Metamorphic Grade Research Articles

Complex sulfur speciation in scapolite – Implications for the role of scapolite as a redox and fluid chemistry buffer in crustal fluids

Scapolite is a common S-, Cl-, and C-bearing mineral in metamorphic terranes, especially those that include meta-evaporites. Fluid interaction with scapolite-bearing rocks can result in albitisation of scapolite, and release of volatile components such as Cl−, CO32–, SO2 and/or SO42−. Hence, scapolite may play an important role in buffering the oxidation state, salinity and sulfur speciation of the rock and fluids in such terranes, and since Cl, S, and the redox state of S are vital for the transport and deposition of metals, scapolite may exert an important influence on hydrothermal mineralisation processes. We investigated the oxidation state of S and concentrations of S and Cl from 17 scapolite-bearing samples collected from various metamorphic and hydrothermal environments. The highest concentrations of S and Cl in the samples set are respectively 1.11 ± 0.04 wt% S (reported as SO3,n = 10 points measurements) and 3.84 ± 0.17 wt% (n = 11 points measurements). µ-XANES spectroscopy demonstrates that S chemistry in scapolite is complex, S being present in both oxidised (as sulfate and sulfite) and reduced forms (polysulfides and minor sulfide) in most scapolites. These results confirm that scapolite can buffer fluid oxidation state to sulfate/sulfide coexistence, at acidic to neutral pH. The proportion of reduced S in scapolite increases with increasing metamorphic grade from greenschist to amphibolite facies. Since scapolite contains highly reactive polysulfide and sulfite, its breakdown may promote alteration of sulfide minerals and release of hosted metals. Thus, the presence of scapolitic calc-silicate rocks can strongly influence regional syn-orogenic metallogeny and should allow the mobilisation of metals differently from other bulk compositions.

Calcium–aluminum‐rich inclusions in non‐carbonaceous chondrites: Abundances, sizes, and mineralogy

AbstractAs the Sun was forming, calcium–aluminum‐rich inclusions (CAIs) were the first rocks to have condensed in the hottest regions of the solar nebula disk. Carbonaceous chondrites (CCs) contain abundant CAIs but are thought to have accreted in the outer Solar System, requiring that CAIs must have been transported outward. Curiously, CAIs are rare in ordinary, enstatite, rumuruti, and kakangari chondrites, non‐carbonaceous chondrites (NCs), that likely formed in the inner Solar System. Thus, CAI abundances and characteristics can provide constraints on the early dynamical evolution of the disk. In this work, we address whether the hypothesis of an early‐formed proto‐Jupiter “opening a gap” in the disk can explain the dichotomy in the relative abundance of CAIs in CC and NC chondrites. We searched 76 NC meteorite sections to find 232 CAIs which have an average apparent diameter of 46 μm and comprise 0.01 area%, about half the size of and ~200 times less abundant than CC CAIs on average. Unlike CC CAIs, only 4% of the NC CAIs contain melilite and most contain alteration features suggesting that NC CAIs underwent pervasive fluid‐assisted thermal metamorphism on asteroidal parent bodies. However, based on NC CAI populations correlating with meteorite metamorphic grade, we argue that disk dynamics is likely the primary reason behind the existence of small (<100 μm) and rare NC CAIs. Our data support astrophysical models which suggest that, after outward transport of CAIs, formation of a gap in the disk trapped CAIs in the outer Solar System.

Evidence for abundant organic matter in a Neoarchean banded iron formation

Abstract Microbial Fe(II) oxidation has been proposed as a major source of Fe minerals during deposition of banded iron formations (BIFs) in the Archean and Proterozoic Eons. The conspicuous absence of organic matter or graphitic carbon from BIFs, however, has given rise to divergent views on the importance of such a biologically mediated iron cycle. Here, we present mineral associations, major element concentrations, total carbon contents and carbon isotope compositions for a set of lower amphibolite-facies BIF samples from the Neoarchean Zhalanzhangzi BIF in the Qinglonghe supracrustal sequence, Eastern Hebei, China. Graphite grains with crystallization temperatures (~470 °C) that are comparable to that predicted for the regional metamorphic grade are widely distributed, despite highly variable iron (12.9 to 54.0 wt%) and total organic carbon (0.19 to 1.10 wt%) contents. The crystalline graphite is interpreted to represent the metamorphosed product of syngenetic bio-mass, based on its co-occurrence with apatite rosettes and negative bulk rock δ13Corganic values (–23.8 to –15.4‰). Moreover, the crystalline graphite is unevenly distributed between iron- and silica-rich bands. In the iron-rich bands, abundant graphite relicts are closely associated with magnetite and/or are preserved within carbonate minerals (i.e., siderite, ankerite, and calcite) with highly negative bulk rock δ13Ccarb values (–16.73 to –6.33‰), indicating incomplete reduction of primary ferric (oxyhydr) oxides by organic matter. By comparison, only minor graphite grains are observed in the silica-rich bands. Normally, these grains are preserved within quartz or silicate minerals and thus did not undergo oxidation by Fe(III). In addition, the close association of graphite with iron-bearing phases indicates that ferric (oxyhydr)oxides may have exerted a first order control on the abundance of organic matter. Combined, the biological oxidation of Fe(II) in the oceanic photic zone and subsequent burial of ferric (oxyhydr)oxides and biomass in sediments to form BIFs, suggests that a BIF-dependent carbon cycle was important in the Archean Eon. Although significant re-adsorption of phosphorus to ferric (oxyhydr)oxides and the formation of authigenic phosphate minerals at the sediment-water interface would be expected, oxidation of biomass in BIFs may have recycled at least a portion of the P (and other nutrients) released from reactions between organic matter and ferric (oxyhydr)oxides to the overlying water column, potentially promoting further primary productivity.

