Anatectic melt volumes in the thermal aureole of the Etive Complex,Scotland: the roles of fluid‐present and fluid‐absent melting

Pelitic hornfelses within the inner thermal aureole of the Etive igneous complex underwent limited partial melting, generating agmatic micro‐stromatic migmatites. In this study, observed volume proportions of vein leucosomes in the migmatites are compared with modelled melt volumes in an attempt to constrain the controls on melting processes. Petrogenetic modelling in the MnNCKFMASHT system was performed on the compositions of 15 analysed Etive pelite samples using THERMOCALC. Melt modes were calculated at 2.2 kbar (the estimated pressure in the southern Etive aureole) from solidus temperatures to 800 °C for both fluid‐absent and fluid‐present conditions. Volume changes accompanying fluid‐absent melting at 2.2 kbar were also calculated. P–T pseudosections reproduce the zonal sequence of the southern Etive aureole fairly well. The modelled solidus temperatures of silica‐rich pelitic compositions are close to 680 °C at 2.2 kbar and, in the absence of free fluid, melt modes in such compositions rise to between 12 and 29% at 800 °C, half of which is typically produced over the narrow reaction interval in which orthopyroxene first appears. Silica‐poor compositions have solidus temperatures of up to ～770 °C and yield <11.4% melt at 800 °C under fluid‐absent conditions. For conditions of excess H₂O, modelled melt modes increase dramatically within ～13 °C of the solidus, in some cases to >60%; by 800 °C they range from 61 to 88% and from 29 to 74% in silica‐rich and silica‐poor compositions, respectively. Calculated volume changes for fluid‐absent melting are positive for all modelled compositions and reach 4.5% in some silica‐rich compositions by 800 °C. Orthopyroxene formation is accompanied by a volume increase of up to 1.48% over a temperature increase of as little as 2.7 °C, supporting the arguments for melt‐induced ‘hydrofracturing’ as a viable melt‐escape mechanism in low‐P metamorphism. Mineral assemblages in the innermost aureole support previous conclusions that partial melting took place predominantly under fluid‐absent conditions. However, vein leucosome proportions, estimated by image analysis, do not show the expected correlation with grade, and are locally greatly in excess of melt modes predicted by fluid‐absent models, particularly close to the melt‐in isograd. Melting of interlayered psammites, addition of H₂O from interlayered melt‐free rocks, and metastable persistence of muscovite are ruled out as major causes of the excess melt anomaly. The most likely cause, we believe, is that local variations existed in the amount of fluid available at the onset of melting, promoted by focussing of fluid released by dehydration in the middle and outer aureole; however, some redistribution of melt by compaction‐driven flow through the vein channel network cannot be ruled out. The formation of melt‐filled fractures in the inner Etive aureole was assisted by stresses that caused extension at high angles to the igneous contact. The fractures were probably caused either by transient pressure reduction in the diorite magma chamber associated with a second phase of intrusion, or by sub‐solidus thermal contraction in the diorite pluton during the early stages of inner‐aureole cooling.     

Phase equilibria constraints on melting of stromatic migmatites from Ronda (S. Spain): insights on the formation of peritectic garnet

