Partial melting in the Inzie Head gneisses: the role of water and a petrogenetic grid in KFMASH applicable to anatectic pelitic migmatites

A sequence of prograde isograds is recognized within the Dalradian Inzie Head gneisses where pelitic compositions have undergone variable degrees of partial melting via incongruent melting reactions consuming biotite. Three leucosome types are identified. At the lowest grades, granitic leucosomes containing porphyroblasts of cordierite (CRD‐melt) are abundant. At intermediate grades, CRD‐melt mingles with garnetiferous leucosomes (GT‐melt). At the highest grades, CRD‐melt coexists with orthopyroxene‐bearing leucosomes (OPX‐melt), while garnet is conspicuously absent. The prograde metamorphic field gradient is constrained to pressures of 2–3 kbar below the CRD‐melt isograd, and no greater than 4.5 kbar at the highest grade around Inzie Head. A petrogenetic grid, calculated using thermocalc , is presented for the K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (KFMASH) system for the phases orthopyroxene, garnet, cordierite, biotite, sillimanite, H₂O and melt with quartz and K‐feldspar in excess. For the implied field gradient, the reaction sequence predicted by the grid is consistent with the successive prograde development of each leucosome type. Compatibility diagrams suggest that, as anatexis proceeded, bulk compositions may have been displaced towards higher MgO content by the removal of (relatively) ferroan granitic leucosome. An isobaric (P = 4 kbar) T–a_H2O diagram shows that premigmatization fluids must have been water‐rich (a_H2O > 0.85) and suggests that, following the formation of small volumes of CRD‐melt, the system became fluid‐absent and melting reactions buffered a_H2O to lower values as temperatures rose. GT‐ and OPX‐melt formed by fluid‐absent melting reactions, but a maximum of 7–11% CRD‐melt fraction can be generated under fluid‐absent conditions, much less than the large volumes observed in the field. There is strong evidence that the CRD‐melt leucosomes could not have been derived by buoyantly aided upwards migration from levels beneath the migmatites. Their formation therefore required a significant influx of H₂O‐rich fluid, but in a quantity insufficient to have exhausted the buffering capacity of the solid assemblage plus melt. Fluid : rock ratios cannot have exceeded 1 : 30. The fluid was channelled through a regionally extensive shear zone network following melt‐induced failure. Such an influx of fluid at such depths has obvious consequences for localized crustal magma production and possibly for cordierite‐bearing granitoids in general.     

Transformation of two-pyroxene hornblende granulite to garnet granulite involving simultaneous melting and fracturing of the lower crust, Fiordland, New Zealand

Granulite facies gabbroic and dioritic gneisses in the Pembroke Valley, Milford Sound, New Zealand, are cut by vertical and planar garnet reaction zones in rectilinear patterns. In gabbroic gneiss, narrow dykes of anorthositic leucosome are surrounded by fine‐grained garnet granulite that replaced the host two‐pyroxene hornblende granulite at conditions of 750 °C and 14 kbar. Major and trace element whole‐rock geochemical data indicate that recrystallization was mostly isochemical. The anorthositic veins cut contacts between gabbroic gneiss and dioritic gneiss, but change in morphology at the contacts, from the anorthositic vein surrounded by a garnet granulite reaction zone in the gabbroic gneiss, to zones with a septum of coarse‐grained garnet surrounded by anorthositic leucosome in the dioritic gneiss. The dioritic gneiss also contains isolated garnet grains enclosed by leucosome, and short planar trains of garnet grains linked by leucosome. Partial melting of the dioritic gneiss, mostly controlled by hornblende breakdown at water‐undersaturated conditions, is inferred to have generated the leucosomes. The form of the leucosomes is consistent with melt segregation and transport aided by fracture propagation; limited retrogression suggests considerable melt escape. Dyking and melt escape from the dioritic gneiss are inferred to have propagated fractures into the gabbroic gneiss. The migrating melt scavenged water from the surrounding gabbroic gneiss and induced the limited replacement by garnet granulite.     

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.     

