Origin and evolution of a migmatite

The development of a stromatic migmatite exposed east and southeast of Arvika (Western Sweden) is described in four stages beginning with the country rock and following evolution through three areas characterized by low, medium and high amounts of leucosomes (areas L, M, and H, respectively).The country rock is a paragneiss composed of thin, alternating fine- and coarse-grained layers. Composition of the layers varies from granitic (fine) to tonalitic (coarse layers).The bulk of the stromatic migmatite is composed of leucocratic layers of magmatic appearance (leucosomes) and darker layers of gneissic aspect (mesosomes). Petrographical and chemical data (given in the form of Niggli values and K₂O/SiO₂ diagrams) show a close relationship between the fine-grained paragneiss layers and the leucosomes on the one hand and between the coarse-grained layers and the mesosomes on the other.At relatively low temperatures only those gneiss layers with a suitable (granitic) composition are transformed into leucosomes. This process is interpreted to be due to recrystallization of the felsic minerals via partial melting and to the separation of biotite.With increasing metamorphism, leucosomes become broader and more frequent due to partial melting of layers with less suitable composition. Contacts between different generations of leucosome can be recognized in the form of relict melanosomes.These observations favour essentially isochemical melting, followed by later in-situ crystallization. This model of an isochemical layer-by-layer transformation is supported by the preferential formation of hornblende in leucosomes and relict melanosomes, as well as by almost identical compositions of migmatite and country-rock plagioclase.     

Genetic Model for the Stromatic Migmatites of the Rantasalmi-Sulkava Area, Finland

Metasediments of the Rantasalmi-Sulkava area (Finland) showprogressive regional metamorphism with migmatization. The metasedimentsare represented by various types of metapsammites (plagioclase-rich,quartz-rich, and layers of granitic compositions—somerich in microcline and others in plagioclase) and metapelites(dark and light layers). The migmatites of this area are of stromatic type. They consistof leucosomes, mesosomes, and light-coloured plagioclase-richlayers which do not fit the definition of leucosome. Melanosomes,which usually separate leucosomes and mesosomes in stromaticmigmatites, are almost absent. The leucosomes are of three types: (i) quartz-rich; (ii) cordierite-rich;and (iii) granitic. The quartz-rich leucosomes formed firstat subsolidus temperatures through recrystallization. The graniticleucosomes are considered to have developed via partial melting.The cordierite-rich leucosomes are formed—like the graniticones—at supersolidus conditions, but the role of partialmelting is not clear. The mesosomes are the metamorphic portions of the migmatiteswhich are not transformed into leucosomes. They include metapsammiticlayers and light-coloured metapelitic layers, both rich in plagioclase. Besides mineral reactions resulting in new assemblages duringregional metamorphism, the main process changing the protolithsinto migmatites is the conversion of some of the protolith layersinto leucosomes, through (as we believe) an almost isochemicalpartial melting. The migmatites of the Rantasalmi-Sulkava area differ from othermigmatites investigated by the authors in having two differentgenetic types of leucosomes: one formed via partial meltingand the other through subsolidus recrystallization as mentionedabove. The process of migmatization is described and modelledin three steps. Reprint requests to W. Johannes     

The genesis of wadi Magrish migmatites (N-E Sinai)

The migmatite complex of the Magrish area is part of a large crystalline massif south of Elat. The mineralogical composition of the migmatites is very uniform. The components of the melanosome are biotite, quartz and plagioclase, with small amounts of garnet and very rarely sillimanite and those of the leucosome — quartz and plagioclase. On the basis of chemical composition of the migmatites and possible premigmatitic parent rocks, absence of orthoclase in the leucosome, similar composition of plagioclase in the leucosome and in neighbouring melanosome, and Qz:Plag values which do not plot around a cotectic line, it is concluded that migmatisation occurred in a nearly closed system, without the presence of a melt phase. Thus, injection of granitic material, metasomatism or partial anatexis as possible main formation mechanisms are rejected and metamorphic differentiation is favoured.     

