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In this study, we integrated extensive field, petrographic, whole-rock major and trace element, and Nd–Pb–Sr–O isotope, and zircon U–Pb ages, trace element and Lu–Hf isotope data from the Neoarchean Shawmere Anorthosite Complex and surrounding gneisses to unravel their petrogenetic origin and tectonic history. The ~2765 Ma Shawmere Anorthosite Complex is interpreted to have been emplaced into a sequence of interlayered greywacke and basalt deposited in an intra-continental arc rift system above a north-dipping subduction zone. The complex consists mainly of anorthosite, leucogabbro, gabbro, and hornblendite that were emplaced as several batches of magmas and crystal mushes originating from sub-arc mantle sources. In contrast to the previous studies, our field and petrographic data suggest an igneous origin for the most hornblende in the complex, implying hydrous parental magmas. A hydrous magma origin is also consistent with the high-anorthite content (mostly 70–90%) of the plagioclase in the complex. Percolation of hydrous basaltic melts through gabbroic cumulates in crustal magma chambers led to extensive (>50%) replacement of igneous clinopyroxene by igneous hornblende. Continued subduction resulted in the closure of the intra-arc rift system and the intrusion of the complex by tonalite, granodiorite and diorite between 2765 and 2680 Ma in an Andean-type margin. The complex and surrounding gneisses underwent hornblende granulite-facies metamorphism mainly between 2680 and 2620 Ma, overlapping with mid-crustal east-west extension between 2660 and 2640 Ma. The granulite-facies metamorphism is recorded by the replacement of hornblende, plagioclase and clinopyroxene by garnet and the development of a garnet-orthopyroxene-plagioclase metamorphic assemblage with a granoblastic texture. Tectonic rebounding of mid-crustal rocks to upper crustal levels after 2620 Ma led to the formation of an extensive network of extensional fractures and retrograde metamorphism. Migration of CO2-rich hydrous fluids along the extensional fractures and grain boundaries resulted in the precipitation of many metasomatic minerals mainly at the expense of hornblende and plagioclase, including epidote, clinozoisite, tremolite, actinolite, paragonite, margarite, titanite, quartz, calcite, sillimanite, dolomite, and chlorite. Prevalent replacement of hornblende by garnet during prograde metamorphism and metasomatic replacement of hornblende and plagioclase by retrograde mineral assemblages disturbed the Sm–Nd, U–Th–Pb, and Rb–Sr isotope systems. [ABSTRACT FROM AUTHOR]

On the basis of an inorganic concept of the petroleum origin, the phase relationships of crystalline kerogens of black shales and liquid oil at the physicochemical conditions of a typical geobarotherm on the Texas Gulf Coast are considered. At the conditions of the carbon dioxide (CO2) high fluid pressure, the process of oil transformation into kerogens of varying degrees of “maturity” (retrograde metamorphism) takes place with decreasing temperature and hydrogen pressure. Kerogen generation in black shale rocks occurs by the sequential transition through metastable equilibria of liquid oil and crystalline kerogens (phase “freezing” of oil). The upward migration of hydrocarbons (HC) of oil fluids, clearly recorded in the processes of oil deposit replenishment in oil fields, shifts the oil ↔ kerogen equilibrium towards the formation of kerogen. In addition, with decreasing of the hydrogen chemical potential as a result of the process of high-temperature carboxylation and low-temperature hydration of oil hydrocarbons, the “mature” and “immature” kerogens are formed, respectively.The phase relationships of crystalline black shale kerogens and liquid oil under hypothetical conditions of high fluid pressure of the HC generated in the regime of geodynamic compression of silicate shells of the Earth in the result of the deep alkaline magmatism development. It is substantiated that a falling of hydrogen pressure in rising HC fluids will lead to the transformation of fluid hydrocarbons into liquid oil, and as the HC fluids rise to the surface, the HC ↔oil ↔ kerogen equilibrium will shift towards the formation of oil and kerogen.It is round that both in the geodynamic regime of compression and in the regime of expansion of the mantle and crust, carboxylation and hydration are the main geochemical pathways for the transformation of oil hydrocarbons into kerogen and, therefore, the most powerful geological mechanism for the black shale formations.

Magnetic susceptibility is widely applied in geology, but its use in the study of metamorphic rocks has been limited due to the complex nature of metamorphism. In this study, analyzing a partially retrogressed mafic enclave within Archean TTG gneiss from the Jiaobei Terrane, North China craton, we incorporate magnetic susceptibility with conventional metamorphic (petrographic and major element) proxies in order to investigate profiles of metamorphic retrogression. The magnetic susceptibility results show more complete retrogression in the direction parallel to the tectonic fabric and partial retrogression perpendicular to it. This geophysical observation, which is broadly consistent with the petrological observations and P--T estimates, suggests that the availability of fluids and perhaps even non-lithostatic pressure play roles in the preservation of such differential retrogression. This study thus introduces magnetic susceptibility as a novel proxy in this context, revealing its utility in rapidly and quantitatively identifying variable retrogression gradients consistent with, but with more precision than, measured metamorphic pressures. [ABSTRACT FROM AUTHOR]

Segregations of carbonates in association with chlorapatite were discovered in garnet peridotites of the Marun-Keu eclogite–gneiss complex (Polar Urals) for the first time. They contain relics of dolomite, which is replaced by calcite with symplectite (graphic) ingrowths of brucite and bicarbonate. The formation of peridotites with unusual carbonate structures is the result of the evolution of the rocks from this complex at different depths. During deep subduction and progressive metamorphism followed by the formation of garnet peridotites, segregated dolomite underwent solid-phase breakdown into aragonite and magnesite at the pressure peak (≥5 GPa). During exhumation and retrograde metamorphism, garnet peridotites, together with segregations, experienced further transformation; aragonite was replaced by calcite, and magnesite, by brucite and bicarbonate. The presence of carbonates with deep-seated structures confirms that the garnet peridotites and eclogites of the Polar Urals belong to ultrahigh-pressure complexes. [ABSTRACT FROM AUTHOR]

