In-situ migmatite and hybrid diatexite at Mt Stafford, central Australia

Metasedimentary gneisses show a rapid change in grade within a 10-km-wide low- P /high- T  regional aureole at Mt Stafford, Arunta Block, central Australia. Migmatites occur in all but the lowermost of five metamorphic zones, which are characterized by: (1) muscovite–quartz schist; (2) andalusite–cordierite–K-feldspar granofels with small melt segregations; (3) spinel–sillimanite–cordierite–K-feldspar migmatite; (4) garnet–orthopyroxene–cordierite migmatite and minor diatexite; and (5) biotite–cordierite–plagioclase diatexite that shows a transition to granite. A subsolidus unit comprising interbedded sandstone and siltstone is equivalent to bedded migmatite , the main rock type in Zones 2–4. Mesoscopic textures and migmatite classification of this unit vary with grade. In Zone 2, metatexite is developed in siltstone layers that are separated by quartz-rich, unmelted metapsammite layers. Melt segregation was less efficient in Zones 3 and 4, where the dominant migmatite layering is a modified bedding. High proportions of melt were present in Zone 4, in which schlieren migmatite is transitional between bedded migmatite and metapelite-sourced diatexite. The preservation of sedimentary structures and coexistence of melt reactants and products in Zone 4 metapelite imply that melting proceeded in situ without substantial migration of melt. Zone 5 biotite–cordierite–plagioclase diatexite carries rafts of bedded migmatite with strongly resorbed edges, as well as large K-feldspar and quartz augen. This unit of comparatively Ca-rich migmatites is inferred to have been formed by the mixing of locally derived and injected granitic melt.     

Crustal reworking in a shear zone: transformation of metagranite to migmatite

This study uses field, microstructural and geochemical data to investigate the processes contributing to the petrological diversity that arises when granitic continental crust is reworked. The Kinawa migmatite formed when Archean TTG crust in the São Francisco Craton, Brazil was reworked by partial melting at ~730 °C and 5–6 kbar in a regional‐scale shear zone. As a result, a relatively uniform leucogranodiorite protolith produced compositionally and microstructurally diverse diatexites and leucosomes. All outcrops of migmatite display either a magmatic foliation, flow banding or transposed leucosomes and indicate strong, melt‐present shearing. There are three types of diatexite. Grey diatexites are interpreted to be residuum, although melt segregation was incomplete in some samples. Biotite stable, H₂O‐fluxed melting is inferred via the reaction Pl + Kfs + Qz + H₂O = melt and geochemical modelling indicates 0.35–0.40 partial melting. Schlieren diatexites are extremely heterogeneous; residuum‐rich domains alternate with leucocratic quartzofeldspathic domains. Homogeneous diatexites have the highest SiO₂ and K₂O contents and are coarse‐grained, leucocratic rocks. Homogeneous diatexites, quartzofeldspathic domains from the schlieren diatexites and the leucosomes contain both plagioclase‐dominated and K‐feldspar‐dominated feldspar framework microstructures and hence were melt‐derived rocks. Both types of feldspar frameworks show evidence of tectonic compaction. Modelling the crystallization of an initial anatectic melt shows plagioclase appears first; K‐feldspar appears after ~40% crystallization. In the active shear zone setting, shear‐enhanced compaction provided an essentially continuous driving force for segregation. Thus, Kinawa migmatites with plagioclase frameworks are interpreted to have formed by shear‐enhanced compaction early in the crystallization of anatectic melt, whereas those with K‐feldspar frameworks formed later from the expelled fractionated melt. Trace element abundances in some biotite and plagioclase from the fractionated melt‐derived rocks indicate that these entrained minerals were derived from the wall rocks. Results from the Kinawa migmatites indicate that the key factor in generating petrological diversity during crustal reworking is that shear‐enhanced compaction drove melt segregation throughout the period that melt was present in the rocks. Segregation of melt during melting produced residuum and anatectic melt and their mixtures, whereas segregation during crystallization resulted in crystal fractionation and generated diverse plagioclase‐rich rocks and fractionated melts.     

