Calc-silicate Research Articles

Unraveling the polymetamorphism of calc-silicate rocks from 639 to 561 Ma in the Western Gondwana (Passo Feio Complex, Dom Feliciano Belt, South Brazil) based on U-Pb dating in titanite, apatite and zircon

Titanite is a valuable tool for studying polymetamorphism and understanding the evolution of orogenic belts as it potentially records different stages of metamorphic evolution. The combination of geochronology and Zr-in-titanite geothermometry has been widely used in petrochronology studies. However, there is ongoing debate regarding the significance of ages and temperatures obtained from titanite. Here, we conducted the first integrated titanite, zircon, and apatite UPb study, along with Zr-in-titanite geothermometry, on calc-silicate rocks from a polymetamorphic complex of the central Dom Feliciano Belt (Western Gondwana). Our aim was to investigate the impact of various metamorphic events on titanite grains. We carried out a geochronological, geochemical, and petrographic analysis of a calc-silicate sequence of the Passo Feio Complex, which was intruded by the Caçapava do Sul Granitic Complex (CSGC). The metamorphic complex experienced regional, contact, and hydrothermal metamorphism during the Neoproterozoic, but the ages and conditions of these metamorphic events remain widely debated. UPb analyses on over one hundred titanite grains from a K-feldspar-diopside schist revealed three distinct titanite populations. The older group, dated at 639 ± 3/7 Ma (2s; n = 19), was associated with the regional metamorphism event (M1), likely initiated by the primary collision of the Dom Feliciano Belt. The intermediate group exhibited an age of 596 ± 1/6 Ma (2s; n = 91). Given the presence of high-K magmatism and a carbonatite intrusion in the study area, producing zircon UPb ages around 600 Ma, this group was associated with contact metamorphism (M2), belike influenced by these intrusive igneous activities. The youngest titanite population showed an age of 566 ± 3/6 Ma (2s; n = 6), which closely aligns, within analytical error, with the age of 561 ± 1/6 Ma (2s; n = 46) obtained from UPb dating on apatite sourced from a diopside-phlogopite schist. The younger ages observed in both titanite and apatite are attributed to alterations resulting from interactions with hydrothermal fluids (M3) during the cooling phase of the CSGC. Chemical analyses conducted with an electron microprobe assessed the Zr concentrations in fifty titanite grains, examining both bright and dark zones observed in back-scattered electron images. The lowest temperature recorded among the 50 grains was 629 °C, which corresponds to dark BSE zones. In the light zones, the minimum temperature was 639 °C. While temperatures estimated using the Zr content in titanite may not correspond precisely with UPb ages, combining titanite ages with those from other accessory minerals like zircon and apatite, along with microstructural analysis, can provide a more comprehensive understanding of orogenic belt evolution.

Titanite: A source, thermodynamic, and temporal proxy for W-Mo-Cu Skarn mineralization at the Seridó Mineral Province, Northeastern Brazil

New zircon U-Pb geochronology (SHRIMP) and geochemistry combined with geochronology (LA-ICP-MS) of titanite data are provided to investigate the metallic sources (whether igneous or sedimentary) and the temporal evolution of one of the most relevant Precambrian metallogenic provinces of the South American Platform, the Seridó Mineral Province. The granites of Serra de João do Vale pluton, calc-silicate “skarnoid” rocks and paragneiss related to the Bonito W-Mo-Cu exoskarn deposit were investigated. It was analyzed titanites of magmatic (Serra de João do Vale), hydrothermal (calc-silicate rocks) and metamorphic (paragneiss) origin. The magmatic titanite is wedge-shaped with larger grain size (up to 250 μm), richer in HFSE elements, Al and Fe, and manifests negative Eu anomalies (Eu/Eu × averaging 0.31–0.60). The hydrothermal titanite is found alongside epidote and calcite, have subhedral shapes and medium grain size (up to 150 μm). The metamorphic titanite is anhedral, generally smaller than 100 μm and appear as thin coronae in ilmenite or as inclusions in biotite. Both metasomatic and metamorphic titanites manifest positive Eu anomalies (Eu/Eu × averaging 1.20 and 1.06, respectively) and are enriched in Ti, Rb and U in relation to the magmatic. Conventional mineral thermobarometry (Al-in-titanite independent barometer, and Zr-in-titanite thermobarometer) provided pressures between 400 and 550 MPa and temperatures between 720 and 787 °C for the crystallization history of the granite, coeval with the regional peak metamorphism at 568–624 °C and 300–500 MPa (Ti in biotite independent barometer and biotite-muscovite thermobarometer). Conditions for the titanite-epidote calc-silicate assemblage were constrained through thermodynamic modelling in NCKFMAST + COH pseudosection at 439 ± 35 °C and 190 ± 50 MPa. The zircon U-Pb age registered the magmatism between 585.5 and 580.5 Ma. Titanite U-Pb ages from granite and calc-silicate rocks timed the closure of U-Pb system at the isotherm of 550 °C between 562 and 557 Ma. (20 m.y. after the intrusion), which marks the metasomatic process. The average W content in paragneisses and calc-silicate titanites (35.8–35.5 ppm) is 4 times higher than in the granite (6.76 ppm), reaching a maximum of 328 ppm. Mo and Cu mirrors this behavior, with concentrations that exceed nearly double that of titanites from Serra de João do Vale granites, peaking at 308 ppm of Mo and 162 ppm of Cu. Metal contents in titanite disclose the metasiliciclastic rocks of the Jucurutu Formation as, at least partially, a source of metals (mainly for W, but also Cu and Mo) in the skarn mineralization during the terrane exhumation at the end of Ediacaran period.