Isocon analysis of migmatization in the Front Range, Colorado, USA

Isocon analysis has been applied to five sets of leucosome, mafic selvages and immediately adjacent mesosome in the migmatites from a 15-m outcrop in the Colorado Front Range. The results show: (i) mafic selvages formed from the adjacent mesosome by loss of felsic components and therefore the mesosomes are indeed palaeosomes or protoliths; (ii) the leucosomes did not form in a closed system from the palaeosome (in which case the material lost from the palaeosome during selvage formation would become the leucosome). The observed volumes and compositions of leucosomes require that the present leucosome must contain some material in addition to the felsic components lost from the selvages. The materials that must be added are leucotonalitic to granitic in composition, varying greatly in K/(Na + Ca) ratio. The trend in leucosome composition can be reproduced by assuming that a metasomatic exchange, KNa + Ca, modified originally leucotonalitic leucosomes to more K-rich compositions. These leucosomes most likely formed by injection of silicate melts accompanied, or followed, by metasomatism. The trend of leucosome compositions in this study reflects the general trend in the leucosome compositions which have been published from other areas, indicating that the proposed mechanism can be applicable to other regional migmatites.     

Water-assisted migmatization of metagraywackes in a Variscan shear zone, Aiguilles-Rouges massif, western Alps

Migmatitic rocks developed in metagraywackes during the Variscan orogeny in the Aiguilles-Rouges Massif (western Alps). Partial melting took place 320 Ma ago in a 500 m-wide vertical shear zone. Three leucosome types have been recognised on the basis of size and morphology: (1) large leucosomes > 2 cm wide and > 40 cm long lacking mafic selvage, but containing cm-scale mafic enclaves; (2) same as 1 but with thick mafic selvage (melanosome); (3) small leucosomes < 2 cm and < 40 cm) with thin dark selvages (stromatic migmatites). Types 1 + 2 have mineralogical and chemical compositions in keeping with partial melting experiments. But Type 3 leucosomes have identical plagioclase composition (An_19–28) to neighbouring mesosome, both in terms of major- and trace-elements. Moreover, whole-rock REE concentrations in Type 3 leucosomes are only slightly lower than those in the mesosomes, unlike predicted by partial melting experiments. The main chemical differences between all leucosome types can be related to the coupled effect of melt segregation and late chemical reequilibration.

Mineral assemblages and thermodynamic modelling on bulk-rock composition restrict partial melting to  650 °C at 400 MPa. The large volume of leucosome (20 vol.%) thus generated requires addition of 1 wt.% external water. Restriction of extensive migmatization to the shear zone, without melting of neighbouring metapelites, also points to external fluid circulation within the shear zone as the cause of melting.     

High-temperature cordierite migmatites in the north-eastern Grenville Province

Abstract In the northeastern part of the Grenville Province, along the gulf of St Lawrence, cordierite is widespread in the migmatites of Baie Jacques Cartier (BJC) and Baie des Ha! Ha! (BHH). In the BJC area, rafts of mesosome occur in a pervasive network of leucosome consisting of cordierite-bearing pegmatite. In BHH, however, the mesosome and leucosome are well segregated and locally separated by thin biotite –hornblende melanosomes. Leucosomes in the BJC area record the highest temperatures (oxide thermometry = 900°C), whereas leucosomes of BHH and mesosomes of both areas indicate peak temperatures around 800°C (oxide thermometry; biotite–garnet thermometry with fluorine-rich biotite). Peak pressures were constrained at 720 MPa using the Ilm-Sil–Qtz–Grt–Rt (GRAIL) equilibrium. The area is thought to have undergone extensive melting under relatively modest pressures. The highest temperatures recorded in the BJC area are probably related to a pervasive impregnation of this terrane by aluminous granitic melts. Most post-peak P–T estimates for the mesosomes fall on a nearly isobaric, clockwise, P–T path (0.6 MPa/°C) with the exception of the high-temperature leucosomes of BJC, which fall about 100°C away from this path; this is additional evidence for the external origin of these leucosomes. The ultimate source of heat that generated the migmatites is thus though to be an underlying plutonic complex (anorthosite?).     

