Metamorphic Alteration Research Articles

Alteration of primary Cr-spinel mineral composition from the Suru Valley ophiolitic peridotites, Ladakh Himalaya: Their low-temperature metamorphic implications

The present study focuses on the chemical modification of a Cr-spinel from the Suru Valley ophiolitic peridotites exposed near Trespone Village of Kargil district, Ladakh Himalaya. The Suru Valley peridotite is partially serpentinised with the preservation of the spinel and relics of olivine and pyroxene. These peridotites contain characteristic red-brown spinel grains of corroded grain boundaries. While analysing these grains for mineral chemistry, compositional variation was observed with Cr-rich cores rimmed by Cr-poor compositions. Secondary spinel compositions, i.e., ferritchromite and magnetite were observed along the margins and cracks of primary Cr-spinel grains. The primary Cr-spinel cores are identified as Cr-rich and are characterised by higher values of Cr3+# (0.5–0.6) and lower values of Al3+# (0.42–0.54). From primary spinel cores to altered rims it was observed that Cr3+# and Fe3+# increase while Mg2+# decreases due to Mg2+–Fe2+ and Al3+–Fe3+ exchange with surrounding silicates of host peridotite during alteration. On the basis of present spinel mineral chemistry, metamorphic alteration conditions were transitional between greenschist and lower amphibolite similar to most of the Neo-Tethyan ophiolite peridotites.

Shortening of the axial zone, pyrenees: Shortening sequence, upper crustal mylonites and crustal strength

The strength of the lithosphere may be constrained qualitatively by field observations on localized vs distributed modes of deformation and by the mineral assemblages formed during deformation. The internal deformation of the Bielsa basement unit of the Pyrenean Axial zone is investigated through structural, microstructural and thermometric data. In this area, shortening is widely distributed as attested by the folded attitude of the interface between the basement and its sedimentary Triassic cover. Shortening is estimated around 1.7 km (13%) from a regional balanced cross-section and should be considered in pre-Pyrenean reconstructions. Shortening probably occurred before strain localization on crustal ramps as suggested by zircon fission-track analysis. Distributed shortening is characterized at small-scale by low-temperature mylonites and cataclasites. In thin-section, feldspar originally present in the magmatic protolith is partially to totally sericitized. This transformation led to significant weakening of the rock and took place in the 250–350 °C temperature range. Sericitization is ubiquitous, even in un-deformed granodiorites. This shows that the weakening effect of sericitization not only occurs in ultra-mylonites, ultra-cataclasites and phyllonites but also more generally in the upper crust early during the shortening history, with implications for the shortening style. Estimates of the geothermal gradient suggest that inherited thermicity may also have influenced the style of shortening. We propose that the upper crust was very weak before or at the onset of its shortening due to high-thermal gradients and fluid circulation that induced large-scale sericitization in greenschist facies conditions. This has strong implications on the rheological evolution of the upper crust submitted to metamorphic alteration in the greenschist facies and below.

Remnant lenses of komatiitic dykes in Kambalda (Western Australia): Occurrences, textural variations, emplacement model, and implications for other komatiite provinces

Remnants of komatiitic dykes have been intersected for the first time at Kambalda (Western Australia), a classical locality for komatiite-hosted Ni deposits. The dykes consist of metamorphosed lenses, aligned on three planar and parallel surfaces. They are composed of brecciated komatiitic rocks, differentiated lenses with spinifex- and hopper-textured olivine and clinopyroxene cumulates (all now composed of variable amounts of tremolite, chlorite and biotite), and chloritite.Igneous textures (spinifex and cumulate, skeletal chromite, amygdales) are well preserved and their combination with a low carbon content (carbonation is common in more altered komatiites) implies an isochemical metamorphic alteration. The composition of the komatiite dyke near the contact is similar to the Kambalda Komatiite (but lower in MgO) and changes to a clinopyroxene composition in the dyke centre. This is interpreted to be a clinopyroxene cumulate crystallized from a komatiitic basalt. Furthermore, trace elements in whole rock samples from the centre of the dyke record contamination by felsic crust. This means one drill hole intersecting the dyke has a record of contaminated to uncontaminated mantle melts within a few centimetres.The rare occurrence of komatiite dykes compared to sills in all komatiite occurrences worldwide is probably a consequence of three factors – the komatiite melt erupting above its liquidus, the melt's very low viscosity and its laminar flow. In areas of obstacles (e.g., change in lithology), local eddies can cause a local temperature decrease which leads to crystallization of the liquidus phase. A decrease in temperature might be accelerated by locally induced convection cells of pore fluid in the host rock. In the laminar flowing part of the dykes, however, the overall liquidus of the melt is only reached when the eruption ceases. Due to decreasing melt pressure, the fracture closes to adjust to the changing pressure, squeezing the melt out before the liquidus is reached. Such dykes leave no trace of their former existence. These areas of changes in flow direction therefore form chains of komatiite lenses.