Petrogenesis of metatexite and diatexite migmatites determined using zircon U–Pb age,trace element and Hf isotope data,Higo metamorphic terrane,central Kyushu,Japan

Metatexite and diatexite migmatites are widely distributed within the upper amphibolite and granulite facies zones of the Higo low‐P/high‐T metamorphic terrane. Here, we report data from an outcrop in the highest grade part of the granulite facies zone, in which diatexite occurs as a 3 m thick layer between 2 m thick layers of stromatic‐structured metatexite within pelitic gneiss. The migmatites and gneiss contain the same peak mineral assemblage of biotite + plagioclase + quartz + garnet + K‐feldspar with retrograde chlorite ± muscovite and some accessory minerals of ilmenite ± rutile ± titanite + apatite + zircon + monazite ± pyrite ± zinc sulphide ± calcite. Calculated metamorphic P–T conditions are 800–900 °C and 9–12 kbar. Zircon in the diatexite forms elongate euhedral crystals with oscillatory zoning, but no core–rim structure. Zircon from the gneiss and metatexite forms euhedral–subhedral grains comprising inherited cores overgrown by thin rims. The overgrowth rims in the metatexite have lower Th/U ratios than zircon in the diatexite and yield a ²⁰⁶Pb/²³⁸U age of 116.0 ± 1.6 Ma, which is older than the 110.1 ± 0.6 Ma ²⁰⁶Pb/²³⁸U age derived from zircon in the diatexite. Zircon from the diatexite has variable REE contents with convex upward patterns and flat normalized HREE, whereas the overgrowth rims in the metatexite and gneiss have steep HREE‐enriched patterns; however, both types have similar positive Ce and negative Eu anomalies. ¹⁷⁶Hf/¹⁷⁷Hf ratios in the overgrowth rims from the metatexite are more variable and generally lower than values from zircon in the diatexite. Based on U–Pb ages, trace element and Hf isotope data, the zircon rims in the metatexite are interpreted to have crystallized from a locally derived melt, following partial dissolution of inherited protolith zircon during anatexis, whereas the zircon in the diatexite is interpreted to have crystallized from a melt that included an externally derived component. By integrating zircon and petrographic data for the migmatites and pelitic gneiss, the metatexite migmatite is interpreted to have formed by in situ partial melting in which the melt did not migrate from the source, whereas the diatexite migmatite included an externally derived juvenile component. The Cretaceous high‐temperature metamorphism of the Higo metamorphic terrane is interpreted to reflect emplacement of mantle‐derived basalts under a volcanic arc along the eastern margin of the Eurasian continent and advection of heat via hybrid silicic melts from the lower crust. Post‐peak crystallization of anatectic melts in a high‐T region at mid‐crustal depths occurred in the interval c. 116–110 Ma, as indicated by the difference in zircon ages from the metatexite and diatexite migmatites.     

Melt infiltration into quartzite during partial melting in the Little Cottonwood Contact Aureole (UT,USA): implication for xenocryst formation

Melt infiltration into quartzite took place due to generation and migration of partial melts within the high‐grade metamorphic rocks of the Big Cottonwood (BC) formation in the Little Cottonwood contact aureole (UT, USA). Melt was produced by muscovite and biotite dehydration melting reactions in the BC formation, which contains pelite and quartzite interlayered on a centimetre to decimetre scale. In the migmatite zone, melt extraction from the pelites resulted in restitic schollen surrounded by K‐feldspar‐enriched quartzite. Melt accumulation occurred in extensional or transpressional domains such as boudin necks, veins and ductile shear zones, during intrusion‐related deformation in the contact aureole. The transition between the quartzofeldspathic segregations and quartzite shows a gradual change in texture. Here, thin K‐feldspar rims surround single, round quartz grains. The textures are interpreted as melt infiltration texture. Pervasive melt infiltration into the quartzite induced widening of the quartz–quartz grain boundaries, and led to progressive isolation of quartz grains. First as clusters of grains, and with increasing infiltration as single quartz grains in the K‐feldspar‐rich matrix of the melt segregation. A 3D–μCT reconstruction showed that melt formed an interconnected network in the quartzites. Despite abundant macroscopic evidence for deformation in the migmatite zone, individual quartz grains found in quartzofeldspathic segregations have a rounded crystal shape and lack quartz crystallographic orientation, as documented with electron backscatter diffraction (EBSD). Water‐rich melts, similar to pegmatitic melts documented in this field study, were able to infiltrate the quartz network and disaggregate grain coherency of the quartzites. The proposed mechanism can serve as a model to explain abundant xenocrysts found in magmatic systems.     