Stromatic metatexites occurring structurally below the contact with the Ronda peridotite (Ojén nappe, Betic Cordillera, S Spain) are characterized by the mineral assemblage Qtz+Pl+Kfs+Bt+Sil+Grt+Ap+Gr+Ilm. Garnet occurs in low modal amount (2–5 vol.%). Very rare muscovite is present as armoured inclusions, indicating prograde exhaustion. Microstructural evidence of melting in the migmatites includes pseudomorphs after melt films and nanogranite and glassy inclusions hosted in garnet cores. The latter microstructure demonstrates that garnet crystallized in the presence of melt. Re‐melted nanogranites and preserved glassy inclusions show leucogranitic compositions. Phase equilibria modelling of the stromatic migmatite in the MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂–O₂–C (MnNCaKFMASHOC) system with graphite‐saturated fluid shows P–T conditions of equilibration of 4.5–5 kbar, 660–700 °C. These results are consistent with the complete experimental re‐melting of nanogranites at 700 °C and indicate that nanogranites represent the anatectic melt generated immediately after entering supersolidus conditions. The P–T estimate for garnet and melt development does not, however, overlap with the low‐temperature tip of the pure melt field in the phase diagram calculated for the composition of preserved glassy inclusions in garnet in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCKFMASH) system. A comparison of measured melt compositions formed immediately beyond the solidus with results of phase equilibria modelling points to the systematic underestimation of FeO, MgO and CaO in the calculated melt. These discrepancies are present also when calculated melts are compared with low‐T natural and experimental melts from the literature. Under such conditions, the available melt model does not perform well. Given the presence of melt inclusions in garnet cores and the P–T estimates for their formation, we argue that small amounts (<5 vol.%) of peritectic garnet may grow at low temperatures (≤700 °C), as a result of continuous melting reactions consuming biotite.     

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.     

Granulite facies thermal aureoles and metastable amphibolite facies assemblages adjacent to the Western Fiordland Orthogneiss in southwest Fiordland, New Zealand

In southwest New Zealand, a suite of felsic diorite intrusions known as the Western Fiordland Orthogneiss (WFO) were emplaced into the mid to deep crust and partially recrystallized to high‐P (12 kbar) granulite facies assemblages. This study focuses on the southern most pluton within the WFO suite (Malaspina Pluton) between Doubtful and Dusky sounds. New mapping shows intrusive contacts between the Malaspina Pluton and adjacent Palaeozoic metasedimentary country rocks with a thermal aureole ～200–1000 m wide adjacent to the Malaspina Pluton in the surrounding rocks. Thermobarometry on assemblages in the aureole indicates that the Malaspina Pluton intruded the adjacent amphibolite facies rocks while they were at depths of 10–14 kbar. Similar P–T conditions are recorded in high‐P granulite facies assemblages developed locally throughout the Malaspina Pluton. Palaeozoic rocks more than ～200–1000 m from the Malaspina Pluton retain medium‐P mid‐amphibolite facies assemblages, despite having been subjected to pressures of 10–14 kbar for > 5 Myr. These observations contradict previous interpretations of the WFO Malaspina Pluton as the lower plate of a metamorphic core complex, everywhere separated from the metasedimentary rocks by a regional‐scale extensional shear zone (Doubtful Sound Shear Zone). Slow reaction kinetics, lack of available H₂O, lack of widespread penetrative deformation, and cooling of the Malaspina Pluton thermal anomaly within c. 3–4 Myr likely prevented recrystallization of mid amphibolite facies assemblages outside the thermal aureole. If not for the evidence within the thermal aureole, there would be little to suggest that gneissic rocks which underlie several 100 km² of southwest New Zealand had experienced metamorphic pressures of 10–14 kbar. Similar high‐P metamorphic events may therefore be more common than presently recognized.     

P–T–t–d evolution of orogenic middle crust of the Roc de Frausa Massif (Eastern Pyrenees): a result of horizontal crustal flow and Carboniferous doming?

