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Hercynite-rich spinel is a typical mineral of both quartz-bearing and quartz-free metapelitic rocks that form under low-pressure high-temperature conditions in the granulite facies (Sengupta et al., 1991) or upper-amphibolite facies of regional and contact metamorphism (Pattison and Tracy, 1991). Temperature-pressure estimation of metamorphic terrains is necessary to understand how large-scale crust evolution has occurred. High-temperature phase equilibrium cannot be properly evaluated solely based on normal cationic couple geothermobarometers, as ferromagnesian cationic exchange undergoes processes related to retrogression during cooling after peak metamorphism. Instead, key mineral parageneses are preserved and standard petrogenetic grids study based on reaction textures and mineral chemistry can be very helpful in better estimating temperature pressure.Regional GeologyDorbeh's contact aureole is formed in West Azerbaijan province, south of Urmia, north of Oshnavieh, and the Sanandaj-Sirjan zone around Dorbeh diorite. The intrusion is one of the components of the Urmia Plutonic Complex (UPC) which is spread along with granites and alkali syenites/granites at the northwest end of the Sanandaj-Sirjan zone in the south of Urmia. This diorite intruded sedimentary units of shale and carbonate belonging to Permian to Triassic-Jurassic including the Doroud and Ruteh Formations and have metamorphosed them in contact aureole, causing hornfels and calc-silicate to sometimes scarn deposits in the area (Modjarrad and Mohamed, 2015). This intrusion is covered by Miocene units (Ghalamghash, 2009). The observed parageneses in Dorbeh aureole including Cld + Chl, Ctd + And, Alm + Cld, And+Alm, Hc+Alm, Chl + Hc + And + Sil, Alm + And + Sil, Alm + And + Sil + Hc. The peak metamorphism assemblage is Alm + And + Sil + Hc. The reactions responsible for the parageneses productions were introduced in the previous studies (Modjarrad and Mohamed, 2015).Materials and Methods 50 microscopic thin sections were prepared and petrographically examined. A reagent sample with the highest number of parageneses and experienced peak metamorphism was analyzed by JEOL JXA-8200 microprobes electron device for garnet, chlorite, chloritoid, spinel, iron oxide, aluminosilicate, and apatite at the University of Potsdam, Germany. For whole rock chemistry, several metapelite samples were analyzed by XRF at the ALS-Chemex Laboratory of Canada.Discussion and ConclusionsThe main purpose of the present paper is to investigate the parageneses, the mineral chemistry, and the temperature-pressure estimation in the metamorphic contact aureole around Dorbeh intrusive. This intrusion consists of amphibole, plagioclase, and clinopyroxene, is a part of the Urmia Intrusive Complex (UPC), and with dioritic composition was intruded the Upper Cretaceous into the Paleozoic pelitic and limestone sedimentary rocks and metamorphosed them. The metapelite section of the aureole has exhibited interesting parageneses due to the exceptional iron-rich composition of the protolith. The hornfels part of the Dorbeh aureole is dominated by chlorite + chloritoid + garnet + aluminosilicates + spinel + opaque minerals and less quartz in the matrix. Mineral chemistry studies showed that Fe-chlorite between Repidolite to Bronswigite, Fe-chloritoid, Almandine garnet, Hercynitic spinel, and ilmenite are the minerals composition which is in agreement with the Fe-rich content of the parent rock. There was no specific zoning pattern in the metapelites garnet grains. It was also found that there is a reverse relationship between Si content and iron, magnesium, titanium, and chromium in chlorite and aluminosilicate structures. The metamorphic conditions for Dorbeh aureole by the multi-equilibria method by THERMOCALC software have been estimated at 570±7°C, at a pressure of 2.8±0.2 kbar.Detailed investigation of phase relationships of metamorphic rocks can be considered a desirable method along with quantitative data-based methods to evaluate the occurrence of progressive metamorphism. This method is based on direct observation of reaction textures and is more objective in estimating metamorphic conditions by the method of the intersection of equilibrium reactions. Most of the time, secondary cationic exchanges penetrate from the mineral boundary to a considerable stratum and question the accuracy of the temperature and pressure calculated on the cationic couple's geothermometers to a large extent. The contact aureole with higher than normal temperatures specific to the margins of dry mafic intrusives is sometimes associated with the formation of special parageneses such as Grt+Sil+And+Hc. This paragenesis has also been reported from high-temperature (granolithic) regional metamorphic terrains, but its production conditions are more limited in contact aureoles and require a high molar fraction of iron/aluminum or mass dryness and/or aureole. Therefore, the composition of all ferromagnesian minerals is very close to the final iron end member and no specific temperature fluctuation in the aureole is understood from the garnet zoning pattern. Although staurolite has not been seen in the studied sections, the evidence suggests that hercynite in the area was caused by staurolite breakdown. With precision in the composition of analyzed chlorites, it was found that there is a direct relationship between Si and alkaline content in chlorite structure, but titano-ferromagnesian content is in contrast with Si content. An almost similar relationship was observed in the aluminosilicate crystals. The inverse relationship between silica and Fe, Ti, and Cr oxides prevails.