Nd–Sr isotopic constraint to the formation of metatexite and diatexite migmatites,Higo metamorphic terrane,central Kyushu,Japan

ABSTRACT

Metatexite and diatexite migmatites are widely distributed within the upper amphibolite and granulite-facies zones of the Higo low-P/high-T metamorphic terrane. Here we report Nd–Sr isotopic and whole rock composition data from an outcrop in the highest-grade part of the granulite-facies zone, in which diatexite occurs as a 3 m-thick layer between 2 m-thick layers of stromatic-structured metatexite within pelitic gneiss. The metatexite has Nd–Sr isotopes and whole rock compositions similar to those of the gneiss, but the diatexite shows the reverse. The diatexite has a higher ε_Nd(t) and ¹⁴⁷Sm/¹⁴⁴Nd ratio (ε_Nd(t) = ?0.5; ¹⁴⁷Sm/¹⁴⁴Nd = 0.1636) than the gneiss (ε_Nd(t) = ?2.1; ¹⁴⁷Sm/¹⁴⁴Nd = 0.1287) and metatexite (ε_Nd(t) = ?3.1; ¹⁴⁷Sm/¹⁴⁴Nd = 0.1188). The (⁸⁷Sr/⁸⁶Sr)_initial and ⁸⁷Rb/⁸⁶Sr of the diatexite ((⁸⁷Sr/⁸⁶Sr)_initial = 0.70568; ⁸⁷Rb/⁸⁶Sr = 0.416) are lower than those of the gneiss ((⁸⁷Sr/⁸⁶Sr)_initial = 0.70857; ⁸⁷Rb/⁸⁶Sr = 1.13) and metatexite ((⁸⁷Sr/⁸⁶Sr)_initial = 0.70792; ⁸⁷Rb/⁸⁶Sr = 1.11). The metatexite and gneiss show enrichment of Th and depletion of P and Eu and have a similar chondrite-normalized REE pattern, which shows steep LREE–MREE-enriched and gently declining HREE patterns and negative Eu anomalies, whereas the diatexite shows enrichment of Sr and depletion of Th and Y, and exhibits gently declining LREE and steeply declining HREE pattern and weak Eu depletion. The metatexite migmatite is interpreted to have formed by in situ partial melting in which the melt did not migrate from the source, whereas the diatexite migmatite included an externally derived melt with a juvenile component. The Cretaceous high-temperature metamorphism of the Higo metamorphic terrane is interpreted to reflect emplacement of mantle-derived basalts under a volcanic arc along the eastern margin of the Eurasian continent, and mass transfer and advection of heat via hybrid silicic melts from the lower crust.     

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.     

Partial Melting of Aluminous Metagreywackes in the Northern Sierra de Comechingones, Central Argentina

We describe a suite of metamorphic and migmatitic rocks fromthe Sierra de Comechingones (Sierras Pampeanas of Central Argentina)that include unmelted gneisses, migmatites and refractory granulites.The gneisses are aluminous greywackes metamorphosed in the amphibolitegrade and are likely to have been the protoliths for the higher-grademigmatites and granulites. Mineralogical characteristics andmajor and trace element compositions show that metatexite migmatites,diatexite migmatites and granulites are all melt-depleted rocks.The migmatites (both metatexites and diatexites) have undergoneH₂O-fluxed melting and lost     

High‐grade metamorphism and partial melting in Archean composite grey gneiss complexes

Much of the exposed Archean crust is composed of composite gneiss which includes a large proportion of intermediate to tonalitic material. These gneiss terranes were typically metamorphosed to amphibolite to granulite facies conditions, with evidence for substantial partial melting at higher grade. Recently published activity–composition (a?x) models for partial melting of metabasic to intermediate compositions allows calculation of the stable metamorphic minerals, melt production and melt composition in such rocks for the first time. Calculated P?T pseudosections are presented for six bulk rock compositions taken from the literature, comprising two metabasic compositions, two intermediate/dioritic compositions and two tonalitic compositions. This range of bulk compositions captures much of the diversity of rock types found in Archean banded gneiss terranes, enabling us to present an overview of metamorphism and partial melting in such terranes. If such rocks are fluid saturated at the solidus, they first begin to melt in the upper amphibolite facies. However, at such conditions, very little (< 5%) melt is produced and this melt is granitic in composition for all rocks. The production of greater proportions of melt requires temperatures ～800–850 °C and is associated with the first appearance of orthopyroxene at pressures below 8–9 kbar or with the appearance and growth of garnet at higher pressures. The temperature at which orthopyroxene appears varies little with composition providing a robust estimate of the amphibolite–granulite facies boundary. Across this boundary, melt production is coincident with the breakdown of hornblende and/or biotite. Melts produced at granulite facies range from tonalite–trondhjemite–granodiorite for the metabasic protoliths, granodiorite to granite for the intermediate protoliths and granite for the tonalitic protoliths. Under fluid‐absent conditions the melt fertility of the different protoliths is largely controlled by the relative proportions of hornblende and quartz at high grade, with the intermediate compositions being the most fertile. The least fertile rocks are the most leucocratic tonalites due to their relatively small proportions of hydrous mafic phases such as hornblende or biotite. In the metabasic rocks, melt production becomes limited by the complete consumption of quartz to higher temperatures. The use of phase equilibrium forward‐modelling provides a thermodynamic framework for understanding melt production, melt loss and intracrustal differentiation during the Archean.     