Separating metamorphic events in the Fosdick migmatite–granite complex,West Antarctica

The Fosdick migmatite–granite complex in West Antarctica records evidence for two high‐temperature metamorphic events, the first during the Devonian–Carboniferous and the second during the Cretaceous. The conditions of each high‐temperature metamorphic event, both of which involved melting and multiple melt‐loss events, are investigated using phase equilibria modelling during successive melt‐loss events, microstructural observations and mineral chemistry. In situ SHRIMP monazite and TIMS Sm–Nd garnet ages are integrated with these results to constrain the timing of the two events. In areas that preferentially preserve the Devonian–Carboniferous (M₁) event, monazite grains in leucosomes and core domains of monazite inclusions in Cretaceous cordierite yield an age of c. 346 Ma, which is interpreted to record the timing of monazite growth during peak M₁ metamorphism (～820–870 °C, 7.5–11.5 kbar) and the formation of garnet–sillimanite–biotite–melt‐bearing assemblages. Slightly younger monazite spot ages between c. 331 and 314 Ma are identified from grains located in fractured garnet porphyroblasts, and from inclusions in plagioclase that surround relict garnet and in matrix biotite. These ages record the growth of monazite during garnet breakdown associated with cooling from peak M₁ conditions. The Cretaceous (M₂) overprint is recorded in compositionally homogeneous monazite grains and rim domains in zoned monazite grains. This monazite yields a protracted range of spot ages with a dominant population between c. 111 and 96 Ma. Rim domains of monazite inclusions in cordierite surrounding garnet and in coarse‐grained poikiloblasts of cordierite yield a weighted mean age of c. 102 Ma, interpreted to constrain the age of cordierite growth. TIMS Sm–Nd ages for garnet are similar at 102–99 Ma. Mineral equilibria modelling of the residual protolith composition after Carboniferous melt loss and removal of inert M₁ garnet constrains M₂ conditions to ～830–870 °C and ～6–7.5 kbar. The modelling results suggest that there was growth and resorption of garnet during the M₂ event, which would facilitate overprinting of M₁ compositions during the M₂ prograde metamorphism. Measured garnet compositions and Sm–Nd diffusion modelling of garnet in the migmatitic gneisses suggest resetting of major elements and the Sm–Nd system during the Cretaceous M₁ overprint. The c. 102–99 Ma garnet Sm–Nd ‘closure’ ages correspond to cooling below 700 °C during the rapid exhumation of the Fosdick migmatite–granite complex.     

Insights into the complexity of crustal differentiation: K2O‐poor leucosomes within metasedimentary migmatites from the Southern Marginal Zone of the Limpopo Belt,South Africa

Differentiation of the continental crust is the result of complex interactions between a large number of processes, which govern partial melting of the deep crust, magma formation and segregation, and magma ascent to significantly higher crustal levels. The anatectic metasedimentary rocks exposed in the Southern Marginal Zone of the Limpopo Belt represent an unusually well‐exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field. The formation of these migmatites occurred via absent incongruent melting reactions involving biotite, which produced cm‐ to m‐scale, K₂O‐poor garnet‐bearing stromatic leucosomes, with high Ca/Na ratios relative to their source rocks. Field investigation combined with geochemical analyses, and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution: (1) Melting of a portion of the source, with restricted plagioclase availability due to kinetic controls, to produce a magma (melt + entrained peritectic minerals in variable proportions relative to melt); (2) Segregation of the magma at near peak metamorphic conditions into melt accumulation sites (MAS), also known as future leucosome; (3a) Re‐equilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion (H₂O, K₂O), which triggers the co‐precipitation of quartz and plagioclase in the MAS; (3b) Extraction of melt‐dominated magma to higher crustal levels, leaving peritectic minerals entrained from the site of the melting reaction, and the minerals precipitated in the MASs to form the leucosome in the source. The key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction. This results in elevated melt H₂O and K₂O and chemical potential gradient for these components across the leucosome/mafic residuum contact. The combination of all of these processes accurately explains the composition of the K₂O‐poor leucosomes. These findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which, according to the chemical modelling, is <5 years. We demonstrate that in some migmatitic granulites, the leucosomes constitute a type of felsic refractory residuum, rather than evidence of failed magma extraction. This provides a new insight into the ways that source heterogeneity may control anatexis.     

