High-grade Metamorphism Research Articles

The complex tale of Mantiqueira and Juiz de Fora: A comment on “Eoarchean to Neoproterozoic crustal evolution of the Mantiqueira and the Juiz de Fora Complexes, SE Brazil: Petrology, geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes”

In a recent volume of Precambrian Research (vol. 323, pp. 82–101), Kuribara et al. (2019) published a paper on the Eoarchean to Neoproterozoic evolution of the Mantiqueira and Juiz de Fora Complexes of SE Brazil. In this work, Kuribara et al. (2019) suggest that the complexes were previously indicated to be part of the Neoproterozoic to Cambrian Brasiliano Orogeny and that the results of their work instead show that these complexes are fragments of Archaean crust that accreted during the 590–560 Ma Brasiliano orogeny. Kuribara et al. (2019) also present conventional thermobarometry for both complexes that they interpret represents the single metamorphic event that they obtain zircon U-Pb age data for from the Juiz de Fora complex. No metamorphic ages were obtained from the Mantiqueira Complex, instead they have used the metamorphic ages that many workers have obtained from the Mantiqueira Province. In this comment we show that the Archaean crustal input in the Mantiqueira complex has long been known, and the Juiz de Fora Complex has long been considered a Palaeoproterozoic ocean arc. The Mantiqueira Complex records a polymetamorphic history with high-grade metamorphism occurring at ca. 2020–1940 Ma and a later amphibolite facies Brasiliano overprint at 580–540 Ma. The Mantiqueira Province and Mantiqueira Complex are different geological entities, and while the Mantiqueira Complex is affected by the Brasiliano event and occurs within the Mantiqueira Province, the use of metamorphic ages from the Mantiqueira Province is not appropriate for the Mantiqueira Complex. Finally we show that the data presented by Kuribara et al. (2019) actually fits in with the most recent version of the model for the formation of the two complexes.

Molybdenum-isotope signals and cerium anomalies in Palaeoproterozoic manganese ore survive high-grade metamorphism

Molybdenum (Mo) and its isotopes have been used to retrieve palaeoenvironmental information on the ocean–atmosphere system through geological time. Their application has so far been restricted to rocks least affected by severe metamorphism and deformation, which may erase or alter palaeoenvironmental signals. Environmental Mo-isotope signatures can be retrieved if the more manganese (Mn)-enriched rocks are isotopically depleted and the maximum range of δ98Mo values is close to the ~2.7‰ Mo-isotope fractionation known from Mo sorption onto Mn oxides at low temperature. Here, we show that the Morro da Mina Mn-ore deposit in Minas Gerais, Brazil, contains Mn-silicate–carbonate ore and associated graphitic schist that likely preserve δ98Mo of Palaeoproterozoic seawater, despite a metamorphic overprint of at least 600 °C. The extent of Mo-isotope fractionation between the Mn-silicate–carbonate ore and the graphitic schist is similar to modern Mn-oxide precipitates and seawater. Differences in δ98Mo signals are broadly reflected in cerium (Ce) anomalies, which suggest an oxic–anoxic-stratified Palaeoproterozoic ocean.

Monazite and xenotime U–Th–Pb $$_{\mathrm{total}}$$ total ages from basement rocks of the (central) Shillong–Meghalaya Gneissic Complex, Northeast India

Monazite and xenotime are the two most useful and commonly used geochronometers for deciphering ages from metamorphic rocks. The low analytical cost involved in electron probe micro-analyser chemical dating, ease of sample preparation and abundance in metamorphic rocks of wide P–T conditions make monazite and xenotime dating most widely used technique for age determination amongst metamorphic petrologists. This contribution presents age comparisons between coexisting monazite and xenotime in the basement metapelitic rocks of the central part of the Shillong–Meghalaya Gneissic Complex (SMGC). Thermobarometric estimates in the studied samples indicate granulite facies conditions of metamorphism with peak \({P{-}T}\) conditions of \(\sim \)6.5 kbar and \({\sim }750^\circ \hbox {C}\). Results indicate that xenotime in the basement rocks in the central SMGC formed in four discrete geological events while monazite either formed only in the latest Pan-African granulite grade metamorphic event or recrystallised during this event. Monazite in the studied samples yielded a single ubiquitous age of ca. 500 Ma. Xenotime in the study area, although found in only one sample, preserves four distinct ages at \(1153 \pm 29,930 \pm 36,823 \pm 41\; {\hbox {and}}\; 490 \pm 11\;\hbox {Ma}\). Preservation of Grenvillian ages in xenotime from central SMGC marks the eastward extension of Rodinia amalgamation front in the Indian Shield. The Neoproterozoic ages in xenotime from central SMGC suggest that the ca. 820 Ma high-grade metamorphism in the Eastern Indian Tectonic Zone had a wider impact in the SMGC than perceived previously.