Transforming mylonitic metagranite by open-system interactions during melt flow

Gneisses and migmatites of the Gföhl unit (Moldanubian Zone, Bohemian Massif) range from banded mylonitic orthogneiss with recrystallized monomineralic bands, through stromatic (metatexite) and schlieren (inhomogeneous diatexite) migmatite, to isotropic nebulite (homogeneous diatexite). This sequence was classically attributed to increasing degree of anatexis. Under the microscope, the evolution is characterized by progressive destruction of the monomineralic banding that characterizes the original mylonitic orthogneiss. Throughout, the mineral assemblage is biotite–K‐feldspar–plagioclase–quartz ± garnet ± sillimanite, but the mineral compositions exhibit systematic changes with progressive disintegration of the layering. From banded orthogneiss to nebulite, the garnet composition changes systematically, Alm_75→94Prp_17→0.8Grs_2.5→1.2Sps_2→11 and X_Fe = 0.45→0.99 and for biotite, X_Fe = 0.80→1. This is consistent with a decrease in equilibration temperature and pressure of 790 °C and 8.5–6 kbar, to 690 °C and 5–4 kbar respectively. There is also a systematic change of whole‐rock composition, marked by an increase in SiO₂ (71→77 wt%) and X_Fe (0.62→0.85) and by a decrease in Al₂O₃ (16→13 wt%) and CaO (1.50→0.43 wt%). Assuming that the rocks started with the same composition, these systematic changes indicate open‐system behaviour. The predicted consequences of various open‐system processes are assessed using thermodynamic modelling. The observed variations are interpreted as being a consequence of melt flow through, and interaction with the rocks, and, to change the rock composition sufficiently, a large volume of melt must have been involved.     

Tourmaline breakdown in the migmatite zone of the Ryoke metamorphic belt,SW Japan

A sharp line delimitating the distribution of tourmaline (termed as a ‘tourmaline‐out isograd’) is defined in the migmatite zone of the Ryoke metamorphic belt, Japan. The trend of the tourmaline‐out isograd closely matches that of the isograds formed through the regional metamorphism, suggesting that it represents the breakdown front of tourmaline during regional metamorphism. This is confirmed by the presence of the reaction textures of tourmaline to sillimanite and cordierite near the tourmaline‐out isograd. The breakdown of tourmaline would release boron into associated melts or fluids and be an important factor in controlling the behaviour of boron in tourmaline‐bearing high‐temperature metamorphic rocks. Near the tourmaline‐out isograd, large tourmaline crystals occur in the centre of interboudin partitions containing leucosome. In the melanosome of the intervening matrix, reaction textures involving tourmaline are locally observed. These observations imply that tourmaline breakdown is related to a melting reaction and that the boron in the leucosome is derived from the breakdown of tourmaline in the melanosome during prograde metamorphism. Boron released by tourmaline breakdown lowers both the solidus temperature of the rock and the viscosity of any associated melt. Considering that the tourmaline‐out isograd lies close to the schist–migmatite boundary, these effects might have enhanced melt generation and segregation in the migmatite zone of the Ryoke belt. The evidence for the breakdown of tourmaline and the almost complete absence of any borosilicates throughout the migmatite zone suggest that boron was effectively removed from this region by the movement of melt and/or fluid. This implies that the tourmaline‐out isograd can reflect a significant amount of mass transfer in the anatectic zones.     