Structural, petrological and textural studies are combined with phase equilibria modelling of metapelites from different structural levels of the Roc de Frausa Massif in the Eastern Pyrenees. The pre‐Variscan lithological succession is divided into the Upper, Intermediate and Lower series by two orthogneiss sheets and intruded by Variscan igneous rocks. Structural analysis reveals two phases of Variscan deformation. D1 is marked by tight to isoclinal small‐scale folds and an associated flat‐lying foliation (S1) that affects the whole crustal section. D2 structures are characterized by tight upright folds facing to the NW with steep NE–SW axial planes. D2 heterogeneously reworks the D1 fabrics, leading to an almost complete transposition into a sub‐vertical foliation (S2) in the high‐grade metamorphic domain. All structures are affected by late open to tight, steeply inclined south‐verging NW–SE folds (F3) compatible with steep greenschist facies dextral shear zones of probable Alpine age. In the micaschists of the Upper series, andalusite and sillimanite grew during the formation of the S1 foliation indicating heating from 580 to 640 °C associated with an increase in pressure. Subsequent static growth of cordierite points to post‐D1 decompression. In the Intermediate series, a sillimanite–biotite–muscovite‐bearing assemblage that is parallel to the S1 fabric is statically overgrown by cordierite and K‐feldspar. This sequence points to ~1 kbar of post‐D1 decompression at 630–650 °C. The Intermediate series is intruded by a gabbro–diorite stock that has an aureole marked by widespread migmatization. In the aureole, the migmatitic S1 foliation is defined by the assemblage biotite–sillimanite–K‐feldspar–garnet. The microstructural relationships and garnet zoning are compatible with the D1 pressure peak at ~7.5 kbar and ~750 °C. Late‐ to post‐S2 cordierite growth implies that F2 folds and the associated S2 axial planar leucosomes developed during nearly isothermal decompression to <5 kbar. The Lower series migmatites form a composite S1–S2 fabric; the garnet‐bearing assemblage suggests peak P–T conditions of >5 kbar at suprasolidus conditions. Almost complete consumption of garnet and late cordierite growth points to post‐D2 equilibration at <4 kbar and <750 °C. The early metamorphic history associated with the S1 fabric is interpreted as a result of horizontal middle crustal flow associated with progressive heating and possible burial. The upright F2 folding and S2 foliation are associated with a pressure decrease coeval with the intrusion of mafic magma at mid‐crustal levels. The D2 tectono‐metamorphic evolution may be explained by a crustal‐scale doming associated with emplacement of mafic magmas into the core of the dome.     

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.     

Palaeoproterozoic high-T, low-P metamorphism and dehydration melting in metapelites from the Mopunga Range, Arunta Inlier, central Australia

A sequence of psammitic and pelitic metasedimentary rocks from the Mopunga Range region of the Arunta Inlier, central Australia, preserves evidence for unusually low pressure (c. 3 kbar), regional‐scale, upper amphibolite and granulite facies metamorphism and partial melting. Upper amphibolite facies metapelites of the Cackleberry Metamorphics are characterised by cordierite‐andalusite‐K‐feldspar assemblages and cordierite‐bearing leucosomes with biotite‐andalusite selvages, reflecting P–T conditions of c. 3 kbar and c. 650–680 °C. Late development of a sillimanite fabric is interpreted to reflect either an anticlockwise P–T evolution, or a later independent higher‐P thermal event. Coexistence of andalusite with sillimanite in these rocks appears to reflect the sluggish kinematics of the Al₂SiO₅ polymorphic inversion. In the Deep Bore Metamorphics, 20 km to the east, dehydration melting reactions in granulite facies metapelites have produced migmatites with quartz‐absent sillimanite‐spinel‐cordierite melanosomes, whilst in semipelitic migmatites, discontinuous leucosomes enclose cordierite‐spinel intergrowths. Metapsammitic rocks are not migmatised, and contain garnet–orthopyroxene–cordierite–biotite–quartz assemblages. Reaction textures in the Deep Bore Metamorphics are consistent with a near‐isobaric heating‐cooling path, with peak metamorphism occurring at 2.6–4.0 kbar and c. 750–800 °C. SHRIMP U–Pb dating of metamorphic zircon rims in a cordierite‐orthopyroxene migmatite from the Deep Bore Metamorphics yielded an age of 1730 ± 7 Ma, whilst detrital zircon cores define a homogeneous population at 1805 ± 7 Ma. The 1730 Ma age is interpreted to reflect the timing of high‐T, low‐P metamorphism, synchronous with the regional Late Strangways Event, whereas the 1805 Ma age provides a maximum age of deposition for the sedimentary precursor. The Mopunga Range region forms part of a more extensive low‐pressure metamorphic terrane in which lateral temperature gradients are likely to have been induced by localised advection of heat by granitic and mafic intrusions. The near‐isobaric Palaeoproterozoic P–T–t evolution of the Mopunga Range region is consistent with a relatively transient thermal event, due to advective processes that occurred synchronous with the regional Late Strangways tectonothermal event.     