Joveinan marbles and skarns are located 5 km NE of Ghohrood and 140 km NW ofIsfahan in the *central part of Urumieh- Dokhtar magmatic arc. Intrusion of Ghohroodgranitoid into the Cretaceous carbonate rocks caused the contact metamorphism andformation of skarn mineral paragenesis wollastonite, clinopyroxene, garnet, actinolite,epidote, chlorite, calcite, quartz, magnetite and sulfides (iron and copper). Mineralogicalstudies and paragenetic mineral relations confirm that the Joveinan skarn is polygeneticin origin and evolved in two major stages, metamorphism and metasomatism(progressive and retrogressive). In metamorphism stage that occurred immediately afterthe granitoid magma emplacement within carbonate rocks, Joveinan marbles were formed. Metasomatic stage occurred with generation of anhydrous calc-silicatesminerals such as wollastonite, diopsidic-hedenbergitic pyroxene and ugrandite garnets.The main stage of retrograde metasomatic, alteration of primary calc-silicate minerals ofskarn (pyroxene-garnet), caused the formation of epidote, actinolite and chlorite. Theassociation of wollastonite, magnetite and andradite represents that skarn crystallized inover 550 ° C temperature range and low partial pressure of CO2. Joveinan skarn is acalcic skarn that has been formed at shallow depth by oxidative hydrothermal fluids andevolved in different stages.

Large, globally documented negative δ13C excursions that occur across the Permian-Triassic transition are thought to represent the isotopic fingerprint of considerable releases of isotopically light carbon into the atmosphere, with the largest of these excursions (as much as −22.2‰) measured in bulk organic matter in coals and carbonaceous shales from Antarctica. Previous studies have attributed the carbon isotopic excursion (CIE) at the Permian–Triassic boundary to the release of CH 4 from organic-rich rocks intruded by dikes and sills associated with the Siberian Traps flood basalts (Russia) and Longwood-Bluff (New Zealand) intrusions. However, any assessment of paleoatmospheric conditions based on this or other δ13C org excursions must consider the multiple controls on isotopic composition, including maceral content and post-depositional diagenetic and maturation processes. Permian-age coal seams and associated sediments in Antarctica were extensively altered after burial by localized high heat flow and, in some cases, by contact metamorphism associated with Jurassic dikes and sills. Levels of organic maturation reach anthracite, meta-anthracite, and even graphite; such intense thermal alteration had the potential to change the carbon isotopic composition of the bulk organic matter (δ13C org). To evaluate the Permian–Triassic isotopic excursion in Antarctica, it is important to understand the inherent variability in δ13C org in Permian-age coal and organic-rich sedimentary rocks prior to the CIE; thus, 128 samples representing pre-boundary coals and shales were selected from the United States Polar Rock Repository for analysis. Petrographic (maceral analysis and vitrinite reflectance), geochemical (C, H, N), and carbon isotopic analyses were performed. Based on the presence of coarse-grained circular and fine-grained lenticular mosaic in the anisotropic cokes found in samples in close proximity to intrusions, the rank of the samples prior to intrusion is estimated to have been medium volatile bituminous (∼1.2% R max). These highly altered coals and cokes can have random vitrinite reflectances (R r) exceeding 6%. Subsequent to intrusion, alteration beyond the contact aureoles continued due to high heat flow, producing high rank coals with R r > 2%. Values of δ13C org range between −26.5 and –21.1‰ (5.4‰ difference). Significant relationships (at the 1 and 5% levels) exist between δ13C org and the level of organic maturity, organic carbon (C org) content (i.e., size of the carbon reservoir), and total inertinite content, suggesting variability due to these factors should be considered when assessing isotopic excursions and interpretation of Permian-Triassic atmospheric conditions. These findings highlight the importance of incorporating organic petrography into δ13C org studies. • Late Permian Antarctic samples are bituminous to meta-anthracite in rank and coke. • δ13C org values for these samples ranges from −26.5 to −21.1‰ (5.4‰ difference). • δ13C org correlates with C org content, maturation level, and maceral content. • Organic petrography should be incorporated into carbon isotope excursion (CIE) studies. • Considering multiple variables, CIEs should be interpreted with caution. