in pelitic xenoliths. Hence, we postulate it is a retrograde phenomenon, linked to (partial) melt crystallization. This interpretation is in contrast with its earlier attribution to a melt-producing reaction in the Namaqualand migmatites (Waters, 2001). It also differs from the interpretation of precipitation from a liquid (Vernon

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et al., 2008), already rejected by Waters (2001) on the basis of melt composition. A possible reaction could be Bt+ M = Grt + Qtz + Kfs + V, where Kfs is concentrated in the leucosome. This is a suprasolidus decompression dehydration reaction (SDDR, sensu Thompson 2001) in the KFMASH system, likely to occur at T not too far above the solidus, but reactions in more complex chemical systems are also possible. As Waters (2001) noticed, kinetic factors may determine that Qtz growing together with Grt and Kfs is restricted to the leucosome. Similar textures with Crd

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and Qtz (Barbey et al., 1999) may result from an analogous SDDR, namely Bt+ M = Crd + Qtz + Kfs + V. Importantly, fluids released by the above reactions may, rather than completely escaping the system, cause localised retrogression textures such as blocky intergrowths of green Bt and Qtz transecting Kfs (Fig. 2L; figure 2c in Waters, 2001) and the retrogression of Crd to green Bt, Qtz and And (Fig. 2K). Coarse- to fine-grained Bt–Pl intergrowths and local Crd–Qtz intergrowths are far more common in migmatites than in xenoliths.

Fig. 2. Microscopic, plane-polarized light views (except E and J) of key reaction textures in migmatites: (A) corroded Bt separated from Fib by Crd and Kfs (Korppoo, SW Finland); (B) Folded and recrystallized Sil (Korppoo, SW Finland), probably after Ms + Qtz melting; (C) Bt + Fib separated by Crd and/or Grt indicating the disappearance of early assemblage Bt + Sil + Qtz + Pl (Turku); (D) aligned Fib in Grt, separated from Bt (Turku, SW Finland); (E) Bt + Qtz in Kfs (crossed-nichols view); (F) Spl + melt reacting to Grt + Sil ± Bt (Northampton, W Australia); (G) Grt breakdown to a Bt–Crd intergrowth (retrograde; Turku, SW Finland); (H) Grt breakdown to a Bt–Qtz intergrowth; (I) Grt porphyroblast with large Qtz inclusions in leucosome, showing a graphic texture (Sulkava, SE Finland); (J) regrowth of assemblages that had disappeared earlier in a migmatite's history, locally crosscutting the preexisting fabric: Bt–Sil–Qtz aggregate after Crd (Northampton, W Australia) (crossed-nichols view); (K) Crd partly broken down to low-Ti Bt, And and Qtz (Turku, SW Finland); (L) blocky intergrowth of Ti-poor, green Bt and Qtz at Grt–Kfs interface (Turku, SW Finland).

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Fig. 2 (continued).

Similarly, coarse Bt–Qtz intergrowths are common in migmatites, but have not been reported from xenoliths; Bt–Qtz intergrowths in migmatites are often associated with sillimanite, very locally with andalusite (SW Finland migmatites). Finally, fine-grained blocky intergrowths of Bt–Qtz (Fig. 2L; figure 2c in Waters, 2001) and Sil–Qtz (Namaqualand migmatites: figure 2e in Waters, 2001) have not been observed in xenoliths. The fact that they involve mica growth makes it likely that they represent retrogression either through restite–melt back reaction or through the influx of internally (see above) or externally derived hydrous fluids. Their dominance in migmatites supports the hypothesis that these rock types record more retrogression than xenoliths. In a companion paper (Álvarez-Valero and Kriegsman, 2010-this issue), we have given evidence for a single melt-consuming reaction

in NVP xenoliths, producing biotite–plagioclase intergrowths and biotite patches overgrowing ilmenite (see also Álvarez-Valero et al., 2007; Álvarez-Valero and Kriegsman, 2007). That particular reaction texture is very similar to textures with Bt–Qtz overgrowing Ilm observed in granulite-facies migmatites from South Africa (Waters, 2001) and South India (Cenki et al., 2002; Cesare et al., 2008). In both cases, garnet and ilmenite have broken down, biotite overgrows ilmenite and shows a strong Ti increase from corroded garnet to corroded ilmenite, and, in part of the reaction rim, biotite is intergrown with plagioclase (Spain) or quartz (South Africa, India). In all cases, they have been interpreted in terms of melt-consuming reactions producing biotite. The presence of zircon dust, euhedral zircon crystals in the size range of 0.05–0.25 mm, has been noted around garnet in several

L.M. Kriegsman, A.M. Álvarez-Valero / Lithos 116 (2010) 310–320 Table 3 Summary of textural and chemical features in migmatites and xenoliths.