Melting reactions and trace element relationships in selected specimens of migmatitic pelites from New Hampshire and Maine

Leucosomes and melanosomes in selected specimens of migmatitic, sillimanite-zone, pelitic schists are modal and chemical complements formed by segregation within originally homogeneous paleosomes. Systematic bulk chemical and modal variations in melanosomes can be used to infer the reactions by which leucosomes were generated.Trace element variations and relationships in melanosomes and leucosomes indicate that the migmatites behaved as closed systems during leucosome formation. Mass-balance evaluation of trace element relationships in the context of inferred leucosome-forming reactions suggest that trace elements essentially followed the melanosome phases initially containing them, as these phases reacted in leucosome generation. The trace element composition of a leucosome is given by the sum of those of the melanosome phases reacted, minus the trace element contents of any new solid melanosome phases produced by the reactions.Trace element relations are consistent with metamorphic equilibrium during leucosome generation, but suggest that once leucosome was segregated, equilibrium was not maintained between leucosome and melanosome.     

大陆造山带深熔垮塌的岩石学、地球化学证据:以北大别深熔混合岩为例

北大别位于大别造山带的核部,分布着大量的造山带垮塌时期形成的混合岩,其于理解大别造山带的形成和演化有着重要的意义。北大别混合岩的原岩为TTG（D）岩石,因黑云母和角闪石的脱水熔融诱发深熔作用产生。顺层产出的为富斜长石浅色体,主要矿物组成为斜长石+石英+黑云母±钾长石±角闪石。伟晶岩脉或团块为富钾长石浅色体,主要矿物组成为钾长石+石英±斜长石±黑云母±角闪石。暗色体为变晶结构,主要矿物组成为角闪石+黑云母+斜长石+石英±单斜辉石;其中,暗色矿物角闪石和黑云母常常定向排列,具有明显的溶蚀结构;暗色体中浅色矿物颗粒较小,以斜长石和石英为主,指示部分熔融的残余产物。全岩地球化学特征表明,碱金属元素（Na、K等）、大离子亲石元素（Ba、K、La等）和LREE等优先进入酸性熔体,而相容元素和中-重稀土元素等残留在残余体中。浅色体与本区花岗岩相比,二者都有右倾的稀土配分模式,富集LREE,亏损HREE。但浅色体具有明显的Eu正异常,δEu值为2.48~6.55,而花岗岩则有弱的Eu负异常,并且浅色体中大颗粒斜长石相互构成框架结构,含量明显高于正常花岗岩熔体,表明浅色体更可能是熔体早期结晶的产物。     

Shear Zone-hosted Migmatites (Eastern India): the Role of Dynamic Melting in the Generation of REE-depleted Felsic Melts, and Implications for Disequilibrium Melting

In the Ranmal migmatite complex, non-anatectic foliated graniteprotoliths can be traced to polyphase migmatites. Structural–microtexturalrelations and thermobarometry indicate that syn-deformationalsegregation–crystallization of in situ stromatic and diatexiteleucosomes occurred at 800°C and 8 kbar. The protolith,the neosome, and the mesosome comprise quartz, K-feldspar, plagioclase,hornblende, biotite, sphene, apatite, zircon, and ilmenite,but the modal mineralogy differs widely. The protolith compositionis straddled by element abundances in the leucosome and themesosome. The leucosomes are characterized by lower CaO, FeO+MgO,mg-number, TiO₂ , P₂O₅ , Rb, Zr and total rare earth elements(REE), and higher SiO₂ , K₂O, Ba and Sr than the protolith andthe mesosome, whereas Na₂O and Al₂O₃ abundances are similar.The protolith and the mesosome have negative Eu anomalies, butprotolith-normalized abundances of REE-depleted leucosomes showpositive Eu anomalies. The congruent melting reaction for leucosomeproduction is inferred to be 0·325 quartz+0·288K-feldspar+0·32 plagioclase+0·05 biotite+0·014hornblende+0·001 apatite+0·001 zircon+0·002sphene=melt. Based on the reaction, large ion lithophile element,REE and Zr abundances in model melts computed using dynamicmelting approached the measured element abundances in leucosomesfor >0·5 mass fraction of unsegregated melts withinthe mesosome. Disequilibrium-accommodated dynamic melting andequilibrium crystallization of melts led to uniform plagioclasecomposition in migmatites and REE depletion in leucosome. KEY WORDS: migmatite; REE; trace element; partial melting; P–T conditions     