Sulfide anatexis during high-grade metamorphism: a case study from the Bodenmais SEDEX deposit, Germany

Many sedimentary-exhalative (SEDEX) sulfide deposits have been subject to regional metamorphism and, if the metamorphic grade was high enough, this could have resulted in sulfide anatexis. Although experiments and textures indeed showed that some deposits were partially molten, there is an ongoing debate as to the extent to which metamorphosed ore deposits were molten. Since some SEDEX deposits underwent amphibolite to granulite facies metamorphism, not only sulfides but also the host silicate rocks should have reached anatectic conditions. Due to the two immiscible silicate and sulfide melts, the formation of typical mingling and emulsion textures, as already known from magmatic sulfide deposits, should form. To test this hypothesis, we investigate sulfide-silicate textures from the granulite-facies Bodenmais SEDEX deposit (Germany). Textures from Bodenmais are similar to magmatic sulfide deposits including sulfide-matrix breccia, emulsion textures, pegmatitic leucosomes, and massive sulfides overlain by net-textured intergrowths of refractory quartz, which is interpreted to be a relic of silicate anatexis. Minerals crystallized during the interaction of both immiscible melts differ in their chemistry compared to the same minerals found in the adjacent migmatitic host rocks: for example, garnet in sulfides is Mn-rich (spessartine), but Fe-rich (almandine) in the migmatites and sulfide-enclosed cordierite is more enriched in Mg (Mg/(Mg + Fe): 0.84) than migmatitic cordierite (Mg/(Mg + Fe): 0.54). The textures themselves, their spatial arrangement within the deposit, the differences in mineral chemistry, and the observed crystallization sequence provide unequivocal evidence that the sulfides at Bodenmais were molten to a large extent under granulite facies conditions.

Formation pathways of Precambrian sedimentary pyrite: Insights from in situ Fe isotopes

Iron isotope compositions (expressed as δ56Fe) in sedimentary pyrite have been widely used as tracers of redox and chemical evolution of the ocean through geological time. Previous studies mostly built on the mechanical extraction of sulfides from bulk rock samples, and focused on visible macroscopic pyrites, which may introduce a sampling bias. In situ analyses of micropyrite grains can provide new insights into the processes of pyrite formation and their time evolution. Here, we compile ca. 2000 in situ iron isotope compositions of Archean to Paleoproterozoic sedimentary pyrite, from previous literature as well as new data. Contrasting with bulk analyses, micropyrite displays a large and constant range of δ56Fe values, from -4 to +4‰, through time. Micropyrite δ56Fe values are not significantly influenced by metamorphic grade. A bimodal distribution of positive versus negative δ56Fe values can be attributed to two different processes of pyrite formation, Fe (oxyhydr)oxide sulfidation, versus kinetic and possibly microbially mediated pyrite precipitation. These processes are tightly related to rock lithology and thus to sedimentary conditions, and have existed since 3.8 Ga.

Analogue modelling of a Tabberabberan fold-thrust belt in the Cobar region and implications for the origin of Cobar-type mineral deposits

The Cobar Supergroup is a package of Siluro-Devonian sedimentary, volcanic and volcaniclastic rocks in central New South Wales that underwent significant deformation during the Middle Devonian Tabberabberan Orogeny. The Cobar Supergroup, particularly within the Cobar Basin, constitutes one of the most mineral-rich rock packages in New South Wales, being endowed with Au, Cu, Pb, Zn and Ag, mainly in what are commonly referred to as Cobar-type deposits. Inconsistencies in the traditional stratigraphic interpretation of the area have led to a re-evaluation of its structural history, in particular the role of the Rookery Fault, which bounds the eastern side of the basin. Evidence suggests that that structure may dip to the east, rather than the west, as is traditionally assumed, leading to the possibility that Tabberabberan deformation in the area took the form of a west-verging fold-thrust belt. Analogue modelling is employed to examine the consequences of that scenario. Such a mechanism explains the known distribution of stratigraphic units of the Cobar Supergroup and its basement, as well as observed structures, in particular a high strain zone immediately west of the Rookery Fault. The modelling indicates that deeper parts of the crust were thrust up along the Rookery Fault, which, along with substantial burial, led to perturbation of the geotherm and heating of the footwall. This accounts for the higher metamorphic grade immediately west of the Rookery Fault. Furthermore, heating and overpressuring of basement rocks in the footwall led to mobilisation of metal-bearing fluids and transport of them along the high strain zone and subsequent deposition of those metals within favourable structural traps, with variable interaction with basinal fluids, to form Cobar-type deposits. A fold-thrust mechanism with ongoing erosion of the thrust wedge can also explain the distribution of Middle to Upper Devonian rock sequences in the region. KEY POINTS A west-verging fold-thrust model is proposed to explain Tabberabberan deformation of the Cobar Supergroup and its basement. Analogue modelling is employed to demonstrate the fold-thrust model. A fold-thrust model explains the distribution of Ordovician to Upper Devonian stratigraphic units in the Cobar region, as well as associated structures, metamorphic grade distribution and the origin of syn-deformational Au, Cu, Pb, Zn, Ag mineral deposits.

