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Re di Castello is the southernmost unit of the composite Adamello batholith (central Alps, Italy). It was emplaced between 43 and 40 Ma, yet its depth of emplacement is not well constrained. We have studied the contact metamorphism of pelitic country rocks in two localities: the upper Caffaro Valley and the Borzago Valley in the southern and the northern sub‐units of the Re di Castello, respectively. In the Caffaro Valley, the country rocks comprise the Triassic sedimentary Lozio Shale, a carbon‐rich slate–siltstone, which was intruded by quartz‐dioritic magma. Near the contact, the hornfelses display the mineralogical assemblage Kfs‐Crd‐Bt‐Ms‐Pl‐Qz‐Gr, with rare fibrolitic sillimanite in a single sample. Andalusite was never observed throughout the aureole at this locality. In the Borzago Valley, contact metamorphism developed on schists of the pre‐Permian basement with a Variscan, regional metamorphic overprint. The sequence of contact metamorphic mineral assemblages progresses from And‐Bt‐bearing parageneses to the Sil‐Crd‐Bt‐Kfs‐Ms‐Pl‐Qz that characterizes peak conditions, at which incipient melting is also observed. K‐feldspar forms slightly upgrade the first appearance of sillimanite. For samples at both localities, thermodynamic modelling in the NCKFMASHT system failed at predicting a stability field for the sequences of mineral assemblages developed during contact metamorphism. Also the Ti‐in biotite thermometer did not constrain temperatures adequately. The P–T conditions at the thermal peak were thus evaluated by an alternative bathograd‐like approach, considering phase relationships in the simplified NKASH‐C system. To form sillimanite only as product of the incomplete Ms‐Qz breakdown—divariant for the presence of Na in Ms and Kfs and shifted to lower temperature due to the presence of graphite (in the Lozio Shale)—an isobaric path typical of contact metamophism must have crossed above the Msss‐Qz‐Kfsss‐Sil‐And‐fluid invariant point. This constrains an emplacement pressure >3.3 kbar in the Caffaro Valley, and >3.2 kbar in the Borzago Valley. Concerning temperature, the same univariant point also constrains the minimum temperature in the Caffaro Valley as >615°C–620°C, consistent with results of RSCM thermometry on graphite from the Lozio Shale. The evidence of incipient melting in some samples from the Borzago Valley indicates higher temperatures, probably approaching 670°C, near the contact to the intrusion. Assuming an average crust density of 2.7 g cm−3, the estimated pressures correspond to a minimum paleo‐depth of emplacement of about 12 km. These depths are somewhat greater than normally considered and should be regarded as revised constraints on models of the emplacement dynamics of the Adamello batholith and on paleogeographic reconstructions of this part of the Southalpine domain. [ABSTRACT FROM AUTHOR]

Coal rank gradient is fundamental for predicting the degree of coalification in deep reservoirs. This study, based on the measured maximum reflectance of vitrinite of 85 sets, the maximum reflectance of vitrinite /dry ash-free basis volatile yield of 2 213 sets from different coalbed methane blocks' geological exploration reports, and 242 sets of maximum reflectance of vitrinite post-magma intrusion in relation to the coal seam distance from the intrusion, analyzed the coal rank distribution and gradient characteristics under different types of metamorphism. The differences of gradients between deep metamorphism, regional magmatic thermal metamorphism, and contact metamorphism were compared. The results show that the coal rank gradient (Ro, max/100 m) under deep metamorphismin in different coalbed methane blocks in China ranges from 0.013% to 0.068%, with an average of 0.032%, and shows a positive correlation with the average maximum reflectance of vitrinite. Under regional magmatic thermal metamorphism, it ranges from 0.09% to 0.45%, with an average of 0.18%. Under contact metamorphism, it ranges from 20% to 454%, with an average of 106%. In the contact metamorphism induced by magmatic intrusions, the coal rank gradient exhibits a multi-stage variation. The closer the coal seam is to the intrusion, the greater the coal rank gradient. The differences in coal rank gradient under different types of metamorphism are significant, mainly determined by heat source characteristics, with coalification duration, thermal conductivity of rock layers, and maximum burial depth also playing crucial roles. China's coalification process is complex, making the study of coal grade gradient under different types of metamorphism and in different coalbed methane blocks has guiding significance for predicting coal rank of deep reservoir and for the exploration and development of coal/coalbed methane resources.

Abstract Carbonaceous material (CM) undergoes progressive changes that reflect its thermal history. These changes are in general irreversible and provide valuable information for understanding diagenetic and metamorphic processes of crustal rocks. Among various approaches to quantify these changes, the R2 ratio, area ratio of specific peaks in CM Raman spectra, is widely used to estimate the maximum temperature of intermediate- to moderately high-grade metamorphism. The calculation of the R2 ratio requires peak deconvolution of the original spectrum, and the results depend on the details of how this is carried out. However, a clear protocol for selecting appropriate initial conditions has not been established and obtaining a reliable temperature estimate depends at least in part on the experience and skill of the operator. In this study, we developed a Python code that automatically calculates the R2 ratio from CM Raman spectra. Our code produces R2 ratios that are generally in good agreement with those of Aoya et al. (J Metamorph Geol 28:895–914, 2010, https://doi.org/10.1111/j.1525-1314.2010.00896.x ) for the same Raman data, with much less time and effort than was the case in the previous studies. We have confirmed that the code is also applicable to other previous datasets from both contact and regional metamorphic regions. The overall trend of the recalculated data indicates that samples with R2 greater than ~ 0.7 are not sensitive to the changes in CM maturity and thus should not be used for the calibration of an R2-based geothermometer. We propose a modified geothermometer for contact metamorphism that is strictly applicable to samples with R2 from 0.023 to 0.516, with the proviso that a laser with a wavelength of 532 nm should be used. A slight extrapolation of the newly proposed geothermometer up to R2 of 0.57 provides a temperature estimate that is consistent with the geothermometer of Kaneki and Kouketsu (Island Arc 31:e12467, 2022; https://doi.org/10.1111/iar.12467 ); the boundary between the two geothermometers corresponds to a temperature of 391 °C.

Carbonaceous material (CM) undergoes progressive changes that reflect its thermal history. These changes are in general irreversible and provide valuable information for understanding diagenetic and metamorphic processes of crustal rocks. Among various approaches to quantify these changes, the R2 ratio, area ratio of specific peaks in CM Raman spectra, is widely used to estimate the maximum temperature of intermediate- to moderately high-grade metamorphism. The calculation of the R2 ratio requires peak deconvolution of the original spectrum, and the results depend on the details of how this is carried out. However, a clear protocol for selecting appropriate initial conditions has not been established and obtaining a reliable temperature estimate depends at least in part on the experience and skill of the operator. In this study, we developed a Python code that automatically calculates the R2 ratio from CM Raman spectra. Our code produces R2 ratios that are generally in good agreement with those of Aoya et al. (J Metamorph Geol 28:895–914, 2010, https://doi.org/10.1111/j.1525-1314.2010.00896.x) for the same Raman data, with much less time and effort than was the case in the previous studies. We have confirmed that the code is also applicable to other previous datasets from both contact and regional metamorphic regions. The overall trend of the recalculated data indicates that samples with R2 greater than ~ 0.7 are not sensitive to the changes in CM maturity and thus should not be used for the calibration of an R2-based geothermometer. We propose a modified geothermometer for contact metamorphism that is strictly applicable to samples with R2 from 0.023 to 0.516, with the proviso that a laser with a wavelength of 532 nm should be used. A slight extrapolation of the newly proposed geothermometer up to R2 of 0.57 provides a temperature estimate that is consistent with the geothermometer of Kaneki and Kouketsu (Island Arc 31:e12467, 2022; https://doi.org/10.1111/iar.12467); the boundary between the two geothermometers corresponds to a temperature of 391 °C. [ABSTRACT FROM AUTHOR]