Palaeoproterozoic evolution of the Challenger Au deposit, South Australia, from monazite geochronology

Monazite in granulite facies metatexite migmatites (Christie Gneiss) hosting the Challenger Au deposit, South Australia, records a series of growth and resorption stages over a c. 60 Myr period between 2470 and 2410 Ma. A combination of electron microprobe X‐ray mapping and in situ ion‐microprobe dating was used to delineate and date five compositional domains. The oldest prograde metamorphic components are preserved in granoblastic gneisses surrounding the deposit, and as small high‐Y cores in large monazite grains in Au‐bearing migmatites. In metatexite leucosomes, these cores were partially resorbed prior to the growth of large high‐Th monazite domains that crystallized during partial melting and stromatic migmatite development at c. 2443 Ma. Subsequent heating to biotite dehydration conditions (c. 850 °C at 7 kbar) caused further partial melting roughly 10–15 Myr later, giving rise to c. 2428 Ma domains surrounding partly resorbed 2443 Ma grains that were entrained in the higher‐temperature melts. This period of partial melting coincided with isoclinal folding culminating in dextral transpression and represents the most likely window for remobilization of Au‐bearing polymetallic sulphide melts into low‐strain domains. Localized reaction of residual melt with the granulite facies assemblage during cooling gave rise to narrow high‐Y rims dated at 2414 ± 7 Ma. Although monazite from unmineralized granoblastic gneisses and migmatitic ore zones display the same range of U‐Pb dates, monazite in migmatites displays a higher overall Ca + Th + U content, indicating that compositional heterogeneities between ore zones and host rocks developed prior to 2470 Ma, perhaps a consequence of the hydrothermal alteration inferred to have accompanied gold mineralization.     

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.     

Fluids in low-pressure migmatites: a fluid inclusion study of rocks from the Gennargentu Igneous Complex (Sardinia, Italy)

Summary The low-pressure emplacement of a quartz diorite body in the metapelitic rocks of the Gennargentu Igneous Complex (Sardinia, Italy) produced a contact metamorphic aureole and resulted in migmatisation of part of the aureole through partial melting. The leucosome, formed by dehydration melting involving biotite, is characterised by granophyric intergrowth and abundant magnetite crystals. A large portion of the high temperature contact aureole shows petrographic features that are intermediate between quartz diorite and migmatite s.s. (i.e. hybrid rocks). A fluid inclusion study has been performed on quartz crystals from the quartz diorite and related contact aureole rocks, i.e. migmatite sensu stricto (s.s.) and hybrid rocks. Three types of fluid inclusions have been identified: I) monophase V inclusions, II) L + V, either L-rich or V-rich aqueous saline inclusions and III) multiphase V + L + S inclusions. Microthermometric data characterised the trapped fluid as a complex aqueous system varying from H₂O–NaCl–CaCl₂ in the quartz diorite to H₂O–NaCl–CaCl₂–FeCl₂ in the migmatite and hybrid rocks. Fluid salinities range from high saline fluids (50 wt% NaCl eq.) to almost pure aqueous fluid. Liquid-vapour homogenisation temperatures range from 100 to over 400 °C with an average peak around 300 °C. Temperatures of melting of daughter minerals are between 300 and 500 °C. Highly saline liquid- and vapour-rich inclusions coexist with melt inclusions and have been interpreted as brine exsolved from the crystallising magma. Fluid inclusion data indicate the formation of fluid of high iron activity during the low-pressure partial melting and a fluid mixing process in the hybrid rocks.     