Monazite as a monitor for melt‐rock interaction during cooling and exhumation

Granulite facies cordierite–garnet–biotite gneisses from the southeastern Reynolds Range, central Australia, contain both orthopyroxene‐bearing and orthopyroxene‐free quartzofeldspathic leucosomes. Mineral reaction microstructures at the interface of gneiss and leucosome observed in outcrop and petrographically, reflect melt‐rock interaction during crystallization. Accessory monazite, susceptible to fluid alteration, dissolution and recrystallization at high temperature, is tested for its applicability to constrain the chemical and P–T–time evolution of melt‐rock reactions during crystallization upon cooling. Bulk rock geochemistry and phase equilibria modelling constrain peak pressure and temperature conditions to 6.5–7.5 kbar and ~850°C, and U–Pb geochronology constrains the timing of monazite crystallization to 1.55 Ga, coeval with the Chewings Orogeny. Modelling predicts the presence of up to 15 vol.% melt at peak metamorphic conditions. Upon cooling below 800°C, melt extraction and in situ crystallization of melt decrease the melt volume to less than 7%, at which time it becomes entrapped and melt pockets induce replacement reactions in the adjacent host rock. Replacement reactions of garnet, orthopyroxene and K‐feldspar liberate Y, REE, Eu and U in addition to Mg, Fe, Al, Si and K. We demonstrate that distinguishing between monazite varieties solely on the basis of U–Pb ages cannot solve the chronological order of events in this study, nor does it tie monazite to the evolution of melt or stability of rock‐forming minerals. Rather, we argue that analyses of various internal monazite textures, their composition and overprinting relations allow us to identify the chronology of events following the metamorphic peak. We infer that retrograde reactions involving garnet, orthopyroxene and K‐feldspar can be attributed to melt‐rock interaction subsequent to partial melting, which is reflected in the development of compositionally distinct monazite textural domains. Internal monazite textures and their composition are consistent with dissolution and precipitation reactions induced by a high‐T melt. Monazite rims enriched in Y, HREE, Eu and U indicate an increased availability of these elements, consistent with the breakdown of orthopyroxene, garnet and K‐feldspar observed petrographically. Our study indicates that compositional and textural analysis of monazite in relation to major rock‐forming minerals can be used to infer the post‐peak chemical evolution of partial melts during high‐ to ultrahigh‐temperature metamorphism.     

东南极拉斯曼丘陵长英质片麻岩的深熔作用与铁钛氧化物的聚集机制

东南极拉斯曼丘陵高级变质长英质岩石中铁钛氧化物的局部聚集与高级变质作用过程中的深熔作用有关,并非原岩富集这些组分。深熔作用造成惰性组分如铁钛氧化物滞留原地或略有聚集及活动性组分的迁移,而流体挥发组分优先聚集于熔体之中。当体系中水含量较低、处于不饱和状态时,深熔作用过程中形成局部"熔体",其结晶所成的浅色体不具低共结组分,没有熔体结晶结构,不是真正的熔体,可能是(准)熔体。较粗粒的浅色体或伟晶岩也是与深熔作用有关的产物,其形成早于花岗岩脉或岩体,而与花岗质岩浆分异无关。伴随(准)熔体的出现,体系中组分的萃取、分异效果较为明显,即可造成组分分异,形成截然不同的异地、二相分异结构,分别形成固相残留物(组成可以不固定)和(准)熔体相。固相残留体中富铝、铁组分,形成矽线石和铁钛氧化物团块,其中少或无挥发分;与此对应,短距离迁移浅色体中往往贫铁钛组分,可见石榴子石、偶见铁钛氧化物矿物。这种挥发分不饱和状态下的深熔作用基本属于封闭体系,整体失水不显著,高级变质岩中的一些特征矿物如矽线石、石榴子石、堇青石、尖晶石的形成也与这种分异作用有关,但组分迁移范围有限,并可保存组分分异各阶段的产物。拉斯曼丘陵长英质岩系中大量铁钛氧化物和矽线石类矿物组合的形成,反映了临界状态下的局部或差异抬升,变形作用的非均匀性及相伴随的组分分异作用,很可能相当于早期格林维尔期构造的泛非期再活动。      

The processes that control leucosome compositions in metasedimentary granulites: perspectives from the Southern Marginal Zone migmatites,Limpopo Belt,South Africa