A body of dolomite marble near Chotěboř was found in migmatized biotite paragneiss of the Moldanubicum directly on the border with migmatites/orthogneisses of the Kutná Hora Crystalline Complex. The area underwent significant tectonic reworking of rocks along the Přibyslav mylonite zone. Therefore, only small relics of early prograde HT/LP metamorphism at T > 650 - 700 °C were preserved in this marble. They are represented by the assemblage dolomite+calcite+forsterite+Zn-spinel+clinohumite ±Ba-phlogopite and rare baddeleyite. This mineral assemblage underwent strong low-temperature retrograde metamorphism (< 400 - 300 °C) under the conditions of high H2 O activity (XCO2 < 0.10 - 0.15), which was manifested by almost complete serpentinization of forsterite. Partial alteration of clinohumite produced serpentine with numerous inclusions of a TiO2 -mineral and rare sphalerite. The examined serpentine marble (ophicalcite) is lithologically and mineralogically similar to the dolomite marbles found in the Strážek Moldanubicum. [ABSTRACT FROM AUTHOR]

Exhumation and accompanying retrograde metamorphism alter the compositions and textures of metamorphic rocks through deformation, mineral–mineral reactions, water–rock reactions, and diffusion-controlled intra- and inter-mineral atomic mobility. Here, we demonstrate that these processes are recorded in the clumped- and single-isotope (δ13C and δ18O) compositions of marbles, which can be used to constrain retrograde metamorphic histories. We collected 27 calcite and dolomite marbles along a transect from the rim to the center of the metamorphic core-complex of Naxos (Greece), and analyzed their carbonate single- and clumped-isotope compositions. The majority of Δ47 values of whole-rock samples are consistent with exhumation- controlled cooling of the metamorphic complex. However, the data also reveal that water–rock interaction, deformation driven recrystallization and thermal shock associated with hydrothermal alteration may considerably impact the overall distribution of Δ47 values. We analyzed specific carbonate fabrics influenced by deformation and fluid–rock reaction to study how these processes register in the carbonate clumped-isotope system. Δ47 values of domains drilled from a calcite marble show a bimodal distribution. Low Δ47 values correspond to an apparent temperature of 260 °C and are common in static fabrics; high Δ47 values correspond to an apparent temperature of 200 °C and are common in dynamically recrystallized fabrics. We suggest that the low Δ47 values reflect diffusion-controlled isotopic reordering during cooling, whereas high Δ47 values reflect isotopic reordering driven by dynamic recrystallization. We further studied the mechanism by which dynamic recrystallization may alter Δ47 values by controlled heating experiments. Results show no significant difference between laboratory reactions rates in the static and dynamic fabrics, consistent with a mineral-extrinsic mechanism, in which slip along crystal planes was associated with atomic-scale isotopic reordering in the calcite lattice. An intrinsic mechanism (enhanced isotopic reordering rate in deformed minerals) is contraindicated by these experiments. We suggest that Δ47 values of dynamically recrystallized fabrics that form below the diffusion-controlled blocking-temperature for calcite constrain the temperature of deformation. We find that Δ47-based temperatures of static fabrics from Naxos marbles are ∼60–80 °C higher than commonly observed in slowly cooled metamorphic rocks, and would suggest cooling rates of ∼105°CMyr−1. A similar thermal history is inferred for dolomite marbles from the core vicinity, which preserve apparent temperatures up to 200 °C higher than a typical blocking temperature (∼300 °C). This finding could be explained by a hydrothermal event driving a brief thermal pulse and locally resetting Δ47 values. Rapid cooling of the core-complex region is consistent with a compilation of published cooling ages and a new apatite U–Th/He age, associating the thermal event with the emplacement of a granodiorite pluton at ∼12 Ma.

The flake graphite occurrence in Piippumäki, Eastern Finland, as indicated by an airborne electromagnetic anomaly, was located during fieldwork by electromagnetic measurements with Slingram. The anomaly is approximately 0.1 × 1 km in size. The flake graphite is hosted by quartz-feldspar gneiss and amphibolite that have been subjected to retrograde metamorphism. This is observed in thin sections as granulite facies (garnet + cordierite + sillimanite + melt) regressing to greenschist facies (epidote, chlorite, albite, and white mica). The graphite (up to 1 mm large flakes) is found in graphite-bearing layers in the gneiss, and to a minor extent disseminated in the amphibolite. The average total sulfur (TS) is 0.33%, total carbon (TC) is 6.49%, and the average content of graphitic carbon (Cg) is 6.41% for the analyzed graphite-bearing rocks. SEM, XRD, and Raman spectroscopy were used for analyzing the flake graphite, indicating that the graphite is almost defect-free, of high quality, and has not been affected by the retrograde metamorphism. The peak metamorphic temperature of 737 °C was determined by a Raman thermometer, and no temperatures of greenschist facies were observed. A pseudosection was constructed from whole-rock chemical composition and indicated equilibration at ca 5 kbar and 740 °C, which corresponds to the observed mineral assemblages. [ABSTRACT FROM AUTHOR]

Rapid glacio-isostatic rebound in Fennoscandia and Canada that is nonuniform in time and space indicates that there is a layer with strongly decreased viscosity at shallow crustal depths. The upper boundary of the layer is near the depth of 15 km, which corresponds to the maximum depth of earthquake hypocenters in the Precambrian cratons of the Kola Peninsula and Karelia. The position of the lower boundary is less distinct; however, most likely it is located near the base of the crust. The formation of such a layer in the Pliocene–Quaternary occurred due to infiltration of a large volume of mantle fluids into the crust. In many regions, this has led to retrograde metamorphism with rock expansion and a strong decrease in rocks viscosity. [ABSTRACT FROM AUTHOR]