Provenance and Variscan low-grade regional metamorphism recorded in slates from the basement of the (SW Hungary)

A metapelitic sequence with Silurian protolith from the Horváthertelend Unit (Tisza Mega-unit, Hungary) has a K-white mica + chlorite + quartz + albite + anatase ± tourmaline mineral assemblage. Moderately developed disjunctive and well-developed continuous foliations are present. Geochemical results reflect a dominantly felsic source of the protoliths, suggesting an intermediate to acidic volcanic arc in the provenance area. Metasandstone pebbles in the metaconglomerate indicate a recycled sedimentary source. Raman spectroscopy of carbonaceous material indicates a ~ 350‒370 °C peak metamorphic temperature. The Kübler Index (KIBasel) values of the phengitic K-white mica indicate epizonal metamorphism (0.22 ± 0.04 Δ°2θ). The Chlorite ‘Crystallinity’ Index (ChCCIS) suggests metamorphic alteration near to the anchizone‒epizone boundary (0.31 ± 0.06 Δ°2θ). K–Ar ages of K-white mica are interpreted as a result of Variscan metamorphism (c. > 310 Ma) and post-Variscan uplift (c. 290 Ma). The predominance of lydite and slate of Llandoverian age and the overlying coarse-grained metagreywacke and metaconglomerate beds of the Szalatnak Slate Formation show strong lithological similarities with the proximal Silurian sequences in the Małopolska Massif (Kielce Region, Holy Cross Mountains, Poland). The original position of the Horváthertelend Unit is presumably to the northeast from the Bohemian Massif, next to the Upper Silesian Block (Moravo-Silesian Zone) and the Małopolska Terrane.

Does intermediate spreading-rate oceanic crust result from episodic transition between magmatic and magma-dominated, faulting-enhanced spreading?—The Costa Rica Rift example

Ocean-bottom seismograph and multichannel streamer wide-angle seismic data are jointly analysed and compared with reflection images, bathymetric maps and potential field data, to reveal the detailed structure of layer 2 of the oceanic crust formed at the intermediate spreading Costa Rica Rift (CRR). Separate modelling of each wide-angle data set independently reveals a gradual increase in P-wave velocity with distance (hence crustal age) from the ridge axis, with a model derived from their joint inversion, in turn, displaying a pattern of shorter-wavelength structural complexity in addition to a background flow-line trend. Normalizing against a ridge-located reference velocity–depth model reveals that, off-axis, velocity perturbations are correlated with trends in basement roughness and uplift; regions of rougher and uplifted basement correlate with slower layer 2 velocity, <0.5 km s−1 faster than at the ridge axis, and thinner sediment cover, while smoother basement and locations where sediment cover forms a continuous seal over the oceanic basement, are mirrored by regions of relatively higher velocity, 1.0–1.4 km s−1 faster than at the CRR. These velocity variations are interpreted to reflect periodic changes in the degree of magma supply to the ridge axis. Using a combination of global and shipboard magnetic data, we derive a spreading history model for the CRR which shows that, for the past 5 Ma, spreading has been asymmetric. Comparing the seismic model structure with variations in full spreading rate over this period, reveals a correlation between periods of slower spreading and slower layer 2 velocity, basement roughness and uplift, and faster spreading, higher velocity and smoother basement structure. Zones of slower velocity also correlate with lows in the residual mantle Bouguer anomaly, interpreted as most likely reflecting corresponding regions of lower density in the lower crust or upper lithospheric mantle. Using ODP borehole 504B as ground-truth, we show that periods of faster spreading are associated with phases of magmatic accretion, interspersed by phases of increased asymmetric tectonic extension that likely facilitates fluid flow to the deeper crust and results in metamorphic alteration, manifest as the modelled density anomalies. Overall, our study shows that the mode of CRR crustal formation is sensitive to relatively small changes in full spreading rate within the range of 50–72 mm yr−1, that tips the balance between magmatic and magma-dominated crustal formation and/or tectonic stretching, as characterized by significant variation in the fabric and physical properties of layer 2. We further hypothesize that this inherited structure has a direct influence on the subsequent evolution of the crust through secondary alteration. We conclude that descriptive phrases like ‘ocean crust formed at an intermediate-spreading rate’ should no longer be used to describe an actual crustal formation process or resulting crustal structure as, over the full range of intermediate spreading rates, a fine tipping-point dictates an episodic transition between primarily magmatic accretion and magma-dominated crustal formation coupled with enhanced faulting, with their asymmetry recorded in either ridge flank.