P–T–t–D record of crustal‐scale horizontal flow and magma‐assisted doming in the SW Mongolian Altai

The Chandman massif, a typical structure of the Mongolian Altai, consists of a migmatite–magmatite core rimmed by a lower grade metamorphic envelope of andalusite and cordierite‐bearing schists. The oldest structure in the migmatite–magmatite core is a subhorizontal migmatitic foliation S1 parallel to rare granitoid sills. This fabric is folded by upright folds F2 and transposed into a vertical migmatitic foliation S2 that is syn‐tectonic, with up to several tens of metres thick granitoid sills. Sillimanite–ilmenite–magnetite S1 inclusion trails in garnet constrain the depth of equilibration during the S1 fabric to 6–7 kbar at 710–780 °C. Reorientation of sillimanite into the S2 fabric indicates that the S1–S2 fabric transition occurred in the sillimanite stability field. The presence of cordierite, and garnet rim chemistry point to decompression to 3–4 kbar and 680–750 °C during development of the S2 steep fabric, and post‐tectonic andalusite indicates further decompression to 2–3 kbar and 600–650 °C. Widespread crystallization of post‐tectonic muscovite is explained by the release of H₂O from crystallizing partial melt. In the metamorphic envelope the subhorizontal metamorphic schistosity S1 is heterogeneously affected by upright F2 folds and axial planar subvertical cleavage S2. In the north, the inclusion trails in garnet are parallel to the S1 foliation, and the garnet zoning indicates nearly isobaric heating from 2.5 to 3 kbar and 500–530 °C. Cordierite contains crenulated S1 inclusion trails and has pressure shadows related to the formation of the S2 fabric. The switch from the S1 to the S2 foliation occurred near 2.5–3 kbar and 530–570 °C; replacement of cordierite by fine‐grained muscovite and chlorite indicates further retrogression and cooling. In the south, andalusite containing crenulated inclusion trails of ilmenite and magnetite indicates heating during the D2 deformation at 3–4 kbar and 540–620 °C. Monazite from a migmatite analysed by LASS yielded elevated HREE concentrations. The grain with the best‐developed oscillatory zoning is 356 ± 1.0 [±7] Ma (²⁰⁷Pb‐corrected ²³⁸U/²⁰⁶Pb), considered to date the crystallization from melt in the cordierite stability ~680 °C and 3.5 kbar, whereas the patchy BSE‐dark domains give a date of 347 ± 4.2 [±7] Ma interpreted as recrystallization at subsolidus conditions. The earliest sub‐horizontal fabric is associated with the onset of magmatism and peak of P–T conditions in the deep crust, indicating important heat input associated with lower crustal horizontal flow. The paroxysmal metamorphic conditions are connected with collapse of the metamorphic structure, an extrusion of the hot lower crustal rocks associated with vertical magma transfer and a juxtaposition of the hot magmatite–migmatite core with supracrustal rocks. This study provides information about tectono‐thermal history and time‐scales of horizontal flow and vertical mass and heat transfer in the Altai orogen. It is shown that, similar to collisional orogens, doming of partially molten rocks assisted by syn‐orogenic magmatism can be responsible for the exhumation of orogenic lower crust in accretionary orogenic systems.     

Near-isothermal decompression within a clockwise P–T evolution recorded in migmatitic mafic granulites from Clavering Ø, NE Greenland: implications for the evolution of the Caledonides

The high grade rocks (metapelites and metabasites) of Clavering Ø represent the easternmost exposures of granulites in the Palaeozoic Caledonian Orogen of East Greenland. Mafic granulites which occur as sheet‐like bodies and lenses within metapelitic migmatites and orthogneiss complexes have experienced migmatisation and mineral equilibria which define a clockwise P–T path incorporating a near‐isothermal decompression segment. Textures demonstrate the existence of early garnet‐clinopyroxene‐melt assemblages which equilibrated at >8–11 kbar and 850–915 °C. Subsequently, decompression melting led to formation of orthopyroxene‐plagioclase‐melt assemblages at conditions below >8–11 kbar. Continued syn‐deformational decompression is indicated by a combination of both static and syn‐deformational recrystallization textures which generated finer grained orthopyroxene‐plagioclase assemblages. P–T constraints indicate these assemblages equilibrated at c. 5.0–6.5 kbar at 850–915 °C. These data are consistent with the rocks undergoing a stage of rapid tectonic‐induced exhumation involving some 3.0–4.5 kbar (c.10–12 km) uplift as part of a clockwise P–T path in a collisional setting.     