Hydration reactions influence rock density and rheology. For example, volume increases produced in hydration reactions may generate sufficient tensile and shear stress to fracture both the rock undergoing the reaction and the surrounding host rock. We performed in situ dynamic X‐ray synchrotron microtomography experiments to investigate reaction‐induced fracturing. Two experiments on hydration of periclase were performed at 180 or 190 °C, under a confinement of 10 or 80 MPa, a pore fluid pressure of 5 or 75 MPa, and with or without differential stress. The sample assembly consists of a periclase cylinder inserted into a central hole within a serpentinite cylinder. The reaction from periclase to brucite results in a large volume increase (110%), pushing the periclase/brucite against the serpentinite and ultimately breaking it. Using time‐resolved three‐dimensional imaging, we quantify the spatial and temporal distribution of the reaction‐induced fractures. We perform digital volume correlation analysis to obtain the incremental strain tensors throughout the hydration and fracturing process. We use numerical models to assess the distribution of stress within the serpentinite. The digital volume correlation results show mixed‐mode strain localization. The von Mises strain, indicative of shear, increases by a larger percentage than the contractive or dilatative strain components as the reaction‐induced fractures grow. The distribution of von Mises strain follows a power law relationship in the cumulative frequency‐magnitude domain, indicative of long‐range elastic stress interactions during fracturing. This experimental finding sheds insights on the mechanisms of microseismicity measured in areas undergoing active serpentinization. Key Points: Reaction‐induced fracturing of periclase produces self‐sustained hydration to bruciteThe hydration reaction generates sufficient tensile stress to fracture a confining serpentinite cylinderStrain localization in the serpentinite includes both tensile and shear components with a power law distribution of shear strain magnitudes [ABSTRACT FROM AUTHOR]

The eclogites of the Tso Morari Complex, Ladakh, NW Himalayas preserve both garnets with spectacular atoll textures, as well as whole porphyroblastic garnets. Whole garnets are euhedral, idiomorphic and enclose inclusions of amphibole, phengite and zoisite within the cores, and omphacite and quartz/coesite towards the rims. Detailed electron microprobe analyses and back-scattered electron images show well-preserved prograde zoning in the whole garnets with an increase in Mg and decrease in Ca and Mn contents from the core to the rim. The atoll garnets commonly consist of euhedral ring over island/peninsular core containing inclusions of phengite, omphacite and rarely amphibole between the core and ring. Compositional profiles across the studied atoll grains show elemental variations with higher concentrations of Ca and Mn with low Mg at the peninsula/island cores; contrary to this low Ca, Mn and high Mg is observed at the outer rings. Temperature estimates yield higher values at the Mg-rich atoll garnet outer rings compared to the atoll cores. Atoll garnet formation was favoured by infiltration of fluid formed due to breakdown of hydrous phases, and/or the release of structurally bounded OH from nominally anhydrous minerals at the onset of exhumation. Infiltration of fluids along pre-existing fracture pathways and along mineral inclusion boundaries triggered breakdown of the original garnet cores and released elements which were subsequently incorporated into the newly-grown garnet rings. This breakdown of garnet cores and inward re-growth at the outer ring produced the atoll structure. Calibrated geo-thermobarometers and mineral equilibria reflect that the Tso Morari eclogites attain peak pressures prior to peak temperatures representing a clockwise path of evolution. [ABSTRACT FROM AUTHOR]

Exhumation and accompanying retrograde metamorphism alter the compositions and textures of metamorphic rocks through deformation, mineral–mineral reactions, water–rock reactions, and diffusion-controlled intra- and inter-mineral atomic mobility. Here, we demonstrate that these processes are recorded in the clumped- and single-isotope ( δ 13 C and δ 18 O ) compositions of marbles, which can be used to constrain retrograde metamorphic histories. We collected 27 calcite and dolomite marbles along a transect from the rim to the center of the metamorphic core-complex of Naxos (Greece), and analyzed their carbonate single- and clumped-isotope compositions. The majority of Δ 47 values of whole-rock samples are consistent with exhumation- controlled cooling of the metamorphic complex. However, the data also reveal that water–rock interaction, deformation driven recrystallization and thermal shock associated with hydrothermal alteration may considerably impact the overall distribution of Δ 47 values. We analyzed specific carbonate fabrics influenced by deformation and fluid–rock reaction to study how these processes register in the carbonate clumped-isotope system. Δ 47 values of domains drilled from a calcite marble show a bimodal distribution. Low Δ 47 values correspond to an apparent temperature of 260 °C and are common in static fabrics; high Δ 47 values correspond to an apparent temperature of 200 °C and are common in dynamically recrystallized fabrics. We suggest that the low Δ 47 values reflect diffusion-controlled isotopic reordering during cooling, whereas high Δ 47 values reflect isotopic reordering driven by dynamic recrystallization. We further studied the mechanism by which dynamic recrystallization may alter Δ 47 values by controlled heating experiments. Results show no significant difference between laboratory reactions rates in the static and dynamic fabrics, consistent with a mineral-extrinsic mechanism, in which slip along crystal planes was associated with atomic-scale isotopic reordering in the calcite lattice. An intrinsic mechanism (enhanced isotopic reordering rate in deformed minerals) is contraindicated by these experiments. We suggest that Δ 47 values of dynamically recrystallized fabrics that form below the diffusion-controlled blocking-temperature for calcite constrain the temperature of deformation. We find that Δ 47 -based temperatures of static fabrics from Naxos marbles are ∼60–80 °C higher than commonly observed in slowly cooled metamorphic rocks, and would suggest cooling rates of ∼ 10 5 ° C Myr − 1 . A similar thermal history is inferred for dolomite marbles from the core vicinity, which preserve apparent temperatures up to 200 °C higher than a typical blocking temperature (∼300 °C). This finding could be explained by a hydrothermal event driving a brief thermal pulse and locally resetting Δ 47 values. Rapid cooling of the core-complex region is consistent with a compilation of published cooling ages and a new apatite U–Th/He age, associating the thermal event with the emplacement of a granodiorite pluton at ∼12 Ma. [ABSTRACT FROM AUTHOR]

