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Preservation of partially completed metamorphic reactions in the form of partial pseudomorphs is very important as it provides direct insight onto the reaction mechanism and the phases involved in the reaction. The staurolite and andalusite grade rocks in western Maine, USA, contain cordierite porphyroblasts partly pseudomorphed by coarse‐grained muscovite and biotite. The pseudomorphs consist of a cordierite core surrounded by a reaction rim. Modal mineralogy, calculated using the ImageJ processing software based on backscatter images and X‐ray compositional maps, reveals that the core consists of cordierite (53.5%), muscovite (22.8%), biotite (9.1%), quartz (1 0.4%), plagioclase (3.1%) and ilmenite/pyrrhotite and apatite (1.1%) whereas the reaction rim consists of cordierite (1.8%), muscovite (51.6%), biotite (30.4%), quartz (4.3%), plagioclase (10%), garnet (1.2%), ilmenite/pyrrhotite and apatite (0.8%). The net effect of the cordierite breakdown reaction is an increase of 226% in muscovite, 334% in biotite and 323% in plagioclase content and a decrease of 97% in cordierite. The reaction involved exchange of components with the matrix requiring addition of H2O, K+, Na+ and Ti4+ and removal of SiO2, Mg2+ and PO43‐ from the reaction site. PT estimates using the garnet–biotite, Ti‐in‐biotite, Na‐in‐cordierite thermometers and the garnet–biotite–muscovite–plagioclase barometer indicate that cordierite breakdown occurred at ~550°C and 3.5 kbar. thermocalc modelling using the bulk rock composition suggests that cordierite is not stable at these conditions, whereas modelling using a thin section‐derived bulk composition indicates that cordierite stability extends to higher pressures, and most likely that the cordierite breakdown was not PT dependent. The incorporation of Na (up to 0.18 a.f.u.) into the cordierite structure has the effect of stabilizing the cordierite under a variety of H2O activity and limiting the role of fluids into destabilizing it. The cordierite cores contain evidence of plastic and brittle deformation in the form of subgrains and microcracks, which facilitated the infiltration of fluids that destabilized cordierite at constant PT conditions by leaching Na and introducing K. New mica growth along these structural heterogeneities suggests that deformation played an important role promoting breakdown of cordierite to muscovite and biotite.

Silicon carbide reticulated porous ceramics (SiC RPCs) with multi-layered struts were fabricated by polymer replica technique with SiC slurry, followed by infiltrating alumina slurries containing andalusite under vacuum condition. The effects of andalusite addition on the microstructure and mechanical properties of SiC RPCs were investigated, also the residual stress within the multi-layered strut was predicted. Theoretical calculations showed that the residual tensile stress generated in the outer layer of SiC RPCs because of its larger thermal expansion coefficient of infiltration slurry than that of SiC slurry at elevated temperature. Furthermore, the addition of andalusite reduced the thermal expansion coefficient and Young's modulus of infiltration slurries, thereby significantly reducing the residual stress of the outer layer in multi-layered struts. The reduced residual tensile stress within the outer layer was beneficial to eliminate surface cracks on the struts, thus improving the mechanical properties and thermal shock resistance of SiC RPCs.

New petrographic and microstructural observations, mineral equilibria modelling and U/Pb (monazite) geochronological studies were carried out to investigate relationship between deformation and metamorphism across the Rehamna massif (Moroccan Variscan belt). In this area, typical Barrovian (muscovite to staurolite) zones developed in Cambrian to Carboniferous metasedimentary rocks that are distributed around a dome-like structure. First assemblages are characterized by the presence of locally preserved andalusite, followed by prograde evolution culminating at 6 kbar and 620 °C in the structurally deepest staurolite zone rocks. This Barrovian sequence was subsequently uplifted to supracrustal levels, heterogeneously reworked at greenschist facies conditions, which was followed locally by static growth of andalusite, indicating heating to 2.5−4 kbar and 530−570 °C. The 206Pb/238U monazite age of 298.3 ± 4.1 Ma is interpreted as minimum age of peak metamorphic conditions whereas the ages of 275.8 ± 1.7 Ma and 277.0 ± 1.1 Ma date decompression and heating at low pressure, in agreement with previous dating of Permian granitoids intruding the Rehamna massif. The prograde metamorphism occurred during thickening and associated horizontal flow in the deeper crust (S1 horizontal schistosity). The horizontally disposed metamorphic zones were subsequently uplifted by a regional scale antiform during ongoing N-S compression. The re-heating of the massif follows late massive E-W shortening, refolding and retrograde shearing of all previous fabrics coevally with regionally important intrusions of Permian granitoids. We argue that metamorphic evolution of the Rehamna massif occurred several hundred kilometres from the convergent plate boundaries in the interior of continental Gondwanan plate. The tectonometamorphic history of the Rehamna massif is put into Palaeozoic plate tectonic perspective and Late Carboniferous reactivation of (Devonian)-Early Carboniferous basins formed during stretching of the north Gondwana margin and formation of the Palaeotethys Ocean. The inherited heat budget of these magma-rich basins plays a role in the preferential location of this intracontinental orogen. It is shown that rapid transition from lithospheric stretching to compression is characterized by specific HT type of Barrovian metamorphism, which markedly differs from similar Barrovian sequences along Palaeozoic plate boundaries reported from Variscan Europe. This article is protected by copyright. All rights reserved.

This paper investigates the effects of Iranian andalusite and short milling times on alumina–mullite–zirconia composites. Andalusite powder was added at 0, 2.5, 5, and 10 wt% to an alumina–zircon mixture and the raw materials were milled for 1 or 3 h. The sintering of samples was carried out at the temperatures of 1550°C, 1600°C, and 1650°C for 3 h. Microstructural changes, phase composition, physical properties, and mechanical strength of the sintered composites were characterized by scanning electron microscopy, X-ray diffraction, density, and strength measurement tests. Results show that andalusite promoted the decomposition of zircon and accelerated the reaction sintering of alumina–zircon, which leads to the formation of much more mullite phase and improvements to the composites’ thermal shock resistance up to about 50%.

Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond have been recovered, most of which are pale yellow to reddish-orange to colorless. The grains are all 100–200 µm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm−1 and 1333 cm−1, mostly at 1331.51 cm−1 or 1326.96 cm−1. Integration of the mineralogical, petrological and geochemical data for the Dongqiong peridotites suggests a multi-stage formation for this body and similar ophiolites in the Yarlung-Zangbo suture zone. Chromian spinel grains and perhaps small bodies of chromitite crystallized at various depths in the upper mantle, and encapsulated the UHP, highly reduced and crustal minerals. Some oceanic crustal slabs containing the chromian spinel and their inclusion were later trapped in suprasubduction zones (SSZ), where they were modified by island arc tholeiitic and boninitic magmas, thus changing the chromian spinel compositions and depositing chromitite ores in melt channels.

Contact metamorphism of greenschist facies Neoproterozoic turbidites by the Cretaceous Bugaboo Batholith in southeastern British Columbia has resulted in a well-developed contact aureole. The aureole is about 1 km wide and can be divided into three main zones: (i) spotted phyllite zone, extending from the first appearance of spots of cordierite or andalusite to the last occurrence of primary chlorite; (ii) cordierite + andalusite + biotite zone, comprising hornfelses or schists with abundant porphyroblasts of cordierite and andalusite and, at higher grades, fibrolitic sillimanite; and (iii) K-feldspar zone, characterized by hornfelses and schists that, in the inner part of this zone, are variably migmatitic. Four parts of the aureole were examined, three of which are characterized by schists, and one of which (Cobalt Lake area) is characterized by hornfelses and has exceptional exposure and comparatively unaltered rocks. Petrographic, modal, mineral-compositional and whole rock-compositional data were collected from the Cobalt Lake transect, allowing the prograde reaction sequence to be inferred. Notable features of the aureole at Cobalt Lake include: initial development of andalusite and plagioclase at the expense of paragonite-rich white mica; a narrow interval across which cordierite, andalusite and biotite increase markedly at the expense of chlorite; gradual development of andalusite and biotite at the expense of cordierite and muscovite upgrade of chlorite consumption; and near-simultaneous development of andalusite + K-feldspar and sillimanite, the latter indicating a pressure of contact metamorphism of ~3 kbar. In other parts of the aureole, the development of sillimanite downgrade of the initial development of K-feldspar suggests slightly higher pressures of contact metamorphism. Lack of correspondence between the observed sequence of reactions in the aureole and those predicted thermodynamically suggests that modifications to some of the thermodynamic data or activity–composition models may be required. Textural features in the aureole suggest the influence of kinetic factors on metamorphic recrystallization, including: (i) deformation-catalysed reaction in the schists compared to the hornfelses, as indicated by different mineral-growth sequences inferred from microstructures, and (ii) heating rate-controlled recrystallization, as indicated by the decrease in grain size of hornfelses with increasing metamorphic grade.