Origin of K-poor leucosomes in a metasedimentary migmatite complex by ultrametamorphism, syn-metamorphic magmatism and subsolidus processes

Two types of stromatic leucosomes are identified in metasedimentary rocks from the Skagit migmatite complex, North Cascades, Washington state, U.S.A. Both types are trondhjemitic and appear similar in outcrop, but, although both contain low abundances of REE, one type consists of leucosomes that are relatively REE-enriched compared to the other, and contains (1) small (<0.8 mm), Fe-rich garnets that are compositionally and texturally different from mesosome and melanosome garnet; (2) Ti-rich minerals (rutile, titanite) that are not present in the groundmass of the associated mesosomes or melanosomes and (3) CO₂-rich fluid inclusions in quartz. Leucosomes of the second type are REE-depleted compared to the first type, lack garnet and Ti-minerals, and contain only H₂O-rich fluid inclusions. The first type of leucosome is interpreted to have formed by in situ partial melting accompanied, and perhaps initiated, by an influx of water-rich fluid during upper amphibolite facies metamorphism. These conclusions are based on estimates of metamorphic P-T-X_fluid conditions (9–10 kbar, > 700°C, water-rich fluid present), inferences about the origin of the above-listed mineralogical and fluid inclusion features, and modeling of leucosome trace element abundances. The second type of leucosome is interpreted to have formed entirely by subsolidus processes (e.g., metamorphic differentiation) because these leucosomes lack features consistent with an origin by partial melting.

K-poor (tonalitic/trondhjemitic) leucosomes associated with metasedimentary (biotite-bearing) source rocks may form by water-saturated partial melting or by subsolidus processes. Both general leucosome-forming mechanisms may operate at different times during upper amphibolite facies regional metamorphism. Partial melting may be initiated by syn-metamorphic magmatic activity if crystallizing plutons serve as external sources of the water-rich fluid necessary for ultrametamorphism in the middle crust during orogenesis. Large-scale migmatite complexes such as the Skagit migmatites may form at least in part in response to contact effects of plutonism associated with high-grade metamorphism, so, although migmatite complexes are a volumetrically substantial part of many orogenic belts, they may not themselves represent a significant original source of magma for larger-scale igneous bodies.     

What controls partial melting in migmatites?

Abstract The layers of six stromatic migmatites from Northern, Western, and Central Europe display small but systematic chemical and mineralogical differences. At least five of these migmatites do not show any signs of largescale metamorphic differentiation, metasomatism, or segregation of melts. It is concluded, therefore, that the compositional layering observed in most of the investigated migmatites is due to compositional differences inherited from the parent rocks. Almost isochemical partial melting seems to be the most probable process transforming layered paragneisses, metavolcanics, or schists into migmatites.
The formation of neosomes is believed to be caused by higher amounts of partial melts formed due to higher amounts of water moving into these layers. The neosomes have less biotite and more K-feldspar, if K-feldspar is present at all, than the adjacent mesosomes. These differences are small but systematic and seem to control the access of different amounts of water to the various rock portions. Petrographical observations, chemical data, and theoretical considerations indicate a close relationship between rock composition, rock deformation, transport of water, partial melting, and formation of layered migmatites.     

High-Pressure Dehydration Melting of Metapelites: Evidence from the Migmatites of Yaounde (Cameroon)