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– texturally late (often, but not always euhedral) plagioclase generally has a composition around An50–70.

Pelitic xenoliths Pelitic migmatites Inclusions in porphyroblasts Glass/melt inclusions Coronas “Prograde” textures Peak T textures “Retrograde” textures Decompression textures Restitic chemistry Melt loss Can melt composition be estimated? Can reactions be balanced without melt?

Yes Glass Rare Yes Yes Minor Common Yes High Yes Rarely

Yes Crystallized Yes Yes Yes Yes Minor Scale dependent Variable, incomplete With assumptions Some

localities (Fraser et al., 1997) in association with xenotime (Nyström and Kriegsman, 2003) and Bt ± Pl or Bt ± Crd. The accessory phases are attributed to garnet breakdown, where Zr and REE in garnet are derived from garnet, whereas phosphate and Na in Pl are thought to stem from the crystallizing melt. The conversion of garnet to Pl and Crd also requires that some silica from melt is used in the process. This feature is, therefore, considered to be not only diagnostic of garnet breakdown, but may also indicate melt consumption. 3.3.3. Summary The common similarities and main differences between xenolith and migmatite reaction textures seem to support hypotheses (1) and (2). Xenoliths indeed show dominantly melt-producing reactions (hypothesis 1), whereas migmatites show both melt-producing and melt-consuming reactions (hypothesis 2). Table 3 generalizes the primary characteristics for both migmatites and xenoliths, derived from Tables 1 and 2. 3.4. Chemical features Textural observations can be complemented with chemical features, notably zoning patterns (Tracy, 1982). The most useful and most easily measurable compositional variables include Ca and Mn in garnet, Ti in biotite, and Ca/Na in plagioclase. Mn and Ca zoning patterns in garnets are similar in xenoliths and migmatites, with Mn decreasing towards the garnet rim in the case of growth and increasing towards the garnet rim in the case of resorption (e.g., Daniel and Spear, 1999; Spear and Daniel, 2001). However, the clear link to garnet behaviour does not mean it is diagnostic of melt production or consumption. REE patterns in garnet may be more promising when combined with textural information (e.g., Pyle and Spear, 1999, 2000; see Section 3.3). Similarly, Ti in biotite generally increases with decreasing modal abundance (Cesare et al., 2003a), but also reflects local Ti gradients (Cenki et al., 2002; Álvarez-Valero and Kriegsman, 2010-this issue). By implication, Ti contents are not diagnostic either, with one exception: growth of texturally late, low-Ti biotite, often green, is indicative of a retrograde process (e.g., Nyström and Kriegsman, 2003), driven by interaction with externally derived hydrous fluids or with fluids released from the crystallizing in situ melt. A more promising feature may be the Ca/Na ratio of plagioclase. A literature scan on melting in metapelites (e.g., Ramaswamy and Murty, 1974; Waters, 1989, 1991; Kriegsman and Hensen, 1998; Kriegsman, 2001a; Cenki et al., 2002; Scrimgeour et al., 2005; ÁlvarezValero et al., 2007; Diener et al., 2008; Álvarez-Valero and Kriegsman, 2010-this issue) suggests the following: – it is common for more than one generation of plagioclase to be present in pelitic xenoliths and migmatites, – texturally early (often anhedral to clearly corroded) plagioclase generally has a composition around An25–45,