Metaophiolites that consist mainly of serpentinites or metabasites outcrop in the East Thessaly region, Central Greece. These formations, along with some ophiolite outcrops, have been variably emplaced onto the Pelagonian tectonostratigraphic zone as dispersed and deformed thrust sheets. Based upon their estimated metamorphic degree, serpentinites from the metaophiolites and ophiolitic units of East Thessaly have been divided into three groups: Group-1 serpentinites from East Othris, include lizardite and antigorite in balanced amounts, defining greenschist facies metamorphic conditions (~ 320–340 °C, P ≈ 6–7 kbar). Group-2 serpentinites are marked by further prevalence of antigorite over lizardite, suggesting upper-greenschist to lower-blueschist facies metamorphism (~ 340–370 °C, P ≈ 8–10 kbar). Group-3 serpentinites are mainly characterized by the predominance of antigorite corresponding to blueschist facies metamorphism (~ 360–400 °C, P ≈ 11–12 kbar). The chemical composition and mineral chemistry of the East Thessaly serpentinites suggest that their protoliths were highly depleted harzburgites. Group-1 serpentinites exhibit higher Mg/Si ratio values and LOI compared to serpentinite Groups-2 and -3, due to increasing metamorphic conditions of the latter groups. The prominent Cs, U, Pb, As and Sb enrichments point to subduction-related serpentinites that were subjected to fluid/rock interactions. The East Thessaly serpentinites also seem to have undergone deserpentinization retrograde metamorphism (estimated at P < 8 kbar and T < 350 °C). Retrograde metamorphism also had a significant impact on the rodingite intrusions hosted within the serpentinites, forming metarodingites through late-stage derodingitization processes. [ABSTRACT FROM AUTHOR]

Paleozoic ultra-high pressure (UHP) eclogite-bearing mafic boudins are exposed in gneiss in the Xitieshan terrane, North Qaidam. Petrological and thermobarometric studies show that garnet-amphibolite and amphibolite rocks record upper amphibolite-facies and lower amphibolite-facies retrograde metamorphic conditions of 8.1–9.1 kbar, 630–670 °C and 5.0–7.2 kbar, 530–630 °C, respectively. To better constrain the timing of retrogression and exhumation, 40 Ar/ 39 Ar stepwise heating analyses have been performed on minerals that were extracted from these UHP mafic rocks and their country rocks. Amphibole yields disturbed release spectra marked by anomalously old apparent ages in low temperature steps, suggesting the presence of extraneous 40 Ar. Isochron ages of ~ 425 Ma and ~ 419 Ma are obtained for amphibole from the garnet-amphibolite and amphibolite, respectively, which are interpreted as the best estimate for the age of amphibolite-facies retrogression during exhumation. Muscovite from the gneiss yields consistent plateau and isochron ages of ~ 409 Ma. K-feldspar from the same gneiss yields rising staircase-shaped age spectra, which are characterized by younger apparent ages in the first steps (263–281 Ma) and higher plateau ages in the last segment (294–321 Ma), indicating partial diffusive loss of argon from the K-feldspar samples. Biotite from schist yields flat age spectra with a plateau age of ~ 397 Ma that is interpreted as a cooling age. Our new 40 Ar/ 39 Ar data, together with published LA-ICPMS ages of zircon from a coesite-bearing amphibolite indicate that the Xitieshan UHP metamorphic rocks may have experienced two stages of exhumation. During the first stage, from ~ 95–31 km depth, the UHP metamorphic rocks were exhumed with a relatively rapid velocity of ~ 4.0 km/Myr, resulting from buoyancy forces. Subsequently, the rate of exhumation decreased gradually from ~ 1.1 to 0.1 km/Myr between ~ 31–7 km depth. This later gentle exhumation was probably governed by extension and/or erosion. [ABSTRACT FROM AUTHOR]

[Display omitted] • Clockwise P–T evolution is recorded by the South Altyn Tagh felsic gneiss. • The felsic gneiss was exhumed from eclogite-facies to granulite-facies within 20 Myr. • HP–HT eclogite-facies records were preserved in the weakly deformed felsic gneiss. • The HP mineralogical records were further eliminated by the strong deformation. Felsic gneisses are usually the dominant rock types in most high-pressure (HP) and ultrahigh-pressure (UHP) terranes, although their peak metamorphic conditions and complete P–T paths are rarely known. This lack of knowledge is attributed to the high variance of mineral assemblages at HP/UHP conditions of the felsic gneisses, or to the effects of retrograde-related overprinting. Thus, it is difficult to distinguish whether felsic gneisses underwent HP/UHP metamorphism. In this paper, we focus on the felsic gneiss that has been previously documented as having formed under HP granulite-facies metamorphism with peak P–T of > 1.15 GPa and > 860 °C from the Danshuiquan locality in the South Altyn HP – UHP belt. The Danshuiquan felsic gneiss is strongly deformed and is spatially associated with weakly deformed garnet-rich felsic gneiss. On the basis of detailed microstructure observations on mineral assemblages and phase equilibria modelling, three stages of metamorphic evolution for the weakly deformed garnet-rich felsic gneiss are identified. The first stage is represented by garnet cores, revealing a prograde P–T path from 1.5 GPa/750 °C to 2.1 GPa/900 °C. The second stage is characterized by garnet mantles with multiphase inclusions, defining HP eclogite-facies metamorphic conditions of 2.2–2.6 GPa/950–1100 °C. The third stage is represented by the composition of garnet rims, biotite, and plagioclase, suggesting an HP granulite to amphibolite facies decompression cooling process to 1.1–1.3 GPa/710–770 °C. In contrast, the strongly deformed felsic gneiss just records a deformation-related decompression process from 1.1 to 1.2 GPa/750–760 °C to 0.87–0.98 GPa/760–770 °C, and finally to 0.83 GPa/740 °C. Zircon U–Pb dating yields c. 920 Ma protolith ages, c. 500 Ma eclogite-facies ages, and c. 480 Ma granulite to amphibolite facies retrograde ages for the Danshuiquan felsic gneisses. Our results show that the Danshuiquan felsic gneisses underwent a similar metamorphic evolution process to that of Yinggelisayi HP–UHP rocks in the eastern segment of the South Altyn Tagh. Most HP mineralogical records tend to be eliminated by the strong deformation at crustal depths (∼30 km), meaning that the peak metamorphic conditions of the studied felsic rocks in UHP metamorphic terranes are previously underestimated. [ABSTRACT FROM AUTHOR]