Anatexis of metapelitic rocks at the Bandelierkop Quarry (BQ) locality in the Southern Marginal Zone of the Limpopo Belt occurred via muscovite and biotite breakdown reactions which, in order of increasing temperature, can be modelled as: (1) Muscovite + quartz + plagioclase = sillimanite + melt; (2) Biotite + sillimanite + quartz + plagioclase = garnet + melt; (3) Biotite + quartz + plagioclase = orthopyroxene ± cordierite ± garnet + melt. Reactions 1 and 2 produced stromatic leucosomes, which underwent solid‐state deformation before the formation of undeformed nebulitic leucosomes by reaction 3. The zircon U–Pb ages for both leucosomes are within error identical. Thus, the melt or magma formed by the first two reactions segregated and formed mechanically solid stromatic veins whilst temperature was increasing. As might be predicted from the deformational history and sequence of melting reactions, the compositions of the stromatic leucosomes depart markedly from those of melts from metapelitic sources. Despite having similar Si contents to melts, the leucosomes are strongly K‐depleted, have Ca:Na ratios similar to the residua from which their magmas segregated and are characterized by a strong positive Eu anomaly, whilst the associated residua has no pronounced Eu anomaly. In addition, within the leucosomes and their wall rocks, peritectic garnet and orthopyroxene are very well preserved. This collective evidence suggests that melt loss from the stromatic leucosome structures whilst the rocks were still undergoing heating is the dominant process that shaped the chemistry of these leucosomes and produced solid leucosomes. Two alternative scenarios are evaluated as generalized petrogenetic models for producing Si‐rich, yet markedly K‐depleted and Ca‐enriched leucosomes from metapelitic sources. The first process involves the mechanical concentration of entrained peritectic plagioclase and garnet in the leucosomes. In this scenario, the volume of quartz in the leucosome must reflect the remaining melt fraction with resultant positive correlation between Si and K in the leucosomes. No such correlation exists in the BQ leucosomes and in similar leucosomes from elsewhere. Consequently, we suggest disequilibrium congruent melting of plagioclase in the source and consequential crystallization of peritectic plagioclase in the melt transfer and accumulation structures rather than at the sites of biotite melting. This induces co‐precipitation of quartz in the structures by increasing SiO₂ content of the melt. This process is characterized by an absence of plagioclase‐induced fractionation of Eu on melting, and the formation of Eu‐enriched, quartz + plagioclase + garnet leucosomes. From these findings, we argue that melt leaves the source rapidly and that the leucosomes form incrementally as melt or magma leaving the source dumps its disequilibrium Ca load, as well as quartz and entrained ferromagnesian peritectic minerals, in sites of magma accumulation and escape. This is consistent with evidence from S‐type granites suggesting rapid magma transfer from source to high level plutons. These findings also suggest that leucosomes of this type should be regarded as constituting part of the residuum from partial melting.     

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.     

The pressure–temperature–time path of migmatites from the Sikkim Himalaya

A combined metamorphic and isotopic study of lit‐par‐lit migmatites exposed in the hanging wall of the Main Central Thrust (MCT) from Sikkim has provided a unique insight into the pressure–temperature–time path of the High Himalayan Crystalline Series of the eastern Himalaya. The petrology and geochemistry of one such migmatite indicates that the leucosome comprises a crystallized peraluminous granite coexisting with sillimanite and alkali feldspar. Large garnet crystals (2–3 mm across) are strongly zoned and grew initially within the kyanite stability field. The melanosome is a biotite–garnet pelitic gneiss, with fibrolitic sillimanite resulting from polymorphic inversion of kyanite. By combining garnet zoning profiles with the NaCaMnKFMASHTO pseudosection appropriate to the bulk composition of a migmatite retrieved from c. 1 km above the thrust zone, it has been established that early garnet formed at pressures of 10–12 kbar, and that subsequent decompression caused the rock to enter the melt field at c. 8 kbar and c. 750 °C, generating peritectic sillimanite and alkali feldspar by the incongruent melting of muscovite. Continuing exhumation resulted in resorption of garnet. Sm–Nd growth ages of garnet cores and rim, indicate pre‐decompression garnet growth at 23 ± 3 Ma and near‐peak temperatures during melting at 16 ± 2 Ma. This provides a decompression rate of 2 ± 1 mm yr^?1 that is consistent with exhumation rates inferred from mineral cooling ages from the eastern Himalaya. Simple 1D thermal modelling confirms that exhumation at this rate would result in a near‐isothermal decompression path, a result that is supported by the phase relations in both the melanosome and leucosome components of the migmatite. Results from this study suggest that anatexis of Miocene granite protoliths from the Himalaya was a consequence of rapid decompression, probably in response to movement on the MCT and on the South Tibetan detachment to the north.     