U[sbnd]Pb zircon ages and metamorphic conditions of mafic granulites from the basement of the southern Brasilia Orogen, Campinas-SP region

In the southern Brasília Orogen, slices of an intensively reworked Paleoproterozoic basement occur. These are partially covered by a Neoproterozoic sequence of high-metamorphic grade, corresponding to the Socorro–Guaxupé Nappe and intruded by several Neoproterozoic plutonic rocks. In this contribution, we present thermobarometric and geochronological data of the basement rocks in the Campinas city, southern part of the Brasilia Orogen. The occurrence of mafic granulites included as lenses in mylonitic gneisses is described for the first time in a quarry located in the southern part of Campinas. Petrographic studies revealed high temperature paragenesis composed of orthopyroxene, clinopyroxene, garnet, biotite, and plagioclase. We present thermobarometric results with conditions close to the metamorphic peak, at ca. 8.7–10 kbar and 820–890 °C, which characterize these rocks as medium-pressure mafic granulites. UPb zircon ages of 2134 ± 6 Ma and 2156 ± 6 Ma were obtained for the host gneisses and the mafic granulite, respectively and interpreted as the timing of their igneous crystallization. Metamorphic zircon overgrowths in the mafic granulites show a concordia age of 608 ± 3 Ma. This age is coeval with one of the several episodes of melt crystallization reported in the southern Brasilia Orogen. The Paleoproterozoic ages of ca. 2.15 Ga suggest that the protoliths of the gneisses and granulites in the Campinas region are similar to the rocks of the Pouso Alegre Complex, which represents part of the lower São Francisco plate overthrusted by the Brasilia Orogen during the Neoproterozoic orogenic collage in southeastern Brazil.

Widespread occurrences of variably crystalline 13C-depleted graphitic carbon in banded iron formations

Almost all evidence for the oldest traces of life on Earth rely on particles of graphitic carbon preserved in rocks of sedimentary protolith. Yet, the source of carbon in such ancient graphite is debated, as it could possibly be non-biological and/or non-indigenous in origin. Here we describe the co-occurrence of poorly crystalline and crystalline varieties of graphitic carbon with apatite in ten different and variably metamorphosed banded iron formations (BIF) ranging in age from 1,800 to >3,800 Myr. In Neoarchean to Palaeoproterozoic BIF subjected to low-grade metamorphism, 13C-depleted graphitic carbon occurs as inclusions in apatite, and carbonate and arguably represents the remineralisation of syngenetic biomass. In BIF subjected to high-grade metamorphism, 13C-depleted graphite co-occurs with poorly crystalline graphite (PCG), as well as apatite, carbonate, pyrite, amphibole and greenalite. Retrograde minerals such as greenalite, and veins cross-cutting magnetite layers contain PCG. Crystalline graphite can occur with apatite and orthopyroxene, and sometimes it has PCG coatings. Crystalline graphite is interpreted to represent the metamorphosed product of syngenetic organic carbon deposited in BIF, while poorly crystalline graphite was precipitated from C–O–H fluids partially sourced from the syngenetic carbon, along with fluid-deposited apatite and carbonate. The isotopic signature of the graphitic carbon and the distribution of fluid-deposited graphite in highly metamorphosed BIF is consistent with carbon in the fluids being derived from the thermal cracking of syngenetic biomass deposited in BIF, but, extraneous sources of carbon cannot be ruled out as a source for PCG. The results here show that apatite + graphite is a common mineral assemblage in metamorphosed BIF. The mode of formation of this assemblage is, however, variable, which has important implications for the timing of life's emergence on Earth.