The role of plagioclase in melting processes can be complex due to slow diffusion (see, e.g. melting experiments by Johannes and Holtz, 1996), that may lead to partial dissolution of texturally early plagioclase and precipitation of a second plagioclase generation from the melt. If that is the case, the second plagioclase may have higher An values than the first to compensate for low An in melt, as described in Álvarez-Valero and Kriegsman 2010-this issue. The observation that plagioclase compositions are quite often not in agreement with theoretical predictions (“plagioclase problem”: Ashworth, 1985; also Powell and Downes, 1990) can be solved when it is assumed that our observation of a Ca/Na balance between early plagioclase, melt and secondary plagioclase is considered to be more generally valid. In this model, early Pl, that in average pelites at medium to high metamorphic grade has an overall intermediate composition (oligoclase), is dissolved during partial melting and simultaneously produces a low-An melt and a second plagioclase with higher An. During the melt-consuming process, the reverse process could theoretically operate, by which the second plagioclase generation would largely dissolve, while a new rim of intermediate composition would be nucleating on and overgrowing early plagioclase. 3.5. Spatial distribution of melt-producing and melt-consuming reactions Phases formed by incongruent melting are generally reported from the mesosome domains of migmatites, where they enclose relics of the reactant assemblage (e.g., Powell and Downes 1990; Fitzsimons, 1996; Greenfield et al., 1998; Kriegsman and Hensen 1998; Berger and Kalt, 1999). Hence, mesosome is generally described by metamorphic geologists as a restitic rock volume from which melt has segregated. In rare cases, the incongruent phases are restricted to the leucosome, which may be a nucleation feature (e.g. Powell and Downes 1990). Nyström and Kriegsman (2003) noted in their samples that neosome (=melanosome + leucosome) generally shows the same prograde reactions as mesosome, but has commonly been affected by one subsequent prograde reaction. This strongly supports the notion of higher reaction progress and more melt production in melanosome (Mehnert, 1968; Mehnert and Büsch, 1982; Johannes, 1983, 1988; Brown, 1994; Brown et al., 1995). Retrogression was, however, also noted as being most severe in their melanosome samples, in line with observations in other areas (Kriegsman and Hensen, 1998; Spear et al., 1999; Jung et al., 1999). Hence, melanosome may show increased reaction progress both on the prograde (melt-producing) and on the retrograde (melt-consuming) path. Some melt-consuming reactions, namely the SDDR (Section 2), are expected to be concentrated within the leucosome and at the leucosome–melanosome interface, because they are essentially reactions between micas, commonly enriched in melanosome, and crystallizing melt, in adjacent leucosome. One example is given by the coarse, locally graphic, intergrowths of garnet and lobate quartz (Section 3.3). Another example may be thin sillimanite selvages on leucosome, locally observed in granulite-facies migmatites (Kriegsman, 1993). This discussion suggests that hypothesis (3) must be adapted to include leucosome. A better wording is: “melt-consuming reactions have mainly operated at leucosome–melanosome interfaces”. 4. Discussion and applications The conclusion that many reaction textures in migmatites reflect melt-consuming processes and that melanosome may show increased reaction progress both during melt production and melt consumption, has several important implications. First of all, it may give a better understanding of the meaning of trace element patterns in layered migmatites (see Jung and Hellebrand, 2006) by linking chemical and

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isotopic data to textural information (see also Fourcade et al. 1992; Nyström and Kriegsman, 2003). At the same time it may provide better constraints on major and trace element compositions of melts derived from migmatite terrains, if one finds a way of correcting for the retrograde overprint that can be strong (Fourcade et al., 1992). This also leads to an improved interpretation of in situ age patterns. For example, Nyström and Kriegsman (2003) suggested that trace elements contained in the melt may be transferred to accessory phases in the melanosome during restite–melt interaction, often as rims on older grains. As a consequence the youngest zircon material may be represented by new rims on old resorbed zircon in melanosome and not necessarily by leucosome zircon. Recognising melt-producing and melt-consuming reactions also leads to improved P–T paths and thermobarometry (Kriegsman and Hensen, 1998, Spear et al., 1999). For example, textures in metapelitic migmatites from S Germany have been interpreted in terms of FMAS reactions Spl + Qtz = Crd, and Grt + Sil + Qtz = Crd (Kalt et al., 1999), but may, in the presence of leucosome and biotite, also represent meltproducing reaction Bt+ Sil + Grt = Crd+ Kfs + M and melt-consuming reaction Spl + M = Crd + Kfs (± other phases), respectively. Because of the very different slopes in P–T space, the clockwise P–T path attributed to this region may also be an isobaric heating–cooling cycle. We agree with Vernon et al. (2008, p. 441–442) that the term “disequilibrium texture” can be misleading. We also agree that mineral inclusions in porphyroblasts are commonly in equilibrium with their host, but that does not rule out the possibility or even likelihood of a reaction assemblage. The critical observation is not, for example, that biotite, quartz and sillimanite inclusions are stable with a garnet host, but that three-phase inclusions are consistently lacking, indicating that Bt + Sil + Qtz is no longer a stable assemblage. Hence, the early assemblage Bt + Sil + Qtz has been replaced by local assemblages Grt + Bt + Qtz, Grt + Sil + Qtz, and Grt + Bt + Sil, or subsets thereof. These domains may result from the combined effect of original compositional variation and diffusion (e.g., Stüwe, 1997). Several authors (Zeck, 1970; Perini et al., 2009) use the term “erupted migmatites”, referring to the metapelitic xenoliths of the NVP. Our textural comparison, however, shows that such partially melted xenoliths only record the early stages of migmatisation and lack some crucial features pertinent to real migmatites. Notably, they lack migmatitic layering and a clear distinction between mesosome, melanosome and leucosome. The only layering observed generally mimicks older foliations resulting from medium-grade metamorphism and deformation. The absence of migmatitic layering may be related to the absence of deviatorial stresses in xenoliths, because segregation into layers of contrasting viscosity is a process resulting from deformation (Van der Molen, 1985). We conclude that comparative petrology on xenolith and migmatite textures is a useful method to distinguish melt-producing and melt-consuming processes, although care is needed because some overlap exists. In addition, our data corroborate two conclusions reached by Braun and Kriegsman (2001) on remelted migmatites, namely that “final textures and mineral assemblages in migmatites often result from retrograde overprint on prograde textures”; and that “xenoliths are weakly affected by retrograde phenomena and thus give important insights in prograde processes and better constraints on major and trace element behaviour during melting than migmatites”, but now for primary migmatites. Acknowledgements We want to thank M. Brown for the invitation to contribute to this special issue and guest editorial handling by F. Korhonen. Constructive reviews by Ingo Braun and Juan Otamendi are gratefully acknowledged. This research received support from the SYNTHESYS Project http://www.synthesys.info/ which is financed by European Community Research Infrastructure Action under the FP6 “Structuring the