Pegmatite dikes bearing andalusite crosscut foliation S2 in Alpujarride gneisses and schists. Post-S2 andalusite is transposed by a foliation S3, defined by fibrolite, which affects the dikes. The dikes represent highly differentiated granitic magmas with low REE and Zr contents and a positive Eu anomaly. U-Pb SHRIMP dating of magmatic zircons provided Pan-African ages (cores) and late Variscan ages (rims). However, U-rich rims also provided metamorphic Alpine ages, supporting a polyorogenic tectonometamorphic history for pre-Mesozoic Alpujarride rocks.

Experimental data on diffusion in olivine , are used to define certain terms – diffusion coefficient, jump frequency, characteristic distance, random walk – that are useful in a discussion of atom displacements under natural conditions. Examples of atom displacements in two metamorphic terranes of the Canadian Precambrian Shield are then examined, as follows. (i) In a high-grade metamorphic terrane in the Mid-Proterozoic Grenville Province (Otter Lake Area), Mg concentration gradients about dolomite microcrystals in calcite and Na gradients about albite microcrystals in K-feldspar are viewed as stranded Mg–Ca and Na–K interdiffusion gradients, formed by exsolution during slow cooling from ~700 to ~400 °C. (ii) In the Archean Slave Province (Yellowknife area), the crystallization of sillimanite, near andalusite but within crystals of quartz, possibly occurred by coupled Al–Si and oxygen–vacancy interdiffusion in quartz at ~550 °C. And the crystallization of garnet from chlorite occurred by the two-way crystal-boundary diffusion of several kinds of atoms across distances ranging to 3 mm. (iii) In the Otter Lake area, the crystallization of orthopyroxene–hornblende–spinel reaction zones at boundaries between crystals of olivine and plagioclase in metagabbro, evidently occurred by the mechanism of interstitial diffusion, that transported Mg, Fe, Mn and O atoms across the reaction zone from olivine to the plagioclase–(hornblende+spinel) boundary, and Si, Al, Ca and Na atoms from plagioclase to the olivine–orthopyroxene boundary, accompanied by NaSi–CaAl interdiffusion in plagioclase, and the addition of hydrogen and minor Ti, Zn, F, Cl and K from beyond the reaction zone. Also, centimetric reaction zones, with abundant biotite and plagioclase, at boundaries between K-feldspar gneiss and deformed amphibolite dykes, evidently formed by the reaction, strained hornblende (in amphibolite) + K-feldspar (in gneiss)biotite (in amphibolite) + plagioclase (in gneiss), with crystal-boundary diffusion of (Na + Ca) atoms and of K atoms across the reaction zone.

A section of the orogenic middle crust (Orlica-nienik Dome, Polish/Czech Central Sudetes) was examined to constrain the duration and significance of deformation (D) and intertectonic (I) phases. In the studied metasedimentary synform, three deformation events produced an initial subhorizontal foliation S1 (D-1), a subsequent subvertical foliation S2 (D-2) and a late subhorizontal axial planar cleavage S3 (D-3). The synform was intruded by pre-, syn- and post-D-2 granitoid sheets. Crystallization-deformation relationships in mica schist samples document I1-2 garnet-staurolite growth, syn-D-2 staurolite breakdown to garnet-biotite-sillimanite/andalusite, I2-3 cordierite blastesis and late-D-3 chlorite growth. Garnet porphyroblasts show a linear Mn-Ca decrease from the core to the inner rim, a zone of alternating Ca-Y- and P-rich annuli in the inner rim, and a Ca-poor outer rim. The Ca-Y-rich annuli probably reflect the occurrence of the allanite-to-monazite transition at conditions of the staurolite isograd, whereas the Ca-poor outer rim is ascribed to staurolite demise. The reconstructed P-T path, obtained by modelling the stability of parageneses and garnet zoning, documents near-isobaric heating from similar to 4kbar/485 degrees C to similar to 4.75kbar/575 degrees C during I1-2. This was followed by a progression to 4-5kbar/580-625 degrees C and a subsequent pressure decrease to 3-4 kbar during D-2. Pressure decrease below 3kbar is ascribed to I2-3, whereas cooling below similar to 500 degrees C occurred during D-3. In the dated mica schist sample, garnet rims show strong Lu enrichment, oscillatory Lu zoning and a slight Ca increase. These features are also related to allanite breakdown coeval with staurolite appearance. As Lu-rich garnet rims dominate the Lu-Hf budget, the 344 +/- 3Ma isochron age is ascribed to garnet crystallization at staurolite grade, near the end of I1-2. For the dated sample of amphibole-biotite granitoid sheet, a Pb-Pb single zircon evaporation age of 353 +/- 1Ma is related to the onset of plutonic activity. The results suggest a possible Devonian age for D-1, and a Carboniferous burial-exhumation cycle in mid-crustal rocks that is broadly coeval with the exhumation of neighbouring HP rocks during D-2. In the light of published ages, a succession of telescoping stages with time spans decreasing from c. 10 to 2-3Ma is proposed. The initially long period of tectonic quiescence (I1-2 phase, c. 10Ma) inferred in the middle crust contrasts with contemporaneous deformation at deeper levels and points to decoupled P-T-D histories within the orogenic wedge. An elevated gradient of similar to 30 degrees Ckm(-1) and assumed high heating rates of c. 20 degrees CMa(-1) are explained by the protracted intrusion of granitoid sheets, with or without deformation, whereas fast vertical movements (2-3Ma, D-2 phase) in the crust require the activity of deformation phases.

It is generally thought that garnet in metapelites is produced by continuous reactions involving chlo- rite or chloritoid. Recent publications have suggested that the equilibrium temperatures of garnet-in reactions may be significantly overstepped in regionally metamorphosed terranes. The growth of small spessartine-almandine garnet crystals on Mn-siderite at the garnet isograd in graphitic metapel- ites in the Proterozoic Black Hills orogen, South Dakota, demonstrates that Mn-siderite was the prin- cipal reactant that produced the initial garnet in the schists. Moreover, the positions of garnet compositions in isobaric, T-(C/H) pseudosections for the schists show that the temperature of the garnet-in reaction from Mn-siderite was overstepped minimally at the most. In the Black Hills, garnet was initially produced during regional metamorphism beginning at c. 1755 Ma due to the collision of Wyoming and Superior cratons, and was subsequently partially or fully re-equilibrated at more ele- vated temperatures and pressures during intrusion of the Harney Peak Granite (HPG) at c. 1715 Ma. Garnet occurs in graphitic schists in garnet, staurolite and sillimanite zones, the latter being a product of contact metamorphism by HPG. During metamorphism, coexisting fluid contained both CO2 and CH4. In the garnet zone, garnet crystals contain petrographically distinct cores with inclusions of quartz, graphite and other minerals. Centres of the cores have distinctly elevated Y concentrations that mark the positions of garnet nucleation. The elevated Y is thought to have come from the Mn- siderite onto which Y was probably absorbed during precipitation in an ocean. In the upper garnet and staurolite zones, the cores were overgrown by inclusion-poor mantles. Mantles are highly zoned and have more elevated Fe and Mg and lower Mn and Ca than cores. The growth of mantles is attributed to late-orogenic heating by leucogranite magmas and attendant influx of H2O that caused consumption of graphite in rock matrices. A portion of the Proterozoic terrane that includes the HPG is surrounded by four large faults. In this 'HPG block', garnet is inclusion-poor and its compo- sition does not preserve its early growth history. This garnet appears to have re-equilibrated by inter- nal diffusion of its major components and/or recrystallization of an earlier inclusion-rich garnet. It has equilibrated within the kyanite stability range, and together with remnant kyanite in the high- strain aureole of the HPG, indicates that the HPG block had a ≥6 kbar history. The HPG block has undergone decompression during emplacement of the HPG. The decompression is evident in occur- rences of retrograde andalusite and cordierite in the thermal aureole of the HPG. The data support a polybaric metamorphic history of the Black Hills orogen with different segments of the orogen hav- ing their own clockwise P-T-t paths.