Heterogeneity of the Noachian Crust of Mars Using CRISM Multispectral Mapping Data

AbstractWe used Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) multispectral mapping data to assess heterogeneity of primary and secondary mineral compositions of Noachian‐aged regions of Martian crust. Multispectral data corroborate interpretations from CRISM targeted observations and Observatoire pour la Mineralogie, l’Eau, les Glaces et l’Activité global mapping of large horizontal differences in vertical crustal structures, and degree and grade of alteration to secondary minerals. CRISM multispectral data analysis conducted at a combination of high spatial resolution and coverage not available in other data sets also reveals previously unrecognized exposures. At one extreme, basaltic crustal material is minimally altered, mostly to smectite clay; at the other, Al‐enriched alteration products are present, alteration is widespread, and superposed surface materials rich in salts and precipitates, suggesting multiple episodes of alteration. The revealed vertical structure of primary mineralogy is consistent with that inferred in previous studies. Controls on the extent and metamorphic grade of secondary mineral assemblages are proximity to heat sources including large impact basins and inferred magmatic bodies.

Multi-stage metallogeny in the southwestern part of South China, and paleotectonic and climatic implications: A high precision geochronologic study

The South China Block (SCB) is among the large-scale W-Sn mineralized regions of the globe. The Laojunshan W-Sn-dominant ore area (LOA) in the western part of the SCB preserves the records of the tectonic history of the Tethys realm extending through North Vietnam, and Yangtze to Cathaysia blocks, with coeval formation of giant metallic deposits. The prolonged tectonic activities and their control on the genesis and spatio-temporal distribution of giant metallic deposits in the LOA provide a window for a holistic understanding of the tectono-metallogenesis of the SCB. In this study, we present results from a multi-chronologic study to determine the timing of formation of the cassiterite-wolframite-scheelite mineralization. The results suggest three distinct tectono-metallogenic periods in the LOA during the geodynamic evolution of the surrounding tectonic units. The opening of the Proto-Tethys Ocean between the Yangtze-Indochina blocks and the westward Paleo-Pacific subduction beneath the Cathaysia block (420–380 Ma) jointly contributed to the Silurian to early Devonian intracontinental orogeny in the middle of the SCB that involved top-to-the-north thrusting along NE-striking shear zones. This event generated the Dulong-Song Chay granitoids, together with the formation of Xinzhai Sn deposit related to sheared mylonitic granites (ca. 419 Ma) and pegmatites (ca. 389 Ma), which include the early-stage Sn-sulfide skarn (ca. 418 Ma) and the late-stage Sn-bearing schist (ca. 389 Ma). During the Late Permian to Late Triassic (260–220 Ma), with the closure of the Proto-Tethys oceans in the west and ongoing Paleo-Pacific westward subduction in the east, the SCB and Indochina Block (IB) were amalgamated which also marks the time of formation of the Nanwenhe scheelite skarn deposit. The subducted Paleo-Tethys oceanic crust was likely entrained by the nearby rising Emeishan mantle plume (270–259 Ma), which formed the Maguan diabase (ca. 260 Ma) that shows significantly older Re-Os model age of ca. 268 Ma, suggesting that the Nanwenhe mineralization is potentially derived from ca. 260 Ma source. Furthermore, the intraplate shortening induced thin skinned crustal deformation and low grade metamorphism (ca. 230 Ma), with the main stage of scheelite-Sn-Mo mineralization (229.9, 229.8 and 219 Ma) and contemporary formation of the pegmatite (230.7 Ma). The Late Cretaceous involved two episodes of alternate extension and shortening, driven by the subduction polarity change from northwestward subduction of the Okhotomorsk block to northward subduction of the NeoTethys seafloor. The evolution of the LOA consists of the NW–SE transpression ending ca. 100 Ma, the WNW–ESE extension in the earlier episode lasting from 100 Ma to 86 Ma, the WNW–ESE transpression beginning at ca. 85 Ma and the NS extension in the later episode during the latest Cretaceous, which produced the extension-related three periods of Laojunshan granitic magmatism and coeval Sn-W mineralization, with ages in the range of 90–89 Ma, 87–85 Ma and 83–79 Ma. We also evaluate the implications of magmatic-metamorphic-metallogenic degassing on the regional paleoclimatic history.