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     

The evolution of zircon during low‐P partial melting of metapelitic rocks: theoretical predictions and a case study from Mt Stafford,central Australia

Palaeoproterozoic metasedimentary migmatite reflects the highest temperature parts of a regional aureole at Mt Stafford, central Australia, comprising rocks that experienced 500–800 °C at ≈3 kbar. Whole‐rock major element concentrations are correlated with Zr content, psammitic compositions having nearly twice the Zr content of pelitic compositions. Zirconium is concentrated in mesosome compared with leucosome. Zircon is largely detrital, mostly lacking any overgrowth contemporary with migmatite formation. Comparatively small proportions of micro‐zircon (<10 μm) in sub‐solidus rocks are mostly hosted by quartz and plagioclase. Much higher proportions (three to five times) of micro‐zircon in migmatite are hosted by prograde K‐feldspar, cordierite and biotite. T–X and P–T NCKFMASHTZr pseudosections constructed using thermocalc model the distribution of Zr between solid and silicate liquid phases. Half of the detrital zircon (~100 ppm Zr) is predicted to be dissolved into silicate liquid at ≈800 °C and all dissolved by 850 °C, if all zircon is involved in the equilibration volume. Melt segregation at relatively low temperature is predicted to enrich the residuum in Zr, consistent with the observed distribution of Zr between mesosome and leucosome. The limited development of metamorphic zircon rims or overgrowths at Mt Stafford is explained by three concurrent processes: (i) Zr liberated during prograde metamorphism formed micro‐zircon, rather than following the prediction that Zr will partition into silicate liquid; (ii) some detrital zircon was probably armoured by other rock‐forming minerals, reducing Zr content in the effective bulk rock composition; and (iii) small proportions of melt loss during migmatization removed Zr that otherwise would have been available to form metamorphic rims.     

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     

Evolution of structurally contrasting anatectic migmatites in the 3-kbar Ballachulish aureole, Scotland

Abstract Anatectic migmatites of contrasting structural style are found adjacent to the contacts of the Ballachulish Igneous Complex, Argyllshire, Scotland. On the east flank, evidence for migmatization is largely restricted to the local development of millimetre-centimetre scale Kfs + Qtz-rich leucocratic segregations, which accompany fragmentation of brittle hornfels layers and ductile deformation of mm-cm scale semipelitic layers. Large volumes of semipelitic rock rich in feldspar and quartz on the east flank show no migmatitic features, and bedding is usually preserved undisturbed right up to the contact. On the west flank, in contrast, similar semipelitic rocks show widespread migmatitic features and disruption of layering is substantial and widespread over a 400 m wide zone. Within the west-flank migmatites, 1–100 cm scale rigid bedding fragments (schollen) may be suspended and disoriented in a semipelitic matrix that underwent ductile deformation. The P-T conditions on both flanks are in the same range: 3 kbar and 650–700°C. The contrast in gross structural style is believed to result from differences in the volumes of melt produced and differences in the proportion of rock in which the critical melt fraction of the rocks was exceeded. On the east flank, only on a mm-cm scale was enough melt locally accumulated to cause disruption of some layers and segregation of melt. On the west flank, melting proceeded substantially in a broad tract of semipelitic rocks, resulting in larger scale contrasts in rheology that led to the present chaotic structures in this zone. Because migmatization occurred at a pressure too low for muscovite dehydration melting, and at temperatures too low for substantial biotite dehydration melting, the different amounts of melting on the east and west flanks most probably resulted from the introduction of differing amounts of externally derived water. On the east flank, and throughout most of the aureole, the absence of melting even in quartzofeldspathic protoliths indicates that there was no substantial movement of fluid towards or away from the igneous complex during migmatization. The contrasting situation on the west flank may have resulted from devolatilization of underlying quartz diorite magma (? 690–710°C), which released heat and fluids into the overlying quartz- and feldspar-rich semipelites (solidus temperature ? 650–680°C).     