Emerald Deposits: A Review and Enhanced Classification

Although emerald deposits are relatively rare, they can be formed in several different, but specific geologic settings and the classification systems and models currently used to describe emerald precipitation and predict its occurrence are too restrictive, leading to confusion as to the exact mode of formation for some emerald deposits. Generally speaking, emerald is beryl with sufficient concentrations of the chromophores, chromium and vanadium, to result in green and sometimes bluish green or yellowish green crystals. The limiting factor in the formation of emerald is geological conditions resulting in an environment rich in both beryllium and chromium or vanadium. Historically, emerald deposits have been classified into three broad types. The first and most abundant deposit type, in terms of production, is the desilicated pegmatite related type that formed via the interaction of metasomatic fluids with beryllium-rich pegmatites, or similar granitic bodies, that intruded into chromium- or vanadium-rich rocks, such as ultramafic and volcanic rocks, or shales derived from those rocks. A second deposit type, accounting for most of the emerald of gem quality, is the sedimentary type, which generally involves the interaction, along faults and fractures, of upper level crustal brines rich in Be from evaporite interaction with shales and other Cr- and/or V-bearing sedimentary rocks. The third, and comparatively most rare, deposit type is the metamorphic-metasomatic deposit. In this deposit model, deeper crustal fluids circulate along faults or shear zones and interact with metamorphosed shales, carbonates, and ultramafic rocks, and Be and Cr (±V) may either be transported to the deposition site via the fluids or already be present in the host metamorphic rocks intersected by the faults or shear zones. All three emerald deposit models require some level of tectonic activity and often continued tectonic activity can result in the metamorphism of an existing sedimentary or magmatic type deposit. In the extreme, at deeper crustal levels, high-grade metamorphism can result in the partial melting of metamorphic rocks, blurring the distinction between metamorphic and magmatic deposit types. In the present paper, we propose an enhanced classification for emerald deposits based on the geological environment, i.e., magmatic or metamorphic; host-rocks type, i.e., mafic-ultramafic rocks, sedimentary rocks, and granitoids; degree of metamorphism; styles of minerlization, i.e., veins, pods, metasomatites, shear zone; type of fluids and their temperature, pressure, composition. The new classification accounts for multi-stage formation of the deposits and ages of formation, as well as probable remobilization of previous beryllium mineralization, such as pegmatite intrusions in mafic-ultramafic rocks. Such new considerations use the concept of genetic models based on studies employing chemical, geochemical, radiogenic, and stable isotope, and fluid and solid inclusion fingerprints. The emerald occurrences and deposits are classified into two main types: (Type I) Tectonic magmatic-related with sub-types hosted in: (IA) Mafic-ultramafic rocks (Brazil, Zambia, Russia, and others); (IB) Sedimentary rocks (China, Canada, Norway, Kazakhstan, Australia); (IC) Granitic rocks (Nigeria). (Type II) Tectonic metamorphic-related with sub-types hosted in: (IIA) Mafic-ultramafic rocks (Brazil, Austria); (IIB) Sedimentary rocks-black shale (Colombia, Canada, USA); (IIC) Metamorphic rocks (China, Afghanistan, USA); (IID) Metamorphosed and remobilized either type I deposits or hidden granitic intrusion-related (Austria, Egypt, Australia, Pakistan), and some unclassified deposits.

Pressure‐temperature‐time evolution of ultrahigh‐temperature granulites from the Trivandrum Block, southern India: Implications for long‐lived high‐grade metamorphism

The Southern Granulite Terrane in India comprises a number of Archean to Neoproterozoic magmatic arcs and supracrustal units that underwent latest Neoproterozoic to Cambrian high‐grade metamorphism. Here, we present a new petrological and geochronological data from khondalites in the western part of the Trivandrum Block and discuss pressure‐temperature‐time (P–T–t) path of the block for unravelling the duration and heat source of high‐grade metamorphism. Phase equilibria modelling of the khondalite indicates peak P–T condition of 920°C–1,030°C and 6.0–7.6 kbar, suggesting ultrahigh‐temperature (UHT) metamorphism. Prograde and retrograde P–T conditions of ~750°C/~7 kbar and ~750°C/~4 kbar, respectively, were also obtained, based on which a clockwise P–T path with geotherm‐parallel slow cooling is inferred. Zircon and monazite U–Pb geochronology and rare‐earth elements (REE) patterns suggest that heavy rare‐earth elements (HREE)‐depleted zircons grew together with garnet during prograde partial melting at >810°C or during fluid activity at around 582 ± 17 Ma, which was followed by peak UHT metamorphism at 555.1 ± 8.1 Ma as inferred from the dominant monazite ages. Relatively HREE‐enriched zircons (527.3 ± 8.0 Ma) and monazites (501.9 ± 8.5 Ma) were probably formed by garnet breakdown during retrograde metamorphism. The growth of HREE‐enriched zircons at 489 ± 12 Ma might be related to later fluid infiltration and hydration of garnet to form biotite at <770°C/~4 kbar. Our results suggest that high‐grade metamorphism continued at least 90 Myr, from 582 to 489 Ma, suggesting a long‐lived thermal event possibly related to the input of radiogenic heat from the crust and/or magmatic heat from syntectonic to post‐tectonic intrusions.