Rubidium and potassium isotopic variations in chondrites and Mars: Accretion signatures and planetary overprints

As moderately volatile elements, isotopes of Rb and K can trace volatilization processes in planetary bodies. Rubidium isotopic data are however very scarce, especially for non-carbonaceous meteorites. Here, we report combined Rb and K isotopic data (δ87/85Rb and δ41/39Κ) for 7 ordinary, 6 enstatite, and 4 Martian meteorite falls to understand the causes for the variations in volatile abundances and isotopic compositions. Bulk Rb and K isotopic compositions of planetary bodies are estimated to be (Table 1): Mars +0.10 ± 0.03 ‰ for Rb and −0.26 ± 0.05 ‰ for K, bulk OCs -0.12-0.24+0.15 ‰ for Rb and -0.72-0.41+0.28 ‰ for K, bulk ECs +0.02-0.26+0.29 ‰ for Rb and -0.33-0.23+0.37 ‰ for K. The bulk K isotopic compositions of subgroup OCs are estimated to be -0.72-0.55+0.26 ‰ for H chondrites, -0.71-0.39+0.23 ‰ for L chondrites, and -0.77-0.30+0.63 ‰ for LL chondrites. A broad correlation between the Rb and K isotopic compositions of planetary bodies is observed. The correlation follows a slope that is consistent with kinetic evaporation and condensation processes, suggesting volatility-controlled mass-dependent isotope fractionation (as opposed to nucleosynthetic anomalies).Individual ordinary and enstatite chondrites show large Rb and K isotopic variations (−1.02 to +0.29 ‰ for Rb and −0.91 to −0.15 ‰ for K). Samples of lower metamorphic grades display correlated elemental and isotopic fractionations between Rb and K, while samples of higher metamorphic grades show great scatter, suggesting that chondrite parent-body processes have decoupled the two elements and their isotopes at the sample scale. Several processes could have contributed to the observed isotopic variations of Rb and K, including (i) chondrule “nugget effect”, (ii) volatilization during parent-body thermal metamorphism (heat-induced vaporization and gas transport within parent bodies), (iii) thermal diffusion during parent-body metamorphism, and (iv) impact/shock heating. Quantitative modeling of the first two processes suggests that neither of them could produce isotopic variations large enough to explain the observed isotopic variations. Volatilization during parent-body thermal metamorphism [the scenario (ii)], which has been commonly invoked to explain the isotopic variations of volatile elements, is gas transport-limited and its effect on isotopic fractionations of moderately volatile elements should be negligible. Modeling of diffusion processes suggests that (iii) could produce K isotopic variation comparable to the observed variation. The large isotopic variations in non-carbonaceous meteorites are thus most likely due to diffusive redistribution of K and Rb during metamorphism and/or shock-induced heating and vaporization.

Geology, Structure, Metamorphism and Deformation of Botechaur-Chaurjhari Area of Western Nepal

The Lesser Himalayan unit of the Nepal Himalaya, especially the western and central parts comprise numerous metamorphic crystalline thrust sheets occupying the cores of large synclinorium. Despite having complex geological settings, some areas in western parts of Nepal lack detailed geological mapping with credible information. This study aims to explore one least explored section; with emphasis on lithostratigraphy, structural setting, deformation, metamorphism, and magmatism. Detailed geological mapping along the Botechaur-Chaurjhari section revealed three tectonostratigraphic units separated by the Main Boundary Thrust (MBT) and the Mahabharat Thrust (MT); the Siwaliks (Middle Siwalik), autochthonous units of the Lesser Himalaya (Dandagaon Phyllite, Nourpul Formation and Dhading Dolomite) and the Lesser Himalayan Crystalline units (Kalagaon Formation and Chaurjhari Formation) from south to north respectively. From the microstructural analysis and measurements of shear sense indicators, the MT shows a top-to-the-south sense of shear indicated by asymmetric boudins, parasitic folds, and thrusting indications in blasto-mylonitic augen gneiss and asymmetric porphyroblast of garnet and the shearing in the garnet. The area also displays an inversion of metamorphism, supporting the development and existence of thrust. The metamorphic grade ranges from chlorite grade to kyanite grade having metamorphic facies assemblage of zeolite to amphibolite facies. Five phases of deformation (D1 to D5) are prevalent within the area, two of which (D1 and D2) belong to the pre-Himalayan (pre-Tertiary) orogeny and the rest of the deformation syn-to post-Himalayan.

Exceptional preservation of organic matter and iron-organic colloidal mineralization in hydrothermal black smoker-type sulfide mineralization from the Paleoarchean seafloor