Corundum Anorthosites-Kyshtymites from the South Urals, Russia: A Combined Mineralogical, Geochemical, and U-Pb Zircon Geochronological Study

Kyshtymites are the unique corundum-blue sapphire-bearing variety of anorthosites of debatable geological origin found in the Ilmenogorsky-Vishnevogorsky complex (IVC) in the South Urals, Russia. Their mineral association includes corundum-sapphire, plagioclase (An61–93), muscovite, clinochlore, and clinozoisite. Zircon, churchite-(Y), monazite-(Ce), and apatite group minerals are found as accessory phases. Besides, churchite-(Y) and zircon are also identified as syngenetic solid inclusions within the sapphires. In situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb zircon geochronology showed the ages at about 290–330 Ma linked to the Hercynian orogeny in IVC. These ages are close to those of the syenitic and carbonatitic magmas of the IVC, pointing to their syngenetic origin, which is in agreement with the trace element geochemistry of the zircons demonstrating clear magmatic signature. However, the trace element composition of sapphires shows mostly metamorphic signature with metasomatic overprints in contrast to the geochemistry of zircons. The reason for this discrepancy can be the fact that the discrimination diagrams for sapphires are not as universal as assumed. Hence, they cannot provide an unambiguous determination of sapphire origin. If it is true and zircons can be used as traces of anorthosite genesis, then it can be suggested that kyshtymites are formed in a magmatic process at 440–420 Ma ago, most probably as plagioclase cumulates in a magma chamber. This cumulate rock was affected by a second magmatic event at 290–330 Ma as recorded in zircon and sapphire zoning. On the other hand, Ti-in-zircon thermometer indicates that processes operated at relatively lower temperature (&lt;900 °C), which is not enough to re-melt the anorthosites. Hence, zircons in kyshtymites can be magmatic but inherited from another rock, which was re-worked during metamorphism. The most probable candidate for the anorthosite protolith is carbonatites assuming that metamorphic fluids could likely leave Al- and Si-rich residue, but removed Ca and CO2. Further, Si is consumed by the silicification of ultramafic host rocks. However, kyshtymites do not show clear evidence of pronounced metasomatic zonation and evidence for large volume changes due to metamorphic alteration of carbonatites. Thus, the obtained data still do not allow for univocal reconstruction of the kyshtymite origin and further investigations are required.

Chlorine and lithium behavior in metasedimentary rocks during prograde metamorphism: A comparative study of exhumed subduction complexes (Catalina Schist and Schistes Lustrés)

Lithium and chlorine concentrations and isotopic compositions of well-characterized metasedimentary suites from the Schistes Lustrés and Lago di Cignana (Western Alps) and the Catalina Schist (southern California, USA) were compared to determine Li and Cl behavior during prograde subduction-zone metamorphism. New data were obtained for Cl from both suites and for Li from the Western Alps. The Catalina Schist contains lawsonite-albite to amphibolite grade rocks recording peak temperatures of 200 to 750 °C and depths of 15 to 45 km, with the higher-grade units representing relatively warm subduction P-T paths. Bulk Cl concentrations decrease from ~100–500 μg/g to ~10–25 μg/g across metamorphic grades, largely due to Cl loss from chlorite and white mica, with most of this loss occurring at the lower grades. This loss mirrors the previously documented decreases in N, B, Cs, As, and Sb concentrations, with relatively little loss at higher metamorphic grade, and contrasts with the behavior of Li showing only modest loss and change in δ7Li values across all grades. Metapelitic rocks from the Western Alps representing a relatively “cool” geothermal gradient of ~8 °C/km, record peak temperatures of 350 to 550 °C and depths of up to 90 km. In this suite, both Cl and Li concentrations are uniform across grade, showing no obvious change in concentration. Lithium concentrations range to somewhat higher values compared to those previously measured in the Catalina Schist. Chlorine isotope compositions of the Schistes Lustrés overlap with those measured in the Catalina Schist, whereas Li isotope compositions of the Schistes Lustrés samples range to somewhat higher values than those from the Catalina Schist. Both suites show significant isotopic heterogeneity within single metamorphic grades and no systematic change in δ37Cl or δ7Li values with increasing grade. These heterogeneities are interpreted to be inherited from the protolith. Lithium isotope compositions correlate well with the chemical index of alteration (CIA), an observation also supporting minimal metamorphic alteration of the isotopic composition reflecting the protolith. These data demonstrate that, although some Cl is retained to great depths during subduction, most Cl is lost from metasedimentary rocks at shallow depths, similar behavior to that of B, Cs, As, and Sb. In contrast, Li loss is minimal across the full range of grade, more similar in behavior to Rb, Ba, and K.