Monazite-bearing Alpine clefts located in the Sonnblick region of the eastern Tauern Window, Austria, are oriented perpendicular to the foliation and lineation. Ion probe (SIMS) Th-Pb and U-Pb dating of four cleft monazites yields crystallization ages of different growth domains and aggregate regions ranging from 18.99 ± 0.51 to 15.00 ± 0.51 Ma. The crystallization ages obtained are overlapping or slightly younger than zircon fission track ages but older than zircon (U-Th)/He cooling ages from the same area. This constrains cleft monazite crystallization in this area to ~300-200 °C. LA-ICP-MS data of dated hydrothermal monazites indicate that in graphite-bearing, reduced host lithologies, cleft monazite is poor in As and has higher La/Yb values and U concentrations, whereas in oxidised host rocks opposite trends are observed. Monazites show negative Eu anomalies and variable La/Yb values ranging from 520 to 6050. The positive correlation between Ca and Sr concentration indicates dissolution of plagioclase or carbonates as the source of these elements. The data show that early exhumation and cleft formation in the Tauern is related to metamorphic dome formation caused by the collision of the Adriatic with the European plate and that monazite crystallization in the clefts occurred later. Our data also demonstrate that hydrothermal monazite ages offer great potential in helping to constrain the chronology of exhumation in collisional orogens. [ABSTRACT FROM AUTHOR]

Ophiolitic rocks are found abundantly in the area of Yaguaneque, which is part of the Moa Baracoa massif. Dike rocks occur inside basic bodies (gabbro). Dike rocks contain tremolite-actinolite, chlorite, epidote, clinochlore and zoisite corresponding to green schist facies, indicating that metasomatic processes occurred in the basic rocks as well as retrograde metamorphism. Rodingites in association with gabbros occur in the form of lens or dikes in light color, with thicknesses varying from 30 to 70 cm and up to 1,1 m in length. The mineralogical composition is: clinochlore, calcic amphiboles (tremolite-actinolite), zoisite, epidote and chlorite. It is inferred that these rocks are the product of calcic metasomatism, contemporaneous to the serpentinization of ultrabasic rocks in two events: the first one associated with hydrothermal metamorphism as evidenced by the preservation of magmatic textures, the second one, during the tectonic emplacement, under the P-T conditions associated with greenschist facies, which resulted in retrograde metamorphism where supposedly garnets were transformed into chlorites. Gabbros are box rocks having the following mineralogical composition: plagioclase (anorthite), clinopyroxenes (diopside) and subordinately, olivine which occurs in the shape of relictic grains with an alteration edge to chlorite. [ABSTRACT FROM AUTHOR]

Near-surface reaction of CO2-bearing fluids with silicate minerals in peridotite and basalt forms solid carbonate minerals. Such processes form abundant veins and travertine deposits, particularly in association with tectonically exposed mantle peridotite. This is important in the global carbon cycle, in weathering, and in understanding physical-chemical interaction during retrograde metamorphism. Enhancing the rate of such reactions is a proposed method for geologic CO2 storage, and perhaps for direct capture of CO2 from near-surface fluids. We review, synthesize, and extend inferences from a variety of sources. We include data from studies on natural peridotite carbonation processes, carbonation kinetics, feedback between permeability and volume change via reaction-driven cracking, and proposed methods for enhancing the rate of natural mineral carbonation via in situ processes (''at the outcrop'') rather than ex situ processes (''at the smokestack''). [ABSTRACT FROM AUTHOR]

Gabbroic rocks and amphibolites were collected from the KR03-01-D10 dredge site located on the West Arm Rise of the Godzilla Megamullion, close to the Parece Vela Rift which appears to correspond to the termination area of a detachment fault, the Philippine Sea. The gabbroic rocks and amphibolites reveal the occurrence of a high hydrothermal activity in the lower crust close to a paleo-ridge. In the gabbroic rocks, plagioclase compositions of both porphyroclasts and matrix were transformed into sodium-rich compositions close to albite. Amphiboles are of secondary rather than igneous origin based on their microstructural occurrences. In the amphibolites, anorthite contents of porphyroclasts and matrix plagioclase are relatively lower than those of the gabbroic rocks, whereas the chemical compositions of amphibole within the amphibolites are similar to those of amphibole within the gabbroic rocks. Amphibolites represent the product of retrograde metamorphism associated with hydrothermal alteration of the gabbroic body by the reaction: clinopyroxene + calcic plagioclase + fluid → amphibole + sodic plagioclase. The estimated temperatures of the amphibolites derived from the amphibole thermobarometer and the gabbroic rocks derived from the hornblende-plagioclase geothermometer show ∼700-950°C and 650-840°C, respectively. The hydrothermal alteration recorded in the gabbroic rocks possibly occurred under high- T conditions; the rocks were then metamorphosed to the amphibolites during a retrogressive stage. Our study indicates that amphibolitization took place with various degrees of deformation. It may imply that the hydrothermal activity increased as the Godzilla Megamullion developed as an oceanic core complex in the paleo-ridge. [ABSTRACT FROM AUTHOR]

Abstract: Petrological analysis reveals the volume of aqueous fluids infiltrated into high-pressure–ultrahigh-pressure metamorphic (HP–UHPM) rocks during exhumation of the Kokchetav massif, Kazakhstan. The mode and extent of retrograde hydration in the massif are highly variable depending on lithology. The volume of aqueous fluids infiltrated during retrograde hydration was calculated based on a comparison of the peak and present mineral assemblages obtained by observations and thermodynamic calculations. Eclogite and metapelite absorbed about 1.0wt.% and 0.2wt.% of water during retrograde metamorphism, respectively, whereas orthogneiss and leptite showed no change in water content despite nearly complete retrograde recrystallisation. Based on the change in water content and the spatial coverage of each lithology, the volume of aqueous fluids infiltrated during retrograde metamorphism of the HP–UHPM unit was calculated as 0.1% of the total mass of the massif. This amount of fluid is fully covered by the dehydration of the underlying Daulet Suite during contact metamorphism by the emplacement of the HP–UHPM unit at the mid-crustal level. Our results suggest that the fluid infiltration at the exhumation stage is tectonically inevitable, and even a small amount of water infiltration less than 1wt.% effectively enhances retrograde recrystallisation in permeable lithologies such as pelitic and felsic schists/gneisses. [Copyright &y& Elsevier]