Fossil organic matter (OM) in Paleoarchean rocks is an invaluable tracer of ancient life, yet it is often contentious due to low preservation potentials of its original organic molecular characteristics under generally high metamorphic grades. This study reports on exceptionally preserved OM within black smoker-type sulfide mineralizations from the 3.24 billion-years-old Sulphur Springs volcanic-hosted massive sulfide deposit, East Pilbara Terrane, northwestern Australia. Fine scale mineralogical and organic molecular variations – documented by SEM and TEM techniques, Raman and FTIR Spectroscopy, as well as ToF-SIMS analysis – trace the formation of millimetre-scale pyritic hydrothermal orifices. Overmature OM (≳200–250 °C) enriched in aromatic hydrocarbons occurs within earliest formed colloform-banded pyrite that could have precipitated at high temperature upon mixing of hot hydrothermal fluids with cold seawater. In contrast, mature OM (within the catagenesis window; ∼100–150 °C) with lower proportions of aromatic hydrocarbons occasionally occurs alongside pyrite within barite near the inner paleofluid conduits of the hydrothermal orifices. A deposition of this OM generation from cooler fluids enriched in sulfate during waning hydrothermal activity is corroborated by its association with original mineralogy, particularly the occurrence of isolated nanoinclusions of OM within hydrothermal barite away from grain boundaries or fissures and cracks, which renders impossible an origin from post-depositional hydrocarbon migration. When compared to the overmature OM within colloform-banded pyrite, the better-preserved inner OM not only contains less abundant, smaller, and less ordered aromatic hydrocarbons, but also higher amounts of aliphatic molecules with longer chain lengths and higher proportions of carbonyl and amide functional groups. Although the latter characteristics are consistent with contributions by microbial biomass, abiotic origins are equally plausible because hydrothermal processes can produce OM with similar organic molecular attributes. Irrespective of these uncertainties on the ultimate sourcing of OM, common intergrowths of the cooler OM with nanoscopic iron oxides – hematite and lesser magnetite – hint at its hydrothermal deposition from colloidal particles that have been stabilized by iron-organic complexation. Further research is required to ascertain the significance of this process for the hydrothermal cycling and release of organic carbons and bound iron into the Paleoarchean oceans. Collectively, our results show that ancient marine hydrothermal systems can preserve OM with differing degrees of thermal degradation, which allows for insights into its sourcing, cycling, and deposition.

Early Mylonitization in the Nevado-Filábride Complex (Betic Cordillera) during the High-Pressure Episode: Petrological, Geochemical and Thermobarometric Data

In the western part of the Sierra de los Filabres area, there are fine-grained metamorphic rock bands, showing a field aspect simitar to slates, as previously described in the geological literature of the studied region. They are variable in thickness, from millimeters to tens of meters and appear intercalated in the schist succession. The geochemical resemblance between both types of rocks (major, minor and trace elements), determined by a statistical approach and the comparison of depositional condition indices, points to a similar sedimentary origin of the protolith but different clay content. Mineral facies and illite “crystallinity” indices in the so-called slates indicate that they followed the same metamorphic path and reached the same metamorphic grade than schists. According to compositional zoning detected in micas and garnets present in both lithologies and the P-T conditions deduced from garnets, the mineral nucleation and growth episode of the main mineral paragenesis in these fine-grained schists was more remarkable during the high-pressure event, with a no significant effect of the latter low-pressure–high-temperature episode. In contrast, the coarse-grained schists developed higher size minerals during the low-pressure–high-temperature episode. A differential mylonitization process during the metamorphism is proposed to justify the discrepant field appearance and the contrasting response of both types of metapelitic rocks to the latter metamorphic event.

Juxtaposition of different-grade metamorphic rocks in an ancient orogen: Evidence from the Chengde Complex of the Trans-North China Orogen, North China Craton

Juxtaposition of different-grade metamorphic slices is a typical feature of Phanerozoic orogens but is relatively scarce in Precambrian orogens. Here, we focus on the Chengde Complex at the northern segment of the late Paleoproterozoic Trans-North China Orogen, the North China Craton as a case, to explore how different-grade orogeny-related metamorphism was juxtaposed in Precambrian orogens. The Trans-North China Orogen is a typical Precambrian collisional orogen and records abundant information about the Paleoproterozoic orogeny. High-pressure (HP) mafic granulite and two types of amphibolite samples were collected from the Chengde Complex, and we conducted a combined study involving litho-structural assemblage investigation, metamorphic petrology, and geochronology to decipher their metamorphic P-T−t history. HP mafic granulite and amphibolite occur as enclaves or dikes within felsic gneisses. HP mafic granulite records clockwise P-T paths with isothermal decompression (ITD) segments, and the peak metamorphic P-T conditions are 13.0−14.9 kbar/790−830 °C. Peak metamorphic P-T conditions retrieved from garnet-bearing amphibolite are 8.3 kbar/675 °C, whereas peak metamorphic P-T conditions retrieved from garnet-free amphibolite are 4.0−5.5 kbar/500−510 °C. Zircon U-Pb dating of HP granulite and amphibolite yield different prograde (1914−1871 Ma), peak (1869−1816 Ma), and retrograde (ca. 1830 Ma) metamorphic ages, and all of these data indicate that these metamorphic rocks were diachronously transferred to different depths in the subduction channel and subsequently exhumed at shallower crustal levels. Therefore, we conclude that the Chengde Complex is composed of imbricate rocks with different metamorphic grades and ages, and such an unordered juxtaposition of diverse metamorphism could also be a typical feature of Precambrian orogens.