European Research Area” Programme. Research project: Textures in crustal xenoliths versus textures in migmatites, at the National Museum of Natural History/NATURALIS in Leiden. A-V wants to thank the JAEDoc research program (CSIC-2007). References Álvarez-Valero, A.M., Cesare, B., Kriegsman, L.M., 2005. Formation of elliptical garnets in a metapelitic enclave by melt-assisted dissolution and reprecipitation. Journal of Metamorphic Geology 23, 65–74. Álvarez-Valero, A.M., Cesare, B., Kriegsman, L.M., 2007. Formation of melt-bearing spinel– cordierite–feldspars coronas after garnet in metapelitic xenoliths. Reaction modelling and geodynamic implications. Journal of Metamorphic Geology 25, 305–320. Álvarez-Valero, A.M., Kriegsman, L.M., 2007. Crustal thinning and mafic underplating beneath the Neogene Volcanic Province (Betic Cordillera, SE Spain): evidence from crustal xenoliths. Terra Nova 19, 266–271. Álvarez-Valero, A.M., Kriegsman, L.M., 2008. Partial crustal melting beneath the Betic Cordillera (SE Spain): the case study of Mar Menor volcanic suite. Lithos 101, 379–396. Álvarez-Valero, A.M., Kriegsman, L.M., 2010. Chemical, petrological and mass balance constraints on the textural evolution of pelitic enclaves. Lithos 116, 300–309 (this issue). Álvarez-Valero, A.M., Waters, D.J., 2010. Partialy melted xenoliths as a window into sub-volcanic processes: evidence from the Neogene magmatic province of the Betic Cordillera, SE Spain. Journl of Petrology. doi:10.1093/petrology/egq007. Ashworth, J.R., 1985. Introduction. In: Ashworth, J.R. (Ed.), Migmatites. Glasgow, Blackie and Son, pp. 36–85. Barbey, P., Marignac, C., Montel, J.M., Macaudiere, J., Gasquet, D., Jabbori, J., 1999. Cordierite growth textures and the conditions of genesis and emplacement of crustal granitic magmas; the Velay granite complex (Massif Central, France). Journal of Petrology 40, 1425–1441. Berger, A., Kalt, A., 1999. Structures and melt fractions as indicators of rheology in cordierite-bearing migmatites of the Bayerische Wald (Variscan Belt, Germany). Journal of Petrology 40, 1699–1719. Braun, I., Kriegsman, L.M., 2001. Partial melting in crustal xenoliths and anatectic migmatites: a comparison. Physics and Chemistry of the Earth 26, 261–266. Brown, M., 1994. The generation, segregation, ascent and emplacement of granite magma: the migmatite-to-crustally-derived granite connection in thickened orogens. Earth Science Reviews 36, 83–130. Brown, M., Averkin, Y.A., McLellan, E.L., Sawyer, E.W., 1995. Melt segregation in migmatites. Mechanisms and Consequences of Melt Segregation from Crustal Protoliths: In: Brown, M., Rushmer, T., Sawyer, E.W. (Eds.), Journal of Geophysical Research, 100, pp. 15655–15679. Carmichael, D.M., 1969. On the mechanism of prograde metamorphic reactions in quartzbearing pelitic rocks. Contributions to Mineralogy and Petrology 20, 244–267. Cenki, B., Kriegsman, L.M., Braun, I., 2002. Melt-producing and melt-consuming reactions in anatectic granulites: P–T evolution of the Achankovil cordierite gneisses, South India. Journal of Metamorphic Geology 20, 543–561. Cesare, B., Cruciani, G., Russo, U., 2003a. Hydrogen deficiency in Ti-rich biotite from anatectic metapelites (El Joyazo—SE Spain): crystal-chemical aspects and implications for high-temperature petrogenesis. American Mineralogist 88, 583–595. Cesare, B., Gómez-Pugnaire, M.T., 2001. Crustal melting in the Alboran Domain: constraints from enclaves of the Neogene Volcanic Province. Physics and Chemistry of the Earth 26, 255–260. Cesare, B., Gómez-Pugnaire, M.T., Rubatto, D., 2003b. Residence time of S-type anatectic magmas beneath the Neogene Volcanic Province of SE Spain: a zircon and monazite SHRIMP study. Contributions to Mineralogy and Petrology 146, 28–43. Cesare, B., Maineri, C., Baron Toaldo, A., Pedron, D., Acosta-Vigil, A., 2007. Immiscibility between carbonic fluids and granitic melts during crustal anatexis: a fluid and melt inclusion study in the enclaves of the Neogene Volcanic Province of SE Spain. Chemical Geology 237, 433–449. Cesare, B., Rubatto, D., Gomez-Pugnaire, M.T., 2009. Do extrusion ages reflect magma generation processes at depth? An example from the Neogene Volcanic Province of SE Spain. Contributions to Mineralogy and Petrology 157, 267–279. Cesare, B., Salvioli Mariani, E., Venturelli, G., 1997. Crustal anatexis and melt extraction during deformation in the restitic enclaves at El Joyazo (SE Spain). Mineralogical Magazine 67, 15–27. Cesare, B., Satish-Kumar, M., Cruciani, G., Pocker, S., Nodari, L., 2008. Formation of glassbearing spinel–cordierite–feldspars coronas after garnet in metapelitic xenoliths. Reaction modelling and geodynamic implications. American Mineralogist 93, 327–338. Clemens, J.D., Droop, G.T.R., 1998. Fluids, P–T paths and the fates of anatectic melts in the Earth's crust. Lithos 44, 21–36. Cruciani, G., Franceschelli, M., Elter, F.M., Puxeddu, M., Utzeri, D., 2008. Petrogenesis of Alsilicate-bearing trondhjemitic migmatites from NE Sardinia, Italy. Lithos 102, 554–574. Daniel, C.G., Spear, F.S., 1999. The clustered nucleation and growth processes of garnet in regional metamorphic rocks from north-west Connecticut, USA. Journal of Metamorphic Geology 17, 503–520. Diener, J.F.A., White, R.W., Powell, R., 2008. Granulite facies metamorphism and subsolidus fluid-absent reworking, Strangways Range, Arunta Block, central Australia. Journal of Metamorphic Geology 26, 603–622. Ellis, D.J., Obata, M., 1992. Migmatite and melt segregation at Cooma, New South Wales. Transactions of the Royal Society Edinburgh Earth Sciences 83, 95–106. Fitzsimons, I.C.W., 1996. Metapelitic migmatites from Brattstrand Bluffs, East Antarctica— metamorphism, melting and exhumation of the mid crust. Journal of Petrology 37, 395–414.