Emplacement and metamorphism of the mafic rocks from the Chencai terrane within the Cathaysia Block: Implications for the Paleozoic orogenesis of the South China Block

Understanding the Early Paleozoic tectonic evolution of the South China Block (SCB) is critical in constraining its relationship with (super-) continent configurations, such as the Rodinia and Gondwana. High-grade mafic and supracrustal rocks occurring in the Chencai terrane of the Cathaysia Block are the uplifted lower crustal components of the Kwangsian (Early Paleozoic) Orogen which extensively reshaped many parts of the SCB. Two amphibolites, three biotite–bearing amphibolites, one norite and one mafic granulite were collected from the Chencai terrane. In previous studies, these rocks (or their protoliths) have been suggested to belong to the same cumulate series and the mafic granulite records metamorphic P-T conditions of a high-geothermal-gradient environment (∼30℃/km). Our SIMS zircon U–Pb results indicate that protoliths of these meta–mafic rocks were emplaced at ∼434–445 Ma and most of them have experienced subsequent (almost synchronous) metamorphism at 428–439 Ma. The norite sample (434 Ma), which experienced the least crustal contamination and the least metamorphic alteration, has the lowest zircon δ18O values of ∼ +6.7‰ (V–SMOW), whereas the mafic granulite which experienced the highest grade metamorphism has the highest zircon δ18O values of ∼ +9.4‰ (V–SMOW). Zircon δ18O values of other samples mostly scatter between them. Most metamorphic zircons have younger U–Pb ages and higher δ18O values than magmatic zircons. High δ18O crustal materials (e.g. sedimentary rocks) have possibly been involved during the metamorphism which resulted in the contrasting δ18O values of magmatic and metamorphic zircons. The large variations in δ18O values shown by magmatic zircons of each sample (like the amphibolite and biotite-bearing amphibolite) indicate that the high-δ18O-value crustal materials (sedimentary rocks) must have started to affect these mafic rocks since the magmatic stage. Zircon 176Hf/177Hf values of each of these samples scatter within small ranges. Initial ɛHf(t) values of most zircons fall in between the CHUR line and the DM linear model line, indicating that these mafic rocks were derived from an enriched mantle source. Combined with previous geochronological, metamorphic and geochemical data, we propose that these Paleozoic mafic magmatism might have contributed thermal energy to the later thermal relaxation of the orogeny. This episode of mafic magmatism may represent a transition of the Kwangsian Orogen from early compression to post-orogenic extension. Metamorphism of these mafic rocks and the synchronous anatexis indicated by the high grade supracrustal rocks from Chencai terrane, might be related with extension rather than orogenic compression.

Акцессорные зональные хромиты из архейских коматиитов Карельского кратона: признаки метаморфических преобразований

SEM and LA-ICP-MS analyses were used for the study of accessory chromite in Archean (3.0—2.8 Ga) komatiites from the most representative greenstone belts of the Karelian Craton. Zoned chromite, subjected to varying degrees of metamorphic alteration, was identified in the komatiites. Two types of zoned chromite, differing in the composition of their cores, were recognized. Al-rich zoned chromite of type I shows the low extent of metamorphic alteration and has cores with Cr# 0.70—0.86, Mg# 0.03—0.12, and Fe# 0.06—0.23. It is enriched in Zn, Co, and Mn but strongly depleted in Ga, Ni, and V. The study reveals that the primary magmatic distribution of minor and trace elements in the cores was strongly redisturbed during prograde metamorphism of greenschist to low amphibolite facies. Zoned chromite with the low Al 2 O 3 content of type II has cores with higher Fe# (0.42—0.49), Cr# (0.88—1), and lower Mg# (0—0.04) as compared to cores of Al-rich chromite, that corresponds to the more intensive metamorphic alteration. Since the primary composition of studied zoned chromite was significantly obliterated during metamorphism, established patterns can be used as a guide for the reconstruction of metamorphic processes.

ONCE AGAIN ON THE RELATIONSHIP BETWEEN SPECIES DIVERSITY AND BEDROCK COMPOSITION: A CASE STUDY OF THE UNIQUE MRAMORNY ISLAND, VON’GA BAY, WHITE SEA

Сarbonate rocks of unique Mramorny (Marble) Island (65.420921 N, 34.631167 E) constitute a ca. 5-40 m thick lens-shaped body thrown into isoclinal folds. They are associated, interbedded and deformed jointly with amphibolites and are cut by granite-aplites. Their major minerals are dolomite (ca. 70-90%), calcite, biotite and hornblende. Isotopic data show that Mramorny Island carbonate rocks are endogenously igneous carbonatites. Geochronology data date the magmatic stage of carbonatite formation to 2420±20 Ma and metamorphic alterations of this rock to 1845±15 Ma. During the survey in 2018, 82 species of vascular plants, 25 mosses and 44 lichens were recorded from the island. The vascular plant flora was not very specific, but richer in species compared to other White Sea islands of similar size. High frequencies and abundances were noted also for such calciphilous species as Botrychium lunaria , Draba incana and Saxifraga cespitosa . The moss flora was not rich, but the proportion of calciphilous species was quite high – there occurred Brachythecium glareosum , Brachythecium turgidum , Distichium inclinatum and Ditrichum flexicaule . For Brachythecium glareosum Mramorny Island is the only location among White Sea islands where the species has been found. The calciphilous lichens encountered were Bilimbia sabuletorum , Caloplaca stillicidiorum , Candelariella aurella , Cladonia pocillum , Fuscopannaria praetermissa , and Xanthoria elegans . Five species ( Rhodiola rosea, Brachythecium glareosum, Brachythecium turgidum, Distichium inclinatum and Ramalina subfarinacea ) are listed in the regional Red Data Book. The unique geological structure of the island has had the most pronounced effect on the bryoflora, less so on the lichen flora, and the weakest effect on the flora of vascular plants. The latter can be attributed to the small size of the island and the peculiarities of its relief, which leads to an almost unhindered transport of carbonatite weathering products into the sea by rainfall and seawater.