New data favor the view previously substantiated by geological, geochemical and isotopic data that the orebodies of the Belogorskoe deposit are the Triassic erosion products of the laterite weathering crust after gabbroids, which were metamorphosed and partially regenerated in the Late Cretaceous–Paleogene. The source for the formation of the Belogorskoe deposit was shown to be the products of exogenic destruction of rocks, the isotopic and geochemical characteristics of which are close to those of Cambrian gabbroids of the Vladimiro-Aleksandrovsky massif (southern part of the Okrainsko-Sergeevsky terrane). It has been found that the Belogorskoe deposit consists of rocks and ores, the primary (magmatic) REE distribution in which was variably modified as a result of interaction between sediments (protoliths) and seawater (presumably during the Late Jurassic–Early Cretaceous accretion), as well as between their metamorphosed analogues and hydrothermal solutions (in the Late Cretaceous–Paleogene). The influence of gabbroids (as a source of material) on the chemical and mineral composition of the Belogorskoe deposit agrees with the data reported in a paper on the enrichment of orebodies in such elements as Fe and Mn characteristic of ultramafic rocks and the presence of Au–Ag–Pd–Pt, Ni–Co, and Bi mineralization in them. The orebodies of the Belogorskoe deposit contain accessory minerals and mineral varieties that are rare in nature and poorly studied. They include an unusually Th-rich variety of zircon, baddeleyite, gudmundite, a large group of bismuth compounds, including Bi2Te, (Ag,Pb)BiS2, as well as coloradoite, lafossaite, sanbornite, perovskite, and the compound InPO4. There is also a large group of rare and unusual noble metal compounds: copper gold, platinum gold, jonassonite, disordered solid solutions of Cu, Ag and Au (Au-based), Pt–Pd, Pt–Ag intermetallides, and other rare minerals and mineral varieties. [ABSTRACT FROM AUTHOR]

Gem sapphire is commonly retrieved from primary and secondary deposits associated with alkali basaltic fields, but its source rocks are rarely preserved. The Eifel (Rhenish Massif, western Germany), although not producing gem sapphire, shares many petrologic and geochemical similarities with such fields worldwide. Due to the young age of volcanic deposits and active quarrying, sapphire-bearing rocks are readily accessible, along with detrital sapphire from modern sediments. Here, oxygen isotope and trace element compositions are reported for 223 sapphire grains, and rutile and zircon inclusions in sapphire were dated indicating crystallization synchronous with Paleogene–Quaternary volcanism. Endmembers in δ18O range are sapphires from syenites representing mantle-derived differentiated melts with minor crustal contamination (~4–6‰) and contact metamorphic mica schists (>10‰) as purely crustal source rocks. Intermediate values between ~6 and 10‰ require variable degrees of mantle-crust hybridization. Lower crustal granulite sources are dismissed based on their oxygen isotopic compositions being lower than most sapphire crystals. Diffusion modelling of sharp oxygen isotopic zonation in compositionally zoned crystals precludes crystal residence at >900 °C over the lifetime of evolved magma reservoirs in the Eifel (c. 50 ka). This argues against direct mantle or lower crustal sapphire origins. Instead, low temperature residence is consistent with sharp δ18O gradients, coexisting andalusite, and fluid inclusion barometry. Hence, Eifel sapphire crystallization is attributed to contact metamorphic aureoles around upper crustal (5–7 km) magma bodies where phonolite, trachyte, and carbonatite melts differentiated from mafic parental magmas, and reacted with metasedimentary wall rocks. [ABSTRACT FROM AUTHOR]

Abstract In the Carajás Province, many Au and Cu deposits are hosted in Archean metabasic rocks belonging to the Itacaiúnas Supergroup. These metabasic rocks underwent seawater metamorphism of greenschist facies concomitantly to the volcanic activity and banded iron deposition. In the Carajás Province, synkinematic granite magmatism occurred at ca. 2.75 Ga as represented by the Estrela, Serra do Rabo, Planalto, and Igarapé Gelado granites. Emplacement at relatively shallow levels created thermal aureoles of albite-epidote hornfels to hornblende hornfels facies. Metabasic xenoliths show assemblages of hornblende hornfels. Regional stresses and lateral expansion of the syntectonic granites promoted high-temperature low-pressure metamorphism and ductile deformation in the host metabasic rocks. Dehydration reactions increased the fluid pressure permitting hydraulic fracturing and veining in the metabasaltic rocks. These conditions increased the kinetic energy of minerals so that pressure solution and dissolution of minerals were enhanced. The material transfer caused by the advective flow of supersaturated fluids leads to the formation of amphibole-bearing veins and subordinate quartz-bearing and plagioclase-bearing veins. Progressive reactions during thermal metamorphism promoted the replacement of actinolite to ferropargasite/edenite and Na-plagioclase to Ca-plagioclase. Silica produced in these reactions was transported by fluids and deposited in veins that crosscut the amphibolites.

In the Carajás Province, many Au and Cu deposits are hosted in Archean metabasic rocks belonging to the Itacaiúnas Supergroup. These metabasic rocks underwent seawater metamorphism of greenschist facies concomitantly to the volcanic activity and banded iron deposition. In the Carajás Province, synkinematic granite magmatism occurred at ca. 2.75 Ga as represented by the Estrela, Serra do Rabo, Planalto, and Igarapé Gelado granites. Emplacement at relatively shallow levels created thermal aureoles of albite-epidote hornfels to hornblende hornfels facies. Metabasic xenoliths show assemblages of hornblende hornfels. Regional stresses and lateral expansion of the syntectonic granites promoted high-temperature low-pressure metamorphism and ductile deformation in the host metabasic rocks. Dehydration reactions increased the fluid pressure permitting hydraulic fracturing and veining in the metabasaltic rocks. These conditions increased the kinetic energy of minerals so that pressure solution and dissolution of minerals were enhanced. The material transfer caused by the advective flow of supersaturated fluids leads to the formation of amphibole-bearing veins and subordinate quartz-bearing and plagioclase-bearing veins. Progressive reactions during thermal metamorphism promoted the replacement of actinolite to ferropargasite/edenite and Na-plagioclase to Ca-plagioclase. Silica produced in these reactions was transported by fluids and deposited in veins that crosscut the amphibolites. [ABSTRACT FROM AUTHOR]

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.

• Carbonaceous material in recent sediments record short-lived contact metamorphism. • Such record is preserved in low-grade metasediments contacting basalts in mélanges. • The studied sediment-basalt mélanges formed prior to subduction. • Mixed lithologies experienced different intensity in deformation during subduction. • Such mélanges represent a potential geological record of ridge-trench interactions. Mélanges, intriguing rock units often found in accretionary complexes, consist of basalt lenses embedded in a highly sheared sedimentary matrix. The origin of mélanges remains a subject of vigorous debate, with consequences on our understanding of subduction processes. A first line of thought interprets mélanges as mixed lithologies intertwined by convergent tectonics. Supporters of this interpretation regard mélanges as fossilized witnesses of the lower- and upper-plate interface, with their rheological properties reflecting seismogenic subduction zones. However, a second line of thought is to consider that basalts and sediments were mixed prior to subduction by sedimentary and/or magmatic processes, this mix being only later incorporated into the accretionary wedge. In this study, we present evidence supporting the pre-subduction mixing interpretation for mélanges from two paleo-accretionary complexes: the Kodiak complex in Alaska and the Shimanto Belt in Japan. In modern seafloor sediments in contact with basaltic submarine magmas, we show that the crystallinity of carbonaceous particles in sediments increases toward basalts, indicating a ∼1 cm-thick aureole of contact metamorphism. Intriguingly, a comparable aureole of increased crystallinity is observed in four mélanges from the two paleo-accretionary complexes. Basalts were thus emplaced onto and into sediments by magmatism rather than by tectonics, challenging the notion of mélanges explored in this study as formed along the plate boundary interface. Moreover, the studied mélanges are made of mid-ocean ridge basalts, and deposition ages of mélange sediments coincide with proposed ridge subductions. This implies that the mid-ocean ridges at the trench were the source of the magmas that intruded into and extruded onto the clastic sediments and contributed to form the multilayered basalt-sediments architecture. [ABSTRACT FROM AUTHOR]