L.M. Kriegsman, A.M. Álvarez-Valero / Lithos 116 (2010) 310–320 Fourcade, S., Martin, H., de Bremond d'Ars, J., 1992. Chemical exchange in migmatites during cooling. Lithos 28, 43–53. Fraser, G., Ellis, D., Eggins, S., 1997. Zirconium abundance in granulite-facies minerals, with implications for zircon geochronology in high grade rocks. Geology 25, 607–610. Gil Ibarguchi, J.I., Martínez, F.J., 1982. Petrology of garnet–cordierite–sillimanite gneisses from the El Tormes Thermal Dome, Iberian Hercynian Fold belt (W Spain). Contributions to Mineralogy and Petrology 80, 14–24. Grapes, R.H., 1986. Melting and thermal reconstitution of pelitic xenoliths, Wehr Volcano, East Eifel, West Germany. Journal of Petrology 27, 343–396. Greenfield, J.E., Clarke, G.L., Bland, M., Clark, D.J., 1996. In-situ migmatite and hybrid diatexite at Mt. Stafford, central Australia. Journal of Metamorphic Geology 14, 413–426. Greenfield, J.E., Clarke, G.L., White, R.W., 1998. A sequence of partial melting reactions at Mt Stafford, central Australia. Journal of Metamorphic Geology 16, 363–378. Grew, E.S., Yates, M.G., Wilson, C.J.L., 2008. Aureoles of Pb(II)-enriched feldspar around monazite in paragneiss and anatectic pods of the Napier Complex, Enderby Land, East Antarctica: the roles of dissolution-reprecipitation and diffusion. Contributions to Mineralogy and Petrology 155, 363–378. Guernina, S., Sawyer, E.W., 2003. Large-scale melt-depletion in granulite terranes: an example from the Archean Ashuanipi Subprovince of Quebec. Journal of Metamorphic Geology 21, 181–201. Hanchar, J.M., Miller, C.F., Wooden, J.L., Bennett, V.C., Staude, J.-M.G., 1994. Evidence from xenoliths for a dynamic lower crust, Eastern Mojave Desert, California. Journal of Petrology 35, 1377–1415. Hand, M., Dirks, P.H.G.M., 1992. The influence of deformation on the formation of axial– planar leucosomes and the segregation of small melt bodies within the migmatitic Napperby Gneiss, central Australia. Journal of Structural Geology 14, 591–604. Harangi, S., Downes, H., Kósa, L., Szabó, C., Thirlwall, M.F., Mason, P.R.D., Mattey, D., 2001. Almandine garnet in calc-alkaline volcanic rocks of the Northern Pannonian Basin (Eastern-Central Europe): geochemistry, petrogenesis and geodynamic implications. Journal of Petrology 42, 1813–1843. Hölttä, P., Väisänen, M., Rastas, J., 1994. Excursion in the Turku area. In: Pajunen, M. (Ed.), High Temperature–Low Pressure Metamorphism and Deep Crustal Structures. Program for the meeting of IGCP project 304 ‘‘Deep Crustal Processes’’, Finland, September 16–20, 1994: Geol. Survey of Finland, Rep. K31.4/–94./1, pp. 13–17. Janák, M., Hurai, V., Ludhová, L., O'Brien, P.J., Horn, E.E., 1999. Dehydration melting and devolatilization during exhumation of highgrade metapelites: the Tatra Mountains, Western Carpathians. Journal of Metamorphic Geology 17, 379–395. Johannes, W., 1983. On the origin of layered migmatites. In: Atherton, M.P., Gribble, C.D. (Eds.), Migmatites, Melting and Metamorphism. Shiva Geol. Ser. Shiva, Nantwich, UK, pp. 234–248. Johannes, W., 