Discrimination of Clinozoisite–Epidote Series by Raman Spectroscopy: An application to Bengal Fan Turbidites (IODP Expedition 354)

Epidote group minerals are one of the three most abundant kinds of heavy minerals in orogenic sediments, the other two being amphibole and garnet. They resist diagenesis better than amphibole and resist weathering in soils better than garnet. Their chemical composition and optical properties vary markedly and systematically with temperature and pressure conditions during growth. Useful information on the metamorphic grade of source rocks can thus be obtained by provenance analysis. In this study, we combine optical, SEM–EDS, and Raman analyses of nine standard crystals of epidote group minerals collected from different rock units exposed in the European Alps and Apennines and develop a Raman library for efficient discrimination of epidote, clinozoisite, zoisite, and allanite by establishing clear user-oriented relationships among optical properties, chemical composition, and Raman fingerprint. This new library allows us to distinguish and reliably determine, directly from their Raman spectrum, the chemical compositions of epidote group minerals during routine heavy mineral analyses of sand/sandstone and silt/siltstone samples down to the size of a few microns. The validity of the approach is illustrated by its application to 41 Bengal Fan turbidites collected from five cores during IODP Expedition 354 and ranging in grain size from medium sand to fine silt.

Lithological and structural controls of disseminated-type orogenic gold mineralization in high-grade metamorphic turbidites from the Central Zone of the Damara Belt, Namibia

We report the characteristics and controls of low-grade gold-sulfide disseminations in the Twin Hills prospects, the Central Zone of the Damara Belt in Namibia, that combines to a large (>12 km strike length) zone of mineralization. The host rocks are subvertical, tightly folded and transposed, amphibolite facies metaturbidites (cordierite-biotite metapelites and quartz-biotite metapsammites) that form the subvertical limb of a regional-scale syncline. Sulfide and gold grains are finely dispersed in the host rocks or form networks of quartz-sulfide veinlets. Zones of economic-grade gold mineralization are associated with fine-scale quartz-sulfide (pyrrhotite > arsenopyrite ≫ pyrite) vein stockworks. Morphology and deformational textures of the mineralized vein networks suggest their formation during flexural-slip folding and the progressive transposition of the metaturbidites into the regional, ENE trending steep foliation. The more competent metapsammites promoted fracturing while ductile strain during flexural-slip was localized into the schistose metapelites. As a result, economic-grade mineralization is best developed in well-bedded, compositionally heterogeneous parts of the metaturbidites that experienced pronounced strain partitioning. Metapelite-dominated packages, in contrast, show mainly disseminated sulfide mineralization, fewer and variably transposed vein networks and only sub-economic gold grades. The association of disseminated and vein-type mineralization suggests that pervasive fluid flow was channelized within the lithologically heterogeneous packages that provided fracture permeabilities and hydraulic gradients for more focused fluid flow. Higher gold grades correspond to subtle (5-15°) deflections of bedding and the regional foliation that may relate to dilational jog geometries. On a prospect-scale, occurrence of economic-grade gold mineralization coincides with the inflection and change of the vergence of regional-scale first-order folds. The style of mineralization shares similarities with turbidite-hosted orogenic gold deposits. The differences in gold grades and styles of mineralization can be explained by syn-tectonic (D2) fluid flow, the different metamorphic grade (amphibolite versus mid- to lower-greenschist) and ductility of wall rocks that prevented the formation of larger, brittle, high-permeability structures.

Country-wide exploration for graphite- and sulphide-rich black shales with airborne geophysics and petrophysical and geochemical studies

Black shales host critical raw materials such as graphite and cobalt and occur in the vicinity of many types of sulphide deposits. We report the procedure for country-wide mapping of graphite- and sulphide-rich rocks and the chemical and petrophysical data of 319 samples we selected from sulphide occurrences and mines in Finland. Even though black shales are rarely outcropped in glaciated and deeply weathered terrains, they can be traced with geophysical surveys.In the Precambrian of Finland, where the metamorphic grade varies from greenschist to granulite facies, systematic airborne geophysical surveys revealed stratigraphy-related, coupled magnetic and electrically conductive patterns. Electrical conductivity was related to the graphite and sulphide contents, producing continuous and bending electromagnetic anomaly patterns. The magnetic anomalies, if present, resulted from ferrimagnetic monoclinic pyrrhotite.The petrophysical properties of black shales varied in our sample set. The densities were mainly between 2700 and 3000 kg/m3, with the mean density ~ 2800 kg/m3, where the amount of graphite had a reducing effect on density whereas sulphides increased it. The average magnetic susceptibilities were about 6000·10−6 (SI), but they showed wide variation, depending on the abundance of ferrimagnetic monoclinic pyrrhotite. The electrical conductivity of black shales appeared to be positively related to the abundance of monoclinic pyrrhotite. Conductivity variation, 1–105 1/Ωm was based on laboratory determinations of apparent resistivities.We correlated an airborne magnetic and electromagnetic survey with petrophysical and chemical data from altogether 319 drill core samples containing >1 % graphitic C and >1 % S. The samples were selected during 2009–2011 from 102 drill cores all over Finland except for the Talvivaara–Outokumpu region, which was studied during previous projects. The black-shale-hosted Talvivaara Ni–Zn–Co–Cu deposit is currently being mined.The maximum graphite concentration in the country-wide sample set was 34.3 %, and the median value was 5.7 %, a lower value than reported from the Talvivaara black-shale-hosted sulphide deposit (7.6 %). S, Co, Cu, Fe and Ni concentrations were also lower in our sample set on average than in the Talvivaara ore. However, the maximum concentration in our sample set was 397 mg/kg for Co, 0.36 % for Cu, 40.8 % for Fe and 0.28 % for Ni.The developed black shale mapping procedure can be directly applied in other parts of the world in terrains with greenschist to granulite facies regional metamorphism. Information on basic petrophysical properties, i.e., density, magnetic and electric properties, are needed to explain geophysical anomalies. If the metamorphic grade is lower than greenschist facies and there is no graphite, sulphides will increase the electromagnetic properties. If ferrimagnetic pyrrhotite exists, susceptibility increases, as well as remanence.The black shale database covering the whole of Finland is used not only in exploration and bedrock mapping, but also in regional planning and for environmental risk analyses, because sulphide-rich black shales may cause acid rock drainage when exposed to weathering and the quality of surface water and groundwater may suffer from black shale bedrock and glacial till. The scale limitations given by airborne geophysics may request detailed studies in selected sites.