The migmatites of Yaound? consist essentially of anatectic metapelitickyanite-garnet gneisses characterized by granulite-facies mineralassemblages. Several types of migmatitic rocks have been recognized:(1) leucosomes associated with garnet-rich melanosomes, conformablewith the regional metamorphic layering; some leucosomes aregranitic in composition whereas some others are granodioriticand characterized by low K and Rb and by the lack of HREE fractionation;(2) quartzo-feldspathic differentiations without the relatedmelanosomes, occurring as veins conformable with or cross-cuttingthe regional metamorphic layering or along shear-zones, andcorresponding mineralogi-cally to granitic or quartz-rich v?ins;(3) garnet-rocks mainly composed of garnet with abundant accessories,occurring as intrusive bodies within the migmatitic series. Structural and petrographic data suggest that the migmatitesare not derived from the surrounding granulite-facies gneissesbut that both types of rock result from a single dehydrationmelting event. The formation of migmatites or gneisses, interpretedin terms either of absence of melt extraction or of shear-inducedmelt segregation, is ascribed to variations in strain distributionwithin the metamorphic pile. The chemical characteristics of the rocks and petrogenetic modellingsuggest that the migmatites of Yaounde arose from the superimpositionof the following events: (1) subsolidus differentiation of biotite-gneisses;(2) dehydration melting of biotite-gneisses at temperaturesaround 800?C (P=10–12 kbX leading to low amounts of melt(F<0?2), which was either tectonically segregated (migmatites)or not (granulite-facies gneisses); (3) injection of anatecticmaterial comprising both partial melts and garnet-rich residues,corresponding to high melt fractions (F>0?5) and probablyformed at higher temperatures (850?C) and at deeper structurallevels. The REE signature of equilibrium partial melts (9?3<Ce_N/Yb_N78;l?2<Yb_N<5?4) indicates that granitic magmas cannot bederived from dehydration melting of biotite-bearing metapelitesonly. Several other possibilities are discussed.     

Spatial distribution of melt-bearing structures in anatectic rocks from Southern Brittany, France: implications for melt transfer at grain- to orogen-scale

In this study, we present quantitative spatial information on the one- and two-dimensional distribution of inferred melt-bearing structures in anatectic supracrustal rocks of the Southern Brittany Migmatite Belt, south of the transcurrent South Armorican Shear Zone (SASZ); based on these data, we infer the mechanism of melt extraction from partially molten crust. Former melt-bearing structures include foliation-parallel leucosomes and cross-cutting granitic leucosomes that infill inter-boudin partitions and extensional shear surfaces, as well as discordant dykes of granite. Petrographic (i.e., mineralogical and microstructural) continuity of granite from structure to structure suggests that they once formed a continuous melt-bearing network. Measurements along one-dimensional line traverses perpendicular to layering of stromatic migmatite exposed in clean, sub-horizontal outcrop surfaces provide information about thickness and spacing distributions of foliation-parallel leucosomes. Most leucosome thicknesses fall in the range of 1–10 mm, with upper limits around 20–30 mm. The number of thicker layers decreases abruptly with increasing thickness, which is inconsistent with scale-invariance. This suggests that leucosome formation was controlled by short-range melt movement along grain boundaries to form melt-rich layers constrained by pre-existing compositional layering. Spacing distributions also are not scale-invariant; however, the large percentage of leucosomes (40–60%) in these line traverses suggests that spacing distributions may be controlled in part by impingement of leucosomes, making it difficult to derive genetic information from these data. Qualitative observation of inferred melt-bearing structures in mutually perpendicular two-dimensional exposures from the same outcrop reveals anisotropy of the leucosome network related to a well-developed sub-horizontal quartz–feldspar lineation reflecting stretching associated with transcurrent movement along the SASZ. Analysis of these two-dimensional distributions using the box-counting method corroborates the observed anisotropy, but indicates that leucosome morphology (and perhaps distribution) is not scale-invariant. The applicability of the box-counting method, or of fractal analysis, to understanding melt movement in migmatites is discussed in light of these results. Based on the anisotropy of melt-bearing structures, we infer that melt-movement in structures now represented by layer-parallel leucosomes was primarily sub-horizontal. These layers fed steeply dipping structures now represented by cross-cutting leucosomes, in particular those developed at inter-boudin partitions, and granite dykes. The formation and orientation of these steeply dipping structures was in part controlled by far-field stresses related to dextral displacement along the SASZ. Melt extraction is inferred to have occurred along these steeply dipping structures; extracted melt accumulated in plutons at higher crustal levels, such as the Quiberon, Sarzeau, and Guérande granites.     