Synorogenic exhumation is important for understanding the evolution of mountain belts but is often difficult to discern because traditional thermochronology methods mostly date the final stages of exhumation. Intrusions emplaced during orogenesis provide an alternate method to elucidate these exhumation patterns since they preserve the timing and depth of their emplacement. We apply this concept the southern Omineca belt of the Canadian Cordillera by combining new and existing zircon U Pb and mica Ar/Ar ages from 43 intrusions as well as new and existing estimates of emplacement depth from 23 contact aureoles. Seventeen new zircon U Pb laser ablation inductively coupled plasma mass spectrometry results are presented in the paper, spanning the existing range of previous ages for four episodes of Phanerozoic magmatism in this region: Middle Jurassic (174–154 Ma), mid-Cretaceous (118–95 Ma), Late Cretaceous (78–76 Ma), and Paleocene (66–60 Ma). This new work has resolved two new distinct periods of Cretaceous magmatism at ∼100 and ∼ 76 Ma compared to what was previously considered to be a single mid-Cretaceous event (Bayonne magmatic suite). Furthermore, the Fry Creek batholith, previously considered to be emblematic of the Bayonne suite, the adjacent Shoreline stock, the ∼60 Ma Nelson stock located 40 km to the southwest, and the 55–62 Ma Ladybird magmatic suite form part of a newly identified phase of 66–55 Ma Paleocene magmatism in the region. Compiled intrusion ages were then combined with estimates of emplacement depth determined from metamorphic mineral assemblages in contact aureoles developed in pelitic host rocks. Jurassic intrusions typically have higher pressure staurolite + andalusite-bearing contact aureoles (3.3–4.0 kbar) corresponding to depths of 12.5–15 km, whereas Cretaceous and Paleocene intrusions mainly have lower pressure andalusite + cordierite-bearing contact aureoles (2.3–2.9 kbar) corresponding to depths of 8.5–11 km. These data indicate regional exhumation of 2–6 km occurred between the Middle Jurassic (180–160 Ma) and mid-Cretaceous to Paleocene (120–60 Ma) periods of Cordilleran orogenesis. In contrast, pluton emplacement depths changed by <2 km, suggesting minimal exhumation occurred in the southern Canadian Cordillera between the mid-Cretaceous and Paleocene. This extended lull in exhumation may be related to the partitioning of compressional deformation east into the foreland along a basal decollement beneath the study area during this time. • Emplacement age and depth of intrusions indicates exhumation during orogenesis. • New U–Pb geochronology revises magmatic suites in southeastern Canadian Cordillera. • Regional exhumation was 2–6 km from ∼175–100 Ma and 8–11.5 km from ∼60–0 Ma. • Updated regional map with pluton ages and metamorphic grades from contact aureoles. [ABSTRACT FROM AUTHOR]

Abstract Petrographic observations combined with mineral compositional analyses constrain the phase relations of prograde metamorphosed serpentinites in the Bergell contact aureole (Italy). In a 1500 m profile perpendicular to the north-eastern edge of the Bergell intrusion, seven dehydration reactions ran to completion. Three previously undocumented reactions have been identified within 70 m of the intrusive contact: olivine + anthophyllite = orthopyroxene + H2O, tremolite + Cr–Al-spinel = olivine + Mg-hornblende + H2O and chlorite = olivine + orthopyroxene + Cr-Al-spinel + H2O. Petrological analysis indicates that these reactions occur over a narrow range of pressure and temperature, 300 ± 30 MPa and 720 ± 10 °C respectively. Computed phase diagram sections reproduce the observed mineral parageneses with one notable exception. Due to the underestimation of aluminium and sodium contents in Ca-amphibole models, plagioclase is predicted above 700 °C instead of Mg-hornblende. In comparison with natural grains, the aluminium content of computed chlorite compositions is overestimated for low grade parageneses while it is underestimated near the upper thermal stability limit of chlorite. In the computed sections, Fe partitioning relative to Mg between olivine and other silicates, suggests a clear preference for Fe in olivine, that therefore shows lower Mg#s. In contrast, microprobe analyses of natural mineral pairs indicate that orthopyroxene, Mg-hornblende and anthophyllite have lower Mg#s than equilibrium olivine. The inferred thermal profile of the metamorphic aureole is not consistent with simple heat conduction models and indicates a contact temperature of ~ 800 °C, which is 120–230 °C higher than previously estimated. Petrography also reveals extensive retrograde overprint of the prograde parageneses within 200 m of the contact. Retrogression is related to metamorphic fluids that were released by dehydration reactions during contact metamorphism and magmatic fluids expelled from the tonalite intrusion. The thermal gradient between the intrusion and the country rocks induced hydrothermal circulation of these fluids throughout the contact aureole, which beyond peak metamorphic conditions caused retrograde overprint of the prograde parageneses. The proposed phase relations for low and high pressures, and in particular, the transition from tremolite to Mg-hornblende, provides a complete representation of hydration and dehydration processes in serpentinites in subduction zones, along deep oceanic transform faults, and at passive continental margins. The latter has new implications, specifically for subduction initiation.

There is a temporal correlation between the peak activity of the North Atlantic Igneous Province (NAIP) and the Paleocene–Eocene Thermal Maximum (PETM), suggesting that the NAIP may have initiated and/or prolonged this extreme warming event. However, corroborating a causal relationship is hampered by a scarcity of expanded sedimentary records that contain both climatic and volcanic proxies. One locality hosting such a record is Fur Island in Denmark, where an expanded pre- to post-PETM succession containing hundreds of NAIP ash layers is exceptionally well preserved. We compiled a range of environmental proxies, including mercury (Hg) anomalies, paleotemperature proxies, and lithium (Li) and osmium (Os) isotopes, to trace NAIP activity, hydrological changes, weathering, and seawater connectivity across this interval. Volcanic proxies suggest that NAIP activity was elevated before the PETM and appears to have peaked during the body of the δ13C excursion, but decreased considerably during the PETM recovery. This suggests that the acme in NAIP activity, dominated by flood basalt volcanism and thermogenic degassing from contact metamorphism, was likely confined to just ~200 kyr (ca. 56.0–55.8 Ma). The hundreds of thick basaltic ashes in the post-PETM strata likely represent a change from effusive to explosive activity, rather than an increase in NAIP activity. Detrital δ7Li values and clay abundances suggest that volcanic ash production increased basaltic reactive surface area, likely enhancing silicate weathering and atmospheric carbon sequestration in the early Eocene. Signals in lipid biomarkers and Os isotopes, traditionally used to trace paleotemperature and weathering changes, are used here to track seaway connectivity. These proxies indicate that the North Sea was rapidly cut off from the North Atlantic in under 12 kyr during the PETM recovery due to NAIP thermal uplift. Our findings reinforce the hypothesis that the emplacement of the NAIP had a profound and complex impact on Paleocene–Eocene climate, both directly through volcanic and thermogenic degassing, and indirectly by driving regional uplift and changing seaway connectivity. [ABSTRACT FROM AUTHOR]

To improve the geological knowledge of the poorly studied and scarcely accessible Island of Montecristo (Tuscan Archipelago), the Authors performed 1:5000 geological survey, meso- and microstructural study and petrographic analyses on the Messinian pluton, on the associated dyke/vein network and on the remnants of the hornfels cover occurring along the western and southern coasts. The results of this study allow: a) to define a complex system of dykes and hydrothermal veins crossing the pluton and its hornfels. b) to directly correlate the Montecristo hornfels rocks (here grouped in the “Cala S. Maria-Cala dei Ladri Unit”) with the contact metamorphic aureole of the Monte Capanne pluton of the Elba Island (i.e., the Punta Polveraia-Fetovaia Unit). The Cala S. Maria-Cala dei Ladri Unit is mainly composed of alternating metapelites and metasandstones (whose protolith is likely the Quartzarenitic Member of the Palombini Shales and/or the Val Lavagna Shales of Early-Late Cretaceous age) and of minor metaophiolitic rocks; metaserpentinites were also mapped for the first time. c) to recognize low-medium grade hornfels in the remnants of the contact metamorphic aureole and the ubiquitous presence of basal cataclastic horizons, testifying a role of “roof pendant” plates for the Cala S. Maria-Cala dei Ladri Unit outcrops. d) to reconstruct a complex structural evolution for the emplacement of the pluton and for its contact metamorphic aureole. This evolution started with ductile events, i.e., formation of mylonite in the granitoids and of a pervasive S1 schistosity in the hornfels, which was likely related to flattening processes occurred during the balooning of the magmatic body, followed by ductile D2 folding with the detachment of the recrystallized Ligurian covers due to the rise of the partially cooled pluton, and then by a series of jointing/faulting events, whose lineaments were used as pathways for the intrusion of the different dykes and hydrothermal veins; finally, a NW-SE striking transtensive sinistral shear zone (Cala Maestra-Cala Corfù Fracture Zone) with an about ENE-WSW directed σ1 originated in the south/south-western part of the island. This shear zone represents one of the few witness of regional transcurrent structures in the Tuscan Archipelago and was likely active also during the emplacement of the plutonic body. [ABSTRACT FROM AUTHOR]