UHT metamorphism on Seram,eastern Indonesia: reaction microstructures and P–T evolution of spinel‐bearing garnet–sillimanite granulites from the Kobipoto Complex

The island of Seram, part of the northern limb of the Banda Arc in eastern Indonesia, exposes an extensive Mio‐Pliocene granulite facies migmatite complex (the Kobipoto Complex) comprising voluminous leucosome‐rich diatexites and scarcer Al–Fe‐rich residual granulites. The migmatites are intimately associated with ultramafic rocks of predominantly lherzolitic composition that were exhumed by substantial lithospheric extension beneath low‐angle detachment faults; heat supplied by the lherzolites was evidently a major driver for the granulite facies metamorphism and accompanying anatexis. Residual garnet–sillimanite granulites sampled from the Kobipoto Mountains, central Seram, contain scarce garnet‐hosted inclusions of hercynite spinel (~1.5 wt% ZnO) + quartz (± ilmenite) in direct grain‐boundary contact – an assemblage potentially indicative of metamorphism under ultrahigh‐temperature (UHT) conditions. thermocalc ‘Average P–T’ reactions and melanosome‐specific thermocalc , T–M_O, and P–T pseudosections in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCKFMASHTO) chemical system, supported by Ti‐in‐garnet thermobarometry, are permissive of the rock having experienced a clockwise P–T path peaking at 925 °C and 9 kbar – thus narrowly reaching UHT conditions – before undergoing near‐isothermal decompression to ~750 °C and ~4 kbar. Spinel + quartz assemblages are interpreted to have formed at or just after the metamorphic peak from localized reactions between sillimanite, ilmenite and surrounding garnet. Further decompression of the rock resulted in the formation of complex reaction microstructures comprising cordierite ± plagioclase coronae around garnet, and symplectic intergrowths of cordierite + spinel + ilmenite around sillimanite. Small grains of sapphirine + corundum developed subsequently within spinel by localized quartz‐absent reactions. The post‐peak evolution of the granulites may be related to previously published U–Pb zircon and ⁴⁰Ar/³⁹Ar ages of c. 16 Ma, further substantiating the claim for the Kobipoto Complex granulites having recorded Earth's youngest‐identified episode of UHT metamorphism, albeit at slightly lower temperature and higher pressure than previously inferred. The Kobipoto Complex granulites demonstrate how UHT conditions may be achieved in the ‘modern’ Earth by extreme lithospheric extension, which, in this instance, was driven by slab rollback of the Banda Arc.     

Two successive phases of ultrahigh temperature metamorphism in Rogaland,S. Norway: Evidence from Y‐in‐monazite thermometry