Melt loss and the preservation of granulite facies mineral assemblages

The loss of a metamorphic fluid via the partitioning of H₂O into silicate melt at higher metamorphic grade implies that, in the absence of open system behaviour of melt, the amount of H₂O contained within rocks remains constant at temperatures above the solidus. Thus, granulite facies rocks, composed of predominantly anhydrous minerals and a hydrous silicate melt should undergo considerable retrogression to hydrous upper amphibolite facies assemblages on cooling as the melt crystallizes and releases its H₂O. The common occurrence of weakly retrogressed granulite facies assemblages is consistent with substantial melt loss from the majority of granulite facies rocks. Phase diagram modelling of the effects of melt loss in hypothetical aluminous and subaluminous metapelitic compositions shows that the amount of melt that has to be removed from a rock to preserve a granulite facies assemblage varies markedly with rock composition, the number of partial melt loss events and the P–T conditions at which melt loss occurs. In an aluminous metapelite, the removal of nearly all of the melt at temperatures above the breakdown of biotite is required for the preservation of the peak mineral assemblage. In contrast, the proportion of melt loss required to preserve peak assemblages in a subaluminous metapelite is close to half that required for the aluminous metapelite. Thus, if a given proportion of melt is removed from a sequence of metapelitic granulites of varying composition, the degree of preservation of the peak metamorphic assemblage may vary widely.     

Behavior of zircon in the upper-amphibolite to granulite facies schist/migmatite transition, Ryoke metamorphic belt, SW Japan: constraints from the melt inclusions in zircon

Behavior of zircon at the schist/migmatite transition is investigated. Syn-metamorphic overgrowth is rare in zircon in schists, whereas zircon in migmatites has rims with low Th/U that give 90.3 ± 2.2 Ma U–Pb concordia age. Between inherited core and the metamorphic rim, a thin, dark-CL annulus containing melt inclusion is commonly developed, suggesting that it formed contemporaneous with the rim in the presence of melt. In diatexites, the annulus is further truncated by the brighter-CL overgrowth, suggesting the resorption and regrowth of the zircon after near-peak metamorphism. Part of the zircon rim crystallized during the solidification of the melt in migmatites. Preservation of angular-shaped inherited core of 5–10 μm in zircon included in garnet suggests that zircon of this size did not experience resorption but developed overgrowths during near-peak metamorphism. The Ostwald ripening process consuming zircon less than 5–10 μm is required to form new overgrowths. Curved crystal size distribution pattern for fine-grained zircons in a diatexite sample may indicate the contribution of this process. Zircon less than 20 μm is confirmed to be an important sink of Zr in metatexites, and ca. 35-μm zircon without detrital core are common in diatexites, supporting new nucleation of zircon in migmatites. In the Ryoke metamorphic belt at the Aoyama area, monazite from migmatites records the prograde growth age of 96.5 ± 1.9 Ma. Using the difference of growth timing of monazite and zircon, the duration of metamorphism higher than the amphibolite facies grade is estimated to be ca. 6 Myr.     

Fluid‐fluxed melting and melt loss in a syntectonic contact metamorphic aureole from the Variscan eastern Pyrenees

Open‐system behaviour through fluid influx and melt loss can produce a variety of migmatite morphologies and mineral assemblages from the same protolith composition. This is shown by different types of granulite facies migmatite from the contact aureole of the Ceret gabbro–diorite stock in the Roc de Frausa Massif (eastern Pyrenees). Patch, stromatic and schollen migmatites are identified in the inner contact aureole, whereas schollen migmatites and residual melanosomes are found as xenoliths inside the gabbro–diorite. Patch and schollen migmatites record D1 and D2 structures in folded melanosome and mostly preserve the high‐T D2 in granular or weakly foliated leucosome. Stromatic migmatites and residual melanosomes only preserve D2. The assemblage quartz–garnet–biotite–sillimanite–cordierite±K‐feldspar–plagioclase is present in patch and schollen migmatites, whereas stromatic migmatites and residual melanosomes contain a sub‐assemblage with no sillimanite and/or K‐feldspar. A decrease in X _Fe (molar Fe/(Fe + Mg)) in garnet, biotite and cordierite is observed from patch migmatites through schollen and stromatic migmatites to residual melanosomes. Whole‐rock compositions of patch, schollen and stromatic migmatites are similar to those of non‐migmatitic rocks from the surrounding area. These metasedimentary rocks are interpreted as the protoliths of the migmatites. A decrease in the silica content of migmatites from 63 to 40 wt% SiO₂ is accompanied by an increase in Al₂O₃ and MgO+FeO and by a depletion in alkalis. Thermodynamic modelling in the NCKFMASHTO system for the different types of migmatite provides peak metamorphic conditions ~7–8 kbar and 840 °C. A nearly isothermal decompression history down to 5.5 kbar was followed by isobaric cooling from 840 °C through 690 °C to lower temperatures. The preservation of granulite facies assemblages and the variation in mineral assemblages and chemical composition can be modelled by ongoing H₂O‐fluxed melting accompanied by melt loss. The fluids were probably released by the crystallizing gabbro–diorite, infiltrating the metasedimentary rocks and fluxing melting. Release of fluids and melt loss were probably favoured by coeval deformation (D2). The amount of melt remaining in the system varied considerably among the different types of migmatite. The whole‐rock compositions of the samples, the modelled compositions of melts at the solidus at 5.5 kbar and the residues show a good correlation.     

