Notes: Reactions producing Al-rich index minerals in the south-eastern part of the Lepontine Dome (Central Alps, Switzerland) are investigated using mineral distribution maps, microstructural observations and equilibrium phase diagrams. The apparent staurolite mineral zone boundary corresponds to the paragonite breakdown reaction Pg + Grt + Qtz = Pl + Al2O3 + W. Equilibrium phase diagrams show that most natural metapelites do not contain staurolite or alumosilicates as long as univalent cations are predominantly accommodated in white mica. For a wide range of metapelitic compositions the paragonite breakdown releases sufficient Al for the formation of these minerals. Rare occurrences of staurolite and kyanite, north of the formerly mapped mineral zone boundaries, coexist with paragonite and are restricted to extremely Al-rich bulk compositions. The stable branch of the kyanite-forming paragonite breakdown reaction above 660 °C yields an additional mapable isograd. The second set of Al-releasing reactions is biotite-producing phengite breakdown. However, these reactions are less suitable to produce well defined reaction isograds in the field as they are more continuous and their progress is strongly dependent on bulk composition. Well developed fibrolite in metapelites does not appear until staurolite starts to breakdown. We conclude that amphibolite facies conditions in the study area were attained by decompression, without substantial heating at low pressures.

Notes: The granitic mylonite zone in the Cretaceous Ryoke metamorphic belt contains deformed amphibolites as thin layers. The amphibolite layers do not exhibit pinch-and-swell or boudinage structures, even when contained in a high-strain granitic mylonite. This mode of occurrence suggests that they were deformed as much as the surrounding granite mylonite. In the highly deformed zone, strongly foliated amphibolites contain Ti-rich brown amphibole porphyroclasts rimmed by Ti-poor green amphibole, titanite and chlorite. These porphyroclasts are elongated, forming shear surfaces defined by preferential distribution of the chlorite and titanite. Porphyroclastic plagioclase in the strongly foliated amphibolites consists of two components: an anorthite-rich core and an anorthite-poor rim. Based on these observations, the mass-balanced reaction occurring during deformation is defined as 〈inlineGraphic alt="inline image" href="urn:x-wiley:02634929:JMG367:JMG_367_mu1" location="equation/JMG_367_mu1.gif"/〉 As the reaction products form a weak interconnected matrix, the strain rate of the amphibolites may be controlled by the rate of dissolution–precipitation through fluids. Weakly foliated amphibolites in the low-strain zone exhibit cataclastic microstructures, whereas the strongly foliated amphibolites do not exhibit such features. These microstructural and chemical changes suggest that high-strain amphibolites were initially deformed by cataclasis, followed by deformation through metamorphic reactions. During the metamorphism/deformation, old plagioclase grains with high Xan were not stable and dissolved, and new plagioclase grains with low Xan crystallized at the old plagioclase rim. Dissolution of old plagioclase and precipitation of new plagioclase occurred normal to and parallel to the foliation, respectively, reflecting incongruent pressure solution due to differential stress and changes in P–T–H2O conditions. The development of incongruent pressure solution is attributed to increased fluid flux in the strongly foliated amphibolites, as evidenced by the greater abundance of hydration-reaction products in the strongly foliated amphibolites than in the weakly foliated ones.

Notes: This paper describes a kinetic study on reaction textures in eclogitic rocks from the Sulu region, eastern China. Some of the eclogitic rocks display a decompressional reaction texture, whereby kyanite grains are surrounded by plagioclase coronas and are never in contact with quartz. The change in mineral parageneses with progress of the reaction was predicted by constructing chemical potential diagrams in a model system. The chemical potential diagrams indicated that the chemical potential of 2Na2O + CaO (2µNa2O + µCaO) in intergranular regions between kyanite and quartz should decrease with decreasing pressure, whereas 2µNa2O + µCaO in intergranular regions between garnet and omphacite should increase with decreasing pressure. Thus, upon decompression, an inequality in chemical potential arises in the rock. To reduce this inequality, garnet and omphacite react to produce amphibole and plagioclase and release Na2O and CaO. Then, the released Na2O and CaO components diffuse into the regions between kyanite and quartz grains and react to produce plagioclase between them. This model also indicates that the chemical potential of SiO2 should decrease around kyanite grains during the progress of the decompressional reaction, and Si-undersaturated conditions should have formed around kyanite grains in spite of the presence of quartz in these eclogitic rocks. Thus, spinel or corundum that are not stable in the system with excess quartz can form as a metastable phase, as observed in eclogitic rocks from the study areas. Phase diagrams in the system with excess quartz should be carefully applied for analysis of such reaction textures.