We study merwinite and products of its retrograde exsolution (monticellite and spurrite) from marbles of a contact metamorphic aureole in the Kochumdek River area (East Siberia). Cooling to 820–880°C (at P ≈ 0.2 kbar and ) has led to pseudomorphic replacement of merwinite by monticellite-spurrite symplectites. Calculations show that most of the components (Ca, Mg, Mn, Fe, and C) are mobile in the reaction of merwinite exsolution while Si is inert. The inert behavior of silicon controls the process of replacement and restricts it to a few local sites. The formation of the monticellite-spurrite symplectites was maintained by the own resources of metacarbonates. Later, limited amounts of the monticellite-spurrite symplectites formed due to fluorine inputs, possibly, from the cooling intrusion. [ABSTRACT FROM AUTHOR]

Detailed sampling at different intervals was performed to better understand the reaction along the contact between limestones and intruded basaltic lava in the Permian Emeishan large igneous province, SW China. The contact metamorphism is characterized by element migrations, the presence of nontronite with poor crystallinity, and large negative anomalies of stable carbon and oxygen isotopes, which indicates that geochemical reaction between basaltic lava and limestones is only marked at the centimeter scale. The CaCO3 content decreases from 93.7 to 95.3% in limestone to 58.2% in the metamorphic limestone while the contents of both major elements (Fe2O3, SiO2, Al2O3, K2O, and Na2O) and trace elements (Ti, Cr, Cu, Mn, V, P, Ba, Co, Nb, Zn, and Zr) increase in the metamorphic limestone based on chemical gradients and thermal effects due to emplacement of basaltic lava. The presence of nontronite presumably derived from smectite transformation and the absence of wollastonite along the contact reflect the very low temperature of metamorphic reaction and the very low-grade contact metamorphism. The depletions in the δ 13C and δ 18O values of the metamorphic limestone achieve 4.066‰–13.792‰ and 7.522‰–9.746‰, respectively. The decreases in stable carbon and oxygen isotopic compositions along the contact relative to the unmetamorphosed limestone indicate that geochemical interaction between basaltic lava and limestone is obvious, suggesting the exchange occurred between the limestone and fluids with low δ13C and δ18O values. In addition, the narrow metasomatically and thermally affected zone demonstrates the low metamorphic reactivity of the limestone, while the limestone behaved as a barrier against fluid percolation. And the amount of crystallization heat of basaltic lava, as well as the interchange of heat with atmosphere, also play non-negligible roles in the centimeter-scale metamorphic reaction. [ABSTRACT FROM AUTHOR]

Infiltration-driven metamorphism has produced the widespread development of forsterite in the siliceous dolomites of the Alta, Utah contact aureole. SIMS (secondary ion mass spectrometry) δ18O analyses show that in most of the middle to outer forsterite zone samples, forsterite, calcite and dolomite are homogenous in δ18O at the grain-scale, but forsterite and calcite exhibit minor intergranular heterogeneities of < 1‰ on the thin-section to hand sample-scales. In contrast, within ~ 300 m of the igneous contact (inner forsterite and periclase zones), grain-scale δ18O heterogeneities in forsterite are as large as 3.0‰, but forsterite crystals do not exhibit systematic center-to-edge decreases in δ18O due to skeletal growth and infilling. With one exception, forsterite, calcite and dolomite in all samples analyzed yield disequilibrium fractionations. The oxygen isotope disequilibrium commonly recorded among forsterite, calcite and dolomite may reflect the rapid growth of forsterite from increased reaction overstepping as temperature increased, or results from 18O/16O depletion in the matrix carbonate minerals during continued fluid infiltration after forsterite crystallization was complete, during the initial stages of cooling in the forsterite zone. In a traverse across the boundary between forsterite- and periclase-bearing marble layers in the periclase zone, forsterite and calcite SIMS δ18O profiles combined with the trend in forsterite textures indicate that these layers were not infiltrated simultaneously, nor continuously during both prograde reactions responsible for forsterite and periclase. Rather, the texture and SIMS data require infiltration and formation of forsterite first in the forsterite marble layer, followed by infiltration and formation of periclase in the periclase marble layer. As a consequence, the δ18O profile recorded by forsterite formed earlier than the δ18O profile recorded by calcite, but both profiles are prograde features—that is, both profiles formed in response to prograde reactions forming first forsterite and then periclase. These grain-scale δ18O profiles further suggest that these layers experienced significant changes in permeability that are linked to their sequential infiltration-reaction history. The asynchronous growth of these index minerals at the outcrop-scale during prograde heating may be more common during contact metamorphism of carbonate lithologies than currently recognized. [ABSTRACT FROM AUTHOR]

P - T path of a contact aureole can provide important insights into the magma emplacement mechanism for nearly concentrically zoned granitic plutons. The Mesoproterozoic Xiamaling formation immediately adjacent to the southern Fangshan granitic Plutons in the Western Hills of Beijing, China, developed three contact metamorphic prograde staurolite zone, sillimanite zone and K-feldspar zone. These zones preserved corroded pink-andalusite wrapped by pale-andalusite, corroded garnet inclusion inside the K-feldspar porphyroblasts and staurolite, which add to well-established sequences of mineral growth. These sequences allow reconstruction of the complete P-T path for each prograde zone on the pseudosection diagram obtained by full system composition-based phase equilibrium modeling The resultant prograde P-T paths are composed of two segments, with the first one isobaric heating and the second distinct compression and heating. The staurolite zone shows the P-T path initiating from ∼2.0 kbar/350 °C to ∼1.8 kbar/510 °C and ending at ∼3.3 kbar/580 °C. The sillimanite zone path began from ∼2.0 kbar/350 °C to ∼1.5 kbar/500 °C then ended at ∼3.8 kbar/580 °C. The K-feldspar zone path started from ∼2.0 kbar/350 °C to ∼1.7 kbar/510 °C, then terminated at ∼3.5 kbar/670 °C, approaching the initial partial melting temperature condition, then documented a retrograde path cooling down to ∼630 °C. The two prograde P-T path segments of the three zones suggest a in situ heating followed by a ductile return flow from ∼7 km down to ∼11 km depth of the metapelites from the Xiamaling formation, responsable to a diapiric emplacement of the magmas for the zoned composite Fangshan Plutons. [Display omitted] • The zoned Fangshan Plutons developed contact aureole in the Xiamaling Formation. • Complete prograde P-T paths were reconstructed for three contact prograde zones. • The strong loading paths reveal wallrock deep downflow and magma diapir emplacement. [ABSTRACT FROM AUTHOR]

Igneous intrusions induce alterations in coal and minerals. Alteration degree depends upon the in-situ coal rank and maceral composition, the intrusion temperature, heat source proximity, and intrusion size. In the Permo-Carboniferous Fengfeng coalfield, coal rank increases over some 50 km distance south to north from high volatile bituminous coal to anthracite. The coal seams were commonly intruded, and the regional elevation of rank was augmented by Jurassic-Cretaceous igneous intrusions, not just burial coalification. The petrographic examination of a semianthracite from the Dashucun mine in the north of the Fengfeng coalfield shows development of coke microstructure and increased reflectance approaching the dike in the No. 2 Seam. Based on 11 samples at 50-cm horizontal intervals, random vitrinite reflectance value (R r) declines from 5.41% at the contact to the background value of 2.05% at 5-m distance from the contact. Towards the dike, vitrinite anisotropy increases and the microtexture shows development of circular, coarse mosaic, and ribbon anisotropic coke textures and devolatilization vacuoles. Natural coke textures respond to an estimated temperature > 570 °C at the intrusion/coal contact. However, the occurrence of coke structures within a semianthracite reveals a local and regional thermal evolution. Coke textures suggest that coal rank was high volatile A bituminous coal at the time of intrusion. This has implications for burial history, heating, and subsidence rates. Assuming a paleogeothermal gradient of 40 °C/km, high volatile A bituminous coal (1% R r) would have been buried to 3 km at the time of the Jurassic intrusion. The intrusion metamorphosed high volatile bituminous coal to natural coke and subsequent larger-scale regional metamorphism then metamorphosed the coal to semianthracite. [ABSTRACT FROM AUTHOR]