In situ LA–ICP–MS U–Pb monazite geochronology from the Boothby Hills in the Aileron Province, central Australia, indicates that the region records more than 80 Ma of high-T, low-P (HTLP) anatectic conditions during the Early Mesoproterozoic. Monazite ages from granulite facies rocks and leucosomes span the interval 1576–1542 Ma. Pegmatites that overprint the regional gneissic fabric and are interpreted to record the last vestiges of melt crystallization give ages between 1523 and 1513 Ma. Calculated P–T pseudosections suggest peak metamorphic conditions in excess of 850 °C at 0.65–0.75 GPa. The retrograde evolution was characterized by a P–T path that involved minor decompression and then cooling, culminating with the development of andalusite. Integration of the geochronological data set with the inferred P–T path trajectory suggests that suprasolidus cooling must have been slow, in the order of 2.5–4 °C Ma−1. In addition, the retrograde P–T path trajectory suggests that HTLP conditions were generated within crust of relatively normal thickness. Despite the long duration over which anatectic conditions occurred, there is no evidence for external magmatic inputs or evidence that HTLP conditions were associated with long-lived extension. Instead, it seems probable that the long-lived HTLP metamorphism was driven to a significant extent by long-lived conductive heating provided by high crustal heat production in voluminous pre-metamorphic granitic rocks. Refereed/Peer-reviewed

Constraints from P-T pseudosections (MnNCKFMASH system), foliation intersection/inflection axes preserved in porphyroblasts (FIAs), mineral assemblages and textural relationships for rocks containing all three Al2SiO5 polymorphs indicate a kyanite→ andalusite→ sillimanite sequential formation at different times rather than stable coexistence at the Al2SiO5 triple point. All three Al2SiO5 polymorphs grew in the Chl, Bt, Ms, Grt, St, Pl and Crd bearing Ordovician Clayhole Schist in Balcooma, northeastern Australia separately along a looped P-T-t-D path that swaps from clockwise to anticlockwise in the tectono-metamorphic history of the region. Kyanite grew during crustal thickening in an Early Silurian Orogenic event followed by decompression/heating, andalusite and fibrolitic sillimanite growth during Early Devonian exhumation.

Terra Nova, 25, 48–56, 2013 Abstract The Palaeoproterozoic Vallecito conglomerate of the Needle Mountains, Colorado, experienced deformation and metamorphism during emplacement of the ∼1.4 Ga Eolus batholith, producing a contact aureole defined by sillimanite near the pluton, andalusite and sillimanite together ∼2 km from the pluton, and andalusite alone at greater distance. In the andalusite–sillimanite zone, sillimanite-rich layers, poor in andalusite, define shear bands. In contrast, regions between shear bands contain abundant andalusite and sillimanite. A deformed sample is enriched in immobile elements, with an estimated major element mass loss of 43% relative to a weakly deformed sample. Modal and compositional variation between deformed and undeformed regions indicates that mass transfer was central to shear band development. This process changed the composition of the deformed sample, facilitated the localization of deformation, catalyzed metamorphism and allowed the assemblage to more rapidly approach equilibrium than weakly deformed assemblages.

The Soursat metamorphic complex (SMC) in northwestern Iran is part of the Sanandaj–Sirjan metamorphic belt. The complex is composed of different metamorphic and plutonic rocks, but is dominated by metapelites composed of garnet, staurolite, kyanite, fibrolite, cordierite, and andalusite. Porphyroblasts in schists have the same fabric, and three stages of schistosity are present. The internal schistosity (Sn) inclusion trails are also offset by conjugate sets of extensional schistosity (Sn+1) and a second (Sn+2) that crenulates (Sn+1). Polyphase metamorphisms are present in the complex. Garnet, staurolite, kyanite, and fibrolite assemblage preserves conditions during the M1 metamorphic event. This assemblage yields a P–t estimate of 645±11°C and 6.5±0.5 kbar. Other samples of the central part of SMC contain cordierite and andalusite (M2) overgrowth that yields a P–t estimate of 532±33°C and 2.1±1.1 kbar.

In the Orlica-Snieznik Dome (NE Bohemian massif), alternating belts of orthogneiss with high-pressure rocks and belts of mid-crustal metasedimentary-metavolcanic rocks commonly display a dominant subvertical fabric deformed into a subhorizontal foliation. The first macroscopic foliation is subvertical, strikes NE-SW and is heterogeneously folded by open to isoclinal folds with subhorizontal axial planes parallel to the heterogeneously developed flat-lying foliation. The metamorphic evolution of the mid-crustal metasedimentary rocks involved successive crystallization of chlorite-muscovite-ilmenite-plagioclase-garnet, followed by staurolite-bearing and then kyanite-bearing assemblages in the subvertical fabric. This was followed by garnet retrogression, with syntectonic crystallization of sillimanite and andalusite parallel to the shallow-dipping foliation. Elsewhere, andalusite and cordierite statically overgrew the flat-lying fabric. With reference to a P-T pseudosection for a representative sample, the prograde succession of mineral assemblages and the garnet zoning pattern with decreasing grossular, spessartine and X-Fe are compatible with a P-T path from 3.5-5 kbar/490-520 degrees C to peak conditions of 6-7 kbar/similar to 630 degrees C suggesting burial from 12 to 25 km with increasing temperature. Using the same pseudosection, the retrograde succession of minerals shows decompression to sillimanite stability at similar to 4 kbar/similar to 630 degrees C and to andalusite-cordierite stability at 2-3 kbar indicating exhumation from 25 km to around 9-12 km. Subsequent exhumation to similar to 6 km occurred without apparent formation of a deformation fabric. The structure and petrology together with the spatial distribution of the metasedimentary-metavolcanic rocks, and gneissic and high-pressure belts are compatible with a model of burial of limited parts of the upper and middle crust in narrow cusp-like synclines, synchronous with the exhumation of orogenic lower crust represented by the gneissic and high-pressure rocks in lobe-shaped and volumetrically more important anticlines. Converging P-T-D paths for the metasedimentary rocks and the adjacent high-pressure rocks are due to vertical exchanges between cold and hot vertically moving masses. Finally, the retrograde shallow-dipping fabric affects both the metasedimentary-metavolcanic rocks and the gneissic and high-pressure rocks, and indicates that the similar to 15-km exhumation was mostly accommodated by heterogeneous ductile thinning associated with unroofing of a buoyant crustal root.

The Mahneshan Metamorphic Complex (MMC) is one of the Precambrian terrains exposed in the northwest of Iran. The MMC underwent two main phases of deformation (D-1 and D-2) and at least two metamorphic events (M-1 and M-2). Critical metamorphic mineral assemblages in the metapelitic rocks testify to regional metamorphism under amphibolite-facies conditions. The dominant metamorphic mineral assemblage in metapelitic rocks (M-1) is muscovite, biotite I, Garnet I, staurolite, Andalusite I and sillimanite. Peak metamorphism took place at 600-620 degrees C and similar to 7 kbar, corresponding to a depth of ca. 24 km. This was followed by decompression during exhumation of the crustal rocks up to the surface. The decrease of temperature and pressure during exhumation produced retrograde metamorphic assemblages (M-2). Secondary phases such as garnet II biotite It. Andalusite II constrain the temperature and pressure of M, retrograde metamorphism to 520-560 degrees C and 2.5-3.5 kbar, respectively. The geothermal gradient obtained for the peak of metamorphism is 33 degrees C km(-1), which indicates that peak metamorphism was of Barrovian type and occurred under medium-pressure conditions. The MMC followed a 'clockwise' P T path during metamorphism, consistent with thermal relaxation following tectonic thickening. The bulk chemistry of the MMC metapelites shows that their protoliths were deposited at an active continental margin. Together with the presence of palaeo-suture zones and ophiolitic rocks around the high-grade metamorphic rocks of the MMC, these features suggest that the Iranian Precambrian basement formed by an island-arc type cratonization.