1988. What controls partial melting in migmatites? In: Tracy, R.J., Day, H.W. (Eds.), Studies in the Genesis and Deformation of Migmatites: Journal of Metamorphic Geology, vol. 6, pp. 451–465. Johannes, W., Ehlers, C., Kriegsman, L.M., Mengel, K., 2003. The link between migmatites and S-type granites in the Turku area, southern Finland. Lithos 68, 69–90. Johannes, W., Holtz, F., 1996. Petrogenesis and Experimental Petrology of Granitic Rocks. Berlin, Springer. 335 pp. Jung, S., Hellebrand, E., 2006. Trace element fractionation during high-grade metamorphism and crustal melting—constraints from ion microprobe data of metapelitic, migmatitic and igneous garnets and implications for Sm–Nd garnet chronology. Lithos 87, 193–213. Jung, S., Hoernes, S., Masberg, P., Hoffer, E., 1999. The petrogenesis of some migmatites and granites (Central Damara Orogen, Namibia): evidence for disequilibrium melting, wall rock contamination and crystal fractionation. Journal of Petrology 40, 1241–1269. Kalt, A., Berger, A., Bluemel, P., 1999. Metamorphic evolution of cordierite-bearing migmatites from the Bayerische Wald (Variscan Belt, Germany). Journal of Petrology 40, 601–627. Kohn, M.J., Spear, F.S., Valley, J.W., 1997. Dehydration melting and fluid recycling during metamorphism: Rangeley Formation, New Hampshire, USA. Journal of Petrology 38, 1255–1277. Kretz, R., 1983. Symbols for rock-forming minerals. American Mineralogist 68, 277–279. Kriegsman, L.M., 1993. Structural and petrological investigations into the Sri Lankan high-grade terrain—geodynamic evolution and setting of a Gondwana fragment. Geologica Ultraiectina 114, 1–208. Kriegsman, L.M., 2001a. Partial melting, partial melt extraction and partial back reaction in anatectic migmatites. Lithos 56, 75–96. Kriegsman, L.M., 2001b. Quantitative field methods for estimating melt production and melt loss. Physics and Chemistry of the Earth 26, 247–253. Kriegsman, L.M., Hensen, B.J., 1998. Back reaction between restite and melt: implications for geothermobarometry and pressure–temperature paths. Geology 26, 1111–1114. Kriegsman, L.M., Nyström, A.I., 2003. Melt segregation rates in migmatites: review and critique of common approaches. In: Vance, D., Müller, W., Villa, I. (Eds.), Geochronology: Linking the Isotopic Record with Petrology and Textures: Geological Society of London Special Publications, vol. 220, pp. 203–212. Mehnert, K.R., 1968. Migmatites and the Origin of Granitic Rocks. Elsevier, Amsterdam. 393 pp. Mehnert, K.R., Büsch, W., 1982. The initial stage of migmatite formation. Neues Jahrbuch für Mineralogie Abhandlungen 145, 211–238. Nyman, M.W., Pattison, D.R.M., Ghent, E.D., 1995. Melt extraction during formation of K-feldspar + sillimanite migmatites, west of Revelstoke, British Columbia. Journal of Petrology 36, 351–372. Nyström, A.I., Kriegsman, L.M., 2003. Prograde and retrograde reactions, garnet zoning patterns, and accessory phase behaviour in SW Finland migmatites, with implications for geochronology. In: Vance, D., Müller, W., Villa, I. (Eds.), Geochronology: Linking the