Petrogenesis of Migmatites in Maine, USA: Possible Source of Peraluminous Leucogranite in Plutons?

In Maine, Siluro-Devonian turbidites were metamorphosed underhigh-T–low-P facies series conditions during deformationwithin a Devonian crustal-scale shear zone system, defined bykilometer-scale straight belts of apparent flattening strainthat anastomose around lozenges of apparent constrictional strain.At upper amphibolite facies grade, metapelites are partiallymelted, the onset of which is recorded by a migmatite front.The resulting migmatites are stromatic or heterogeneous, andsmaller-volume granites form sheets or cylinders according tothe structural zone in which they occur, suggesting that migmatitesand granites record syntectonic melt flow through the deformingcrust. Common leucogranite of the nearby coeval Phillips pluton,which was emplaced syntectonically, was sourced from crustalrocks with geochemical characteristics similar to those of thehost Siluro-Devonian succession. Migmatites have melt-depletedcompositions relative to metapelites. Leucosomes are peraluminousand represent the cumulate products of fractional crystallizationand variable loss of evolved fractionated liquid. Among theheterogeneous migmatites are schlieric granites, the geochemistryof which suggests melt accumulation before fractional crystallizationand loss of the evolved liquid. Smaller-volume granites areperaluminous with a range of chemistries that reflect variableentrainment of residual plagioclase and biotite, accumulationof products of fractional crystallization and loss of most ofthe evolved liquid. Common leucogranite of the Phillips plutonand larger granites in the migmatites have compositions thatsuggest crystallization of evolved liquids derived by fractionalcrystallization of primary muscovite dehydration melts. We inferthat the leucogranite represents the crystallized fugitive liquidfrom a migmatite source similar to that exposed nearby. Watertransported through the shear zone system dissolved in meltwas exsolved at the wet solidus to cause retrogression in sub-solidusrocks and retrograde muscovite growth in migmatites. KEY WORDS: anatexis of pelite; Maine; migmatite; peraluminous granite; plutons     

Hybridization of a Shallow 'I-type' Granitoid Pluton and its Host Migmatite by Magma-Chamber Wall Collapse: the Tokuwa Pluton, Central Japan

The Miocene Tokuwa pluton of ‘I-type’ granitoidaffinity was emplaced discordantly into a Cretaceous to Paleogeneaccretionary complex and induced a contact aureole in whichvarious thermally metamorphosed rocks were developed, includinghornfels, metatexite, diatexite and cordierite-bearing tonalite(Crd-tonalite) of ‘S-type’ granite affinity. Thethermally metamorphosed rocks show low-pressure reaction texturesculminating in partial melting. Peak P–T conditions of3 kbar at 780°C are estimated on the basis of the TWQ thermobarometerfor the garnet-bearing rocks. The rocks in the contact aureoleexhibit a gradual transition from hornfels, through metatexiteand diatexite to Crd-tonalite. The Sr-isotopic composition atthe time of Tokuwa pluton emplacement at 12 Ma decreases systematicallyfrom metatexite (0·7100–0·7112) throughdiatexite (0·7078–0·7094) to Crd-tonalite(0·7067–0·7068); this trend is interpretedin terms of mixing between the Tokuwa magma and the aureolemigmatites. The field relationships, geochemical data, and isotopicdata collectively suggest that the emplacement of the Tokuwapluton triggered partial melting of the surrounding metasedimentaryrocks. Subsequent hybridization of the Tokuwa magma with themetatexite in variable proportions produced the Crd-tonaliteand diatexite. The hybridization was caused by invasion of theTokuwa magma into the migmatite zone, accompanied by gravitationalcollapse of the previously crystallized wall of the magma chamber.The data presented demonstrate that even a relatively low-temperature,shallow, ‘I-type’ granitoid pluton can induce contactanatexis and hybrid ‘S-type’ granitoid formationat the intrusive contact. KEY WORDS: contact metamorphism; hybridization; magma–host-rock interaction; migmatite; ‘S-type’ granitoid     