Metamorphic terranes comprised of blueschist facies and regional metamorphic (Barrovian) rocks in apparent structural continuity may represent subduction complexes that were partially overprinted during syn- to post-subduction heating or may be comprised of unrelated tectonic slices. An excellent example of a composite blueschist-to-Barrovian terrane is the southern Sivrihisar Massif, Turkey. Late Cretaceous blueschist facies rocks are dominated by marble characterized by rod-shaped calcite pseudomorphs after aragonite and interlayered with blueschist that contains eclogite and quartzite pods. Barrovian rocks, which have 40Ar/39Ar white mica ages that are >20 Myr younger than those of the blueschists, are also dominated by marble, but rod-shaped calcite has been progressively recrystallized into massive marble within a ∼200-m transition zone. Barrovian marble is interlayered with quartzite and schist in which isograds are closely spaced and metamorphic conditions range from chlorite to sillimanite zone over ∼1 km present-day structural thickness. Andalusite, kyanite and prismatic sillimanite are present in muscovite-rich quartzite; in one location, all three are in the same rock. Andalusite pre-dates Barrovian metamorphism, kyanite is both pre- and syn-Barrovian and sillimanite is entirely Barrovian. Muscovite with phengitic cores and relict kyanite in quartzite below the staurolite-in isograd are evidence for pre-Barrovian subduction metamorphism preserved at the low-T end of the Barrovian domain; above the staurolite isograd, all evidence for subduction metamorphism has been erased. Some regional metamorphism may have occurred during exhumation, as indicated by syn-kinematic high-T minerals defining the fabric of L-tectonite. Quartz microstructures in lineated quartzite reveal a strong constrictional fabric that may have formed in a transtensional bend in the plate boundary. Transtension accounts for the closely spaced isograds and development of a strong constrictional fabric during exhumation.

The sequential growth of biotite, garnet, staurolite, kyanite, andalusite, cordierite and fibrolitic sillimanite, their microstructural relationships, foliation intersection axes preserved in porphyroblasts (FIAs), geochronology, P–T pseudosection (MnNCKFMASH system) modelling and geothermobarometry provide evidence for a P–T–t–D path that changes from clockwise to anticlockwise with time for the Balcooma Metamorphic Group. Growth of garnet at ∼530 °C and 4.6 kbar during the N–S-shortening event that formed FIA 1 was followed by staurolite, plagioclase and kyanite growth. The inclusions of garnet in staurolite porphyroblasts that formed during the development of FIAs 2 and 3 plus kyanite growth during FIA 3 reflect continuous crustal thickening from c. 443 to 425 Ma during an Early Silurian Benambran Orogenic event. The temperature and pressure increased during this time from ∼530 °C and 4.6 kbar to ∼630 °C and 6.2 kbar. The overprinting of garnet-, staurolite- and kyanite-bearing mineral assemblages by low-pressure andalusite and cordierite assemblages implies ∼4-kbar decompression during Early Devonian exhumation of the Greenvale Province.

Five lengthy periods involving multiple phases of cordierite and andalusite growth were revealed by detailed studies of foliation inflection/intersection axes (FIA) preserved in porphyroblasts in schists from the Arkansas River region in Colorado, USA. The regionally consistent character of the succession of five different FIA trends enabled the relative timing of each FIA with respect to the next to be determined. The FIA succession from first to last is: FIA 1 trending W-E, FIA 2 trending SSW-NNE, FIA 3 trending NNW-SSE, FIA 4 trending NW-SE and FIA 5 trending SW-NE. For four of the FIA sets, samples were found containing monazite grains preserved as inclusions. These were dated on an electron microprobe. The ages obtained concur exactly with the FIA succession, with FIA 1 at 1506 ± 15 Ma, FIA 2 at 1467 ± 23 Ma, FIA 3 at 1425 ± 18 Ma, FIA 4 not dated and FIA 5 at 1366 ± 20 Ma. These ages are directly reflected in a succession of plutons in the surrounding region dated by other isotopic approaches, suggesting that deformation, metamorphism and pluton emplacement occurred together episodically, but effectively continuously, for some 140 Ma.

A series of thermal-structural-gneiss domes (briefly TSG domes) are developed in the Chinese Altaides. Sericite-chlorite zone, biotite zone, garnet zone, staurolite zone, kyanite (andalusite) zone, sillimanite - cordierite (sillimanite - garnet) zone, migmatite zone and migmatic granite - gneiss field are developed from the low-grade metamorphic area to the centre of the TSG domes. The succession of the formation and evolution of the progressive metamorphic zone, migmatite zone and migmatic granite-gneiss corresponds to the spatial sequence from the outer part to the centre of the TSG domes. The peak temperature of the metamorphism and granitization increases progressively from 400 °C to 800 °C while the pressure decreases progressively from 1.05 GPa to 0.10 GPa from the biotite zone to the migmatic granite-gneiss field. The metamorphism of the orogenic belt may be described by the pressure-temperature-space-time model (PTst). In the main episode of orogeny, the deep heat flow and structural flow upsurged along a series of the centres of the regional thermodynamic anomalies, giving rise to the progressive metamorphism, granitization, and the differential uplift and the formation of TSG domes.

The distribution of metapelitic mineral assemblages in the Nelson aureole, British Columbia, generally conforms to what is predicted from phase equilibria. However, in detail, the sequence and spacing of isograds, mineral textures and mineral compositions and mineral chemical zoning do not. Two of the main disequilibrium features in the aureole are: (i) delay in the onset and progress of several reactions, i.e. overstepping in temperature; and (ii) unreactivity of staurolite and especially garnet porphyroblasts when they are reactants in prograde reactions. The thermal overstepping is ascribed to difficulty of nucleation of the product porphyroblasts and sluggishness of dissolution of porphyroblasts when they are reactants. The extent to which these kinetic barriers delay the onset of reaction is related to the reaction affinity of each reaction, defined herein as the Gibbs free-energy difference between the thermodynamically stable, but not-yet-crystallized, products and the metastable reactants. For oversteps in temperature (ΔT), reaction affinity is, in turn, related to the difference in entropy (ΔS) between these two states through the relation A = ΔT * ΔS. Mineral reactions which release large quantities of H2O, such as chlorite-consuming reactions, have a higher entropy change per unit of temperature overstep, and therefore a higher reaction affinity, than those which release little or no H2O, such as the chlorite-free staurolite-consuming reaction. Thermal overstepping is consequently expected to be less for the former than for the latter, as was estimated in the aureole where 0 to 30 °C overstepping was required for garnet, staurolite and andalusite growth from a muscovite + chlorite-bearing precursor rock and ∼70 °C overstepping was required for the growth of Al2SiO5 from a staurolite-bearing, chlorite-free precursor. In all cases, reaction progress was strongly influenced by the presence or absence of fluid, with presence of fluid lowering kinetic barriers to nucleation and growth and therefore the degree of thermal overstepping. Textural features of rocks from the nearly coincident garnet, staurolite and andalusite isograds are suggestive of a fluid-catalysed ‘cascade effect’ in which reaction took place in a narrow temperature interval; several competing muscovite + chlorite-consuming reactions, some metastable, appear to have occurred in parallel. Metamorphic reaction, fluid release and possibly fluid presence in general in the aureole were episodic rather than continuous, and in several cases well removed from equilibrium conditions. The extent to which these findings apply to regional metamorphism depends on several factors, a major one being enhanced deformation, which is expected to lower kinetic barriers to nucleation and growth.

Phase equilibrium modelling and monazite microprobe dating were used to characterize the polymetamorphic evolution of metapelites from the northern part of the Vepor Unit, West Carpathians. Three generations of garnet and associated metamorphic assemblages found in these rocks correspond to three distinct metamorphic events related to the Variscan orogeny, a Permian phase of crustal extension and the Alpine orogeny. Variscan staurolite-bearing and Alpine chloritoid-bearing assemblages record medium-temperature and medium-pressure regional metamorphisms reaching 540–570 °C/5–7.5 kbar and 530–550 °C/5–6.5 kbar respectively. The Permian metamorphic assemblage involves garnet, andalusite, sillimanite, biotite, muscovite, plagioclase and corundum and locally forms silica-undersaturated andalusite-biotite-spinel coronas around older staurolite. The transition from andalusite to sillimanite indicates a prograde low-pressure and medium-temperature metamorphism characterized by temperature increase from 500 to 650 °C at ∼3 kbar. As accessory monazite is abundant in the rocks, an attempt was made to derive its age of formation by means of electron microprobe-based Th-U-Pb chemical dating. Despite the polymetamorphic nature of the metapelites, the monazite yielded uniform Permian ages. Microstructures confirm that monazite was formed in relation to the low-pressure and medium-temperature paragenesis, and the weighted average ages obtained for two different samples are 278 ± 5 and 275 ± 12 Ma respectively. The virtual lack of Variscan and Alpine monazite populations points to interesting aspects concerning the growth systematics of monazite in metamorphic rocks.