Orthophosphate and biotite chemistry from orthopyroxene-bearing migmatites from California and South India: The role of a fluid-phase in the evolution of granulite-facies migmatites

Migmatites from Cone Peak, California, USA and the Satnur-Sangam road, Southern Karnataka, India contain coarser grained orthopyroxene-bearing leucosomes with subordinate biotite in finer grained hornblende-biotite-pyroxene-bearing hosts. At both localities the leucosomes are enriched in quartz and feldspar and have a higher ratio of pyroxene to hornblende + biotite compared to the host rocks. Biotite grains in leucosomes along the Satnur-Sangam road are concentrated at the margins of orthopyroxene grains and have lower abundances of Ti, Fe, and Cl and a higher abundance of F than biotite grains from the host rock. Fluorapatite grains in all rocks from both localities contain monazite inclusions similar to those produced experimentally by metasomatically induced dissolution and reprecipitation. Some fluorapatite grains at both localities are partially rimmed by allanite. The only compositional differences found between fluorapatite grains in the leucosomes and host rocks were higher concentrations of Cl in grains in leucosomes from Cone Peak. The mineralogies of the rocks suggest that the leucosomes formed by dehydration melting reactions that consumed feldspar, quartz, hornblende, and biotite and produced orthopyroxene. Allanite rims at the margins of fluorapatite grains may have formed by the later retrogression of monazite rims formed by incongruent dissolution of fluorapatite in the melt. Biotite grains at the margins of orthopyroxene crystals in the leucosomes from the Satnur-Sangam road apparently formed by retrogression of orthopyroxene upon the solidification of the anatectic melt. A similar high-grade retrogression did not affect orthopyroxene crystals at Cone Peak, indicating that H₂O was removed from the crystallizing leucosomes probably in a low H₂O activity fluid. Compositional differences between the paleosome and neosomes at Cone Peak are best explained by metasomatic interaction with concentrated brines while elevated Cl concentrations in fluorapatites in the leucosome suggest interaction with a Cl-bearing fluid. Brines may have been responsible for an exchange of elements between the host rock along the Satnur-Sangam road and zones of melt generation now marked by leucosomes, but fluid flow appears to have been less vigorous than at Cone Peak.     

Two-stage growth of porphyroblastic biotite and garnet in the Barrovian metapelites of the Imjingang belt, central Korea

Porphyroblastic biotite and garnet in the Barrovian metapelites of the Imjingang belt, Korea, were investigated to unravel the sequence and mechanism of mineral growth. Poikiloblastic biotite contains straight inclusion trails (S_i) discontinuous to the major foliation, and develops clear zones at the grain margin. These microstructures suggest an initial growth of biotite between two contractional deformations (D_n−1 and D_n) followed by an overgrowth during D_n. Although garnet poikiloblasts contain variable S_i patterns, their major growth is likely to have occurred during D_n on the basis of compositional relationships among variable garnet types. Early poikiloblasts of both minerals were formed by chemical replacement of the matrix that consisted mainly of chlorite, muscovite and quartz. Subsequent growth of biotite was governed by a crack-filling mechanism, and was accompanied by the production of extensional cracks inside or around biotite, providing fluid pathways. The overgrowth of garnet was favoured at the biotite–garnet interface, and the consequence was a partial replacement of inclusion-poor garnet after biotite subsequent to D_n. In addition, clear zones and pressure shadows as well as the matrix around biotite porphyroblasts were replaced by garnet, suggesting an inheritance of various pre-existing microstructures in the S_i pattern of garnet. Further attention is thus required for any attempt to delineate the microstructural interaction between deformation and metamorphism, particularly in a sample containing early-grown porphyroblasts. Microstructural evidence for the two-stage growth of biotite and garnet is present up to the kyanite zone, indicating that this growth mechanism is prevalent during progressive metamorphism of Barrovian metapelites.     

Constraints on the timing and conditions of high‐grade metamorphism,charnockite formation and fluid–rock interaction in the Trivandrum Block,southern India