Multi-stage Melting in the Lower Crust of the Serre (Southern Italy)

The lower-crustal section exposed in the Serre, southern Italy,consists mainly of Al-rich metasediments, which underwent granulite-faciesmetamorphism, partial melting and melt extraction. The paperconsiders the formation of melts in metapelites and metagreywackes.Leucosomes and host rocks have been studied to investigate themelting process. Biotite-rich and biotite-free melanosomes withscarce felsic components are present; the biotite-rich typesare widespread in the upper part of the section and the twotypes may occur side by side in the lower part. Na-rich andK-rich leucosomes including residual phases are interspersedwithin the metasediments; on the whole they do not show geochemicalsignatures suggestive of magmatic fractionation. Leucotonalitictypes prevail among the sampled leucosomes, which generallyare rare earth element (REE) depleted with positive Eu anomalieswhereas the host rocks are REE enriched with overall negativeEu anomalies. Melanosomes and migmatites show restitic chemistries.The precursor metagreywackes underwent depletion in Na₂O andenrichment in K₂O. The precursor metapelites document generaldepletion in Na₂O and they may be enriched or depleted in K₂O.All the characteristics of the migmatites and of their componentsreflect a two-stage melting: (1) H₂O-present melting, involvingmainly plagioclase, and (2) dehydration melting of micas. Allthe metasediments underwent H₂O-present melting, forming mostlysodic melts which, owing to their removal from the source asfast as they formed, did not accumulate in such proportionsas to allow migration and mostly remained within the lower-crustalmetasediments; metapelites also underwent variable dehydrationmelting, depending on chemical features and physical conditions,forming larger volumes of mobile granitic melts, most of whichmigrated far from the source. Extractions of 57–66 vol.% of total melts (sodic + potassic) from the most residual metapeliticmelanosomes and of about 27–44 vol. % of potassic meltsfrom metapelitic migmatites have been calculated. Higher volumesof the extracted melts have been calculated for the metapelitesof the lower part of the section; the most depleted metagreywackesunderwent melt extraction of about 9–13 vol. %. The two-stagemelting occurred during the prograde metamorphism and continuedduring the isothermal decompression. KEY WORDS: Calabria; lower crust; multi-stage melting     

REE distribution in some layered migmatites: constraints on their petrogenesis

The REE distributions in mesosomes, neosomes, leucosomes and melanosomes of four layered migmatites have been investigated. In one example (Arvika migmatites) the REE patterns in adjacent paragneisses, the presumed parent rock of the migmatites, were also determined. REE patterns of neosomes and mesosomes of Arvika migmatites are similar to the finegrained layers and coarse-grained layers, respectively, observed in the adjacent paragneiss. This is in agreement with the layer-by-layer paragneiss-migmatite transformation model.

The REE patterns of mesosomes and neosomes indicate that these lithologies may have been closed systems (for REE) during the formation of the migmatites. No indication of metasomatic reactions, melt segregation or injection could be detected. Within the neosomes, leucosomes are depleted and melanosomes enriched in REE contents. This is interpreted to be due to separation and concentration of accessory minerals (monazite, epidote, allanite, zircon, sphene, apatite, garnet) into the melanosomes. The behaviour of accessory minerals during migmatite formation is closely allied to that of biotite, which is also concentrated in the melanosomes.     

Melt segregation in the continental crust: distribution and movement of melt in anatectic rocks