Advantages and Limitations of Combined Diffusion-Phase Equilibrium Modelling for Pressure–Temperature–Time History of Metamorphic Rocks

ABSTRACT This paper presents and discusses the results of phase diagram (Perple_X) and diffusion modelling (CZGM, or Compositional Zoning and its Modification by diffusion) to constrain the P–T path of metamorphism. The approach is based on the best fits between the zoning profile in measured garnet and that obtained by the intersections of garnet isopleths calculated by phase diagram modelling using whole rock bulk composition. The model was applied to garnets in natural rocks of various metamorphic grades, which were formed within different geotectonic environments. To compare the sequence of compositional change during Barrovian-type metamorphism, well-studied pelitic rocks from garnet–staurolite, kyanite–sillimanite, and sillimanite-K-feldspar metamorphic zones were selected. Garnets with two-stepped core and rim profiles that were formed during two different metamorphic stages or events were used for pressure–temperature (P–T) path constraint of each stage or event. For high-grade rocks, in which the original zoning profile in garnet was severely modified, the diffusion of the initial zoning profile was quantified to estimate the timescale of the metamorphic event. These rocks include high- to ultra-high-pressure rocks, which were subjected to thermal overprinting during collisional orogenesis. The results of the application of this approach allow for deciphering the reason why the calculated profile by phase diagram modelling does not fit with that of the measured garnet from low-grade rocks, in which garnet has preserved the original compositional zoning. This includes garnets whose nucleation was shifted from the garnet-in boundary to higher temperatures and pressures, as well as garnet crystallised during different metamorphic stages or events. Finally, the P–T paths in high-grade rocks were constrained after the multicomponent diffusion in garnet was quantified, and this was used for further P–T-time path constraint of metamorphism in the rocks.

Major-element geochemistry of pelites

Abstract Pelites (shales and mudstones) are arguably the most important rock type for interpreting metamorphism. Their significance derives from their widespread occurrence and the range of mineral assemblages they develop at different conditions of pressure and temperature. We compiled a global database of 5729 major-element whole-rock analyses of pelites from different metamorphic grades (shales to granulite-facies paragneisses) to (1) determine an average composition, (2) examine the range and variability in their composition, and (3) assess if there is evidence for grade-related geochemical changes. Median values are given instead of average values to eliminate the effect of extremes. The median worldwide pelite is as follows (anhydrous, values in wt%): SiO2 = 64.13, TiO2 = 0.91, Al2O3 = 19.63, FeOtotal = 6.85, MnO = 0.08, MgO = 2.41, CaO = 0.65, Na2O = 1.38, and K2O = 3.95. The median XMg = MgO/(MgO + FeOtotal) in moles is 0.39. The median XFe3+ = 2 × Fe2O3/(2 × Fe2O3 + FeO) in moles was measured in 1964 samples and is 0.23. On an Al2O3-FeO-MgO (AFM) diagram, the median worldwide pelite plots within a strong clustering of analyses between = projected molar MgO/(MgO + FeOtotal) = 0.30–0.55 (median = 0.42) and AMs = molar [Al2O3 – (3 × K2O)]/[Al2O3 – (3 × K2O) + FeOtotal + MgO] = 0.0–0.4 (median = 0.19). Pelites show a continuous decrease in volatile content with increasing meta-morphic grade and a decrease in XFe3+ from the diagenetic to biotite zone. Lower median SiO2 values and higher median Al2O3 and AMs values in the porphyroblast and subsolidus sillimanite or K-feldspar zones, as well as higher median MnO values in the garnet zone, may reflect sampling bias or metasomatism.