REE patterns and trustworthiness of stable carbon isotopes of Salitre Formation, Irecê Basin (Neoproterozoic), São Francisco Craton

Neoproterozoic limestones have been subject to variations in their original isotopic ratios, which has led to studies to verify the reliability of isotopic data obtained in these rocks. A Brazilian example of Neoproterozoic carbonate succession is found in Irecê Basin (BA) (Salitre Formation, São Francisco Supergroup). The present research aims to evaluate the reliability of the δ13C, mainly through REE patterns of samples gathered in Upper Nova America Unit. This unit represents an intermediate section included in a thick carbonate cover that represents the Salitre Formation. X-Ray fluorescence and inductive plasma mass spectrometry (ICP-MS) techniques were used to obtain elemental geochemistry. X-ray diffraction, including the fraction < 4 μm, and microscope with coupled fluorescence were used to verify the presence of carbonate, organic matter, siliciclastic phases and their proportions. Samples are mainly calcitic and have REE patterns with enrichment of light REEs, strong positive La anomalies, slightly positive Ce anomalies, variety of Gd anomalies, absence of strong Y anomaly and Y/Ho ratio of 31.5 ± 1.5. These anomalies indicate a mixture of oceanic and fresh waters during precipitation. Ce positive anomalies indicate anoxic states of early digenetic conditions of the environment. δ13C values have little contrast around 0‰. As a result, calcareous from Nova América Unit precipitated under shallow depths on carbonate platform. High concentration of Zr and SiO2 suggests high rates of weathering on marginal belts of São Francisco craton through high energy input of detrital material by freshwater. On the other hand, low concentration of Al2O3 indicates deposition of clay minerals occurred in a mild area of the plataform. The Mn/Sr and Fe/Sr ratios did not indicate the presence of significant diagenetic or metamorphic alterations. From the petrographic point of view, the samples do not present preserved organic matter, but they are recrystallized instead, sometimes dolomitized and dissolution features are present. The results indicate that Nova América limestones were precipitated on shallow carbonate restricted platform close to the coastline prone to fresh water and detrital input, therefore, δ13C does not correspond to the isotopic composition of the global ocean.

Petrogenesis and geodynamic setting of the Neoarchaean Haines Gabbroic Complex and Shebandowan greenstone belt, southwestern Superior Province, Ontario, Canada

The 2722 ± 2 Ma Archaean megacrystic anorthosite-bearing Haines Gabbroic Complex in the Wawa subprovince of the western Superior Province formed contemporaneously with the spatially associated Greenwater Assemblage of the Shebandowan greenstone belt, Ontario, Canada. Both field and petrographic observations indicate that the Haines Gabbroic Complex and the spatially and temporally associated ultramafic to intermediate volcanic and intrusive rocks of the Greenwater and Burchell assemblages experienced extensive metamorphic recrystallization and alteration as a consequence of greenschist-facies metamorphism. These metamorphic processes resulted in pervasive replacement of igneous minerals by metamorphic counterparts. Metamorphic alteration also mobilised LILE and transition metals and slightly reset the U-Th-Pb and Sm-Nd isotope systems. Consequently, the Haines Gabbroic Complex and Greenwater Assemblage have respective Sm-Nd errorchron ages of 2765 +110/‐280 Ma and 3108 +120/‐380 Ma.Major and trace element compositions suggest that the least altered anorthosites, leucogabbros and gabbros of the Haines Gabbroic Complex are not cogenetic with the Greenwater and Burchell assemblages and have distinct parental magmas and depleted mantle sources. The calc-alkaline andesites of the Burchell Assemblage, the tholeiitic basalts and gabbros of the Greenwater and Burchell assemblages, the transitional gabbros of the Greenwater and Burchell assemblages and the picrites, serpentinites and peridotites of the Greenwater Assemblage are all petrogenetically distinct and have different parental magmas and depleted mantle sources. The pyroxenites are genetically distinct from the other Greenwater Assemblage lithologies, the Burchell Assemblage and the Haines Gabbroic Complex. The Haines Gabbroic Complex crystallised from hydrous, Ca- and Al-rich tholeiitic parental magmas that fractionated from more primitive, hydrous boninitic parental magmas.The initial εNd values of the Haines Gabbroic Complex (εNd = +3.0 to +4.1) and the Greenwater (εNd = +1.8 to +4.1) and Burchell (εNd = +2.0 to +3.7) assemblages are indicative of variably depleted mantle sources. The major and trace element characteristics of the Haines Gabbroic Complex and the Greenwater and Burchell assemblages and geological relationships indicate that they formed in a Japan-style mature intra-oceanic continental arc with a large volume of TTGs. Intra-oceanic arc rifting was most likely triggered by slab rollback, followed by mantle plume emplacement. On the basis of rock association and geochemical data, we suggest that the Haines Gabbroic Complex and the Greenwater and Burchell assemblages represent a dismembered subduction-related Archaean ophiolite that marks a Neoarchaean suture zone between the Wawa and Wabigoon subprovinces.