Eoarchean to Neoproterozoic crustal evolution of the Mantiqueira and the Juiz de Fora Complexes, SE Brazil: Petrology, geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes

The Mantiqueira and the Juiz de Fora Complexes (MQC and JFC) in the Araçuaí-Ribeira orogenic system, southeastern Brazil, have been regarded as parts of the Neoproterozoic to Cambrian Brasiliano Orogeny formed by a series of collisions of the São Francisco and the Congo Cratons. Here we report new petrological, geochemical, and zircon U-Pb and Lu-Hf isotopic data on felsic to mafic orthogneisses from the complexes with a view to evaluate the timing and P-T conditions of the collisional event as well as protolith formation. Metamorphic P-T estimates for orthogneisses indicate amphibolite-facies conditions of 650–730 °C and 5.8–9.1 kbar from the MQC, whereas slightly higher (upper amphibolite- to lower granulite-facies) conditions of 680–770 °C and 6.0–7.7 kbar were obtained from the JFC. Geochemical, zircon U-Pb geochronological, and Lu-Hf isotopic data of felsic orthogneisses indicate that the MQC was formed by a series of Neoarchean (ca. 2.67 and 2.61 Ga) volcanic arc magmatisms with magma sourced from Eoarchean to Paleoarchean (ca. 3.82–3.42 Ga) crustal components. In contrast, slightly younger U-Pb ages of ca. 2380 Ma and ca. 2050–1960 Ma were obtained from the JFC marking the timing of volcanic arc magmatism as Early to Middle Paleoproterozoic, with magma sourced from Paleoarchean (∼3.45 Ga) crust with addition of juvenile crustal components. The recycled ancient crustal components were also supplied for late Neoproterozoic (ca. 600 Ma) continental arc magmatism of the JFC. The high-grade metamorphism is possibly related to the final collision of the Congo and São Francisco Cratons at 590–560 Ma, which broadly compares with the formation of the Kuunga Orogen.

Origin and thermometry of graphites from Itapecerica supracrustal succession of the southern Sao Francisco Craton by C isotopes, X-ray diffraction, and Raman spectroscopy

ABSTRACTIn the mines of the Nacional de Grafite Company around Itapecerica (Minas Gerais), located in the southern Sao Francisco Craton, occurs a supracrustal succession of high-grade metamorphic rocks including quartzite, garnet-biotite gneiss, and graphite schist formed in the Palaeoproterozoic (2.0 Ga). During metamorphic processes, organic matter was progressively transformed into graphite. From four graphite samples of three different mines (two samples from high-grade metamorphic graphite schist and two generated by hydrothermal recrystallization of the graphite schist), the origin and formation temperature of this mineral was obtained by C isotopes, X-ray diffraction (XRD), and Raman spectroscopy. The values of δ13C range between −21.23 and −27.89 ‰, indicating that the source of the graphite was a primitive biogenic carbon material. High-grade metamorphic graphites show average temperatures around 729°C, while hydrothermal recrystallizated graphites (vein-graphites) show temperatures around 611°C by XRD, which correspond to granulite- to amphibolite facies conditions. The hydrothermal process with percolation of C-O-H fluids leads to a decrease in the crystal size along stacking direction (Lc(002)) when compared with the previously formed high-grade metamorphism graphites. An update of the current tectonic model about the collisional process during Rhyacian-Orosirian orogeny in the Sao Francisco Craton is proposed to insert the formation of the Itapecerica graphite-rich metasedimentary sequence.

High-Grade Metamorphism and Anatexis in the Teletskoe–Chulyshman Belt (Gorny Altai): U–Pb Geochronology, P–T Estimates, and Thermal Tectonic Model

Abstract —A model of the formation of the Teletskoe–Chulyshman metamorphic belt (TCMB) in Gorny Altai has been elaborated. The estimated pressure (not exceeding 3–4 kbar) and temperature (about 740 °C) indicate an increased regional crustal thermal gradient equal to 60–90 °C/km during the formation of the metamorphic belt. The age of migmatites of the Chulyshman complex has been evaluated at 483.9 ± 5.7 Ma (Early Ordovician) by U/Pb (SHRIMP) zircon dating. The paleogeodynamic setting of the TCMB formation and the protolith nature are identified based on the geochemical and petrochemical parameters of the metamorphic rocks. Structural parameters and numerical modeling show that the Chulyshman migmatite–gneiss complex is an apical part of the thermal-dome structure formed under the thermal impact of a magmatic basic heat source in the base of the crust and displaced to the relevant depths via thrusts and crustal extrusion during the Early Ordovician accretion–collision event. Matching the metamorphism parameters and the numerical-modeling results for the crustal thermal regime, we determined the rate of the anatectic front displacement along the thrust to be at least 6 cm/yr.