For petrographic studies of glacial deposits, semi-quantitative X-ray diffraction analysis was tested as a time-saving method. Reproducible mounts with random orientation were produced with an acetate cement as mounting medium. For simplicity the number of minerals was reduced through electromagnetic separation. Weichsel deposits in SE Scania were sampled. Anatase, andalusite, apatite, rutile, zircon (‘nonmagnetic fraction’) and amphibole, epidote, ilmenite (‘magnetic fraction’) were chosen for semi-quantitative evaluation of diffraction patterns. Standard deviation was calculated for selected samples. With the method employed here, it is possible to distinguish or correlate samples of a group of glacial deposits from relatively similar heavy-mineral associations. The Baltic tills and underlying glaciofluvial sands differ consistently from the NE tills, for example through a higher rutile and a lower apatite content. If there is no time or technical possibility for optical investigations, useful petrographic parameters may be rapidly recognized by the method described. Minerals usually not identified optically (c.g. ilmenite) may likewise be chosen for this analytical technique.

The main mesoscopic features of the ductile deformation in the Hongusan region of the Ryoke Belt can be explained in terms of a single deformation phase, Dm. The associated stretching lineation is roughly east–west and has an associated top-to-the-west sense of shear. This information fills an important gap in our knowledge about the deformational history of the Ryoke metamorphic belt. Andalusite porphyroblasts are well-developed in the Hongusan area and their microstructure can be used to link the deformational history with the metamorphism. However, the andalusite porphyroblasts show both intertectonic and post-tectonic microstructures. Some porphyroblasts have an intertectonic core and post-tectonic rim. Analysis of associated metamorphic minerals and bulk-rock chemistry suggests that the distinct types of microstructure can be explained in terms of distinct formation reactions related to regional and contact metamorphism. The intertectonic microstructure shows the presence of a deformation pre-dating Dm, referred to as Dc. These two deformation stages are sufficient to account for all the main deformation features. The distribution of post-tectonic andalusite growth shows a very broad thermal effect around an intrusion in the area. This can help place constraints on the thermal history of the Ryoke metamorphic belt.

Monazite petrogenesis in the Nelson contact aureole is the result of allanite breakdown close to, but downgrade and therefore independent of, major phase isograds involving cordierite, andalusite and staurolite. The development of garnet downgrade of the staurolite and andalusite isograds does not appear to affect the onset of the allanite-to-monazite reaction but does affect the textural development of monazite. In lower pressure, garnet-absent rocks, allanite breakdown results in localized monazite growth as pseudomorphous clusters. In higher pressure, garnet-bearing rocks, allanite breakdown produces randomly distributed, lone grains of monazite with no textural relationship to the original reaction site. Fluids liberated from hydrous phases (chlorite, muscovite) during garnet formation may have acted as a flux to distribute light rare earth elements more widely within the rock upon allanite breakdown, preventing the localized formation of monazite pseudomorphs. Despite these textural differences, both types of monazite have very similar chemistry and an indistinguishable age by electron microprobe chemical dating (157 ± 6.4 Ma). This age range is within error of isotopic ages determined by others for the Nelson Batholith. Garnet from the garnet, staurolite and andalusite zones shows euhedral Y zoning typified by a high-Y core, low-Y collar and moderate-Y annulus, the latter ascribed to allanite breakdown during garnet growth in the garnet zone. The cause of the transition from high-Y core to low-Y collar, traditionally interpreted to be due to xenotime consumption, is unclear because of the ubiquitous presence of xenotime. Accessory phase geothermometry involving monazite, xenotime and garnet returns inconsistent results, suggesting calibration problems or a lack of equilibration between phases.

Monazite electron microprobe U–Th–Pb and garnet Sm–Nd isotopic data from metapelitic assemblages in the Willyama Supergroup in the southern Curnamona Province, south-central Australia, indicate that the terrain underwent regional greenschist to amphibolite-grade metamorphism during the c. 500 Ma Delamerian Orogeny. The Delamerian-aged mineral assemblages include prograde garnet–staurolite and kyanite-bearing associations that overprint andalusite- and sillimanite-bearing assemblages that are interpreted to have developed during the c. 1600 Ma Olarian Orogeny. Importantly, the development of secondary kyanite-bearing assemblages in the southern Curnamona Province has been used previously to suggest that the Olarian Orogeny followed an anticlockwise P–T evolution. If such assemblages are the product of c. 500 Ma metamorphism, then the anticlockwise P–T path is an apparent path, due to the overprint of a distinct metamorphic cycle c. 1100 Ma later. Making such distinctions is therefore extremely important when using the textural and metamorphic evolution of polycyclic terrains to model the thermal behaviour of the crust during orogeny. This study highlights the utility of in situ geochronology, linking age data to petrologically important phases and assemblages.

Low-pressure anatexis, whereby rocks melt in place after passing through the andalusite stability field, develops under more restricted conditions than does low-pressure metamorphism. Our thermal modelling and review of published work indicate that the following mechanisms, operating alone, may induce anatexis in typical pelitic rocks without inducing wholesale melting in the lower crust: (i) magmatic advection by pervasive flow; (ii) crustal-scale detachment faulting; and (iii) the presence of a high heat-producing layer. Of these, only magmatic advection by pervasive flow and crustal-scale detachment faulting have been shown quantitatively to provide sufficient heat to cause widespread melting. Combinations of the above mechanisms with pluton-scale magmatic advection, shear heating, removal of the lithospheric mantle, or with each other provide additional means of developing suitable high temperatures at shallow crustal levels to generate low-pressure anatexis.

Andalusite porphyroblasts are totally pseudomorphosed by margarite–paragonite aggregates in aluminous pelites containing the peak mineral assemblage andalusite, chlorite, chloritoid, margarite, paragonite, quartz ± garnet, in a NW Iberia contact area. Equilibria at low P–T are investigated using new KFMASH and (mainly) MnCNKFMASH grids constructed with Thermocalc 3.21. P–T and T–X pseudosections with phase modal volume isopleths are constructed for compositions relatively richer and poorer in andalusite to model the assemblages in an andalusite-bearing rock that contains a thin andalusite-rich band (ARB) during retrogression. Their compositions, prior to retrogression, are used in the modelling, and have been retrieved by restoring the pseudomorph-forming elements into the current-depleted matrix, except for Al2O3 which is assumed to be immobile. Compositional differences between the thin band and the rest of the rock have not resulted in differences in andalusite porphyroblast retrogression. The absence of chloritoid resorbtion implies either a pressure increase at constant reacting-system composition, or that its composition changed during retrogression at constant pressure, by becoming enriched in the progressively replaced andalusite porphyroblasts. T–X pseudosections at 1 kbar model this latter process using as end-members in X, first, the restored original rock and ARB compositions, and, then the same process, taking into account the change in composition of both as retrogression proceeded. The MnNCKFMASH pseudosections of rocks with different Al contents facilitate making further deductions on the rock-composition control of the resulting assemblages upon retrogression. Andalusite eventually disappears in relatively Al-poor rocks, resulting, as in this study, in a rock formed by chloritoid–chlorite as the only FM minerals, plus margarite–paragonite pseudomorphs of andalusite. In rocks richer in Al, chlorite would progressively disappear and a kyanite/andalusite–chloritoid assemblage would eventually be stable at retrograde conditions. The Al-silicate, stable during retrogression in Al-rich rocks, indicates pressure conditions and hence the tectonic context under which retrogression took place.

Andalusite is easily converted to mullite and silica on heating. A better understanding of the mullitization mechanisms provides new information on use of this mineral in refractory castables. By using specific particle-size distributions for andalusite-based high-alumina castables, the primary mullite formation can be effectively enhanced by a secondary mullite reaction within castables matrices. The influence of ultrafine andalusite grains on thermomechanical properties of specimens is underlined by testing hot modulus of rupture in combination with mineralogical and microscopic analyses. The results demonstrate that andalusite has great promise as a component of high-alumina no-cement- or ultra-low-cement-containing castables.