Mineral composition and rock provenance of a prehistoric stone axe head from the surroundings of Nevestino village, Kyustendil District, SW Bulgaria

This paper presents a petrographical study of a stone axe head which is an exhibit of the Historical Museum in the town of Kyustendil, SW Bulgaria, and is of supposed Bronze Age origin. The methods used comprise optical and electron microscopy, and electron microprobe analysis. The results show that the stone axe head is made of microgabbro rock. This rock in mineral composition, mineral chemistry, and metamorphic alteration is similar to gabbro rocks of the Struma Diorite Formation, which crops out on large areas in SW Bulgaria. Thus, it is proposed that the rock provenance of the studied stone tool is the Vlahina Planina mountain or Osogovo Planina one since the nearest outcrops of the Struma Diorite Formation there are in a distance of ~ 4–5 km away from the place of finding. Use of local rock for production of an every-day tool reveals good knowledge of local natural materials by prehistoric people and suggests a settled way of life.

Metamorphic evolution of sulphide-rich chromitites from the Chernichevo ultramafic massif, SE Bulgaria

The upper mantle rocks of the metamorphosed ophiolite of Chernichevo, Rhodope Metamorphic Complex in southern Bulgaria, host small chromite ores with unusual mineralization of base-metal sulphides rich in platinum-group elements. Mineralogical and chemical data indicate that after their formation in the mantle the Chernichevo’s chromite ores were modified by the intrusion of an alkaline mafic melt, which resulted in the precipitation of a suite of metasomatic minerals (sulphides, calcite, apatite and ilmenite), accompanied by an increase in FeO, TiO2, Ga, Zn, Vn, Mn, and especially Ti and Fe2O3 contents in the chromite. The degree of chemical modification and abundance of metasomatic minerals are positively correlated and mark the extent of reaction of the chromitite with the intruding melt. Sulphide segregation promoted the concentration of high amounts of PGEs (up to 3661 ppb), particularly Pt and Pd, yielding chromite ores with a typical flat to positive-sloped chondrite-normalized pattern. Subsequently, the chromite ores were deformed and metamorphosed together with their host rocks at ultra-high pressure (UHP) (>2.5 GPa, >1200 °C) to be later retrograded under eclogite and finally hydrous amphibolite-facies conditions, giving rise to three microstructural types. Metamorphism of the most metasomatized (i.e., sulphide-rich) chromitites at temperatures >700 °C within the conditions of UHP and eclogite-facies resulted in the formation of (1) non-porous recrystallized chromite, consisting of a granoblastic microstructure made-up of coarse-grained blasts and finer-grained chromite neoblasts. In contrast, hydrous metamorphism on the less metasomatized (i.e., sulphide-poor) chromitite under the conditions of amphibolite-facies (ca. 482–483 °C) resulted in the formation of (2) partly altered chromite, characterized by unaltered cores surrounded by Fe2+-rich and Al-depleted porous chromite containing abundant clinochlore, and (3) porous chromite corresponding to a chromite that was entirely transformed by the metamorphic alteration to Fe2+-rich and Al-depleted porous chromite. During metamorphism magmatic Ni-Fe-Cu sulphides originally formed during the metasomatic event in the mantle were altered, resulting in a major leaching of Cu-rich sulphides, leading to significant remobilization of Pt and Pd.

GEOLOGICAL POSITION AND AGE OF TUXEN-RASMUSSEN LAYERED GABBROID INTRUSION (WEST ANTARCTICA)