Fluid-induced metamorphism and anatexis of refractory Ni-Co-Cu sulphides in subduction-related rocks

The role of metamorphism on refractory sulfides is not well constrained. Although experiments have displayed the effectiveness of high grade metamorphism, namely granulite facies metamorphism, on sulfide anatexis, its role in the presence of other variables is still poorly understood. Rocks from the Bay Islands Accretionary Complex in Honduras and the Ildeus-Lucha Complex in Russia exhibit extensive metamorphism. Sulfide mineralization is prolific in these rocks suggesting that metamorphism has played an important role in re-concentrating these sulfides during amphibolite and granulite facies metamorphism.

U-Pb SHRIMP dating of basement rocks of the Iriri-Xingu domain, Central Amazonian province, Amazonian craton, Brazil

Abstract The Iriri-Xingu domain, located in the central part of the Amazonian craton, consists of extensive occurrences of Paleoproterozoic volcanic rocks and granites with published ages of ca. 1990 to 1840 Ma, which show a strong crustal contribution for their magmas. Exposures of basement rocks are small and rare. Samples from two areas were dated in this work by U-Pb SHRIMP in zircon. In the northern Maribel area, a high-grade pelitic paragneiss presented an age of 2160 ± 8 Ma and the leucosome of a migmatitic orthogneiss, probably a diatexite, crystallized at 2149 ± 20 Ma. These Rhyacian ages suggest that it is part of the Bacaja domain, related to the Trans-Amazonian cycle. The Morro Grande area occurs in the central part of Iriri-Xingu domain, where a high-grade muscovite gneiss with a protolith of 2120-2180 Ma is dated at 1982 ± 7 Ma, and the leucosome of a migmatitic orthogneiss is crystallized at 1979 ± 8 Ma. These are the first records of high-grade metamorphism at ca. 1980 Ma in the Central Brazil shield and may be related to the generation of this Orosirian felsic magmatism, which is widespread throughout the nearby Ventuari-Tapajos province.

High-grade metamorphism during Neoproterozoic to Early Palaeozoic Gondwana assembly, exemplified from the East African Orogen of northeastern Mozambique

Northeastern Mozambique exposes a deeply eroded high-grade segment of the East African Orogen, situated at the critical intersection of the Pan-African orogenic belts, showing widespread granulite facies lithologies. This study documents and characterizes metamorphism attributed to the multistage Neoproterozoic and Early Palaeozoic events, relating to a polyphase Pan-African history of outboard nappe stacking, nappe emplacement onto Mesoproterozoic crust, and compressional and extensional deformation along the Lurio belt: 1) A Neoproterozoic nappe stack (Cabo Delgado Nappe Complex) records high-pressure granulite facies: The highest peak conditions reach P = 1.45 GPa and T > 800 °C are in Grt-Cpx-Pl-bearing mafic granulites of the Xixano Complex. A post-peak isothermal decompressional P-T evolution is supported by garnet chemical zoning pattern, pseudsection modelling and garnet corona textures. 2) The underlying Mesoproterozoic gneiss complexes record amphibolite facies to intermediate-pressure granulite facies, the high-grade metamorphism is documented by Grt-Opx-Cpx-Pl-bearing mafic granulites and charnockitic gneisses in the southern part of the Unango and Marrupa complexes. This metamorphism is attributed to crustal thickening related to overriding of the nappe complex and shortening along the Lurio belt during the early Palaeozoic Kuunga orogeny. 3) The Ocua Complex coring the Lurio belt is a locus of Early Palaeozoic high-pressure metamorphism. A minimum of P = 0.79 ± 0.13 GPa at T = 760 ± 98 °C is constrained by Grt-Opx-Cpx-Pl-Qz assemblage in mafic granulite from the western limb while high-pressure granulite metamorphism was reached in the eastern part of the complex. The high-pressure granulite facies conditions is followed by isothermal decompression.