Petrographic observations combined with mineral compositional analyses constrain the phase relations of prograde metamorphosed serpentinites in the Bergell contact aureole (Italy). In a 1500 m profile perpendicular to the north-eastern edge of the Bergell intrusion, seven dehydration reactions ran to completion. Three previously undocumented reactions have been identified within 70 m of the intrusive contact: olivine + anthophyllite = orthopyroxene + H2O, tremolite + Cr–Al-spinel = olivine + Mg-hornblende + H2O and chlorite = olivine + orthopyroxene + Cr-Al-spinel + H2O. Petrological analysis indicates that these reactions occur over a narrow range of pressure and temperature, 300 ± 30 MPa and 720 ± 10 °C respectively. Computed phase diagram sections reproduce the observed mineral parageneses with one notable exception. Due to the underestimation of aluminium and sodium contents in Ca-amphibole models, plagioclase is predicted above 700 °C instead of Mg-hornblende. In comparison with natural grains, the aluminium content of computed chlorite compositions is overestimated for low grade parageneses while it is underestimated near the upper thermal stability limit of chlorite. In the computed sections, Fe partitioning relative to Mg between olivine and other silicates, suggests a clear preference for Fe in olivine, that therefore shows lower Mg#s. In contrast, microprobe analyses of natural mineral pairs indicate that orthopyroxene, Mg-hornblende and anthophyllite have lower Mg#s than equilibrium olivine. The inferred thermal profile of the metamorphic aureole is not consistent with simple heat conduction models and indicates a contact temperature of ~ 800 °C, which is 120–230 °C higher than previously estimated. Petrography also reveals extensive retrograde overprint of the prograde parageneses within 200 m of the contact. Retrogression is related to metamorphic fluids that were released by dehydration reactions during contact metamorphism and magmatic fluids expelled from the tonalite intrusion. The thermal gradient between the intrusion and the country rocks induced hydrothermal circulation of these fluids throughout the contact aureole, which beyond peak metamorphic conditions caused retrograde overprint of the prograde parageneses. The proposed phase relations for low and high pressures, and in particular, the transition from tremolite to Mg-hornblende, provides a complete representation of hydration and dehydration processes in serpentinites in subduction zones, along deep oceanic transform faults, and at passive continental margins. The latter has new implications, specifically for subduction initiation. [ABSTRACT FROM AUTHOR]

The article provides new data on geochemistry and mineralogy of rare-earth elements (REE) in rocks of structural-material complexes of the Bashkir megaanticlinorium, which underwent metamorphic transformations of various nature: contact metamorphism (Suran section); syn- and postgenetic contact-dislocation metamorphism (Shatak complex) and hydrothermal metamorphism (Uluelga-Kudashmanovo zone). It has been established that when a magmatic melt is exposed to sediments, the latter are enriched with REEs with the formation of rare earth minerals (monazite, allanite, xenotime et al.). The study of the chemical composition of monazites and allanites showed that all variations of oxides in the composition of the former are due to isomorphous Ce-Ca-Th substitutions in the structure of minerals, but redistribution of these elements was an independent process characteristic of each structural-material complex. The study of allanites made it possible to establish the presence of isomorphism according to the Ca↔Ce, La, Nd principle, as well as the sharp difference between the characterized minerals in the amount of MgO, Fe* and MnO from analogues from other regions, which indicates the presence of a regional component in the chemical compositions of minerals altogether, geotectonic settings of mineralization formation. The temperature regimes of mineral-forming processes with metamorphic transformations of rocks calculated from chlorite and muscovite compositions (344-450°C – Suran section, 402-470°C – Shatak complex, 390-490°C – Uluelga-Kudashmanovo zone) indicate the possibility of stable coexistence of the association monazite-allanite. It was established that when a magmatic melt on the sedimentary substrate of the frame, the lanthanides enrich the exocontact rocks with the formation of newly formed REE-mineral associations. At the same time, the processes of formation of rare-earth mineralization are largely determined by the physicochemical parameters and thermobaric conditions of the accompanying and subsequent metamorphism.

We report contrasting pressure–temperature–time (P–T–t) paths of migmatites developed in the highest‐grade metamorphic zone (Grt–Crd zone) and the contact metamorphic zone (Crd–Kfs zone) of the Mikawa area, Ryoke belt, southwest Japan to discuss the complex P–T–D–t evolution of the middle crust that experienced pulsed granitoid intrusions. In the Grt–Crd zone, sillimanite‐grade high‐T metamorphic condition prevailed from ca. 97 to 87 Ma, followed by cooling to ~500 °C, ~4 kbar. The intrusion of gneissose granitoids below the Grt–Crd zone isobarically reheated the Grt–Crd zone rocks again to the sillimanite‐grade high‐T condition at ca. 84 Ma. This was followed by ca. 71–70 Ma contact metamorphism. Ductile deformation that formed and folded the foliation of migmatites started before ca. 89 Ma and continued at least until ca. 84 Ma in the Grt–Crd zone. On the other hand, ca. 74 Ma age of the Crd–Kfs zone migmatite developed around the Inagawa Granodiorite in addition to ca. 70 Ma age of a syn‐tectonic pegmatite vein revealed that the intrusion of "75–69 Ma granitoids" caused partial melting and locally triggered low‐strain ductile deformation in their contact aureoles. Comparison with other areas of the Ryoke belt suggests that plutono‐metamorphic evolution of the Mikawa and Aoyama areas are similar with each other in that ca. 80 Ma reheating events (i.e., contact metamorphism) are observed, while absence of separate reheating event postdating peak metamorphism in the Yanai area is a rather uncommon feature in the Ryoke belt. [ABSTRACT FROM AUTHOR]

Volatile emissions to the atmosphere associated with the Siberian Traps eruptions at the Permian-Triassic boundary were sourced from the outgassing of primary magmas and the sedimentary host rocks into which they were intruded. Halogens in volcanic gases may have played an important role in environmental degradation and in stratospheric ozone destruction. Here we investigate how halogens behave during the interaction between salts and basalt magma emplaced as sills and erupted as lava. We present whole-rock, trace, and halogen concentrations for a suite of samples from three locations in the Siberian Traps Large Igneous Province, including basalt lavas erupted, and dolerites intruded into both organic-bearing shales and evaporites. Dolerites are enriched in Cl, Br, and I; their enrichment in Cl is similar to MORB and OIB that have been inferred to have assimilated seawater. The dolerites exhibit halogen compositional systematics, which extend towards both evaporites and crustal brines. Furthermore, all analyzed samples show enrichment in Rb/Nb; with the dolerites also showing enrichment in Cl/K similar to MORB and OIB that have been inferred to have assimilated seawater. We infer that samples from all three locations have assimilated fluids derived from evaporites, which are components of crustal sedimentary rocks. We show that up to 89% of the chlorine in the dolerites may have been assimilated as a consequence of the contact metamorphism of evaporites. We show, by thermal modeling, that halogen transfer may occur via assimilation of a brine phase derived from heating evaporites. Halogen assimilation from subcropping evaporites may be pervasive in the Siberian Traps Large Igneous Province and is expected to have enhanced emissions of Cl and Br into the atmosphere from both intrusive and extrusive magmatism.