The latest geological research carried out during seasonal work in 2017 revealed that metamorphosed layered gabbroids, which partially outcrop along the Antarctic Peninsula Coast on Cape Tuxen and Rasmussen Island, compose sheet-like intrusive body of total dimensions more than 3 km2. Significant part of the Tuxen-Rasmussen Gabbroid Intrusion (TRGI) is submerged under Waddington Bay. Main objectives of the study were the identification of geological position of the TRGI and the clarification of its geological age. In addition to the field methods of the geological research the samples collected during this and the previous years were studied using optical and electron microscopy techniques in order to identify petrographic and mineralogical features of the gabbroids. Results of the studies confirmed previous observation that TRGI was embedded in Upper Jurassic Volcanic Group (UJVG) of Antarctic Peninsula and it was responsible for contact metamorphism of the UJVG and suffered contamination by volcanic material. Gabbroids and volcanic rocks were later intruded by granites of Late Cretaceous age, which belong to the Andean Intrusive Suite (AIS) of Graham Land. Authors define geological age of TRGI as Early Cretaceous. Previous U-Pb isotope datings of the gabbroids are believed to be “rejuvenated”. It was discovered that TRGI strikes in northeastern direction and dips steep in northwestern direction. It is assumed that intrusive body continues underwater and can outcrop at the northern shore of the Waddington Bay and at Barros Rocks direction to the southwest. Petrographical researches showed that gabbroids underwent metamorphic alteration in conditions of epidote-amphibolite facies. They bear, nevertheless, relict structural-textural features and mineral associations of mafic igneous rocks. Authors drew a conclusion that geological position and petrographical peculiarities of TRGI correspond to hypabyssal level of crystallization. Primary igneous origin is proved for the microrhythmic layering of the gabbroids. Relict mineral associations allow to identify the rocks as olivine gabbro-norites. Potential Fe-Ti-V ore specialization is emphasized according to the revealed patterns of crystal fractionation and accumulation of ilmenite and magnetite.

COMPLEX GARNET ORE CONTROL FACTORS FOR THE VYSOTA-181 DEPOSIT

The need for this study arises from the globally growing demand for integrated assessment of mineral resources, particularly in the cases of co-occurrence of minerals with similar or identical processing characteristics. To ensure reliable and full ore body mapping and discrimination of ore types, especially in areas with intensive metamorphic alteration, it is essential to take into account all ore control factors. The main controls of the complex garnet ore formation in the Vysota-181 deposit were metamorphic processes and metasomatism of acid and base facies. The industrial minerals in the complex ore are garnet, kyanite, staurolite, muscovite. Ore control factors for the Vysota-181 deposit can be arranged into three groups: lithological, petrological, and structural. Of primary importance for ore formation are the protolith composition and metasomatism of acid and base facies. Geometallurgical mapping of the deposit reveals three types of ores: kyanite-staurolite-, muscovite-, and garnet-type. The most characteristic control factors for these ore types were identified in this study. The results of the study can be used to as sess ore occurrences and industrial mineral deposits associated with regional Proterozoic metamorphism and metasomatism in the Fennoscandian shield.

Natural Cr3+-rich ettringite: occurrence, properties, and crystal structure

Cr3+-rich ettringite with Cr3+→Al substitution and Cr/(Cr + Al) ratios up to 0.40–0.50 was found in mineral assemblages of the Ma’aleh Adumim area of Mottled Zone (Judean Desert). The Cr3+-rich compositions were the latest in the thaumasite → ettringite–thaumasite solid solution → ettringite → ettringite–bentorite solid solution series. The mineral-forming solution was enriched in Cr3+ and had a pH buffered by afwillite at ~11–12. Chromium was inherited from larnite rocks produced by high-temperature combustion metamorphic alteration of bioproductive calcareous sediments. The Cr/(Cr + Al) ratios are within 0.10–0.15 in most of the analysed crystals. This degree of substitution imparts pink colouration to the crystals, but does not affect their habit (a combination of monohedra and a prism). The habit changes to pyramid faces in coarse and later Cr3+-bearing crystals as Cr/(Cr + Al) ratios increase abruptly to 0.40–0.50. Single-crystal XRD analysis of one Cr-free and two Cr3+-rich samples and their structure determination and refinement indicate that the Cr-rich crystals (with Cr/(Cr + Al) to 0.3) preserve the symmetry and metrics of ettringite. The Ca–O bonding network undergoes differentiation with increase of Cr3+ concentration at octahedral M sites. The compression of Ca2 and expansion of Ca1 polyhedra sub-networks correlates with the degree of Cr3+→Al substitution.

Differential metamorphic effects on nitrogen isotopes in kerogen extracts and bulk rocks

The last decade has seen a steady rise in the number of publications on nitrogen isotopes in sedimentary rocks, which have become an established tool for investigating the evolution of life and environmental conditions. Nitrogen is contained in sedimentary rocks in two different phases: bound to kerogen or substituted in potassic minerals (mostly K-bearing phyllosilicates and feldspars). Isotopic measurements and interpretations typically focus either on kerogen extracts alone or on bulk rocks that include both phases. The community is split about which sample type more accurately captures the original composition of biomass. To address this question, we combined nitrogen isotopes and carbon-to-nitrogen ratios with carbon-to-hydrogen ratios, which act as an independent proxy for metamorphic alteration. Our results reveal that metamorphism drives kerogen-bound nitrogen isotopically lighter while silicate-bound nitrogen becomes heavier. For rocks up to greenschist facies, the isotopic effect of this internal partitioning (up to 3–4‰) is larger than the isotopic effect of metamorphic nitrogen loss from the system (up to 1–2‰). The opposite may be true for higher metamorphic grades. We conclude that for low-grade sedimentary rocks with more than 60% of their total nitrogen residing in the silicate phase the primary isotopic composition of the biomass is best approximated by bulk rock measurements, whereas for high-grade rocks the kerogen extract may be the more accurate proxy. The isotopic difference between nitrogen phases can serve as a rough indicator of the degree of metamorphic alteration.