Notes: 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.

Notes: Nine marble horizons from the granulite facies terrane of southern India were examined in detail for stable carbon and oxygen isotopes in calcite and carbon isotopes in graphite. The marbles in Trivandrum Block show coupled lowering of δ13C and δ18O values in calcite and heterogeneous single crystal δ13C values (− 1 to − 10‰) for graphite indicating varying carbon isotope fractionation between calcite and graphite, despite the granulite facies regional metamorphic conditions. The stable isotope patterns suggest alteration of δ13C and δ18O values in marbles by infiltration of low δ13C–δ18O-bearing fluids, the extent of alteration being a direct function of the fluid-rock ratio. The carbon isotope zonation preserved in graphite suggests that the graphite crystals precipitated/recrystallized in the presence of an externally derived CO2-rich fluid, and that the infiltration had occurred under high temperature and low fO2 conditions during metamorphism. The onset of graphite precipitation resulted in a depletion of the carbon isotope values of the remaining fluid+calcite carbon reservoir, following a Rayleigh-type distillation process within fluid-rich pockets/pathways in marbles resulting in the observed zonation. The results suggest that calcite–graphite thermometry cannot be applied in marbles that are affected by external carbonic fluid infiltration. However, marble horizons in the Madurai Block, where the effect of fluid infiltration is not detected, record clear imprints of ultrahigh temperature metamorphism (800–1000 °C), with fractionations reaching 〈2‰. Zonation studies on graphite show a nominal rimward lowering δ13C on the order of 1 to 2‰. The zonation carries the imprint of fluid deficient/absent UHT metamorphism. Commonly, calculated core temperatures are  〉 1000 °C and would be consistent with UHT metamorphism.

Notes: The frequency of occurrence of minerals in 1876 samples of Sanbagawa pelitic schist in central Shikoku is summarized on the basis of microscopic observation accompanied, in part, by use of an electron microprobe. All samples contain quartz, plagioclase, phengite, chlorite and graphite. More than 90% of samples contain clinozoisite, titanite and apatite. Garnet is present in 95% of samples from the garnet zone, and biotite is present in 64% of samples from the albite-biotite zone. Calcite is found in about 40% of samples of the pelitic schist collected from outcrop, but occurs in 95% of the pelitic schist from drill cores. Calcite was apparently ubiquitous in the pelitic schist during the Sanbagawa metamorphism, but must have been dissolved recently by the action of surface or ground water. The mineral assemblages of the Sanbagawa pelitic schist have to be analyzed in the system with excess calcite, quartz, albite (or oligoclase), clinozoisite, graphite and fluid that is composed mainly of H2O, CO2 and CH4. In the presence of calcite, reactions that produce garnet, rutile, oligoclase, biotite and hornblende, some of which define isograds of the metamorphic belt, should be written as mixed volatile equilibria that tend to take place at lower temperature than the dehydration reactions that have been proposed. The presence of calcite in pelitic schist suggests that fluid composition is a variable as important in determining mineral assemblages as pressure and temperature. Thus Ca-bearing phases must be taken into account to analyze the phase relations of calcite-bearing pelitic schist, even if CaO content of Sanbagawa pelitic schist is low. As calcite is a common phase, the mineral assemblages of the biotite zone pelitic schist may contravene the mineralogical phase rule and warrant further study.

Notes: 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.