Garnet-bearing schists and migmatites sampled from the high-T, low-P Wongwibinda Complex in the New England Fold Belt, northern New South Wales, contain S1 and S2 assemblages that are inferred to have formed within error of each other at T = 700 and 650°C, respectively, and P = 400 and 380 MPa, respectively. Garnet grains commonly display a zoning profile that includes a flat unzoned interior with narrow (<350 μm) rims of variable composition. We interpret the unzoned cores as resulting from elemental homogenisation at peak D1 metamorphic conditions and the narrow rims (with increased Mn) as resorbed grain edges that formed during retrograde conditions (D2 and thereafter). The retrograde overprint is nearly pervasive across the complex and is most notable nearer to shear zones and intrusive rocks that cut S1, including the Hillgrove Plutonic Suite. A gravity traverse across the complex determined the Wongwibinda Fault is best modelled with a dip of 65° towards the west but did not identify any substantial concealed mafic plutons, suggesting that the heat source for the shallow crustal thermal perturbation is not imaged beneath the complex today. [ABSTRACT FROM AUTHOR]

Outgrowths of zircon and xenotime are abundant on detrital zircon in a greenschist facies regional metamorphic slate from the Scottish Highlands. Back-scattered electron images reveal that ca. 3-μm wide zircon outgrowths are porous, inclusion-rich and contain fine-grained intergrowths with xenotime. Focused ion beam milling, transmission electron microscopy and low-voltage scanning transmission electron microscopy show that the outgrowth microstructure is complex, composed of mis-orientated nano-crystalline zircon and a poorly crystalline zircon–xenotime complex. Well-defined micron-sized xenotime is also present within the outgrowth. Micro-textures show that the outgrowth developed in a polyphase history during cooling and exhumation. Accommodation space for the outgrowth is probably generated by a combination of thermal decompaction and the migration of defects in adjacent quartz driven by the force of crystallisation. Zircon may be unique in its ability to record textural evidence of events during uplift and exhumation that are not recorded in major silicate phases. [ABSTRACT FROM AUTHOR]

Abstract: The peak metamorphic conditions and processes of the Cretaceous Sanbagawa HP metamorphic belt, central Shikoku, Japan, were examined based on petrological, thermodynamic and geochronological studies in the Asemi-gawa area, where garnetite (garnet>50 wt.%) is included in mafic schists of the oligoclase–biotite zone. The representative mineral assemblage in the garnetite is Grt+Qtz+Phe+Ep+Chl+Rt. The peak and retrograde pressure–temperature (P–T) conditions were estimated by thermodynamic calculations using THERMOCALC ver. 3.31 (Holland and Powell, 1998) and THERIAK-DOMINO ver. 15.05.08 (De Capitani, 1994). The estimated peak and retrograde P–T conditions of the garnetite are in the range of P =15–19 kbar and T =500–520 °C, and P =7–11 kbar and T =460–510 °C, respectively. The P–T data indicate that the whole package of the oligoclase–biotite zone in the Asemi-gawa area once reached eclogite facies (ca. 48–60 km depth) and underwent retrograde metamorphism under epidote–amphibolite facies conditions (ca. 22–35 km depth) during the exhumation. A zircon U–Pb age was determined using Nano-SIMS for zircons extracted from pelitic schist of the oligoclase–biotite zone. Only zircon rims yielded a concordia age of 85.6±3.0 Ma (weighted mean age) that is considered to be the time of the retrograde recrystallization. These results imply that the eclogite–facies rocks in the Sanbagawa belt, which have been considered to be tectonic blocks or nappes, are an integral part of the Sanbagawa belt, and formed together with the surrounding schists under a specific thermal gradient at depth in the subduction zone. [Copyright &y& Elsevier]

Abstract: Retrograde metamorphism played the dominant role in changing the low-field rock magnetic properties and density of 198 specimens of variably retrograded eclogites from the main borehole of the Chinese Continental Scientific Drilling Project (CCSD) and from surface outcrops in the Donghai area in the southern part of the Sulu UHP belt, China. Bulk magnetic susceptibility (κ) of unretrogressed UHP eclogite is controlled by whole-rock chemical composition and ranges from 397 to 2312 μSI with principal magnetic susceptibility carrying minerals paramagnetic garnet, omphacite, rutile and phengite. Partially retrograded eclogites show large variations in magnetic susceptibility between 804 and 24,277 μSI, with high mean magnetic susceptibility values of 4372±4149 μSI caused by appreciable amounts of Fe-Ti oxide minerals such as magnetite, ilmenite and/or titanohematite produced by retrograde metamorphic reactions. Completely retrograded eclogites have lower susceptibilities of 1094±600 μSI and amphibolite facies mineral assemblages lacking high magnetic susceptibility minerals. Jelínek''s corrected anisotropy (P j ) of eclogites ranges from 1.001 to 1.540, and shows a positive correlation with low-field magnetic susceptibility (κ). Arithmetic mean bulk density (ρ) shows a steady decrease from 3.54±0.11 g/cm3 (fresh eclogite) to 2.98±0.06 g/cm3 (completely retrograded eclogite). Retrograde metamorphic changes in mineral composition during exhumation appear to be the major factor causing variations in low field magnetic susceptibility and anisotropy. Retrograde processes must be taken into account when interpreting magnetic surveys and geophysical well logs in UHP metamorphic terranes, and petrophysical properties such as density and low-field magnetic susceptibility could provide a means for semi-quantifying the degree of retrogression of eclogite during exhumation. [Copyright &y& Elsevier]