Petrogenetic and geodynamic origin of the Neoarchean Doré Lake Complex, Abitibi subprovince, Superior Province, Canada

The Neoarchean (ca. 2728 Ma) anorthosite-bearing Dore Lake Complex in the northeastern Abitibi subprovince, Quebec, was emplaced into an association of intra-oceanic tholeiitic basalts and gabbros known as the Obatogamau Formation. The Obatogamau Formation constitutes the lower part of the Roy Group, which is composed of two cycles of tholeiitic-to-calc-alkaline volcanic and volcaniclastic rocks, siliciclastic and chemical sedimentary rocks, and layered mafic-to-ultramafic sills. In this study, we report major and trace element results, and Nd, Sr, Pb and O isotope data for anorthosites, leucogabbros, gabbros and mafic dykes from the Dore Lake Complex and spatially associated basalts and gabbros of the Obatogamau Formation to assess their petrogenetic origin and geodynamic setting. Field and petrographic observations indicate that the Dore Lake Complex and associated volcanic rocks underwent extensive metamorphic alteration under greenschist facies conditions, resulting in widespread epidotization (20–40%) and chloritization (10–40%) of many rock types. Plagioclase recrystallized mainly to anorthite and albite endmembers, erasing intermediate compositions. Metamorphic alteration also led to the mobilization of many elements (e.g., LILE and transition metals) and to significant disturbance of the Rb–Sr and U–Pb isotope systems, resulting in 1935 ± 150 and 3326 ± 270 Ma errorchron ages, respectively. The Sm–Nd isotope system was less disturbed, yielding an errorchron age of 2624 ± 160 Ma. On many binary major and trace element diagrams, the least altered anorthosites and leucogabbros, and the gabbros and mafic dykes of the Dore Lake Complex plot in separate fields, signifying the presence of two distinct magma types in the complex. The gabbros and mafic dykes in the Dore Lake Complex share the geochemical characteristics of tholeiitic basalts and gabbros in the Obatogamau Formation, suggesting a possible genetic link between the two rock associations. Initial eNd (+2.6 to +5.0) and δ18O (+6.1 to +7.9‰) values for the Dore Lake Complex and gabbros of the Obatogamau Formation (eNd = +2.8 to +4.0; δ18O = +7.3 to 8.0‰) are consistent with depleted mantle sources. All rock types in the Dore Lake Complex and the Roy Group share the trace element characteristics of modern arc magmas, suggesting a suprasubduction zone setting for these two lithological associations. On the basis of regional geology and geochemical data, we suggest that the Dore Lake Complex and the Obatogamau Formation represent a dismembered fragment of a suture zone, like many Phanerozoic ophiolites, resulting from closure of a back-arc basin between 2703 and 2690 Ma.

The use of chromium reduction in the analysis of organic carbon and inorganic sulfur isotope compositions in Archean rocks

One of the most serious issues with deciphering the evolution of organisms and their biogeochemical environments from the ancient rock record is the difficulty in obtaining well-preserved samples. Although not much can be done to avoid diagenetic and metamorphic alteration when they have occurred, alteration due to weathering can be avoided by working on drill core samples. This implies however that the amount of sample is limited, which may in turn restrain the number of possible chemical and isotopic analyses that can be performed. In order to save sample we show here that the chemical protocol used for the sulfur sulfide extraction (for later sulfur isotope analyses) is also suitable to decarbonate samples (for later organic carbon isotope analyses). In the case of carbonated rocks, both sulfur sulfide extraction and decarbonation require high amounts of sample so that coupling them may save a significant amount of sample and time. In addition it allows both organic carbon (TOC and δ13C) and sulfur isotope composition measurements to be performed on the exact same powder, which is essential when trying to understand couplings between S and C cycles in heterogeneous samples. We thus tested the efficiency of the acidic chromium solution, commonly used to extract sulfur from sulfide, for sample decarbonation on various Archean rocks. Our results show that no significant carbon isotope fractionation is caused by this new decarbonation protocol, even for the samples with low organic carbon content. The chromium solution seems to be perfectly adapted for the analysis of organic matter in the ancient rock record, at least when the rock samples have experienced low greenschist facies metamorphism. Further tests will be needed to verify if this protocol can also be used for less mature organic matter.