Magnetic fabric development in the Lower Seve thrust from the COSC-1 drilling, Swedish Caledonides

The COSC-1 project drilled the several hundred meters thick basal shear zone of the Lower Seve nappe with mylonites in mica schists, amphibole schists and gneisses. In zones of high magnetic susceptibility from 1910 to 2450 m we studied magnetic and petrographic fabrics, and magnetic mineralogy. Borehole imaging allowed for geographic reorientation of the samples and offered the opportunity to study anisotropy of magnetic susceptibility (AMS) in relation to tectonic evolution of the Seve thrust. We measured AMS at room temperature, added low-temperature and field-dependent AMS for a subset of samples, and compared magnetic with petrographic fabrics. Triaxial and prolate magnetic fabrics with degree of anisotropy (P′) up to 3.2 together with abundant S-C fabrics and strain partitioning around porphyroclasts indicate dominant simple shear until 2300 m. Magnetite and ilmenohematite mimic the rock fabric due to fabric parallel alignment and/or magnetic interaction and either contribute to increase or decrease of P′, depending on the dominating rock fabric elements. Field-dependency of pyrrhotite and magnetite in kmax-direction further increases P′. Homogeneous and oblate petrographic and magnetic fabrics in the greenschist-grade overprinted rocks below 2300 m with subhorizontal kmax-kint-girdle distributions indicate dominant flattening. AMS depicts shear fabrics including magnetite and ilmenohematite, and is additionally increased by retrograde magnetite-rutile intergrowth in ilmenohematites. We interpret that shape and degree of AMS are controlled by (a) tectonic deformation and strain, (b) alteration and magnetic grain interaction, and (c) field-dependency of deformed pyrrhotite and/or magnetite. We observed that all petrographic and magnetic subfabrics are coaxial, and lineations are mainly E-W to SE-NW directed confirming the transport direction of the Caledonian allochthonous. From our microstructural and AMS results we suggest that thrusting of the Lower Seve unit commenced under simple shear conditions at higher metamorphic grades and subsequently switched to more pure shear under greenschist-grade conditions.

Magmatic and metamorphic imprints from the root of an Archean continental arc: Evidence from the Qianhuai microblock in the North China Craton

The Archean tectonic framework of the North China Craton (NCC) involved the assembly of several micro-continents along zones of ocean closure during the late Neoarchean, followed by high-grade metamorphism associated with the final assembly and cratonization of continental blocks during Late Paleoproterozoic. Here we investigate a metamorphosed igneous suite comprising pyroxenite, gabbro and trondhjemite in association with charnockite from the margin of the Qianhuai micro-block with a view to understand the magmatic and metamorphic history and crustal evolution in the NCC during late Neoarchean. We present new data from field investigations, petrology, geochemistry and zircon U-Pb, REE and Lu-Hf analyses. Mineral phase equilibria modeling shows that the pyroxenite formed at P-T conditions of 1.5–1.6 GPa and 900–1000 °C, possibly representing underplating of high temperature basaltic magma at the base of the continental arc, with the subsequent metamorphic conditions estimated as 1.2–1.4 GPa and 840–870 °C from the metagabbros. Both metagabbro and pyroxenite show an anticlockwise P-T path characterized by initial heating followed by moderate to high-pressure metamorphism and cooling. The base of a thickened arc is a viable tectonic settings to account for this scenario. Geochemical data suggest that the pyroxenite-gabbro-trondhjemite association represent the fractionated sequence from a common mafic magma, whereas the charnockite corresponds to anhydrous lower crustal melts. The high degree of hydration of the mafic and ultramafic units suggest retrograde metamorphism under hydrous conditions associated with slab subduction. The mafic-ultramafic rocks fall within the field of island arc tholeiite and the trondhjemite falls in the field of oceanic arc basalt. Geochemical data indicate arc signature of the rocks and the subduction processes involved in the generation of precursor basaltic melts that underplated the overriding arc crust and triggered lower crustal melting.The U-Pb data on magmatic zircon grains from the trondhjemite, metagabbro, pyroxenite and charnockite yield emplacement ages of 2481 Ma, 2449 Ma and 2464 and 2480 Ma and suggest subduction-related magmatism during Neoarchean-early Paleoproterozoic transition, followed by high grade metamorphism at ca. 1.94 Ga as indicated by the lower intercept ages. Zircon REE data confirm the magmatic crystallization history. The zircon Lu-Hf data show consistently positive εHf(t) values (0.5–11.7) and plot between CHUR and new crust line suggesting that depleted mantle sources contributed significantly to arc building and crustal growth. The computed model ages suggest that the magma source involved Neoarchean-Mesoarchean juvenile components. Our study provides important insights into magmatic and metamorphic processes at the base of an Archean continental arc and contributes to the understanding of the formation and growth of micro-continents within the NCC.