高级变质岩中深熔作用的相平衡研究

深熔作用在高级变质岩中非常普遍并受到广泛关注。自20世纪90年代以来,随着变质相平衡研究的突破性发展,利用THERMOCALC程序和视剖面图方法可以定量研究固相线以上的熔体形成、熔体分馏和退变质反应。变质沉积岩中的熔融作用主要有三种机制饱和水固相线上的熔融、白云母脱水熔融和黑云母脱水熔融。在模拟泥质岩石的KFMASH体系和NCKFMASH体系中的相平衡计算表明,NCKFMASH体系中铁镁矿物的相平衡关系受KFMASH亚体系中矿物相平衡关系的控制,但KFMASH亚体系中固相线位置要比实际的高50~60℃。因此,模拟泥质岩石的固相线以上的相平衡关系最好在NCKFMASH或组分更多的体系中进行。相平衡研究表明麻粒岩相岩石的保存与熔体丢失有关;混合岩的形成过程包括部分熔融作用、不同程度熔体分凝与汲取和不同程度的逆反应和退变反应。     

From migmatites to granites in the Pan-African Damara orogenic belt,Namibia

The Swakop River exposes a unique structural section into the root of the Pan-African Damara orogenic belt (DOB) in Namibia formed as a result of collision between the Congo and the Kalahari cratons from ca. 550 to 500 Ma. The Central Zone of the Damara orogenic belt is characterized by amphibolite to granulite facies metamorphism accompanied by intense partial melting. Three tectonic units are defined in the Central Zone based on the proportion and distribution of the granitic fraction, namely (1) a lower unit dominated by diatexites and comprising plutons of homogeneous granites, (2) a middle unit composed by metatexites with mainly a metasedimentary protolith, and (3) an upper unit corresponding to metamorphic rocks with intrusive leucogranitic sills and laccoliths. The increase in the granitic fraction with structural depth is suggesting an increase in the degree of partial melting and implies a relative inefficiency of magma mobility from the source to higher structural levels. The transition from metatexites of the middle unit to diatexites and granites of the lower unit is interpreted as reflecting the former transition from partially molten rocks to a crustal-scale magmatic layer. Mushroom-shaped granitic plutons in the lower unit are consistent with their emplacement as diapirs and the development of gravitational instabilities within the magmatic layer. In the middle unit, granitic veins concordant and discordant to the synmigmatitic foliation localized in structurally-controlled sites (foliation, boudin’s necks, shear zones, fold hinges) indicate that, within the partially molten zone, deformation plays the dominant role in melt segregation and migration at the outcrop scale. Melt migration from the partially molten zone to the intrusive zone is related to the build-up of an interconnected network of dikes and sills with diffuse contacts with the migmatitic hosts in the middle unit. In contrast, the upper unit is characterized by homogeneous leucogranitic plutons in sharp intrusive contact with genetically unrelated host rocks suggest that part of the melt fraction has migrated upward from its source to an intrusive zone.     

Delamerian Glenelg tectonic zone,western Victoria: Geology and metamorphism of stratiform rocks

The Cambro‐Ordovician Glenelg tectonic zone of western Victoria is a distinctive metamorphic‐igneous segment of the Delamerian Orogenic Belt comprising two northwest‐striking regional metamorphic segments of andalusite‐sillimanite type prograding towards an axial granitic batholith. The second of five deformations (D₂) was most significant, producing isoclinal folds, transposition and a pervasive regional foliation (S₂). Southwest of the central batholith, biotite to migmatite zones contain mainly quartzo‐feldspathic rock (turbiditic metagreywacke, quartzo‐feldspathic schist and migmatite), plus less common metaquartzite and calc‐silicate rocks and minor metapelite. Metagabbro, metadolerite and amphibolite typically have the chemistry of mid‐ocean ridge basalts. Serpentinite pods and sheets were tectonically introduced to low‐grade areas. Northeast of the central batholith, quartzo‐feldspathic rock occupies the sillimanite and migmatite zones exclusively, with a regional concentration of pegmatites adjacent to the zone boundary. Gross interleaving of quartzo‐feldspathic schist, migmatite, pegmatite and muscovite‐bearing granitic rock is characteristic. Peak metamorphic conditions of 550 MPa at 640°C leading to migmatite formation were established by D₂ time and accompanied by tonalite‐granodiorite and pegmatite emplacement. Subsequently, the thermal high contracted to the northeast culminating in the more extensive syn‐, post‐D4 to pre‐D5 granitic magmatism.     