In the Hlinsko region (Variscan Bohemian Massif, Czech Republic) a major extensional shear zone separates low-grade metasedimentary series (Hlinsko schists) and high-grade rocks of the Moldanubian terrane (Svratka Crystalline Unit). During late-Variscan extension, a tonalite intruded syntectonically into the normal ductile shear zone, and caused contact metamorphism of the overlying schists. Concurrent syntectonic sedimentation of a flysch series took place at the top of the hangingwall schists. In order to decipher the detailed petrological evolution of the Hlinsko unit situated in the hangingwall of this tectonic contact, a phase diagram approach and pctrogenetic grids, calculated with the thermocalc computer program, were used. The crystallization/deformation relationships and the paragenetic analysis of the Hlinsko schists define a P-T path with an initial minor increase in pressure followed by cooling. Calculated pseudosections constrain this anticlockwise P-T evolution to the upper part of the andalusite field between 0.36 and 0.40 GPa for temperatures ranging from 570 to 530 -C. A low uHZ0 is required to explain the presence of andalusite-biotite-bearing assemblages, and could be related to the presence of abundant graphite. In contrast, the footwall rocks of the Svratka Crystalline Unit record decompression from around 0.8 GPa at a relatively constant temperature, followed by cooling. Thus, the footwall and the hangingwall units display opposite, but convergent P-Thistories. Decompression in the footwall rocks is related to a rapid exhumation. We propose that the inverse, anticlockwise P-T path recorded in the hangingwall pelites is related to the rapid, extension-controlled sedimentation of the overlying flysch series.

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+H2OAnd+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.

Kyanite replaces andalusite in a belt of Ordovician and Silurian pelitic rocks that form a narrow synform pinched between high-grade antiforms in NW Variscan Iberia. Kyanite occurs across the belt in Al-rich, black pelites in assemblages I: kyanite–chloritoid–chlorite–muscovite and II: kyanite–staurolite– chlorite–muscovite. In I, kyanite occurs in the matrix and in kyanite–muscovite aggregates that pseudomorph earlier andalusite porphyroblasts. The aggregates are found across the belt and can still be recognized in assemblage II and even in III: andalusite–staurolite–biotite–muscovite, this latter being a hornfelsic Silurian schist where kyanite is relic and staurolite occurs in the matrix, and is resorbed inside new massive pleochroic andalusite. KFMASH and MnKFMASH pseudosections have been constructed using Thermocalc for Al-rich and Al-poorer compositions from the belt. Chloritoid zoning in Al-rich rocks containing assemblage I, plus chloritoid–chlorite thermometry complemented with garnet–chlorite thermometry in Al-poorer lithologies, mean that the path is one of increasing pressure and temperature. Conditions prior to assemblage I, with earlier andalusite stable, are those of the andalusite–chloritoid– chlorite field as testified by chloritoid enclosed in andalusite porphyroblast rims. The passage from assemblage I to II implies a prograde path within the kyanite field. Assemblage III represents peak conditions, indicating a prograde staurolite-consuming reaction across a KFMASH field, leading eventually to a locally found andalusite–biotite–muscovite hornfels. The lowest pressure stages are recorded by cordierite–biotite in Al-poor pelites. Garnet-bearing MnKFMASH assemblages in Al-poorer pelites record conditions similar to assemblages II and III. The replacement of andalusite by kyanite in assemblage I is attributed to downdragging of andalusite-bearing rocks into a synform as testified by the strained andalusite porphyroblasts affected by a subvertical crenulation cleavage. Prograde metamorphism in the eastern contact of the belt is due to heat transferred to the belt from the ascending high grade antiform across the Vivero fault.

Spinel-cordierite symplectites partially replacing andalusite occur in metapelitic rocks within the cores of several country rock diapirs that have ascended into the upper levels of layered mafic/ultramafic rocks in the Bushveld Complex. We investigate the petrogenesis of these symplectites in one of these diapirs, the Phepane dome. Petrographic evidence indicates that at conditions immediately below the solidus the rocks were characterized by a cordierite-, biotite- and K-feldspar-rich matrix and 5-10 mm long andalusite porphyroblasts surrounded by biotite-rich fringes. Phase relations in the MnNCKFMASHT model system constrain the near-solidus prograde path to around 3 kbar and imply that andalusite persisted metastably into the sillimanite + melt field, where the fringing relationship between biotite and andalusite provided spatially restricted equilibrium domains with silica-deficient effective bulk compositions that focused suprasolidus reaction. MnNCKFMASHT pseudosections that model these compositional domains suggest that volatile phase-absent melting reactions consuming andalusite and biotite initially produced a moat of cordierite surrounding andalusite; reaction progressed until all quartz was consumed. Spinel is predicted to grow with cordierite at around 720 ! C. Formation of the aluminous solid products was strongly controlled by the receding edge of andalusite grains, with symplectites forming at the andalusite-cordierite moat interface. Decompression due to melt-assisted diapiric rise of the floor rocks into the overlying mafic/ultramafic rocks occurred close to the thermal peak. Re-crossing of the solidus at P ! 1.5-2 kbar, T > 700 ! C resulted in preservation of the symplectites. Two features of the silica-deficient domains inhibited resorption of spinel. First, the cordierite moat armoured the symplectites from reaction with crystallizing melt in the outer part of the pseudomorphs. Second, an up-T step in the solidus at low-P, which may be in excess of 100 ! C higher than the quartz-saturated solidus, resulted in high-T crystallization of melt on decompression. Even in metapelitic rocks where melt is retained, preservation of spinel is favoured by decompression.

Based on first-principles density functional calculations, we have investigated the high-pressure polymorphism of phosphorus nitrides (P 3 N 5 ). In addition to the recently-determined a and y phases, our study predicts three new meta-stable high pressure phases existing in the 5 to 35 GPa range, with structures related to the Al 2 SiO 5 mineral polymorphs sillimanite, andalusite, and kyanite, and a new thermodynamically stable phase with a structure related to V 3 O 5 , that should form above 35.5 GPa. All these phases contain octahedrally coordinated phosphorus. By providing new insights on the different P-N bonding coordination within different pressure ranges and on the structural connection between isoelectronic oxides and nitrides, our predictive theoretical study will serve as a guideline for future experiments to explore synthesis of novel dense main group nitrides under high-pressure conditions.

The Priest pluton contact aureole in the Manzano Mountains, central New Mexico preserves evidence for upper amphibolite contact metamorphism and localized retrograde hydrothermal alteration associated with intrusion of the 1.42 Ga Priest pluton. Quartz–garnet and quartz–sillimanite oxygen isotope fractionations in pelitic schist document an increase in the temperatures of metamorphism from 540 °C, at a distance of 1 km from the pluton, to 690 °C at the contact with the pluton. Comparison of calculated temperature estimates with one-dimensional thermal modelling suggests that background temperatures between 300 and 350 °C existed at the time of intrusion of the Priest pluton. Fibrolite is found within 300 m of the Priest pluton in pelitic and aluminous schist metamorphosed at temperatures >580 °C. Coexisting fibrolite and garnet in pelitic schist are in oxygen isotope equilibrium, suggesting these minerals were stable reaction products during peak metamorphism. The fibrolite-in isograd is coincident with the staurolite-out isograd in pelitic schist, and K-feldspar is not observed with the first occurrence of fibrolite. This suggests that the breakdown of staurolite and not the second sillimanite reaction controls fibrolite growth in staurolite-bearing pelitic schist. Muscovite-rich aluminous schist locally preserves the Al2SiO5 polymorph triple-point assemblage – kyanite, andalusite and fibrolite. Andalusite and fibrolite, but not kyanite, are in isotopic equilibrium in the aluminous schist. Co-nucleation of fibrolite and andalusite at 580 °C in the presence of muscovite and absence of K-feldspar suggests that univariant growth of andalusite and fibrolite occurred. Kyanite growth occurred during an earlier regional metamorphic event at a temperature nearly 80 °C lower than andalusite and fibrolite growth. Quartz–muscovite fractionations in hydrothermally altered pelitic schist and quartzite are small or negative, suggesting that late isotopic exchange between externally derived fluids and muscovite, but not quartz, occurred after peak contact metamorphism and that hydrothermal alteration in pelitic schist and quartzite occurred below the closure temperature of oxygen self diffusion in quartz (

Porphyroblastic schists in the thermal aureole of the Victor Harbor Granite at Petrel Cove, in the southern Adelaide Fold Belt, South Australia, preserve a record of sequential cordierite, andalusite, staurolite, fibrolite, chlorite and muscovite growth (along with biotite + plagioclase + quartz + ilmenite) during progressive deformation. A P-T pseudo-section appropriate to biotite-saturated assemblages in KFMASH shows that the sequence of mineral reactions records increasing pressure of at least 1 kbar (from c. 3 to c. 4 kbar) during cooling from around 580 °C. Heating at pressures below c. 3 kbar is inferred for growth of early formed cordierite porphyroblasts, and is attributed in part to the thermal effects of granite emplacement, while the pressure increase is attributed to tectonic burial accruing from ongoing deformation. The 'anticlockwise' P-T path is consistent with convergent deformation being focussed as a consequence of heating, as to be expected for a lithospheric rheology that is strongly temperature dependent.