Abstract: Textural observations, chemical zoning, and 196 in situ LA-ICP-MS 232Th/208Pb and 238U/206Pb dates of monazite inclusions in Greater Himalayan garnets demonstrate that inclusions are frequently composite grains composed of multiple generations of monazite. Cenozoic prograde and retrograde monazite are commonly intergrown, and both of these components are frequently intergrown with lower Paleozoic monazite. Because zones in most inclusions are too small to date, and because some inclusions show little zoning of Y and Th, chemical characterization alone is often insufficient to guide interpretation of the crystallization significance of Th/Pb dates. Comparison of 238U/206Pb and 232Th/208Pb dates thus is critical for interpreting the tectonic significance of dates derived from these composite inclusions. Most Cenozoic monazite crystallized between 42–29 and 22–12 Ma, and these age ranges may record prograde and retrograde metamorphism of Greater Himalayan rocks, respectively. Dates younger than ∼12 Ma are discordant, without exception. The <12 Ma dates come from the eastern side of the Annapurna Range and record limited monazite growth during retrograde metamorphism and metasomatism of these Greater Himalayan rocks. Microcracks allow communication between the interior and exterior of garnet, affecting monazite inclusions by facilitating Pb loss, dissolution, recrystallization, and intergrowth of younger monazite. [Copyright &y& Elsevier]

Abstract: Parallel studies on rock magnetic properties, petrology and mineralogy were conducted on 16 eclogite samples from the ZK703 hole and magnetic susceptibilities and densities of 41 eclogite samples with different degree of retrograde metamorphism (from fresh eclogite to fully-retrograded eclogite) from the Chinese Continental Scientific Drilling (CCSD) near the ZK703 hole, located at Donghai, southern Sulu ultrahigh-pressure metamorphic belt, eastern China. Results show: (1) that the high-field slopes obtained from the hysteresis loops (the paramagnetic fraction χ para) and density have a positive correlation with the volume concentration of garnet+omphacite, a typical mineral assemblage used to semi-quantify the degree of retrograde metamorphism. The low-field slopes obtained from hysteresis loops (the ferrimagnetic susceptibility fraction χ ferri), saturation isothermal remanent magnetization M rs and saturation magnetization M s have a positive correlation with the volume concentration of symplectite, a mineral related to retrograded metamorphism. Therefore they could be potential indicators for quickly semi-quantifying the degree of retrograde metamorphism of the eclogite units. (2) The dominant magnetic carriers in retrograded eclogites are magnetite particles in pseudo-single domain grain size region. (3) The P–T conditions during the retrograde (decompressional) process could first increase the concentration of magnetite, which can reach up to 3% for extensively retrograded eclogite and then was dissolved for fully-retrograded eclogite. Therefore, change in the magnetite contents during the retrograde process is the major factor controlling the magnetism of retrograde eclogites. [Copyright &y& Elsevier]

Abstract: The Hongtoushan Archaean Cu–Zn Volcanic-hosted Massive Sulphide (VHMS) deposit occurs in the Hunbei granite–greenstone terrane, Liaoning Province of NE China. The ores were metamorphosed (3.0–2.8 Ga) to upper amphibolite facies at temperatures between 600 and 650 °C, and underwent three phases of deformation. The deformation has transposed the stratiform orebodies into a Y-shaped configuration. Ore minerals in the deposit were originally composed of pyrite, pyrrhotite, chalcopyrite and sphalerite. Prograde metamorphism obliterated almost all the sedimentary textures in the ores, and the present ore textures were formed substantially during peak and post-peak metamorphism. Intensive remobilisations have occurred in the Hongtoushan deposit at various scales. There are more than 30 oreshoots inside the main massive sulphide orebodies. The oreshoots range from 2 to 30 m in length, 0.1 to 1.5 m in width, and 2 to 30 m in depth extension. Most run parallel or sub-parallel to the strike of the massive sulphide ore layers with only a few being developed in metamorphic rocks in the immediate vicinity of the sulphide orebodies. Almost all the sulphide and gangue minerals in the oreshoots have been affected by intense shearing. Pyrite is characterised by cataclastic flow; pyrrhotite, chalcopyrite and sphalerite exhibit strong plasticity. Hydrothermal alteration minerals in the oreshoots are dominated by deformation-free hornblende, actinolite, epidote, clinozoisite, biotite, muscovite, albite, carbonate and quartz. Cu, Au and Ag in the ore mylonites average 11 wt.%, 1.74 and 235 g/t, respectively. Compared to the massive sulphide ores, these oreshoots show remarkable enrichment of these elements with enrichment coefficients of 5.3, 5.0 and 4.6, respectively. We draw the conclusion that these oreshoots are actually ore mylonites overprinted by late-stage fluids. The presence of annealing textures in pyrrhotite and the absence in pyrite suggest that formation and succeeding annealing of the ore mylonite took place over the temperature range from 550 to 450 °C. Such a temperature range, combined with the geology and mineralogy, indicates that these ore mylonites and the derivative oreshoots were formed during upper-greenschist-facies retrograde metamorphism. Similarity of the ore mylonites to the massive ores in metal varieties and lead isotope ages suggest that Cu, Au and Ag of the overprinting fluids on the ore mylonites were derived mainly from the nearby massive sulphide ores. However, lead isotope compositions of the ore mylonites also suggest that an external source, such as the metamorphic host rocks, has also contributed to their metal budget. [Copyright &y& Elsevier]

Hydration reactions are direct evidence of fluid–rock interaction during regional metamorphism. In this study, hydration reactions to produce retrograde actinolite in mafic schists are investigated to evaluate the controlling factors on the reaction progress. Mafic schists in the Sanbagawa belt contain amphibole coexisting with epidote, chlorite, plagioclase and quartz. Amphibole typically shows two types of compositional zoning from core to rim: barroisite → hornblende → actinolite in the high-grade zone, and winchite → actinolite in the low-grade zone. Both types indicate that amphibole grew during the exhumation stage of the metamorphic belt. Microstructures of amphibole zoning and mass-balance relations suggest that: (1) the actinolite-forming reactions proceeded at the expense of the preexisting amphibole; and (2) the breakdown reaction of hornblende consumed more H2O fluid than that of winchite, when one mole of preexisting amphibole was reacted. Reaction progress is indicated by the volume fraction of actinolite to total amphibole, Yact, with the following details: (1) reaction proceeded homogeneously in each mafic layer; (2) the extent of the hornblende breakdown reaction is commonly low ( Yact < 0.5), but it increases drastically in the high-grade part of the garnet zone ( Yact > 0.7); and (3) the extent of the winchite breakdown reaction is commonly high ( Yact > 0.7). Many microcracks are observed within hornblende, and the extent of hornblende breakdown reaction is correlated with the size reduction of the hornblende core. Brittle fracturing of hornblende may have enhanced retrograde reaction progress by increasing of influx of H2O and the surface area of hornblende. In contrast to high-grade rocks, the winchite breakdown reaction is well advanced in the low-grade rocks, where reaction progress is not associated with brittle fracturing of winchite. The high extent of the reaction in the low-grade rocks may be due to small size of winchite before the reaction. [ABSTRACT FROM AUTHOR]