Criteria for the recognition of partial melting

Partial melting changes rocks from single phase (solid) to two phase (solid+melt) systems. The bulk viscosity decreases as the melt fraction increases and this effect raises the rate of deformation and heat transfer, as well as causing crustal differentiation. Therefore, it is important to be able to recognise which rocks have partially melted.Macroscopic textures provide the simplest criteria for recognising partial melting. Melting and deformation are generally synchronous, and when the melt fraction retained is low (<20%) metatexite migmatites are formed. Typically, these are morphologically complex because the melt fraction is squeezed out of the deforming matrix and collects in whatever dilatant sites are present. The presence of melanosome layers and patches provides the best evidence of where the melt formed, and the leucosomes where it collected. Diatexite migmatites can be easily recognised by the presence of a flow foliation, schlieren, enclaves and vein like leucosomes, and are evidence of a high melt fraction and pervasive partial melting. For the unusual case of melting without synchronous deformation, rounded neosome patches containing both the melt and solid fractions of the melt-producing reaction develop and, as the degree of melting increases these enlarge, to form diatexite migmatites. In both cases the characteristic feature is an increased grainsize and loss of pre-migmatization structures. Migmatite textures are robust, they survive later deformation well.Microscopic textures such as: (1) thin films of quartz, plagioclase and K-feldspar along brain boundaries that represent crystallized melt and, (2) melt-solid reaction textures, also provide good criteria for identifying partially melted rocks. However, these textures are fragile and easily destroyed by deformation. The identification of mineral assemblages from which melt-forming reactions can be inferred is another reliable critera for recognising partial melting, but post-migmatization rehydration in granulite terranes can destroy this evidence.Whole rock geochemistry can be used to model the partial melting process, but problems in identifying the palaeosome and an unmodified melt compositions can restrict its application. However, whole rock geochemistry coupled with good field based control, can be used to deduce what processes have occurred in a terrane where the rocks have partially melted.Variations in field appearance, texture and composition are, in large part a consequence of whether, or not, and when, the melt-fraction separated from the solid fraction.     

A sequence of partial melting reactions at Mt Stafford, central Australia

Metasedimentary gneisses show a rapid change in grade in a 10  km wide low- P /high- T  regional aureole at Mt Stafford in the Arunta Block, central Australia. Migmatite occurs in all but the lowermost of five metamorphic zones, which grade from greenschist (Zone 1) through amphibolite (Zones 2–3) to granulite facies (Zones 4–5). The sequence of partial melting reactions inferred for metapelitic rocks is dependant upon protolith, temperature and fluid conditions. The metapelite solidus in Zone 2 reflects vapour-present melting at P ≈3  kbar and T  ≈640  °C, melting having initially been controlled by the congruent breakdown of the assemblage Crd–Kfs–Bt–Qtz. At slightly higher temperature, andalusite in leucosome formed via the reaction Kfs+Qtz+Bt+H₂O→And+melt; And+melt having been stabilized by the presence of boron. Sillimanite coaxially replaces andalusite in the high-grade portion of Zone 2. In Zone 3, large aluminosilicate aggregates in leucosome are armoured by Spl–Crd±Grt symplectites. Garnet partially pseudomorphs biotite, cordierite or spinel in high-grade portions of Zone 3. Zone 4 Grt–Crd–Opx-bearing metapsammite assemblages and garnet-bearing leucosome reflect T  ≈800  °C and P =2.2±0.9  kbar. In the model KFMASH system the principal vapour-absent melting step reflected significant modal changes related to the breakdown of the As–Bt tie-line and the establishment of the Spl–Crd tie-line; the bulk rock geochemistry of migmatite samples straddle the Spl–Crd tie-line. The aluminous bulk-rock composition of the common bedded migmatite restricted its potential to witness garnet-forming and orthopyroxene-forming reactions, minor textural and modal changes in and above Zone 3 reflecting biotite destablization in biotite-poor assemblages.