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The mineralogy of thermometamorphic granites is relatively simple, making it possible to track the spatial distribution of chemical and mineralogical variations in these rocks and investigate the processes that underpin these metamorphic reactions. We have undertaken a detailed investigation of metagranites from the contact aureole that fringes a quartz diorite intrusion of Late Permian age, emplaced into Carboniferous peraluminous granites of the Gennargentu Igneous Complex (Sardinia, Italy). New data are presented including the petrography of metagranites within a 500 m zone adjacent to the quartz diorite intrusion, the compositions of minerals and bulk-rocks, and the oxygen isotope compositions of separated minerals. We have used these data to assess the mobility of elements, expressed as Δoxide, in the aureole, and the physical conditions of fluid-assisted thermometamorphism. Modal variations and the oscillatory zoning of plagioclase demonstrate that the shallow (P ≤200 MPa) quartz diorite intrusion was emplaced through a number of magmatic injections. The border zone of the quartz diorite intrusion presents evidence of two main processes: hybridization between andesite and rhyolite magmas and volatile saturation of the mingled magma. Modal differences in the contact zone with respect to the protolith (i.e. peraluminous granite), variations in mineral composition, temperature constraints and ΔK2O, ΔNa2O, ΔSiO2 and ΔAl2O3 indicate that a relatively large volume of the host granite (up to 400 m from the contact) was metasomatized by high-temperature (650–350°C) fluids derived from the mingled zone of the quartz diorite intrusion. In detail, the metasomatic K2O-rich fluid reacted with albite to form K-feldspar, and triggered the recrystallization of quartz and plagioclase to higher calcium concentrations. The progressive increase in the MgO/(MgO + FeO) of chlorite closer to the contact indicates that this phase also recrystallized. The iron released during chlorite recrystallization was buffered by hematite formation in the pores of metasomatic K-feldspar. The Gennargentu metagranites provide evidence that metasomatic fluids can play a major role in driving metamorphic reactions in contact aureoles. For instance, the expected increase of Ca in plagioclase owing to thermal equilibration was not achieved in the high-T zone of the aureole because of fluid-assisted removal of cations. We conclude that caution should be taken when interpreting the processes that underpin contact metamorphism in terms of thermally driven, ionic diffusion alone, because the role of fluids may be significant, if not overwhelming, in the domains closest to the magmatic source. [ABSTRACT FROM AUTHOR]

Lunar granulitic breccias are a group of clast-bearing impact-melt lithologies that have been brecciated, and then metamorphosed at high temperature (~1000 °C) to generate annealed (granoblastic to poikiloblastic) textures. They are ubiquitous among lunar samples, but occur in small volumes, typically as clasts in other lunar rocks. We have determined major, minor, and trace element diffusion profiles in olivines, orthopyroxenes, and clinopyroxenes from one Apollo 16 (60035), three Apollo 17 (77017, 78155, 79215), and two paired lunar meteorites (NWA 3163 and NWA 4881) by means of electron microscopy and electron probe microanalysis. The results show that relic igneous clasts exhibit an absence of, or limited, major element zoning, yet retain minor and trace element profiles. We exploit this characteristic to estimate the duration of high-temperature metamorphism responsible for their recrystallization. To achieve this we have completed pyroxene thermometry, element linescans, X-ray mapping, and modeling of heating and cooling of hanging wall and footwall lithologies juxtaposed with a hot body. The high equilibration temperatures, moderately high siderophile contents, and time scales of metamorphism of the lunar granulites indicate that they were metamorphosed in relatively near-surface settings. Diffusion calculations indicate that most granulitic breccias were heated for 13 000-300 000 yr. We conclude that they formed above or beneath superheated impact-melt sheets associated with medium-size (100-200 km) craters. [ABSTRACT FROM AUTHOR]