Potential safety evaluation method based on temperature variation during gas adsorption and desorption on coal surface

The mechanism of gas adsorption and desorption is very important for mine safety. Meanwhile, the gas adsorption or desorption process may lead to temperature or heat variation. In order to reveal this phenomenon, the relationship between gas adsorption (and desorption) and temperature of three different coal samples was experimentally investigated under different temperatures, coal ranks and equilibrium pressures. It’s found that gas adsorption quantity, gas adsorption rate and temperature variation decreased with temperature. Additionally, the variation of equilibrium pressures and the gas adsorption quantity were larger when the particle size was smaller. The more the equilibrium pressure is, the more the maximum temperature variation and the rate of temperature variation are. On the other hand, the higher the temperature is, the more the desorption quantity is, but the desorption rate and the maximum temperature variation of dropping increase with the temperature increasing. And the desorption quantity and the rate of desorption temperature increased when the particle sizes became larger. Besides, the gas adsorption quantity of coal samples with high metamorphic grade was larger under same equilibrium pressure, the temperature variation of dropping of ZX-2# is as much as the ML-8#. As a result, ZX-2# coal can accumulate a large amount of heat energy and it belongs to soft coal, which is easy to cause the outbursts. It also can be concluded that the bigger the equilibrium pressure is, the bigger the desorption quantity and desorption rate are. And desorption temperature decline greatly with the increase of equilibrium pressures.

Fluid-driven resetting of titanite following ultrahigh-temperature metamorphism in southern Madagascar

LA-ICP-MS U-Pb dates and EPMA trace-element maps of titanite were collected from calc-silicate gneisses of the Ikalamavony (peak T: 750–800 °C) and Anosyen (peak T: 900–950 °C) domains in southern Madagascar to evaluate how titanite responds to high-temperature metamorphism, cooling, and retrogression. Fluid-mediated replacement of precursor titanite by titanite domains of different composition (interface-coupled dissolution-precipitation, ICDR) was the primary mechanism by which titanite was reset following high-grade metamorphism. Comparison of titanite U-Pb dates (530–490 Ma) with independent petrology and thermochronology indicates that the alteration occurred at temperatures as low as 300–500 °C. Apparent Zr temperatures (temperatures calculated assuming titanite–quartz–zircon equilibrium) in altered titanite are less than or equal to the metamorphic peak, but higher than the inferred alteration temperature, implying that Zr was removed, but that titanite–quartz–zircon equilibrium was not achieved during alteration. Although evidence for ICDR was observed in titanite samples separated by 10's to ~100 km, differences in U-Pb dates among samples, and even among titanite grains in the same thin section suggest that alteration at any given time was localized.

The nature and significance of the Faroe-Shetland Terrane: Linking Archaean basement blocks across the North Atlantic

Core samples of the continental basement rocks that underlie the eastern Faroe-Shetland Basin (FSB) and its inshore margins west of Shetland reveal a suite of predominantly granodioritic to granitic orthogneisses (including TTG), together with lesser volumes of foliated granitoids and subordinate dioritic to mafic gneisses and amphibolites. A small area of lithologically similar gneisses also crops out onshore at North Roe/Uyea west of the Caledonian front in Shetland. In the core samples, coarse grained gneissose textures and mineralogies consistent with upper amphibolite facies metamorphism are overprinted by a weak, but ubiquitous, static greenschist facies retrogression. Later structures include widespread epidote-quartz veining, and local developments of mylonite, cataclasite, pseudotachylite and phyllonite. Regions associated with the Rona Ridge oil fields (e.g. Clair, Lancaster) also preserve extensive brittle fracturing and associated low temperature hydrothermal mineralization (quartz, adularia, calcite, pyrite/chalcopyrite) with significant fracture-hosted hydrocarbons. New and published U-Pb zircon analyses from the gneisses offshore give a relatively narrow range of Neoarchaean protolith ages (ca 2.7–2.8 Ga) spread over a geographic area of 60,000 km2 west of Shetland. Detrital zircon data from overlying Triassic-Cretaceous sedimentary sequences in the FSB suggest an equivalent limited range of Neoarchaean source materials. Hf isotopic data indicate involvement of Palaeo- to Mesoarchaean crustal sources. Overall, our findings suggest that a major phase of Neoarchaean crustal formation and associated high grade metamorphism dominates basement rocks in the region. They are similar in age and lithology to the protoliths of the nearest onshore Lewisian Complex of NW Scotland (Rhiconich Terrane). However, they lack geochronological or textural evidence for the widespread Proterozoic Laxfordian events (ca 1.7–1.8 Ga) which are widespread in Scotland. This suggests that the Precambrian rocks west of Shetland – the Faroe-Shetland Terrane – can be correlated with the Archaean rocks of the Central Greenland-Rae Craton and that a northern limit of Proterozoic reworking lies just offshore from the north coast of Scotland.