Incipient charnockites have been widely used as evidence for the infiltration of CO₂‐rich fluids driving dehydration of the lower crust. Rocks exposed at Kakkod quarry in the Trivandrum Block of southern India allow for a thorough investigation of the metamorphic evolution by preserving not only orthopyroxene‐bearing charnockite patches in a host garnet–biotite felsic gneiss, but also layers of garnet–sillimanite metapelite gneiss. Thermodynamic phase equilibria modelling of all three bulk compositions indicates consistent peak‐metamorphic conditions of 830–925 °C and 6–9 kbar with retrograde evolution involving suprasolidus decompression at high temperature. These models suggest that orthopyroxene was most likely stabilized close to the metamorphic peak as a result of small compositional heterogeneities in the host garnet–biotite gneiss. There is insufficient evidence to determine whether the heterogeneities were inherited from the protolith or introduced during syn‐metamorphic fluid flow. U–Pb geochronology of monazite and zircon from all three rock types constrains the peak of metamorphism and orthopyroxene growth to have occurred between the onset of high‐grade metamorphism at c. 590 Ma and the onset of melt crystallization at c. 540 Ma. The majority of metamorphic zircon growth occurred during protracted melt crystallization between c. 540 and 510 Ma. Melt crystallization was followed by the influx of aqueous, alkali‐rich fluids likely derived from melts crystallizing at depth. This late fluid flow led to retrogression of orthopyroxene, the observed outcrop pattern and to the textural and isotopic modification of monazite grains at c. 525–490 Ma.     

Structural,petrological and chemical analysis of syn‐kinematic migmatites: insights from the Western Gneiss Region,Norway

Evidence of melting is presented from the Western Gneiss Region (WGR) in the core of the Caledonian orogen, Western Norway and the dynamic significance of melting for the evolution of orogens is evaluated. Multiphase inclusions in garnet that comprise plagioclase, potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies. The multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting, such as segregations in mafic rocks, leucosomes and pegmatites hosted in mafic rocks and in gneisses. Based on field studies, these lithologies are found in three structural positions: (i) as zoned segregations found in high‐P (ultra)mafic bodies; (ii) as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss; and (iii) as undeformed pegmatites cutting the main Caledonian structures. Segregations post‐date the eclogite facies foliation and pre‐date the amphibolite facies deformation, whereas leucosomes are contemporaneous with the amphibolite facies deformation, and undeformed pegmatites are post‐kinematic and were formed at the end of the deformation history. The geochemistry of the segregations, leucosomes and pegmatites in the WGR defines two trends, which correlate with the mafic or felsic nature of the host rocks. The first trend with Ca‐poor compositions represents leucosome and pegmatite hosted in felsic gneiss, whereas the second group with K‐poor compositions corresponds to segregation hosted in (ultra)mafic rocks. These trends suggest partial melting of two separate sources: the felsic gneisses and also the included mafic eclogites. The REE patterns of the samples allow distinction between melt compositions, fractionated liquids and cumulates. Melting began at high pressure and affected most lithologies in the WGR before or during their retrogression in the amphibolite facies. During this stage, the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward‐buoyant crust. Partial melting of both felsic and mafic sources at temperatures below 800 °C implies the presence of an H₂O‐rich fluid phase at great depth to facilitate H₂O‐present partial melting.     

Contrasting behaviour of rare earth and major elements during partial melting in granulite facies migmatites, Wuluma Hills, Arunta Block, central Australia

Low‐P granulite facies metapelitic migmatites in the Wuluma Hills, Strangways Metamorphic Complex, Arunta Block, preserve evidence of polyphase deformation and migmatite formation which is of the same age of the c. 1730 Ma Wuluma granite. Mineral equilibria modelling of garnet‐orthoproxene‐cordierite‐bearing assemblages using thermocalc is consistent with peak S3 conditions of 6.0–6.5 kbar and 850–900 °C. The growth of orthopyroxene and garnet was primarily controlled by biotite breakdown during partial melting reactions. Whereas orthopyroxene in the cordierite‐biotite mesosome shows enrichment of heavy‐REE (HREE) relative to medium‐REE (MREE), orthopyroxene in adjacent garnet‐bearing leucosome shows depletion of HREE relative to MREE. There is no appreciable difference in major element contents of minerals common to both the mesosome and leucosome. The REE variations can be satisfactorily explained by decoupling of major element and REE partitioning, in the context of appropriate phase‐equilibria modelling of a prograde path at ～6 kbar. Sparse garnet nucleii formed at ～760 °C, along with concentrated leucosome development and preferentially partitioned HREE. Further heating to ～800 °C at constant or subtly increasing pressure conditions additionally stabilized orthopyroxene and decreased the garnet mode. Orthopyroxene in the leucosome inherited an REE pattern consequent to the partial consumption of garnet, it being distinct from the REE pattern in mesosome orthoproxene that was mostly controlled by biotite breakdown. Such within‐sample variability in the enrichment of heavy REE indicates that caution needs to be exercised in the application of common elemental partitioning coefficients in spatially complex metamorphic rocks.     