Notes: Metre to tens-of-metre wide, steeply dipping, greenschist facies shear zones that cut blueschists and eclogites of the Combin and Zermatt–Saas Zones at Täschalp and in adjacent areas of the western Alps were sites of extensive recrystallization driven by fluid flow and deformation. Rb–Sr data imply that these shear zones formed at 42–37 Ma with a systematic younging of structures northward toward, and into, the hangingwall of the Mischabel Structure. Shearing commenced at 400–475 °C and 400–500 MPa and continued as pressures and temperatures fell to 300–350 °C and 300–350 MPa. Individual shear zones were active for 2–3 Myr with later lower grade stages of shearing concentrated into narrow zones. Fluids that infiltrated the shear zones were water rich (XH2O 〉 0.9). Alteration zones around albite veins and at the margins of serpentinite bodies are penecontemporaneous with these shear zones and formed at approximately the same conditions. The eclogites were exhumed from c. 64 km at 44 Ma to 14–16 km at 42–41 Ma implying exhumation rates of 2–5 cm yr−1. Rapid exhumation was probably achieved by extension aided by buoyancy, following subduction of continental crust, and rapid erosion. The shear zones form part of a regional-scale extensional system responsible for a significant portion of the exhumation of the subducted oceanic crust.

Notes: High-T, low-P metamorphic rocks of the Palaeoproterozoic central Halls Creek Orogen in northern Australia are characterised by low radiogenic heat production, high upper crustal thermal gradients (locally exceeding 40 °C km−1) sustained for over 30 Myr, and a large number of layered mafic-ultramafic intrusions with mantle-related geochemical signatures. In order to account for this combination of geological and thermal characteristics, we model the middle crustal response to a transient mantle-related heat pulse resulting from a temporary reduction in the thickness of the mantle lithosphere. This mechanism has the potential to raise mid-crustal temperatures by 150–400 °C within 10–20 Myr following initiation of the mantle temperature anomaly, via conductive dissipation through the crust. The magnitude and timing of maximum temperatures attained depend strongly on the proximity, duration and lateral extent of the thermal anomaly in the mantle lithosphere, and decrease sharply in response to anomalies that are seated deeper than 50–60 km, maintained for 〈5 Myr in duration and/or have half-widths 〈100 km. Maximum temperatures are also intimately linked to the thermal properties of the model crust, primarily due to their influence on the steady-state (background) thermal gradient. The amplitudes of temperature increases in the crust are principally a function of depth, and are broadly independent of crustal thermal parameters. Mid-crustal felsic and mafic plutonism is a predictable consequence of perturbed thermal regimes in the mantle and the lowermost crust, and the advection of voluminous magmas has the potential to raise temperatures in the middle crust very quickly. Although pluton-related thermal signatures significantly dissipate within 〈10 Myr (even for very large, high-temperature intrusive bodies), the interaction of pluton- and mantle-related thermal effects has the potential to maintain host rock temperatures in excess of 400–450 °C for up to 30 Myr in some parts of the mid-crust. The numerical models presented here support the notion that transient mantle-related heat sources have the capacity to contribute significantly to the thermal budget of metamorphism in high-T, low-P metamorphic belts, especially in those characterised by low surface heat flow, very high peak metamorphic geothermal gradients and abundant mafic intrusions.

Notes: The late Palaeozoic western Tianshan high-pressure /low-temperature belt extends for about 200 km along the south-central Tianshan suture zone and is composed mainly of blueschist, eclogite and epidote amphibolite/greenschist facies rocks. P–T conditions of mafic garnet omphacite and garnet–omphacite blueschist, which are interlayered with eclogite, were investigated in order to establish an exhumation path for these high-pressure rocks. Maximum pressure conditions are represented by the assemblage garnet–omphacite–paragonite–phengite–glaucophane–quartz–rutile. Estimated maximum pressures range between 18 and 21 kbar at temperatures between 490 and 570 °C. Decompression caused the destabilization of omphacite, garnet and glaucophane to albite, Ca-amphibole and chlorite. The post-eclogite facies metamorphic conditions between 9 and 14 kbar at 480–570 °C suggest an almost isothermal decompression from eclogite to epidote–amphibolite facies conditions. Prograde growth zoning and mineral inclusions in garnet as well as post-eclogite facies conditions are evidence for a clockwise P–T path. Analysis of phase diagrams constrains the P–T path to more or less isothermal cooling which is well corroborated by the results of geothermobarometry and mineral textures. This implies that the high-pressure rocks from the western Tianshan Orogen formed in a tectonic regime similar to ‘Alpine-type’ tectonics. This contradicts previous models which favour ‘Franciscan-type’ tectonics for the southern Tianshan high-pressure rocks.