Petrologic analysis is used to determine the intensity and extent of the thermal effects of a post-kinematic granite on its surrounding rocks to understand the thermal evolution of an accretionary prism in Central Kyushu, Japan. The relatively young (14 Ma) Okueyama granite represents a single magma chamber, which solidified as a zoned pluton, and is therefore suitable for the study of the thermal effects by a single heat source. The Lower Shimanto Group country rock, an accretionary prism of Cretaceous age, has undergone contact metamorphism, to form an orthopyroxene zone, cordierite zone, and biotite zone in order of decreasing metamorphic grade. Singular value decomposition (SVD) analysis of composition matrices of relevant mineral assemblages results in the following isograd reactions 0.475BtCrd = 0.538Opx + 0.179BtOpx + 0.665Kfs + 0.075Crd + 0.067Ilm, for the orthopyroxene isograd, and 0.612BtBt + 0.218MsBt + 0.063Ilm = 0.292KfsCrd + 0.686BtCrd + 0.137CrdCrd, for the cordierite isograd. Peak metamorphic temperatures for each zone were determined using ternary feldspar and garnet-biotite thermometers, yielding a peak thermal gradient of 0.071 °C/m. This thermal gradient yields isograd temperature conditions of 740 °C for the orthopyroxene isograd and 550 °C for the cordierite isograd, respectively. [ABSTRACT FROM AUTHOR]

Abstract: The metamorphic evolution of a key sector of the western Mediterranean internal Alpine orogenic belt (southern Calabrian Peloritani Orogen) is identified and described by means of P–T pseudosections calculated for selected metapelite specimens, showing evidence of multi-stage metamorphism. Attention focused on the two lowermost basement nappes of the Aspromonte Massif (southern Calabria), which were differently affected by poly-orogenic multi-stage evolution. After a complete Variscan orogenic cycle, the upper unit (Aspromonte Peloritani Unit) was involved in a late-Alpine shearing event. In contrast, the several underlying metapelite slices, here grouped together as Lower Metapelite Group, show exclusive evidence of a complete Alpine orogenic cycle. In order to obtain reliable P–T constraints, an integrated approach was employed, based on: a) garnet isopleth thermobarometry; and b) theoretical predictions of the P–T stability fields of representative equilibrium assemblages. This approach, which takes into account the role of the local equilibrium volumes in controlling textural developments, yielded reliable information about P–T conditions from early to peak metamorphic stages, as well as estimates of the retrograde trajectory in the pseudosection P–T space. According to inferred detailed P–T paths, the evolution of the Aspromonte Peloritani Unit is characterised by a multi-stage Variscan cycle, subdivided into an early crustal thickening stage with P–T conditions ranging from 0.56±0.05 GPa at 570±10 °C to 0.63–0.93 GPa at 650–710 °C (peak conditions) and evolving to a later crustal thinning episode in lower P–T conditions (0.25 GPa at 540 °C), as documented by the retrograde trajectory. Conversely, the prograde evolution of the rocks of the Lower Metapelite Group shows evidence of a HP-LT early Alpine multi-stage cycle, with P–T evolving from 0.75–0.90 GPa at 510–530 °C towards peak conditions, with pressure increasing northwards from 1.12±0.02 GPa to 1.24±0.02 GPa, and temperatures of 540–570 °C. A late-Alpine mylonitic overprint affected the rocks of both the Aspromonte Peloritani Unit and the Lower Metapelite Group. This overprint was characterised by an initial retrograde decompression trajectory (0.75±0.05 GPa at 570–600 °C), followed by a joint cooling history, ranging from 0.38±0.14 at temperature from 450 to 520 °C. These inferred results were then used: a) to interpret the Lower Metapelite Group as a single crystalline basement unit exclusively affected by a complete Alpine orogenic cycle, according to the very similar features of P–T paths, comparable petrography and analogous structural characteristics; b) as a tool for more reliable correlations between the Aspromonte Massif, the other Calabrian terranes and the north African Orogenic Complexes. They may therefore consider a contribution to the geodynamic modelling of the western Mediterranean. [Copyright &y& Elsevier]