Trace element behaviour during migmatization. Evidence for a complex melt-residuum-fluid interaction in the St. Malo migmatitic dome (France)

The St. Malo migmatitic dome represents an interesting example wherein migmatites arise from the anatexis of the surrounding gneisses. Petrographical and chemical data suggest that leucosome compositions are compatible with partial melting of the quartzo-feldsphathic fraction of the parent gneiss. The contribution of the incongruent melting of biotite to the melt does not exceed 5% of the parent rock.Petrogenetic modelling based on experimental data and assuming non modal batch melting show that the K, Rb, Ca, Sr, U and Th chemical patterns of these migmatites result in fact from the interaction of several mechanisms, namely: equilibrium partial melting, mixing between melts and refractory minerals (biotite and accessories), melt removal and late hydrothermal alteration. Zr, Y and Th which are mostly hosted in accessory minerals are significantly withheld from the melts and remain stored in melanosomes (metatexites) except when leucosomes are affected by mixing (diatexites). U is frequently enriched in the leucosomes as well as in some melanosomes suggesting external supply.     

Chemical characteristics of migmatites: accessory phase distribution and evidence for fast melt segregation rates

 Equilibration between melt and solid is inhibited by rapid melt extraction and by restricted equilibration (armouring, slow dissolution). When segregation occurs by channelised migration along high-porosity pathways, melt migration is more rapid than trace element diffusion rates in silicates and faster than accessory phase dissolution rates. Evidence for channelised flow and deformation-enhanced melt segregation into boudin necks, fractures and micro-shears at low melt fractions is present in the Moine Kirtomy Migmatitie Suite (KMS) in Sutherland, Scotland. Melt migration distances are on a metre to tens of metres scale. Concordant leucosomes in stromatic migmatities in the KMS have low Zr contents, low LREE (light rare-earth element) and H (heavy) REE contents and positive Eu anomalies. REE patterns of this type can be produced by removal of leucosome before complete equilibration with source due to the inhibited dissolution of LREE- and HREE-bearing accessory phases in water-undersaturated melts. Melting in the KMS, however, occurred at or near the wet granite solidus, leaving biotite as a residual phase. Detailed back-scattered electron imaging shows that REE-bearing accessory phases remained as residual phases, and were concentrated in the melanosome and at the melanosome-leucosome boundary. Irregularly shaped patches of diatexite contain a small proportion of excess Zr, consistent with entrainment of melanosome-schlieren enriched in zircon. These data indicate that deformation-enhanced melt extraction led to the rapid migration of small melt fractions from the melting site on a time-scale less than that required to saturate the melt in Zr. Leucosomes were thus prevented from equilibrating with accessory phases before extraction. Received: 12 July 1995 / Accepted: 4 March 1996     

The Role of Partial Melting and Fractional Crystallization in Determining Discordant Migmatite Leucosome Compositions

Anatectic migmatite leucosomes in the Quetico MetasedimentaryBelt (Superior Province) are discordant to the host rock layering.Two morphological varieties within the anatectic leucosome suiteare distinguished. The first type show little compositionalor textural variation either across, or along, the leucosomes.In contrast, the second variety exhibits both compositionaland textural variations in a single leucosome, typically withinternal cross-cutting relationships. Major-oxide contents varycomparatively little in the Quetico anatectic leucosome suite,but there is a considerable range in the incompatible element(REE, Hf, Zr, Y and Th) concentrations. In particular La contentsrange from 1.8 to 78.1 p.p.m. and the La/Yb ratios from 9.1to 101.9. Samples with high REE contents have negative Eu anomalies,whereas those with low total REE abundances have positive Euanomalies, which indicate that feldspar fractionation was importantin their petrogenesis. Three samples which have no Eu anomalies,and which are taken not to have experienced significant feldsparfractionation, are regarded as the closest approximation toa primary melt composition. Petrographic evidence indicates that only the most aluminousbulk compositions in the host rocks have melted, with cordieriteand biotite as the principal residual phases. Batch partialmelting models indicate that the three leucosomes without Euanomalies could have been derived from 40–80 per centpartial melting of the aluminous metasediments, but garnet musthave been a residual phase. Since the residuum from 40 per centpartial melting is more mafic than any of the rocks currentlyexposed in the area, it is concluded that the melting whichgave rise to the leucosomes occurred at greater depth. Crystallization models indicate that the observed range of leucosomecompositions can be derived by crystal fractionation of meltcompositions similar to the three leucosomes lacking Eu anomalies(i.e. the assumed primary melts). Samples with high abundancesof incompatible elements and negative Eu anomalies representfractionated melts, whereas those with low levels of REE andpositive Eu anomalies represent cumulates. Leucosome composition,morphology and texture can be related to crystallization history,notably the timing of crystallization with respect to leucosomeintrusion. In particular, those leucosomes that exhibit compositionaland textural zoning are interpreted to have undergone crystalfractionation during intrusion. Although a suite of migmatite leucosomes may be derived by partialmelting, it is concluded that the trace-element compositionof any particular leucosome depends, to a great extent, uponits segregation and crystallization history. Indeed, the primarymelt composition may not be preserved.     