The reaction muscovite+cordieritebiotite+Al2SiO5 +quartz+H2O is of considerable importance in the low pressure metamorphism of pelitic rocks: (1) its operation is implied in the widespread assemblage Ms + Crd +And± Sil + Bt + Qtz, a common mineral assemblage in contact aureoles and low pressure regional terranes; (2) it is potentially an important equilibrium for pressure estimation in low pressure assemblages lacking garnet; and (3) it has been used to distinguish between clockwise and anticlockwise P–T paths in low pressure metamorphic settings. Experiments and thermodynamic databases provide conflicting constraints on the slope and position of the reaction, with most thermodynamic databases predicting a positive slope for the reaction. Evidence from mineral assemblages and microtextures from a large number of natural prograde sequences, in particular contact aureoles, is most consistent with a negative slope (andalusite and/or sillimanite occurs upgrade of, and may show evidence for replacement of, cordierite). Mineral compositional trends as a function of grade are variable but taken as a whole are more consistent with a negative slope than a positive slope. Thermodynamic modelling of reaction 1 and associated equilibria results in a low pressure metapelitic petrogenetic grid in the system K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) which satisfies most of the natural and experimental constraints. Contouring of the Fe–Mg divariant interval represented by reaction 1 allows for pressure estimation in garnet-absent andalusite+cordierite-bearing schists and hornfelses. The revised topology of reaction 1 allows for improved analysis of P–T paths from mineral assemblage sequences and microtextures in the same rocks.

Andalusite–staurolite–biotite hornfels metamorphosed beneath the mafic layered rocks of the Bushveld Complex, South Africa, preserves a detailed record of the relative timing of porphyroblast growth and metamorphic reactions. The sequence inferred from microstructures shows considerable overlap of the period of growth of porphyroblasts of staurolite, cordierite, biotite and andalusite, and the persistence over a similar interval of the reactant porphyroblastic phase chloritoid. This is inconsistent with calculations of equilibrium phase relations, and implies that disequilibrium processes controlled the prograde reaction sequence, despite the slow heating rates involved (1 °C per 10 000 yr). The early appearance of cordierite by a metastable reaction and its subsequent disappearance indicates that delayed nucleation of porphyroblastic phases, rather than simply sluggish reaction, is required to account for the sequence of growth. The predicted reactions for the first appearance of andalusite and staurolite have low entropy of reaction, and do not occur until they have been overtaken in terms of reaction affinity by high-entropy devolatilisation reactions involving the breakdown of chlorite. Once the porphyroblasts have nucleated, metastable chloritoid-breakdown reactions also contribute to their growth. The implied magnitude of the critical overstepping for andalusite nucleation is around 5 kJ mole−1 (equivalent to 40 °C for the chlorite-breakdown reaction), and that for other phases is expected to decrease in the order andalusite>staurolite>cordierite. Coupling between nucleation rate, crystal growth rates and the resulting grain size distribution suggests that the rate constants of natural reactions are at least an order of magnitude lower than those measured in the laboratory. Pseudomorphs after chloritoid and cordierite conserve volume but not Al or other species of low mobility, suggesting a breakdown mechanism controlled by an interface process such as the slow dissolution of the refractory porphyroblast phase, rather than by a transport step.

Rocks of the Snake Creek Anticline are mainly pelitic schists, psammitic schists and quartzites that were metamorphosed during multiple high-T/low-P events extending from D1 to D5, with the metamorphic peak occurring late to post-D3. Albitites are widespread, but are concentrated in five areas. They are typically fine- to medium-grained, and consist of albite, with or without combinations of quartz, biotite, staurolite, cordierite, garnet, andalusite, sillimanite, kyanite, gedrite and tourmaline. From the presence or absence of albite inclusions in porphyroblasts, the albitites are interpreted as forming early in the D3 event as a result of infiltration of external fluids. Psammitic schists and quartzites were preferentially altered, but pelitic schists were also albitized in localities where the alteration was more extreme, with the replacement of muscovite total and the replacement of quartz and biotite variable. Structural controls on albitization include fracturing and syn-D3 shear zones in fold hinges. Biotite schists with abundant porphyroblasts (combinations of staurolite, garnet, andalusite and cordierite) occur adjacent to albitites, and it is argued that they formed by the addition of Fe and Mg sourced from the albitites. In several albitite-rich areas, cordierite grew early in D3 and was partly or entirely replaced during or after D3 by combinations of biotite, andalusite, tourmaline, staurolite and sillimanite. A postulated P–T–d path involved an increase in pressure (with or without a decrease in temperature) subsequent to early D3 albitization, followed by an increase in temperature up to the metamorphic peak (late D3 to early D4. The metamorphism was contemporary in part with the emplacement of the Williams Batholith (c. 1550–1500 Ma), which probably supplied the Na-rich fluids.

The exceptional andalusite-kyanite-andalusite sequence occurs in Al-rich graphitic slates in a narrow pelite belt on the hangingwall of a ductile normal fault in NW Variscan Iberia. Early chiastolite is replaced by Ky-Ms-Pg aggregates, which are overgrown by pleochroic andalusite near granites intruded along the fault. Slates plot in AKFM above the chloritoid-chlorite tie-line. Their P-T grids are modelled with Thermocalc v2.7 and the 1998 databases in the NaKFMASH and KFMASH systems. The univariant reaction Ctd+And/Ky=St+Chl+Qtz+H2O ends at progressively lower pressure as F/FM increases and A/AFM decreases, shrinking the assemblage Cld-Ky-Chl, and opening a chlorite-free Cld-Ky trivariant field on the low temperature reaction side. This modelling matches the observed absence of chlorite in high F/FM rocks, which is restricted to low pressure in the andalusite stability field. The P-T path deduced from modelling shows a first prograde event in the andalusite field followed by retrogression into the kyanite field, most likely coupled with a slight pressure increase. The final prograde evolution into the andalusite field can be explained by two different prograde paths. Granite intrusion caused the first prograde part of the path with andalusite growth. The subsequent thermal relaxa- tion, together with aH2O decrease, generated the retrograde andalusite-kyanite transformation, plus chlorite consumption and chloritoid growth. This transformation could have been related to folding in the beginning, and aided later by downthrowing due to normal faulting. Heat supplied by syntectonic granite intrusion explains the isobaric part of the path in the late stages of evolution, causing the prograde andalusite growth after the assemblage St-Ky-Chl. Near postectonic granites, a prograde path with pressure decrease originated the assemblage St-And-Chl.

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.

Metapelitic rocks from the Marble Hall Fragment, enclosed in the granites of the magmatic Bushveld Complex, record a two-stage, low-pressure, high-temperature metamorphism. An early paragenesis containing chiastolitic andalusite, cordierite, biotite and quartzigarnet crystallized in most rocks and equilibrated at 550-600 uC, 0.2 GPa. It was transformed during the second, peak event into various parageneses that commonly coexist within a single thin section. These include garnet-cordierite-biotite- K-feldspar-quartz, sillimanite-cordierite-K-feldspar-quartz and spectacular quartz-undersaturated cordierite-spinel symplectites replacing the chiastolite porphyroblasts. Based on a detailed phase diagram analysis, we argue that these parageneses result from rapid heating at an approximately constant pressure to temperatures of more than about 720 uC. At these temperatures, the internally buffered activity of water was reduced by incipient water-saturated partial melting, while only minor quantities of melt were produced. Subsequent dry conditions inhibited large-scale equilibration and, together with local inhomogeneities in mineral distribution, led to the development of contrasting parageneses and symplectite textures. No signs of widespread fluid-absent melting of biotite were found, and so the temperature probably did not exceed 760 uC. The peak metamorphic event is attributed to the emplacement of the hot Nebo granite, whereas the early metamorphism was probably caused by the intrusion of one of the phases of the Rustenburg Layered Suite. We infer the conditions of development of the cordierite-spinel intergrowths and we show that, although symplectites are commonly associated with retrograde processes (cooling and/or decompression), they can record a prograde metamorphic evolution. Furthermore, our contribution emphasizes the importance of the concept of reduced equilibration volume for the understanding and interpretation of some particular textures and parageneses in common rocks.