Previous studies of metapelitic rocks from the core of the southern Brittany metamorphic belt suggest a complex clockwise P–T evolution. We use pseudosections calculated for an average subaluminous metapelite composition in the MnNCKFMASH system and average P–T calculations to investigate in more detail the metamorphic evolution of these rocks. For migmatites, sequential occurrence of kyanite, kyanite + staurolite and sillimanite suggests that a prograde evolution to P > 8 kbar at T ≈ 625°C was followed by a P decrease to around 6 kbar at 650°C. Subsequent heating and burial led to melting as a result of the incongruent breakdown of first muscovite then biotite; at the metamorphic peak (P ≈ 8 kbar, T ≈ 800°C), around 25 mol % melt and >20 mol % garnet are predicted. The retrograde evolution began with conductive cooling, allowing crystallization of melt and retrograde replacement of garnet by reaction to biotite and sillimanite. Subsequent near-isothermal decompression from around P ≈ 6 kbar to P ≈ 4 kbar was associated with widespread development of cordierite and a second episode of melt generation. Microstructural relations in upper amphibolite-facies metapelites from the unit structurally overlying the migmatites are consistent with growth of staurolite, andalusite and white mica along the retrograde path. On the basis of phase relations that account for the quantity of H2O required to saturate a given assemblage, an influx of an H2O-rich volatile phase is implied, which we infer to have been derived from crystallizing melt in underlying granites. The close correspondence of predictions from pseudosections to thermobarometric results is encouraging, and the complementary approach used has allowed tighter constraints than before to be placed on the P–T–X evolution of this sector of the Variscides. [ABSTRACT FROM AUTHOR]

Abstract The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H[sub 2] O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two-pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg-rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic. [ABSTRACT FROM AUTHOR]

Inclusions composed of relict coesite and retrograde quartz in garnet of a pyrope-quartzite from the Dora Maira massif were examined in transmission electron microscopy. Although coesite has suffered very little plastic deformation, the Burgers vectors of dislocations could be identified. They are [100], [001], [110] (i.e., a, c, and a+b), and other symmetrically almost identical vectors. The (110) plane could be identified as a slip plane. The dislocations are very straight, pointing to a high Peierls stress. Some small prismatic dislocation loops with Burgers vector [010] are observed, possibly formed by water-related defects. Retrograde quartz occurs at the margin of coesite with a palisade texture and as veins within coesite. Palisade quartz exhibits a high density of dislocations pinned on bubbles, and Brazil twins parallel to {1011} planes. The quartz veins are aligned parallel to the (100) and (021) composition planes of twins in coesite crystals. These observations strongly suggest that back-transformation of coesite into quartz starts at grain and twin boundaries, and that coesite loses water to quartz during transformation. [Copyright &y& Elsevier]

Abstract This work supports a growing body of evidence that the Ashe Metamorphic Suite (AMS) of the eastern Blue Ridge province in North Carolina has an ensimatic origin and is part of a subduction-related accretionary mélange, marking the Taconic suture between the North American craton and the Inner Piedmont. In a palinspastic reconstruction, the thrust fault at the base of the AMS appears to have intercepted the greatest depths (i.e. highest-P metamorphic rocks) beneath parts of the AMS now exposed adjacent to the Grandfather Mountain window. The greatest volume of mafic rock is found in these same areas. We suggest that the nascent, subduction-related, basal thrust fault was deflected downward by an obstacle in the form of an isolated, mafic volcanic edifice on the oceanic crust–a sea mount. Pelitic and mafic rocks dominate the AMS. North of the Grandfather Mountain window, retrograded eclogite occurs in the amphibolite near the base of the AMS. Textures and mineralogy indicate that an original eclogite assemblage was subjected to the following sequence of parageneses: (a) Eclogite(I) facies: omphacite+garnet+quartz, (b) Eclogite(II) facies: omphacite+garnet+epidote+quartz, (c) Symplectic (diopside+plagioclase)+garnet+epidote+quartz, (d) Amphibolite facies: (diopside+plagioclase)+garnet+epidote+hornblende+quartz, (e) Amphibolite facies: plagioclase+garnet+epidote+hornblende+quartz. P–T conditions, estimated from geothermobarometry applied to relevant mineral compositions, are c.720 °C and c.16 kbar for (b) eclogite(II) facies; c.655 °C and c.8.5 kbar for (e) amphibolite facies. [ABSTRACT FROM AUTHOR]

Alteration phenomena in hornblende crystals of the northwestern Okchon metamorphic belt were investigated in detail using high-resolution transmission electron microscopy (HRTEM). HRTEM images of hornblende commonly show submicroscopic intergrowths of a variety of sheet silicates; chlorite is the most abundant sheet silicate within hornblende, and biotite is also frequently observed along with chlorite. Although the angle between (001) of chlorite and (100) of hornblende is variable, the (001) planes of both chlorite and biotite parallel the c axis of hornblende. Microstructural evidences indicate that both sheet silicates intergrown with hornblende are produced by retrogression of hornblende. Biotite apparently formed by early hydration reaction in the presence of fluids enriched with K+, and chlorite at a later stage of retrograde reaction when K+ was consumed. In addition, hornblende crystals contain numerous etch pits and poorly crystalline silicates as filling materials, which are the product of weathering. Such submicroscopic intergrowths of sheet silicates may result in somewhat high Al contents in the hornblende analysis, producing imprecise result in geothermobarometric calculations. Furthermore, submicroscopic alteration of hornblende deteriorates the Ar retentivity and consequently affects the K−Ar isotopic ages or the40Ar/39Ar age spectra of hornblende. It is thus evident that the HRTEM characterization of hornblende is prerequisite for the correct interpretation of P-T estimates and age data. [ABSTRACT FROM AUTHOR]