Ultra-high-temperature metamorphism in Central Brazil: the Barro Alto complex

The Barro Alto complex, central Brazil, is a layered mafic–ultramafic intrusion, which was subjected to granulite facies metamorphism during the Neoproterozoic. Ultra-high-temperature conditions are recorded by parageneses that occur in some lenses of quartz-bearing rock (metagranite, metapelite and impure quartzite). The peak paragenesis consists of spinel+quartz±cordierite±leucosome (recording the former presence of melt with quartz in excess), which is replaced by either orthopyroxene+sillimanite or garnet+sillimanite. Quartz+biotite±sillimanite±garnet symplectites are ubiquitous and indicate reactions between Fe–Mg phases and melt. Late kyanite porphyroblasts have overgrown these symplectites. The direct replacement of spinel+quartz±cordierite by orthopyroxene+sillimanite or garnet+sillimanite occurred around the [Sa] invariant point, which appears only in a petrogenetic grid with inverted topology. The topology inversion occurs at conditions of high oxygen fugacity or due to the presence of ZnO-bearing spinel. Minimum peak conditions of ultra-high-temperature metamorphism were calculated as c. 980 °C and c. 7.9 kbar. The succession of observed mineral textures can be explained by a near-isobaric cooling P–T  path, with a cooling stage occurring between c. 980 and 750 °C.     

High to ultrahigh temperature contact metamorphism and dry partial melting of the Tasiuyak paragneiss,Northern Labrador

Contact aureoles of the anorthositic to granitic plutons of the Mesoproterozoic Nain Plutonic Suite (NPS), Labrador, are particularly well developed in the Palaeoproterozoic granulite facies, metasedimentary, Tasiuyak gneiss. Granulite facies regional metamorphism (M_R), c. 1860 Ma, led to biotite dehydration melting of the paragneiss and melt migration, leaving behind biotite‐poor, garnet–sillimanite‐bearing quartzofeldspathic rocks. Subsequently, Tasiuyak gneiss within a c. 1320 Ma contact aureole of the NPS was statically subjected to lower pressure, but higher temperature conditions (M_C), leading to a second partial melting event, and the generation of complex mineral assemblages and microstructures, which were controlled to a large extent by the textures of the M_R assemblage. This control is clearly seen in scanning electron microscopic images of thin sections and is further supported by phase equilibria modelling. Samples collected within the contact aureole near Anaktalik Brook, west of Nain, Labrador, mainly consist of spinel–cordierite and orthopyroxene–cordierite (or plagioclase) pseudomorphs after M_R sillimanite and garnet, respectively, within a quartzofeldspathic matrix. In addition, some samples contain fine‐grained intergrowths of K‐feldspar–quartz–cordierite–orthopyroxene inferred to be pseudomorphs after osumulite. Microstructural evidence of the former melt includes (i) coarse‐grained K‐feldspar–quartz–cordierite–orthopyroxene domains that locally cut the rock fabric and are inferred to represent neosome; (ii) very fine‐ to medium‐grained cordierite–quartz intergrowths interpreted to have formed by a reaction involving dissolution of biotite and feldspar in melt; and (iii) fine‐scale interstitial pools or micro‐cracks filled by feldspar interpreted to have crystallized from melt. Ultrahigh temperature (UHT) conditions during contact metamorphism are supported by (i) solidus temperatures >900 °C estimated for all samples, coupled with extensive textural evidence for contact‐related partial melting; (ii) the inferred (former) presence of osumilite; and (iii) titanium‐in‐quartz thermometry indicating temperatures within error of 900 °C. The UHT environment in which these unusual textures and minerals were developed was likely a consequence of the superposition of more than one contact metamorphic event upon the already relatively anhydrous Tasiuyak gneiss.