We present a detailed study of geochemical composition and ore mineralogy of black shales from Chynín, Czech Republic, representing Ediacaran organic matter-rich sediments, which were subject to regional and contact metamorphism. They are part of the Blovice Accretionaly Complex (BAC) in the Teplá-Barrandian Unit (TBU) and are located close to the contact with the Central Bohemian Pluton (CBP). The black shales were encountered with metasilicites, metabasalts, and basic tuffitic rocks in the CHY-2 drill hole (250 m deep) and are regionally associated with hornfels bodies. The geochemistry of these shales indicates that they correspond to metal-rich black shales deposited under strongly reducing conditions (TOC/Pmolar > 100, high Mo and U values). On the other hand, the lack of a positive link between TOC and redox-sensitive metals (e.g., V, U, Cr, Ni, Mo) and their generally negative correlation with sulfur indicate important late-stage metal and sulfur remobilization. This is reflected in the mineralogical composition of the shales, which documents a thermal event in their history. Abundant framboidal pyrite (pyrite I) was recrystallized into coarse aggregates (pyrite II), locally accompanied by chalcopyrite, sphalerite, and rare molybdenite, pentlandite and breithauptite. Abundant pyrrhotite formed there due to selective desulfurization of pyrite I and II during the contact metamorphism. Locally, this process was also accompanied by the replacement of pyrrhotite by V-Cr-O (karelianite - V2O3 and eskolaite - Cr2O3, mostly with dominant karelianite end-member) and Ti-V-O (vanadium rutile, schreyerite - V2Ti3O9 and a phase with the theoretical composition V4Ti3O12, yet unknown to the mineralogical system). Vanadium-Cr-Ti elemental associations reported from different localities of Neoproterozoic metal-rich black shales, metal-rich black shales, and (meta)silicites in TBU indicate similar sources of these elements but different conditions of their accumulation. [ABSTRACT FROM AUTHOR]

Large and hot intrusions may imprint relatively wide aureoles of ultra‐high temperature (UHT >900°C) contact metamorphism even on granulite facies rocks at mid‐crustal levels, especially on those still somewhat hydrated rocks like biotite‐rich paragneisses. After reaching UHT conditions, even followed by rapid cooling, such contact aureoles record reaction textures and newly formed mineral assemblages. Near intrusive contacts, high‐T to UHT contact aureoles present highly recrystallized hornfelsic rocks exhibiting conchoidal fracture that may also preserve inherited features. This paper describes a contact metamorphic aureole developed under high to ultra‐high temperatures at intermediate P conditions, imposed by a mafic intrusion on granulite facies paragneiss. In the Espírito Santo State (SE Brazil), many post‐collisional intrusions (the 525–480 Ma G5 supersuite) occur in the back‐arc region of the Araçuaí orogen. Among them, a greatly homogeneous norite forms a large intrusion hosted by Al‐rich migmatitic paragneisses previously metamorphosed to granulite facies during the collisional stage. Zircon U‐Pb (SHRIMP) data yield the age of 523 ± 7 Ma for the norite crystallization. This intrusion imprinted a contact metamorphism aureole on the migmatitic paragneiss, forming Al‐rich hornfelses free of biotite, near the intrusive contact, followed by recrystallized garnet‐rich rocks with relict gneissic fabrics progressively enriched in biotite towards the distal aureole. The Al‐rich hornfelses are very tenacious, massive, fine‐grained rocks with conchoidal fracture, resinous lustre, intergrowth of feldspars, simplectites of cordierite and hercynite, and show an increase calcium content in plagioclase towards the intrusion. The hornfelses comprise two mineral assemblages: (I) plagioclase–garnet–orthopyroxene–quartz and (II) plagioclase–garnet–quartz–cordierite–sillimanite–hercynite–K‐feldspar. Calculated temperatures for host rocks range from ~1,010°C, for a hornfels collected at 1 m from the intrusive contact, to ~732°C, for a paragneiss located at 262 m from the intrusion. Using Theriak‐Domino software, phase‐equilibrium modelling for the hornfels assemblage (II) provides temperatures between 970 and 1,010°C, at pressures from 5.6 to 6.0 kbar, characterizing UHT contact metamorphism at mid‐crustal level (21–22 km depth). Neocrystallized zircons with soccer ball morphology aged at 520 ± 19 Ma, and P–T conditions of 845°C at 6.35 kbar for a recrystallized paragneiss record the contact metamorphism at least up to 250 m distance from the intrusion. The presented data constrain a P–T–t peak ~5.8 kbar – 990°C – 520 Ma for the contact metamorphism while the P–T–t peak at 7.5 kbar – 785°C – 560 Ma was related to the collisional metamorphism of regional paragneisses in the literature. Therefore, the pressure–time relation between those metamorphic peaks gives a pressure difference of 1.7 kbar (~6.3 km thickness of continental crust) over 40 Ma, corresponding to low decompression (42.5 bar/Ma) and exhumation (157 m/Ma) rates from the collisional peak to the post‐collisional stage, in the studied region. [ABSTRACT FROM AUTHOR]

Silurian and Devonian marine shales and limestones of the Barrandian Basin host abundant black solid, non-fluorescing bitumens that fill tectonic fractures and veins, and occlude fossil moulds and diagenetic concretions. Solid bitumen, interpreted as thermally degraded petroleum, entered the rocks during several successive episodes of fracture-bound petroleum migration that occurred during deeper burial of the strata. Regional distribution of bitumen reflectance values that range between ∼0.9–2.3% R r , correlate with variations of its FT-IR and Raman spectroscopic characteristics and aromatic hydrocarbon composition, and collectively evidence the maturity trend increasing across the basin from the southwest to the northeast. The reflectance of chitinozoans and graptolites (∼0.8–1.9% R r) in the country rocks and homogenization temperatures of hydrocarbon fluid inclusions document palaeotemperatures ranging between ∼90–150 °C, characteristic of the oil window zone grading into the gas/condensate zone. Although in a basin-wide perspective the averaged values of solid bitumen and zooclast optical reflectance converge and indicate the same northeastern-increasing regional diagenetic trend, solid bitumen reflectance values vary considerably at individual localities and even within some bitumen samples. The wide scatter of optical reflectance values and the heterogeneity of optical properties, which were attributed to the presence of multiple source rocks in the basin, the variable lithology of bitumen host rocks, or other variables, hamper the use of solid bitumen as a simple alternative to zooclast/vitrinite reflectance palaeothermometers in a given basin. On the other hand, the highly anisotropic domain and the mesophase "coking" textures of the solid bitumen that were recognized in the NE part of the basin provide unique evidence on an anomalous, hitherto unrecognized, geologically short-lasting thermal event that affected the Palaeozoic rocks. A line of indirect evidence suggests that the coking of the bitumen was caused by a cryptic intrusion, possibly a concealed branch of the Central Bohemian Pluton, which intruded into the strata during the Variscan orogeny. More rarely occurring semi-solid, vividly yellow fluorescing waxy bitumen, that postdates solid bitumen in some fractures and voids, does not reveal a regional thermal maturation trend. It precipitated from relict waxy oils that migrated through the strata during a post-Neogene uplift of the Barrandian region. • Solid bitumen shows an optical heterogeneity limiting its use as a paleothermometer. • Host rock lithology and different precursor oils affected its optical heterogeneity. • Bitumen "coking textures" evidence a short-lived heating caused by magmatic bodies. [ABSTRACT FROM AUTHOR]

The tectonomagmatic evolution of the Sangilen massif has been described in detail in numerous publications, but little attention was given to heat sources related to the HT/LP metamorphism. Modeling of the magma transport to the upper‐crust levels in West Sangilen shows that the NT/LP metamorphism is related to gabbromonodiorite intrusions. This article is focused on the thermo‐mechanical modeling of melting and lifting of melts in the crust, taking into account the density interfaces. The model of the Erzin granitoid massif shows that in case of fractional melting, the magma ascent mechanism is fundamentally different, as opposed to diapir upwelling – percolation take place along a magmatic channel or a system of channels. An estimated rate of diapiric rise in the crust amounts to 0.8 cm/yr, which is more than an order of magnitude lower than the rate of melt migration in case of fractional melting (25 cm/yr). In our models, a metamorphic thermal ‘anticline’ develops in stages that differ, probably, due to the modes of crust melting: batch melting occurs at the first stage, and fractional melting takes place at the second stage. It is probable that the change of melting modes from melting conditions in a ‘closed’ system to fractional melting conditions in ‘open’ systems is determined by tectonic factors. For the Sangilen massif, we have estimated the degrees of melting in the granulite, granite, and sedimentary‐metamorphic layers of the crust (6, 15, and 5 vol. %, respectively).