The Milyang pyrophyllite deposit, which is embedded in the Late Cretaceous Yuchon Group of the Kyongsang Supergroup, is one of the largest hydrothermal clay deposits in the Kyongsang basin, southeast Korea. Host rocks of the deposit are porphyritic andesite lava and minor andesitic lapilli tuff. In the Milyang district, a hydrothermally altered zone is about 2 × 3 km in extent; we can recognize the concentric arrangement of advanced argillic, propylitic, and sericitic alteration zones from the central to peripheral parts of the zone. The Milyang pyrophyllite deposit forms a part of the advanced argillic alteration zone. The Milyang pyrophyllite deposit is subdivided into the following four zones based on mineral assemblages: the pyrophyllite zones 1, 2, 3, and the silicified zone. The pyrophyllite zone 1, which occupies the central part of the deposit, comprises mainly pyrophyllite, kaolinite, and diaspore without quartz. Diaspore nodules often concentrate in beds 40–50 cm thick. Andalusite, dumortierite, and tourmaline locally occur as network veins, crack-filler, or small spherulitic spots. The Al2O3 content of the ore ranges from 27 to 36 wt%. The pyrophyllite zone 2, which constitutes a major part of the deposit, comprises mainly pyrophyllite, kaolinite, and quartz. The Al2O3 content of the ore ranges from 15 to 24 wt%. The pyro-phyllite zone 3 is the hematite-rich marginal facies of the deposit. The silicified zone, which occurs as beds and septa, is mostly composed of quartz with minor pyrophyllite and kaolinite; the SiO2 contents range from 79 to 90 wt%. Comparing chemical compositions of the high-Al ores with those of unaltered host andesite, the Fe, Ca, alkalis, HFSE, and HREE contents are significantly depleted, whereas S, B, As, Sr, and LREE are enriched. The hydrothermal alteration of the Milyang pyrophyllite deposit can be classified into the following four stages: 1) extensive sericitic and propylitic alteration, 2) medium-temperature (200–250°C) advanced argillic alteration, 3) high-temperature (250–350°C or more) advanced argillic alteration, and 4) retrograde low-temperature alteration. The heat and some volatile components such as B and S would be derived from the Pulguksa Granite intruded underneath the deposit.

Ductilely deformed veins consisting of quartz+andalusite, in which the andalusite is partially replaced by fibrous sillimanite, locally occur in garnet–sillimanite schist near a margin of the Nigde metamorphic core complex in south-central Turkey. Mineral assemblages, reaction textures and structural features of the veins record low-pressure–high-temperature deformation during exhumation of mid-crustal rocks. The partial replacement of andalusite by sillimanite may indicate a late-stage increase in temperature and/or fluid pressure, possibly related to Miocene magmatism, during extensional unroofing of the core complex. Aluminosilicate-bearing veins are observed at the eastern margin of the massif where metapelitic rocks were deformed during unroofing of the core complex. Veins formed in aluminous rocks where deformation-enhanced permeability allowed fluid flow during extensional shear. The cm-scale veins are typically boudinaged and form asymmetric lenses concordant with the host rock foliation and are parallel to the down-dip lineation defined by sillimanite and stretched biotite. Aluminosilicate-bearing boudins record top-to-the-east shear sense, which is compatible with the extensional shear sense displayed by structures in the host rock.

Metapelites from the southern aureole of the Vedrette di Ries tonalite (eastern Alps) were variably overprinted by contact and earlier regional metamorphic events during pre-Alpine and Alpine metamorphic cycles. In these rocks, starting from a primary garnet mica-schist (garnet stage), a complex sequence of transformations, affecting the site of the garnet, has been recognized. In the outermost part of the aureole, the primary garnet sites are occupied by nodules of kyanite (kyanite stage). Closer to the tonalite, kyanite is replaced by staurolite (staurolite stage), which in turn is pseudomorphed by muscovite (muscovite stage). The aggregates of kyanite do not overgrow garnet directly; they post-date a stage (fibrolite stage) represented by the pseudomorphic alteration of garnet into fibrolitic sillimanite plus biotite. A further sericite stage is likely to have occurred between the fibrolite and kyanite stages. Preservation of the sub-spherical garnet shape during all these transformations and persistence of mineralogical and textural relicts from earlier stages were favoured by the very low strain experienced by the rocks since the garnet stage. The textural sequence is in agreement with the metamorphic history of this part of the Austroalpine basement of the Eastern Alps: the garnet and fibrolite stages, and the coeval main foliation of the samples, are referred to the high-grade Hercynian metamorphism; the kyanite stage to the Eo-Alpine metamorphism; the staurolite and muscovite stages to the Oligocene contact metamorphism. It is suggested that kyanite growth as microgranular aggregates took place in polymetamorphic rocks where static, high-P/low-T metamorphism overprinted high-T assemblages that contained sillimanite or andalusite.

Schists from the foothills of the Central Sierra Nevada contain one dominant matrix foliation and yet four phases of growth of both cordierite and andalusite porphyroblasts can be distinguished. These occurred early during four separate deformation events that formed successive steep and shallow foliations. A fifth deformation event pre-dates the growth of all porphyroblasts studied. The multiple phases of porphyroblast growth allow correlation of structures across and along the region. A repeated pattern of deformation, in terms of the curvature of earlier foliations against the overprinting one, allows samples containing porphyroblasts with simpler inclusion trail geometries to be interpreted with confidence. The large-scale fold structures in this region formed before or during the second of the five deformation events recorded by the porphyroblasts. However, the matrix foliation is predominantly a product of the fourth deformation, which has commonly reactivated or re-used older foliations, and is dominated by east-side-up shear. The intervening third deformation produced locally intense foliations and was accompanied by top-to-the-east shear. The very weak fifth deformation produced weak crenulations with subhorizontal axial planes and was coaxial. Multiple phases of episodic but synchronous growth of cordierite and andalusite were produced by the KFMASH univariant equilibrium Ms+Chl+Qtz=And+Crd+Bt+H2O. The rocks crossed this reaction at a pressure just below the intersection with the KFMASH divariant equilibrium Ms+Chl+Qtz=Crd+Bt+H2O; the latter being overstepped in favour of the former as there is no evidence for cordierite growth prior to andalusite in these rocks. Subsequent multiple episodes of synchronous growth of cordierite and andalusite indicate that the possible variation in P–T during subsequent deformations was not large. This requires the high-amplitude macroscopic fold to form prior to porphyroblast growth and then be simply tightened and modified by the younger deformations.

Restricted occurrences of early, syn- and late-kinematic kyanite adjacent to large domal batholiths in the Archean granite–greenstone terrane of the east Pilbara craton, Australia, are considered to result from partial convective overturn of the crust. The analogue models of Dixon & Summers (1983) and thermo-mechanical models of Mareschal & West (1980), involving gravitionational overturn of dense greenstone crust that initially overlay sialic basement, successfully explain the geometry, dimension, kinematics and strain patterns of the batholiths and greenstone rims. Application of these models suggests that andalusite and sillimanite are the stable aluminosilicate polymorphs in domal crests and rims, where prograde clockwise P–T–t paths, with small pressure changes, should be recorded. Both aluminosilicates are predicted to overprint kyanite, which is observed locally around the east Pilbara domes. Kyanite is the predicted aluminosilicate polymorph in the deeper parts of domal rims and within sinking greenstone keels, reflecting rapid, near-isothermal burial. The narrow zones of kyanite-bearing schists adjacent to some batholiths in the Pilbara craton are metamorphosed, highly strained equivalents of altered felsic volcanic rocks in the low-grade greenstone succession, dragged to mid-crustal depths (6 kbar) during greenstone sinking. The schists rebounded as an arcuate tectonic wedge along the southern Mount Edgar batholith rim, during the later stages of doming, and were juxtaposed against regional, greenschist facies, low-strain greenstones. Thus, kyanite was preserved: if the walls had remained at depth, it would have been overprinted by the higher-temperature aluminosilicate polymorphs during thermal recovery. Kyanite growth in the Pilbara craton is unlikely to have resulted from ballooning of plutons, mantled gneiss doming, metamorphic core complex formation, or early crustal overthickening. The typical subvertical foliations and lineations of the tectonic wedge suggest that subvertical fabrics extended to mid-crustal depths (c. 20 km) before rebound, providing a three-dimensional glimpse of Archean dome-and-keel structures. The general occurrence of large granitoid domes in Archean granite–greenstone terranes, restriction of rare kyanite to the adjacent, high-strain batholith margins, and its absence from the batholiths, suggest that partial convective overturn of the crust may have been a common process at this early stage of Earth history.