SHRIMP U-Pb Geochronology of the Socorro Batholith and implications for the Neoproterozoic evolution in SE Brazil

The Socorro Batholith is one of the most expressive granite manifestations associated with the Neoproterozoic evolution in SE Brazil, occupying large areas (~1,200 km 2 ) in the southern portion of the Socorro-Guaxupe Nappe. A U-Pb zircon SHRIMP dating program was developed to determine the ages of the main components of this batholith, identified in previous detailed mapping projects. High-K calc-alkaline (HKCA) porphyritic biotite-hornblende granites with relatively low (60–67 wt%) SiO 2 are the most voluminous component of this and other large “syn-tectonic” batholiths in the SGN (Agua Limpa and Pinhal-Ipuiuna) and neighboring domains located south of it in the Apiai and Sao Roque Domains of the Ribeira Fold Belt. Two samples collected in widely separated localities at the northern and southern part of the Socorro batholith yield similar ages of magmatic crystallization, respectively 610.1 ± 7.0 and 608.3 ± 6.6 Ma. A more fractioned (> 72 wt% SiO 2 ) granite reported in the literature as related to a younger event (“Socorro II magmatism”, as opposed to the previous “Socorro I” HKCA granites) yield a precise age that is clearly older (624.4 ± 3.6 Ma), and contemporary to anatectic granites and migmatites that were produced during a prolonged period of high-grade metamorphism (635–605 Ma) that affected the SGN. Our data thus indicates that at least part of the HKCA magmatism that constitutes the Socorro batholith post-dates the high-P metamorphism associated to continental collision, and may have been a source of heat and volatiles to the high-T metamorphism responsible for partial melting of the upper portions of the crustal section represented by the SGN. Two charnockitic rocks that show transitional contacts with granites of the Socorro batholith were also dated. The Socorro Charnockite is aged 641.6 ± 4.1 Ma, which overlaps those of regional orthogneisses (in part also of charnockitic character) considered as associated with a pre-collisional tectonics (subduction-related?). However, it is reported to transition to granites that are very similar to the HKCA granites of the Socorro batholith, which are yet undated. The Atibaia Charnockite has distinct geochemical affinity (lower mg# and Sr content; higher Zr), a younger age (633.3 ± 6.2 Ma), and may signal a different tectonic setting at the end of the period of plate consumption as yet poorly characterized.

Structural analysis combined with new geothermobarometric and geochronological results of the Além Paraíba shear zone, between Três Rios and Bananal, Ribeira Orogen, SE Brazil

The Além Paraíba Shear Zone (APSZ) is composed of subvertical mylonites trending ENE/WSW with subhorizontal stretching lineation indicative of dextral movement. In the western part of the area mapped in this paper the shear zone, defined as an elongated body of mylonites, dies out. This constrains the possible displacement along the zone. The mylonites are of relatively high metamorphic grade, product of deformed orthogneisses, granulites, paragneisses and associated syn-tectonic granitoids. These rocks reached granulite facies conditions at temperatures and pressures in excess of 850 °C and 8 kbar. Geothermobarometric results indicate that the progressive ductile deformation related to the shear zone occurred under amphibolite facies conditions at temperatures of 714 ± 23 °C to 562 ± 38 °C and pressures between 7.1 ± 0.8 kbar and 4.3 ± 1.1 kbar. Zircon U-Pb dates by LA-ICPMS were obtained from three samples, one from a mylonitic orthogneiss with age 583.6 ± 7.1 Ma, one from a mylonitic paragneiss with age 591.5 ± 6.1 Ma, and one from a pegmatitic vein that cuts the mylonites obliquely and yields an age of 597.7 ± 9.4 Ma. Monazite crystals from the mylonitic orthogneiss were also analyzed by LA-ICPMS U-Pb, resulting in a spread of concordant dates ranging from 611 to 553 Ma. Taking these data together, we estimate the age of peak metamorphism to be between 595 and 590 Ma. The age of the monazite crystals suggests continuous crystallization and/or re-equilibration after the metamorphic peak, probably catalyzed by the activity of the shear zone and/or fluid migration under amphibolite facies conditions. Since the APSZ affects a 535 Ma granite, we argue that the shear zone was probably active in the time span between 580 and 530 Ma, after metamorphic peak conditions but still at relatively high temperatures. The temporal coincidence with the main collision in the Ribeira Orogen supports the idea that the shear zone is the result of oblique collision.