319

Isotopic Record with Petrology and Textures: Geological Society of London Special Publications, vol. 220, pp. 213–230. Oliver, N.H.S., Bodorkos, S., Nemchin, A.A., Kinny, P.D., Watt, G.R., 1999. Relationships between zircon U–Pb SHRIMP ages and leucosome type in migmatites of the Halls Creek Orogen, Western Australia. Journal of Petrology 40, 1553–1575. Palmeri, R., 1997. P–T paths and migmatite formation: an example from Deep Freeze Range, northern Victoria Land, Antarctica. Lithos 42, 47–66. Patiño Douce, A.E., Harris, N., 1998. Experimental constraints on Himalayan Anatexis. Journal of Petrology 39, 689–710. Perini, G., Cesare, B., Gómez-Pugnaire, M.T., Ghezzi, L., Tommasini, S., 2009. Armouring effect on Sr–Nd isotopes during disequilibrium crustal melting: the case study of frozen migmatites from El Hoyazo and Mazarrón, SE Spain. European Journal of Mineralogy 21, 117–131. Powell, R., Downes, J., 1990. Garnet porphyroblast-bearing leucosomes in metapelites: mechanisms, phase diagrams, and an example from Broken Hill, Australia. In: Ashworth, J.R., Brown, M. (Eds.), High-temperature Metamorphism and Crustal Anatexis. Unwin Hyman, London, pp. 105–123. Preston, R.J., Dempster, T.J., Bell, B.R., Rogers, G., 1999. The petrology of mullite-bearing peraluminous xenoliths, implications for contamination processes in basaltic magmas. Journal of Petrology 40, 549–573. Pyle, J.M., Spear, F.S., 1999. Yttrium zoning in garnet; coupling of major and accessory phases during metamorphic reactions. Geological Materials Research 1, 1–49. Pyle, J.M., Spear, F.S., 2000. An empirical garnet (YAG)-xenotime thermometer. Contributions to Mineralogy and Petrology 138, 51–58. Raase, P., Schenk, V., 1994. Petrology of granulite-facies metapelites of the Highland Complex, Sri Lanka; implications for the metamorphic zonation and the P–T path. Precambrian Research 66, 265–294. Ramaswamy, A., Murty, S., 1974. Minerals from the charnockite series of Amaravathi, Andhra Pradesh, South India. Mineralogical Magazine 39, 807–810. Sawyer, E.W., 1999. Criteria for the recognition of partial melting. Physics and Chemistry of the Earth (A) 24, 269–279. Sawyer, E.W., 2010. Migmatites formed by water-fluxed partial melting of a leucogranodiorite protolith: microstructures in the residual rocks and source of the fluid. Lithos 116, 273–286 (this issue). Scrimgeour, I.R., Kinny, P.D., Close, D.F., Edgoose, C.J., 2005. High-T granulites and polymetamorphism in the southern Arunta Region, central Australia: evidence for a 1.64 Ga accretional event. Precambrian Reasearch 142, 1–27. Sederholm, J.J., 1913. Die Entstehung der migmatitischen Gesteine. Geologische Rundschau 4, 174–185. Sederholm, J.J., 1934. On migmatites and associated pre-Cambrian rocks of southwestern Finland: part III. The Åland Islands. Bulletin de la Commission Géologique de Finlande, vol. 107. 68 pp. Sighinolfi, G.P., Gorgoni, C., 1978. Chemical evolution of high-grade metamorphic rocks— anatexis and remotion of material from granulite terrains. Chemical Geology 22, 157–176. Spear, F.S., 1991. On the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling. Journal of Metamorphic Geology 9, 379–388. Spear, F.S., Daniel, C.G., 2001. Diffusion control of garnet growth, Harpswell Neck, Maine, USA. Journal of Metamorphic Geology 19, 179–195. Spear, F.S., Kohn, M.J., Cheney, J.T., 1999. P–T paths from anatectic pelites. Contributions to Mineralogy and Petrology 134, 17–32. Stípská, P., Schulmann, K., Powell, R., 