Metamorphic patterns and zircon U–Pb dating of the Xilingol complex in Inner Mongolia, China: Implications for rifting metamorphism and tectonic evolution in eastern Central Asian Orogenic Belt

The Xilingol complex is one of the maximal outcropped metamorphic terranes in the eastern segment of the Central Asian Orogenic Belt. Detailed petrological and geochronological study of the Xilingol complex will provide valuable insights into the orogenic process and closure of the Paleo-Asian Ocean. We present a comprehensive study of petrology, pseudosection modelling, and zircon U-Pb geochronology of representative rocks collected in Daqing Pasture, including two-mica quartz schist, epidote amphibolite, biotite plagioclase gneiss and quartzite. The Xilingol complex in Daqing Pasture can be preliminarily divided into the north and south segments based on field and petrological observations. The north segment comprises two-mica quartz schist, biotite quartz schist, amphibolite and minor epidote amphibolite, whereas the south segment consists mainly of biotite gneiss, amphibolite and quartzite, with varying degrees of migmatization developed. Combined pseudosection modelling and composition isopleths of garnet and biotite constrain a peak P–T condition of ~660 °C, ~5.5 kbar for the garnet-bearing two-mica quartz schist in the north segment, corresponding to low amphibolite facies metamorphism. The garnet-bearing biotite plagioclase gneisses in the south segment are interpreted to experience high amphibolite facies metamorphism at a peak condition of ~750–790 °C, ~5.5–8.0 kbar, followed by nearly isobaric cooling. The variations of peak P–T estimates within Daqing Pasture, coupled with the spatial heterogeneities of lithological associations between different outcropped terranes, reveal an overall northward decreasing metamorphic grade for the Xilingol complex. Metamorphic zircons from three biotite plagioclase gneisses in the south segment yield apparent 206 Pb/ 238 U age spreads of 397–348 Ma, 363–294 Ma and 341–301 Ma, respectively. These partially overlapping zircon ages are an indication of discrete metamorphic events involving localized thermal metamorphism at depth in the Middle Devonian and long-lasting Buchan-type metamorphism in the Carboniferous, with intermediate data probably related to a mixture of multiple growth domains. Five metamorphic zircons with low Th/U ratios from the epidote amphibolite in the north segment yield a concordia age of 284.8 ± 1.9 Ma, reflecting the diachronism of the Late Paleozoic Buchan-type metamorphism. Summarizing all U-Pb data of inherited magmatic zircons and metamorphic zircons, a distinct U-Pb age continuum is retrieved to elucidate a successive process from sedimentary diagenesis of forearc sedimentary formations to deep burial in the Early Devonian, and then to subsequent multi-stage metamorphic overprinting from the Middle Devonian to the Permian. The isograd patterns, P–T–t paths (clockwise paths and long-term duration) and spatial distribution of metamorphic terranes of the Xilingol complex are more compatible with an active continent rift setting rather than paired metamorphic belts in the arc-basin system, revealing post-collisional extension of the Early-Middle Paleozoic orogenic lithosphere of eastern CAOB in the Late Paleozoic. • The Xilingol complex underwent long-lasting Buchan-type metamorphism in the Carboniferous to Permian. • Metamorphic patterns of the Xilingol complex are compatible with continent rift setting. • Zircon U-Pb age continuum reveals a successive process from diagenesis to deep burial to multiple metamorphic episodes.

Affinity and Petrogenesis of the Huzyk Creek Metal-Enriched Graphite Deposit: A Metamorphosed Metalliferous Black Shale in the Trans-Hudson Orogen Of Manitoba, Canada

ABSTRACT The Huzyk Creek area is situated along the boundary between the Reindeer Zone and the Superior Boundary Zone of the Paleoproterozoic Trans-Hudson Orogen, where the Precambrian rocks are overlain by Phanerozoic cover. Two drill holes intersect graphite schist that is enriched in V, as well as U, Zn, Mo, and Cu, and is hosted by a metamorphosed wacke-mudstone sequence interleaved with variably altered mafic rocks. Whole-rock lithogeochemistry and Sm-Nd isotope chemistry suggest that the wacke-mudstone package is related to the turbidite-derived Burntwood Group of the Kisseynew Domain and was likely deposited relatively proximal to the Flin Flon arc-collage. A model is proposed in which redox-sensitive metals were leached from rocks of the Flin Flon arc-collage during weathering under oxidizing conditions. The metals were transported in oxygenated surface run-off draining the arc-collage and discharged into the Kisseynew Basin. Shallow waters of the Kisseynew Basin were likely oxygenated and biologically productive; however, the basin was likely euxinic at mid-depths. The mixing of the metal-enriched, oxygenated water with organic matter and euxinic water resulted in the reduction of the redox-sensitive metals and the formation of insoluble organometallic complexes and particles. A highstand, or period of tectonic quiescence, likely halted turbidite deposition and allowed for the settling organic and metal-rich particles to create relatively thick deposits. Burial and metamorphism resulted in the organic-rich material being transformed into graphite, while Mo, Cu, and Zn were partitioned into sulfides. The mineral hosts of V and U are not known at this time. The model calls for the fractionation of redox-sensitive metals from the water column shortly after discharge into the Kisseynew Basin and implies that graphitic horizons in relatively close proximity to the Flin Flon arc-collage have a greater potential for metal enrichment than graphite deposits farther removed from the arc. This model could apply to basins of similar metamorphic grade, age, and tectonic setting around the globe.