Abstract: Detrital zircon U–Pb age data from ultrahigh-temperature metasedimentary rocks (950°C and 10–12kbar) in the Rauer Group and a granulite grade metasedimentary rock (850°C and 7–8kbar) in the Bolingen Islands in Prydz Bay, East Antarctica, indicate the existence of a Neoproterozoic (Ediacaran) cover sequence that was deposited on basement ranging in age between approximately 2800Ma and 940Ma. The Neoproterozoic succession has a maximum depositional age of approximately 600Ma with derivation from igneous and metamorphic sources of Archaean, Palaeoproterozoic and Neoproterozoic age. The minimum depositional age is constrained to be approximately 575Ma based on existing (Th+U)–Pb monazite metamorphic ages from the UHT metasedimentary rocks. The identification of Late Neoproterozoic protoliths to the UHT granulites in the Rauer Group demonstrate that ultrahigh-temperature metamorphism in Prydz Bay must be Ediacaran to Cambrian in age. The timing of basin development (ca. 600–575Ma) is coincident with the amalgamation of eastern Gondwana. The palaeogeography of the Prydz Bay region during the amalgamation of Gondwana is uncertain, but in our proposed reconstruction we suggest that inherited zircons were derived from Indo-Antarctic crust. Ocean closure is inferred to have occurred along a suture zone to the southeast of Prydz Bay and the Prince Charles Mountains. Our palaeogeographic model invites further work on the possibility of a shared history between India and a large portion of East Antarctica prior to the final amalgamation of eastern Gondwana during the Ediacaran–Cambrian. [Copyright &y& Elsevier]

Combined petrographic, structural and geochronological study of the Malashan dome, one of the North Himalayan gneiss domes, reveals that it is cored by a Miocene granite, the Malashan granite, that intruded into the Jurassic sedimentary rocks of Tethys Himalaya. Two other granites in the area are referred to as the Paiku and Cuobu granites. New zircon SHRIMP U-Pb and muscovite and biotite 40Ar-39Ar dating show that the Paiku granite was emplaced during 22.2–16.2 Ma (average 19.3 ± 3.9 Ma) and cooled rapidly to 350–400 °C at around 15.9 Ma. Whole-rock granite chemistry suggests the original granitic magma may have formed by muscovite dehydration melting of a protolith chemically similar to the High Himalayan Crystalline Sequence. Abundant calcareous metasedimentary rocks and minor garnet-staurolite-biotite-muscovite ± andalusite schists record contact metamorphism by three granites that intruded intermittently into the Jurassic sediments between 18.5 and 15.3 Ma. Two stages of widespread penetrative ductile deformation, D1 and D2, can be defined. Microstructural studies of metapelites combined with geothermobarometry and pseudosection analyses yield P– T conditions of 4.8 ± 0.8 kbar at 550 ± 50 °C during a non-deformational stage between D1 and D2, and 3.1–4.1 kbar at 530–575 °C during syn- to post-D2. The pressure estimates for the syn- to post-D2 growth of andalusite suggest relatively shallow (depth of ∼15.2 km) extensional ductile deformation that took place within a shear zone of the South Tibetan Detachment System. Close temporal association between intrusion of the Malashan granite and onset of D2 suggests extension may have been triggered by the intrusion of the Malashan granite. [ABSTRACT FROM AUTHOR]

Abstract: Nitrogen content and isotopic composition of carbonaceous-rich metasediments were determined by on-line and sealed-tube combustion using ultra-high vacuum line and static mass spectrometer adapted to analyse nitrogen nanomoles. Accurate measurements showed that nitrogen amount released by on-line combustion technique was underestimated to various extents. As a result, the nitrogen isotopic composition was not correctly determined. In contrast, sealed-tube combustion appeared to yield the most reproducible and accurate measurements, except for nitrogen depleted carbonaceous matter (semi-graphite to graphite transition) containing less than around 60 ppm of nitrogen, which were contaminated during their extraction from the rock. In view of that, a preliminary sealed-tube investigation of the organic nitrogen content and isotopic composition in a homogenous series of low-grade metasediments was undertaken: in spite of an important nitrogen loss, the carbonaceous matter nitrogen isotopic composition remains about the same during the meta-anthracite and semi-graphitisation stages. Inferences on the process of organic nitrogen mineralization during carbonaceous matter metamorphism can be drawn and several paleo-biogeochemical implications envisaged. [Copyright &y& Elsevier]