Variations in the pressures and temperatures of metamorphism reflect the complexity of collisional orogenic systems. In the north‐west Sanandaj–Sirjan Zone of western Iran, migmatites in the Boroujerd area record earlier dynamothermal metamorphism followed by the development of a contact metamorphic aureole induced by the Jurassic Boroujerd Batholith. Integrated micro‐textural observations, mineral compositions, and the results of phase equilibrium modelling indicate that peak metamorphic temperatures reached 640–660°C in andalusite schists and ~700°C in cordierite–sillimanite migmatites. The pressure does not exceed 3.5 kbar, implying a maximum depth of ~13 km. A hairpin clockwise P–T path in the Boroujerd region was inferred from mineral relations (e.g., replacement of andalusite1 by sillimanite in the migmatites in the heating stage, which was followed by a retrograde episode with growth of andalusite2) and P–T pseudosections. Metamorphism of the Boroujerd area was coeval with voluminous intermediate–mafic magmatism, which provided the heat necessary for partial melting. This study demonstrates that metamorphic sequences of the north‐west Sanandaj–Sirjan Zone developed under similar metamorphic conditions in the Lower to Middle Jurassic and record an extensional back‐arc setting related to the subduction of Neo‐Tethys. [ABSTRACT FROM AUTHOR]

Euhedral andradite crystals were found in trachyandesitic (latitic) lavas of the volcanic Andahua Group (AG) in the Central Andes. The AG comprises around 150 volcanic centers, most of wich are monogenetic. The studied andradite is complexly zoned (enriched in Ca and Al in its core and mantle, and in Fe in this compositionally homogenous rim). The core-mantle regions contain inclusions of anhydrite, halite, S- and Cl-bearing silicate glass, quartz, anorthite, wollastonite magnetite and clinopyroxene. The chemical compositions of the garnet and its inclusions suggest their contact metamorphic to pyrometamorphic origin. The observed zoning pattern and changes in the type and abundance of inclusions are indicative of an abrupt change in temperature and subsequent devolatilization of sulfates and halides during the garnet growth. This process is interpreted to have taken place entirely within a captured xenolith of evaporite-bearing wall rock in the host trachyandesitic magma. The devolitilization of sediments, especially sulfur-bearing phases, may have resulted in occasional but voluminous emissions of gases and may be regarded as a potential hazard associated with the AG volcanism. [ABSTRACT FROM AUTHOR]

• Massive CO 2 mobilisation from ophicarbonate around shallow intrusions. • Mixed H 2 O-CO 2 fluids dominate metamorphic carbon degassing at collisional plate boundaries. • Metamorphic ultramafic rocks with >50% tremolite or diopside likely were ophicarbonate. • Metamorphic CO 2 degassing may be highly relevant for the long-term global carbon cycle. The processes of carbon mobilisation at convergent plate boundaries are hotly debated. Recent findings suggest that carbon release along subduction geothermal gradients may well be more relevant than previously thought; however, it has remained difficult to achieve steady state atmospheric CO 2 conditions over geological time scales based on current volatile cycle models. Here, we report on meta-ophicarbonate rocks in the contact metamorphic aureole of the Bergell intrusion, Val Malenco, European Alps, that reached T-P conditions of at least 650 °C at 0.35 GPa. We demonstrate by combined field evidence, geochemistry, and closed-system thermodynamic modelling that over 50% of the rock carbonate has been mobilised locally in response to reactive fluid flow upon progressive isobaric heating from 350 to >650 °C. Despite complete mineral transformation at the antigorite + calcite devolatilisation reaction, the resulting tremolite-ophicarbonate preserves the original ophicarbonate texture by olivine-chlorite clasts, representing the former antigorite-serpentinite fraction, embedded in a monomineralic tremolite matrix formed from the calcite fraction that often exceeded 50 vol%. Closed system thermodynamic modelling based on an ophicarbonate composition of 80 wt% serpentinite +20 wt% calcite reveals that rock-buffered fluid X CO2 values evolve from <0.09 to as high as ∼0.16, and the total H 2 O-CO 2 fluid fraction released may be as high as 14 wt%; values that are highly sensitive to bulk ophicarbonate composition. Major to trace element geochemistry reveals a history of peridotite melt depletion followed by ocean floor hydration and ophicarbonate formation, consistent with the ocean continent transition (OCT) setting of the mantle rocks in the Mesozoic. This is demonstrated by positive anomalies in B, U, As, Sb, Bi, and W, and by bulk rock and notably carbonate primitive mantle normalised REE patterns that are basically identical to that of calcite precipitated from Jurassic seawater except for its negative Eu anomaly. Prograde metamorphic tremolite and diopside possess enrichments notably in Sr and REE inherited from reactant calcite, as is also supported by a good match between measured and modelled silicate mineral compositions. Specific geochemical characteristics of the prograde meta-ophicarbonate rocks imply fluid mediated, open system processes. Antigorite dehydration fluid ingress, likely produced in neighbouring antigorite-serpentinites, may have shifted bulk tremolite ophicarbonate rock compositions towards higher SiO 2 /MgO ratios along with an increase in B contents and depletion in fluid mobile elements such as As, Sb, and Sr. The massive, contact metamorphic CO 2 mobilisation documented here occurred at P-T conditions that are similar to those that can be achieved in collisional orogenic settings, whose clockwise P-T-t paths are often characterised by further heating upon initial decompression. They thus evolve at large angle to devolatilisation reactions and silicate-carbonate rock-buffered fluid X CO2 isopleths, thus fostering carbon mobilisation towards metamorphic peak temperatures at moderate to low pressures (in the region of between >700 °C – 1.0 GPa and >600 °C – 0.3 GPa). Barrovian metamorphism, too, can reach T-P conditions of 800 °C – 1 GPa. Amagmatic CO 2 mobilised this way will eventually reach the atmosphere via dispersion in the groundwater table and diffuse outgassing. Despite the huge uncertainties associated with quantification of such metamorphic carbon fluxes, geological time scale global carbon cycling models should more rigorously explore the significance of these contributions. [ABSTRACT FROM AUTHOR]

Abstract To assess thermal and kinetic influences on atomic mobility and mineral (neo)crystallization, clumped‐isotope abundances of calcite and dolomite were measured alongside dolomite cation ordering and U–Pb dates, across metamorphic grade within the c. 35–30 Ma Alta stock contact metamorphic aureole, Utah, USA. Average Δ47 values of dolomite inside the metamorphic aureole reflect the blocking temperature of dolomite (300°C–350°C) during cooling from peak temperatures. Dolomite Δ47 values outside the metamorphic aureole record a temperature of ∼160°C. At the talc isograd, dolomite Δ47 values abruptly change, corresponding to a decrease of ∼180°C over

A rhyolite magma body within the Krafla geothermal system that was encountered at a depth of 2.1km during drilling of the IDDP-1 borehole is producing high temperature metamorphism within a conductive boundary layer (CBL) in adjacent host rocks. Cuttings recovered during drilling within a few meters of the intrusive contact in IDDP-1 are mainly comprised of granoblastic hornfelses, the rock type which confirms the presence of the CBL at the base of the IDDP-1 bore hole. The two pyroxenes in these hornfelses record temperatures that are in the range of 800-950°C. The minimum heat flow across the CBL is 23Wm-2. Country rocks at distances beyond 30m of the intrusive contact are essentially unaltered, implying that they have been emplaced very recently and/or as yet unaffected by hydrothermal fluid flow. © 2012 Elsevier Ltd.

A rhyolite magma body within the Krafla geothermal system that was encountered at a depth of 2.1km during drilling of the IDDP-1 borehole is producing high temperature metamorphism within a conductive boundary layer (CBL) in adjacent host rocks. Cuttings recovered during drilling within a few meters of the intrusive contact in IDDP-1 are mainly comprised of granoblastic hornfelses, the rock type which confirms the presence of the CBL at the base of the IDDP-1 bore hole. The two pyroxenes in these hornfelses record temperatures that are in the range of 800-950°C. The minimum heat flow across the CBL is 23Wm-2. Country rocks at distances beyond 30m of the intrusive contact are essentially unaltered, implying that they have been emplaced very recently and/or as yet unaffected by hydrothermal fluid flow. © 2012 Elsevier Ltd.