The grain‐ and outcrop‐scale distribution of melt has been mapped in anatectic rocks from regional and contact metamorphic environments and used to infer melt movement paths. At the grain scale, anatectic melt is pervasively distributed in the grain boundaries and in small pools; consequently, most melt is located parallel to the principal fabric in the rock, typically a foliation. Short, branched arrays of linked, melt‐bearing grain boundaries connect melt‐depleted parts of the matrix to diffuse zones of melt accumulation (protoleucosomes), where magmatic flow and alignment of euhedral crystals grown from the melt developed. The distribution of melt (leucosome) and residual rocks (normally melanocratic) in outcrop provides different, but complementary, information. The residual rocks show where the melt came from, and the leucosomes preserve some of the channels through which the melt moved, or sites where it pooled. Different stages of the melt segregation process are recorded in the leucosome–melanosome arrays. Regions where melting and segregation had just begun when crystallization occurred are characterized by short arrays of thin, branching leucosomes with little melanosome. A more advanced stage of melting and segregation is marked by the development of residual rocks around extensive, branched leucosome arrays, generally oriented along the foliation or melting layer. Places where melting had stopped, or slowed down, before crystallization began are marked by a high ratio of melanosome to leucosome; because most of the melt has drained away, very few leucosomes remain to mark the melt escape path — this is common in melt‐depleted granulite terranes. Many migmatites contain abundant leucosomes oriented parallel to the foliation; mostly, these represent places where foliation planes dilated and melt drained from the matrix via the branched grain boundary and larger branched melt channel (leucosome) arrays collected. Melt collected in the foliation planes was partially, or fully, expelled later, when discordant leucosomes formed. Leucosomes (or veins) oriented at high angles to the foliation/layering formed last and commonly lack melanocratic borders; hence they were not involved in draining the matrix of the melting layer. Discordant leucosomes represent the channels through which melt flowed out of the melting layer.     

Migmatite formation at subsolidus conditions–an alternative to anatexis

Abstract This contribution discusses the formation of stromatic high-grade migmatites. Volume considerations require that most of the leucosome material is not added from outside the system. A segregation mechanism is necessary except in those cases where the protolith of the migmatite already had a banded structure. Although partial melting is most often advocated to provide the segregation mechanism, several arguments can be raised against high degrees of melting: mineral compositions and even zoning patterns are similar in both mesosomes and leucosomes; sufficient degrees of melting at reasonable temperatures require more than the available amounts of water; the leucosomes do not always approximate to a minimum melt composition; high degrees of melting cannot occur without an appreciable volume increase; etc. Diffusion works as a segregation mechanism at low temperatures. As diffusion rates increase exponentially with temperature diffusion must become still more important as a segregation mechanism at high temperatures. A model is suggested based on the diffusion of components in response to the gradient δσ/δx, where σ= 1/3∑³_i=1 σ_i is the mean pressure. In homogeneously strained rocks, σ₃ is larger in rock parts rich in incompetent phases than in rock parts depleted in incompetent phases. Accordingly, mechanically competent but chemically incompetent high-volume phases like quartz and feldspars stressed in micadominated parts of a rock (high σσ) migrate to parts of the rock that are depleted in mica (low σ¯). It is suggested that hornblende occurring in many leucosomes may be premigmatitic or early syn-migmatitic and due to its mechanical competency it initiates the segregation. Diffusion occurs along grain boundaries and is enhanced by small amounts of ‘intergranular fluid’;. At the best, semiquantitative estimates of diffusion rates and distances indicate that the process should work over geological times.     

Formation, Crystallization, and Migration of Melt in the Mid-orogenic Crust: Muskoka Domain Migmatites, Grenville Province, Ontario

Migmatitic orthogneisses in the Muskoka domain, southwesternGrenville Province, Ontario, formed during the Ottawan stage(c. 1080–1050 Ma) of the Grenvillian orogeny. Stromaticmigmatites are volumetrically dominant, comprising granodioriticgneisses with 2–5 cm thick granitic leucosomes, locallyrimmed by thin melanosomes, that constitute 20–30 vol.%, and locally 40–50 vol. %, of the outcrops. Patch migmatitesin dioritic gneisses form large (>10 m) pinch-and-swell structureswithin the stromatic migmatites, and consist of decimetre-scale,irregular patches of granitic leucosome, surrounded by medium-grainedhornblende–plagioclase melanosomes interpreted as restite.The patches connect to larger networks of zoned pegmatite dykes.Petrographic and geochemical evidence suggests that the patchleucosomes formed by 20–40% fluid-present, equilibriummelting of the dioritic gneiss, followed by feldspar-dominatedcrystallization. The dyke networks may have resulted from hydraulicfracturing, probably when the melts reached water saturationduring crystallization. Field and geochemical data from thestromatic migmatites suggest a similar petrogenesis to the patchmigmatites, but with significant additions of externally derivedmelts, indicating that they acted as conduits for melts derivedfrom deeper structural levels within the orogen. We hypothesizethat the Muskoka domain represents a transfer zone for meltsmigrating to higher structural levels during Grenvillian deformation. KEY WORDS: migmatite geochemistry; partial melting; melt crystallization; melt transport; Grenville orogen     