The Eocene Saraycık granodiorite, eastern Pontides, Turkey, is a small post-tectonic intrusive body (∼11 km²), cross-cutting tectonically juxtaposed distinct metamorphic and non-metamorphic lithologies. A ∼0.8 to 1-km-wide contact-metamorphic aureole has developed around the intrusion. Disequilibrium is prevalent in all lithologies, even in the inner zones of the contact aureole. Conditions of contact metamorphism are estimated by simple phase equilibria applied to microdomains in the metapelitic lithologies without obvious signs of disequilibrium: Shallow emplacement depths (0.21±0.05 GPa = ∼5-8 km) are suggested by contact-metamorphic mineral assemblages (e.g., andalusite + corundum + K-feldspar), by Al-in-hornblende barometry on granodiorite and by partial melting within the stability field of andalusite near the intrusive contact. Peak temperatures during the contact metamorphism ranged from 430-520 °C at the outer domains of the aureole (∼400 m from the intrusive contact) to ≥730 °C at the immediate contact of the intrusion. Textural evidence for partial melting in the stability fields of both andalusite and sillimanite are observed in metapelitic lithologies at distances ≤30 m from the intrusive contact. Basic lithologies contain mostly foliation-parallel, 2-50-mm-thick layers with skarn-like mineralogy, which probably formed via infiltration of chemically reactive fluids. Prevalence of disequilibrium is predominantly related to temporal variation in temperature and/or fluid composition attending the contact metamorphism, rather than to the presence of some relict minerals from a pre-contact-metamorphic stage. [ABSTRACT FROM AUTHOR]

The Kyffhäuser Crystalline Complex, Central Germany, forms part of the Mid-German Crystalline Rise, which is assumed to represent the Variscan collision zone between the East Avalonian terrane and the Armorican terrane assemblage. High-precision U–Pb zircon and monazite dating indicates that sedimentary rocks of the Kyffhäuser Crystalline Complex are younger than c. 470 Ma and were intruded by gabbros and diorites between 345 ± 4 and 340 ± 1 Ma. These intrusions had magmatic temperatures between 850 and 900°C, and caused a contact metamorphic overprint of the sediments at P–T conditions of 690–750°C and 5–7 kbar, corresponding to an intrusion depth of 19–25 km. At 337 ± 1 Ma the magmatic–metamorphic suite was intruded by granites, syenites and diorites at a shallow crustal level of some 7–11 km. This is inferred from a diorite, and conforms to P–T paths obtained from the metasediments, indicating a nearly isothermal decompression from 5–7 to 2–4 kbar at 690–750°C. Subsequently, the metamorphic–magmatic sequence underwent accelerated cooling to below 400°C, as constrained by garnet geospeedometry and a previously published K–Ar muscovite age of 333 ± 7 Ma. With respect to P–T–D–t data from surrounding units, rapid exhumation of the KCC can be interpreted to result from NW-directed crustal shortening during the Viséan. [ABSTRACT FROM AUTHOR]

The northern margin of the Inland Branch of the Pan-African Damara Orogen in Namibia shows dramatic along-strike variation in metamorphic character during convergence between the Congo and Kalahari Cratons (M3 metamorphic cycle). Low-P contact metamorphism with anticlockwise P–T paths dominates in the western domains (Ugab Zone and western Northern Zone), and high-P Barrovian metamorphism with a clockwise P–T path is documented from the easternmost domain (eastern Northern Zone). The sequence of M3 mineral growth in contact aureoles shows early growth of cordierite porphyroblasts that were pseudomorphed to biotite–chlorite–muscovite at the same time as an