To assess thermal and kinetic influences on atomic mobility and mineral (neo)crystallization, clumped‐isotope abundances of calcite and dolomite were measured alongside dolomite cation ordering and U–Pb dates, across metamorphic grade within the c. 35–30 Ma Alta stock contact metamorphic aureole, Utah, USA. Average Δ47 values of dolomite inside the metamorphic aureole reflect the blocking temperature of dolomite (300°C–350°C) during cooling from peak temperatures. Dolomite Δ47 values outside the metamorphic aureole record a temperature of ∼160°C. At the talc isograd, dolomite Δ47 values abruptly change, corresponding to a decrease of ∼180°C over <50 m in the down‐temperature direction. This observed step in dolomite Δ47 values does not correlate with cation ordering in dolomite or U–Pb dates, neither of which correlate well with metamorphic grade. The short distance over which dolomite Δ47 values change indicates strong temperature sensitivity in the kinetics of dolomite clumped‐isotope reordering, and is consistent with a wide range of clumped‐isotope reequilibration modeling results. We hypothesize that clumped‐isotope reordering in dolomite precedes more extensive recrystallization or metamorphic reaction, such as the formation of talc. Dolomite U–Pb analyses from inside and outside the metamorphic aureole populate a single discordia ∼60 Myr younger than depositional age (Mississippian), recording resetting in response to some older postdepositional, but premetamorphic process. Key Points: Dolomite clumped isotopes record an abrupt ∼180°C decrease, over <50 m, at the talc isograd of the Alta contact metamorphic aureoleThis step feature suggests strong thermal sensitivity in clumped‐isotope reordering that precedes talc neocrystallizationClumped‐isotope geothermometry may provide new insights into processes controlling metamorphism and reaction kinetics in carbonates [ABSTRACT FROM AUTHOR]

Olivine is a major rock-forming mineral in various magmatic and metamorphic rocks and the upper mantle. In this paper, we present the first high-precision analyses of olivine from 15 samples of magnesian skarns and silicate marbles (MSSM) from the collection of the Fersman Mineralogical Museum (Moscow, Russia). Mg# [Mg/(Mg+Fe2+)·100, mol%] of olivine from the samples studied varies from 86 to nearly 100. The main distinctive features of the olivine are anomalously low contents of Co (<51 μg/g), Cr (<5 μg/g), and Ni (<44 μg/g) and high content of B (23–856 μg/g), which correlate with host-rock compositions. Phosphorus (5–377 μg/g) incorporation in olivine is charge balanced by the incorporation of Li (0.15–61 μg/g) and Na (<14.3 μg/g). Y and REE contents exhibit positive correlations with Na, which suggests that REE incorporation into MSSM olivine could occur via charge-balanced coupled substitution with Na at low temperature and low aSiO2 conditions during MSSM formation. The documented compositional features of olivine from magnesian skarns and silicate marbles can help reconstruct the genesis of the host-rocks and identify xenocrysts of MSSM olivine in magmatic rocks. [ABSTRACT FROM AUTHOR]

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The Upper Mustang region of west-central Nepal contains exposures of metamorphosed Tethyan Sedimentary Sequence rocks that have been interpreted to reflect either contact metamorphism related to the nearby Mugu pluton or regional metamorphism associated with the North Himalayan domes. New monazite geochronology results show that the Mugu leucogranite crystallized at c. 21.3 Ma, while the dominant monazite age peaks from the surrounding garnet ± staurolite ± sillimanite schists range between c. 21.7 and 19.4 Ma, generally decreasing in age away from the pluton. Metamorphic temperature estimates based on Ti-in-biotite and garnet–biotite thermometry are highest in the specimens closest to the pluton (648 ± 24°C and 615 ± 25°C, respectively) and lowest in those furthest away (578 ± 24°C and 563 ± 25°C, respectively), while pressure estimates are all within uncertainty of one another, averaging 5.0 ± 0.5 kbar. These results are interpreted to be consistent with contact metamorphism of the rocks in proximity to the Mugu pluton, which was emplaced at c. 18 ± 2 km depth after local movement across the South Tibetan detachment system had ceased. While this new dataset helps to characterize the metamorphic rocks of the Tethyan Sedimentary Sequence and provides new constraints on the thickness of the upper crust, it also emphasizes the importance of careful integration of metamorphic conditions and inferred processes that may affect interpretation of currently proposed Himalayan models. [ABSTRACT FROM AUTHOR]

Columnar-jointed tuffs (bentonites) are located below the Lower Carboniferous Tideswell Dale Sill, Derbyshire, in an abandoned quarry. There are three zones of prismatic joints, columnar joints (∼60 cm thick and mean column width of 4.1 cm) and massive, conchoidally fractured rock sequentially from the dolerite contact downwards. The rocks are very fine-grained (1–10 µm) and consist mainly of sanidine, interlayered illite/smectite, and hematite, with minor quartz, apatite, montmorillonite, anatase and detrital minerals. High K2O and Fe2O3, low Na2O and low MgO are interpreted as due to alteration of felsic volcanic ash. The bentonite was contact-metamorphosed by the sill to temperatures of c. 300 °C. The progression of fracture geometries, thermal considerations and application of theories of column formation indicate that columns formed downwards away from the sill, due to prograde contact metamorphism of the originally clay-rich bentonite. The formation of columnar fracture networks by shrinkage due to igneous heating may have implications for the effectiveness of bentonite as a barrier for disposal of high-level radioactive waste. Regional dimensions include documenting widespread felsic explosive volcanism and inferring a Triassic oxidation event from palaeomagnetic data. [ABSTRACT FROM AUTHOR]

In this paper, we look at the ways in which rock art encapsulates and expresses the tension between tradition and innovation in northern Australia during the period of European colonization. The appearance of new motifs and techniques for producing rock art in the recent past sits alongside the continuation of "traditional" practices reflecting thousands of years of artistic expression. Using case studies from Arnhem Land, we reflect on both ethnographic and archaeological evidence in order to interrogate the ways in which innovation impacted upon and was used by Indigenous groups to navigate contact. Our findings suggest that technological conservatism and the resistance to new technologies by Aboriginal communities is both considered and partial, with the overriding logic being about minimizing the disruption of specific values conceptualised as traditional, rather than eliminating or avoiding all outside influence. [ABSTRACT FROM AUTHOR]

The depth profiles of metamorphic InxAl1-xAs (0.05 and <110> in-plane directions. The x-ray full width at half maximum generally increased with the In composition, but tended lower toward surface. We suggest a possible design strategy for the linear-grade metamorphic buffer layer based on our result. [ABSTRACT FROM AUTHOR]

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.

This paper discusses a research conducted in Dayang Bunting and Tuba Islands, Langkawi, Malaysia, in an effort to promote these islands as educational geosites to explain the process and impact of contact metamorphism to the public. The research activities involves evaluation of microscopic rock samples from the closest to the farthest distance from the metamorphism contact zone. The evaluation involves examining grain size, zonation, shape, and fabric of minerals in rock samples. Pronounced changes were observed from different distances from the contact zone as indicated by the change in grain size, grain shape and zoning in the mineral. These informations are stored in a database that is accessible to the public in the future.

Spurrite-merwinite marbles on the right bank of the Kochumdek River in the Podkamennaya Tunguska basin formed along the top margin of a flood basalt intrusion (Kuzmovka complex) from a marly limestone protolith of the Rhuddanian Lower Kochumdek Subformation, at a pressure of ~200 bars. The contact metamorphic aureole comprises four zones of successively decreasing temperatures marked by the respective mineral assemblages: T ≥ 900 °C (merwinite, spurrite and gehlenite (±rankinite, bredigite); T ≥ 750 °C (spurrite); T ≥ 700 °C (tilleyite, wollastonite, and melilite (Gehl<50)); and ~500-550 °C (diopside, amphibole, and grossular). Very high temperatures at the contact (Tcont > 2/3 Tmelt) result from magma flow along a conduit. The temperature profiles for the Kochumdek metamorphic complex show good fit between measured and geothermometer-derived values at a magma temperature of 1200 °C, an intrusion thickness of ≥ 40 m, a heating time of six months, and a magma flow lifespan within one month. Stagnant magma in a conduit of any thickness cools down and crystallizes rapidly and fails to heat up sediments to the temperatures required for spurrite--merwinite metamorphism (above 790 °C). [ABSTRACT FROM AUTHOR]