Zircon from the polymigmatites of the northwestern Ladoga region: Morphology and geochemistry

This paper reports the results of a geochemical investigation of zircon from a migmatized aluminous gneiss (gn), melanosome (M), and sequential leucosome generations (Lc₂, Lc₃, Lc₄, and Lc₅) from an outcrop in the northwestern Ladoga region. The contents of REE, Y, Ti, Hf, Th, U, and P were determined using a Cameca IMS-4f ion microprobe in 12 zircon grains from the aforementioned rocks, in two-three spots in each grain. All of the specimens show rather uniform REE distribution patterns. More significant variations were observed in the light and medium REE (at smaller variations in the heavy REE), as well as in Ti, Y, Th, and U contents between zircons from the host rocks and from the leucosomes. It was supposed that REE-rich zircons from the gneiss and melanosome (without oscillatory zoning) are relics, whereas rhythmically zoned zircons with lower REE contents crystallized in the gneiss in the presence of dispersed anatectic melt. The contents of most REE and Y increase from core to rim in zircons from the gneiss, melanosome, Lc₂, Lc₄, and Lc₅, which is opposite to the compositional trend of zircons from Lc₃. It was shown that the decrease of HREE and Y content in zircon in the sequence Lc₅ → gn → Lc₂, Lc₃, Lc₄ is related to a decrease in the abundance of these elements in the rocks. The leucosomes do not correspond to a differentiation series of a single melt (there is no variation trends of Rb/Sr, K/Rb, and Rb/Ba in the rock series). The lower Lu/Hf and Sm/Nd values in the leucosomes relative to the host rocks allowed us to suppose that their protolith was gneisses (for Lc₂) and migmatites (for Lc₄ and Lc₃). The similarity of the early migmatites and gneisses to Lc₃ with respect to major and some trace elements and almost identical Lu/Hf and Sm/Nd values support the possibility of the formation of this leucosome generation during the beginning of the diatexis of migmatites, which was promoted by a temperature increase. This resulted in a specific trend in the content of some elements during zircon growth in Lc₃, which is different from the trend of zircons from other leucosomes.     

Segregation of leucogranite microplutons during syn-anatectic deformation: an example from the Taylor Valley, Antarctica

A suite of migmatites in uppermost amphibolite facies schists of the Koettlitz Group exposed in the Taylor Valley, Antarctica, provides direct evidence of the behaviour of partially molten rock during syn-anatectic deformation. The geometry of the migmatites is directly related to their position relative to the hinge of a kilometre-scale antiform. Migmatitic rocks on the fold limbs are characterized by extensional shears and fractures, filled with leucosome material, that intersect the pervasive foliation and millimetre-thick stromatic leucosomes. Vein- and dyke-like leucosomes become more common and thicker from the limb to the hinge region of the antiform. Rocks characterized by high leucosome-to-rock ratios near the antiform hinge are xenolithic in appearance. Major parasitic folds within the hinge contain leucogranite 'microplutons' up to 50 m across beneath refractory 'cap-rock' layers.
Angular boudinage structures in schists surrounded by leucosomes indicate a relatively low yield strength in the leucosome, which is compatible with a molten rather than solid leucosome. Leucogranite-bearing extensional shears and fractures indicate that repeated extensional fracturing and shearing promoted by high fluid (melt) pressure is an important mechanism of melt segregation. Dilation in the hinges of developing folds aids the migration of melt into fold hinges and the development of 10–50-m-wide 'microplutons' of xenolith-rich leucogranite.
Lack of vapour-absent melting and consequent low melt-to-rock ratios allowed the Koettlitz Group to maintain its structural coherency on a kilometre scale. Consequently, leucosome 'microplutons' did not exceed 50 m in width, and therefore observed leucosomes have not contributed to the development of adjacent plutonic-scale granitoids.