andalusite–biotite–muscovite transposed foliation was developed in the matrix. The peak-T metamorphic assemblages and fabrics were overprinted by crenulations and retrograde chlorite–muscovite. The KFMASH P–T pseudosection for metapelites in the Ugab Zone and western Northern Zone contact aureoles indicates tight anticlockwise P–T loops through peak metamorphic conditions of 540–570°C and 2·5–3·2 kbar. These semi-quantitative P–T loops are consistent with average P–T calculations using THERMOCALC, which give a pooled mean of 556 ± 26°C and 3·2 ± 0·6 kbar, indicating a high average thermal gradient of 50°C/km. In contrast, the eastern Northern Zone experienced deep burial, high-P/moderate-T Barrovian M3 metamorphism with an average thermal gradient of 21°C/km and peak metamorphic conditions of c. 635°C and 8·7 kbar. The calculated P–T pseudosection and garnet compositional isopleths in KFMASH, appropriate for the metapelite sample from this region, document a clockwise P–T path. Early plagioclase–kyanite–biotite parageneses evolved by plagioclase consumption and the growth of garnet to increasing XFe, XMg and XCa and decreasing XMn compositions, indicating steep burial with heating. The developed kyanite–garnet–biotite peak metamorphic parageneses were followed by the resorption of garnet and formation of plagioclase moats, indicating decompression, which was followed by retrogressive cooling and chlorite–muscovite growth. The clockwise P–T loop is consistent with the foreland vergent fold–thrust belt geometry in this part of the northern margin. Earlier formed (580–570 Ma) pervasive matrix foliations (M2) were overprinted by contact metamorphic parageneses (M3) in the aureoles of 530 ± 3 Ma granites in the Ugab Zone and 553–514 Ma granites in the western Northern Zone. Available geochronological data suggest that convergence between the Congo and Kalahari Cratons was essentially coeval in all parts of the northern margin, with similar ages of 535–530 Ma for the main phase of deformation in the eastern Northern Zone and Northern Platform and 538–505 Ma high-grade metamorphism of the Central Zone immediately to the south. Consequently, NNE–SSW-directed convergent deformation and associated M3 metamorphism of contrasting styles are interpreted to be broadly contemporaneous along the length of the northern margin of the Inland Branch. In the west heat transfer was dominated by conduction and externally driven by granites, whereas in the east heat transfer was dominated by advection and internally driven radiogenic heat production. The ultimate cause was along-orogen variation in crustal architecture, including thickness of the passive margin lithosphere and thickness of the overlying sedimentary succession. [ABSTRACT FROM AUTHOR]

Quartz–calcite sandstones experienced the reaction calcite + quartz = wollastonite + CO2 during prograde contact metamorphism at P = 1500 bars and T = 560°C. Rocks were in equilibrium during reaction with a CO2–H2O fluid with XCO2 = 0·14. The transition from calcite-bearing, wollastonite-free to wollastonite-bearing, calcite-free rocks across the wollastonite isograd is only several millimeters wide. The wollastonite-forming reaction was driven by infiltration of quartz–calcite sandstone by chemically reactive H2O-rich fluids, and the distribution of wollastonite directly images the flow paths of reactive fluids during metamorphism. The mapped distribution of wollastonite and modeling of an O-isotope profile across a lithologic contact indicate that the principal direction of flow was layer-parallel, directed upward, with any cross-layer component of flow <0·1% of the layer-parallel component. Fluid flow was channeled at a scale of 1–100 m by pre-metamorphic dikes, thrust and strike-slip faults, fold hinges, bedding, and stratigraphic contacts. Limits on the amount of fluid, based on minimum and maximum estimates for the displacement of the wollastonite reaction front from the fluid source, are (0·7–1·9) × 105 cm3 fluid/cm2 rock. The sharpness of the wollastonite isograd, the consistency of mineral thermobarometry, the uniform measured 18O–16O fractionations between quartz and calcite, and model calculations all argue for a close approach to local mineral–fluid equilibrium during the wollastonite-forming reaction. [ABSTRACT FROM PUBLISHER]