2008. Contrasting metamorphic histories of lenses of high-pressure rocks and host migmatites with a flat orogenic fabric (Bohemian Massif, Czech Republic): a result of tectonic mixing within horizontal crustal flow? Journal of Metamorphic Geology 26, 623–646. Stüwe, K., 1997. Effective bulk composition changes due to cooling: a model predicting complexities in retrograde reaction textures. Contributions to Mineralogy and Petrology 129, 43–52. Thompson, A.B., 1974. Calculation of muscovite–paragonite–alkali feldspar phase relations. Contributions to Mineralogy and Petrology 44, 173–194. Thompson, A.B., 2001. Clockwise P–T paths for crustal melting and H2O recycling in granite source regions and migmatite terrains. Lithos 56, 33–45. Tracy, R.J., 1982. Compositional zoning and inclusions in metamorphic minerals. Reviews in Mineralogy 10, 355–397. van Bergen, M.J., 1980. Grandidierite from aluminous metasedimentary xenoliths within acid volcanics; a first record in Italy. Mineralogical Magazine 43, 651–658. van Bergen, M.J., 1983. Polyphase metamorphic sedimentary xenoliths from Mt. Amiata volcanics (central Italy): evidence for a partially disrupted contact aureole. Geologische Rundschau 72, 637–662. van der Molen, I., 1985. Interlayer material transport during layer-normal shortening; part II, boudinage, pinch-and-swell and migmatite at Soendre Stroemfjord airport, West Greenland. Tectonophysics 115, 297–313. Vernon, R.H., White, R.W., Clarke, G.L., 2008. False metamorphic events inferred from misinterpretation of microstructural evidence and P–T data. Journal of Metamorphic Geology 26, 437–449. Vigneresse, J.L., Barbey, P., Cuney, M., 1996. Rheological transitions during partial melting and crystallization with application to felsic magma segregation and transfer. Journal of Petrology 37, 1597–1600. Waters, D.J., 1989. Metamorphic evidence for the heating and cooling path of Namaqualand granulites. In: Daly, J.S., Cliff, R.A., Yardley, B.W.D. (Eds.), Evolution of Metamorphic Belts: Geological Society of London, Special Publication, vol. 43, pp. 357–363. Waters, D.J., 1991. Hercynite–quartz granulites: phase relations and implications for crustal processes. European Journal of Mineralogy 3, 367–386. Waters, D.J., 2001. The significance of prograde and retrograde quartz-bearing intergrowth microstructures in partially melted granulite-facies rocks. In: Kriegsman, L.M. (Ed.), Prograde and Retrograde Processes in Migmatites: Lithos, vol. 56, pp. 97–110.

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L.M. Kriegsman, A.M. Álvarez-Valero / Lithos 116 (2010) 310–320

Wheeler, J., Mangan, L.S., Prior, D.J., 2004. Disequilibrium in the Ross of Mull contact metamorphic aureole, Scotland: a consequence of polymetamorphism. Journal of Petrology 45, 835–853. Whitney, D.L., Irving, A.J., 1994. Origin of K-poor leucosomes in a metasedimentary migmatite complex by ultrametamorphism, syn-metamorphic magmatism and subsolidus processes. Lithos 32, 173–192. Wyllie, P.J., 1961. Fusion of Torridonian sandstone by a picrite sill in Soay (Hebrides). Journal of Petrology 2, 1–37.

Yang, P., Rivers, T., 2002. The origin of Mn and Y annuli in garnet and the thermal dependence of P in garnet and Y in apatite in calc-pelite and pelite, Gagnon terrane, western Labrador. Geological Materials Research 4, 1–35. Zeck, H.P., 1970. An erupted migmatite from Cerro de Hoyazo, SE Spain. Contributions to Mineralogy and Petrology 26, 225–246.