In Rogaland, South Norway, a polycyclic granulite facies metamorphic domain surrounds the late‐Sveconorwegian anorthosite–mangerite–charnockite (AMC) plutonic complex. Integrated petrology, phase equilibria modelling, monazite microchemistry, Y‐in‐monazite thermometry, and monazite U–Th–Pb geochronology in eight samples, distributed across the apparent metamorphic field gradient, imply a sequence of two successive phases of ultrahigh temperature (UHT) metamorphism in the time window between 1,050 and 910 Ma. A first long‐lived metamorphic cycle (M1) between 1,045 ± 8 and 992 ± 11 Ma is recorded by monazite in all samples. This cycle is interpreted to represent prograde clockwise P–T path involving melt production in fertile protoliths and culminating in UHT conditions of ~6 kbar and 920°C. Y‐in‐monazite thermometry, in a residual garnet‐absent sapphirine–orthopyroxene granulite, provides critical evidence for average temperature of 931 and 917°C between 1,029 ± 9 and 1,006 ± 8 Ma. Metamorphism peaked after c. 20 Ma of crustal melting and melt extraction, probably supported by a protracted asthenospheric heat source following lithospheric mantle delamination. Between 990 and 940 Ma, slow conductive cooling to 750–800°C is characterized by monazite reactivity as opposed to silicate metastability. A second incursion (M2) to UHT conditions of ~3.5–5 kbar and 900–950°C, is recorded by Y‐rich monazite at 930 ± 6 Ma in an orthopyroxene–cordierite–hercynite gneiss and by an osumilite gneiss. This M2 metamorphism, typified by osumilite paragenesis, is related to the intrusion of the AMC plutonic complex at 931 ± 2 Ma. Thermal preconditioning of the crust during the first UHT metamorphism may explain the width of the aureole of contact metamorphism c. 75 Ma later, and also the rarity of osumilite‐bearing assemblages in general.     

Microstructure and compositional changes across biotite‐rich reaction selvedges around mafic schollen in a semipelitic diatexite migmatite

Biotite‐rich selvedges developed between mafic schollen and semipelitic diatexite in migmatites at Lac Kénogami in the Grenville Province of Quebec. Mineral equilibria modelling indicates that partial melting occurred in the mid‐crust (4.8–5.8 kbar) in the range 820–850°C. The field relations, petrography, mineral chemistry and whole‐rock composition of selvedges along with their adjacent mafic schollen and host migmatites are documented for the first time. The selvedges measured in the field are relatively uniform in width (~1 cm wide) irrespective of the shape or size of their mafic scholle. In thin section, the petrographic boundary between mafic scholle and selvedge is defined by the appearance of biotite and the boundary between selvedge and diatexite by the change in microstructure for biotite, garnet, plagioclase and quartz. Three subtypes of selvedges are identified according to mineral assemblage and microstructure. Subtype I have orthopyroxene but of different microstructure and Mg# to orthopyroxene in the mafic scholle; subtype II contain garnet with many mineral inclusions, especially of ilmenite, in contrast to garnet in the diatexite host which has few inclusions; subtype III lack orthopyroxene or garnet, but has abundant apatite. Profiles showing the change in plagioclase composition from the mafic schollen across the selvedge and into the diatexite show that each subtype of selvedge has a characteristic pattern. Four types of biotite are identified in the selvedges and host diatexite based on their microstructural characteristics. (a) Residual biotite forms small rounded red‐brown grains, mostly as inclusions in peritectic cordierite and garnet in diatexite; (b) selvedge biotite forms tabular subhedral grains with high respect ratio; (c) diatexite biotite forms tabular subhedral grains common in the matrix of the diatexite; and (d) retrograde biotite that partially replaces peritectic cordierite and garnet in the diatexite. The four groups of biotite are also discriminated by their major element (EMPA) and trace elements (LA‐Q‐ICP‐MS) compositions. Residual biotite is high in TiO₂ and low in Sc and S, whereas retrograde biotite has high Al₂O₃, but low Sc and Cr. Selvedge and diatexite biotite are generally very similar, but selvedge biotite has higher Sc and S contents. Whole‐rock compositional profiles across the selvedges constructed from micro‐XRF and LA‐Q‐ICP‐MS analyses show: (a) Al₂O₃, FeO, MgO and CaO all decrease from mafic scholle across the selvedge and into the diatexite; (b) Na₂O is lowest in the mafic scholle, rises through the selvedge and reaches its maximum about 20–30 mm into the diatexite host; (c) K₂O is lowest in the mafic scholle and reaches its highest value in the first half of the selvedge, then declines before reaching a higher, but intermediate value, about 20 mm into the diatexite. Of the trace elements, Cs and Rb show distributions very similar to K₂O.