Kontrastujące style metamorficznych i metasomatycznych złóż kamieni szlachetnych w Cabo Delgado w północnym Mozambiku

From a conceptual point of view, in some ruby and garnet deposits of the Lalamo Supergroup – Cabo Delgado Nappes Complex, Northern Mozambique – the structural control of the primary mineralization seems to be spatially and geochemically related to the genesis of calcsilicate rocks, also with gemstones. Two main locations, with contrasting paragenesis, were studied in detail – Miteda area and Micute to N’Djekwa sector. Structural and paragenetic analysis, departing from high-resolution cartography, fabric microanalysis, SEM microscopy and mineral identification by X-ray diffractometry, were used with a major incidence in outcrops and small-scale mine diggings. Regional Neoproterozoic formations, especially its metacarbonate members, host Mg, B - rich skarns, bearing dravite and B-Mg-calc-silicate gemstones, especially in Micute and N’Djekwa area. Following the composition of the probable lithological precursor, these minerals, consistently, reveal a geochemical Mg - B signature that could have been inherited from an evaporite type ancestor, inter-stratified with carbonate rocks in the protolithic Lalamo sequence. The focus of hydrothermal circulation in multistage shear zones is invoked as the main pathway to interpret the complex mineralization observed in Miteda. In this case, the structure of a corundum + garnet major spot is coherent with a pull-a- part dilation associated with sinistral shear. There is a relation between red color saturation of ruby and the content of Cr in corundum. Cr can be leached from the meta-ultramafic host-rocks as a byproduct of desilication. Calcsilicate metasomatism could trigger Si sequester, producing metamorphic desilicated fluids. The focus of hydrothermal circulation in shear zones is invoked as the main pathway for the entrapment of metamorphic and metasomatic gem deposits. These occur in corridors of anomalous host-lithologies and vein swarms where peraluminous paragenesis are located inside highly modified meta-ultramafic or metacarbonate rocks. Skarn like lithologies result from reaction with a sedimentary carbonate protolith and gem deposition is mainly metasomatic. Veining affecting metaultramafic sequences of amphibolitic to granulitic facies show occasional, metasomatically affected, eclogite. For the contrasting styles of deposits represented in Micute – N’Djekwa and in Miteda, although the corresponding paragenesis are separated in space, their deposition can be situated in a conceptual model characterized by a geochemical interdependency and time proximity.

Detrital Tourmalines in the Cretaceous–Eocene Julian and Brkini Flysch Basins (SE Alps, Italy and Slovenia)

In the SE Alps, two Cretaceous–Eocene flysch basins, Julian and Brkini, filled with turbidite sediments, are present. This study novelly reports heavy mineral assemblage counts and detrital tourmaline characterization for 11 samples. It is possible to define three different groups, characterized by the presence of (1) a clinopyroxene–epidote–low-ZTR (zircon+tourmaline+rutile; 5%) sample association, (2) a high-ZTR (>48%)–garnet–apatite association and (3) a low-ZTR (<40%)–Cr-spinel–garnet association. Detrital tourmalines from both the Julian and Brkini flysch basins are rather similar in composition, associated with metapelites and metapsammites coexisting or not coexisting with an Al-saturating phase, ferric-iron-rich quartz–tourmaline rocks and calc–silicate rocks; however, their number is drastically different. In fact, even if the percentage of heavy minerals is very low and similar in both basins (0.17–1.34% in weight), in the Julian basin, the number of tourmaline crystals is much lower than that in Brkini (1–14 vs. 30–100), suggesting an important change in the provenance area. Interestingly, the presence of a high amount of tourmaline derived from ferric-iron-rich quartz–tourmaline rocks and calc–silicate rocks makes these two basins different from all the Cretaceous flysch basins of Bosnia and the Northern Dinaric zone, where these supplies are missing or very limited.

The Timescale and Carbon Flux Recorded by Skarn Garnet From Gangdese Arc, Southern Tibet

AbstractContact metamorphism of carbonate rocks in response to fluid infiltration releases carbon dioxide (CO2), potentially affecting Earth's carbon budget over geological timescales. This process was notably active in the Cretaceous when the closure of the Neo‐Tethys Ocean led to extensive arc magmatism in southern Tibet, coincident with a greenhouse climate interval. These arc plutons intersected carbonate sequences and formed widespread calcsilicate rocks. The mineral assemblages of these rocks record both the progression and duration of the CO2 release. In this study, we focus on a representative aureole in southern Tibet that was metamorphosed by a ∼64 Ma granitoid pluton of the Gangdese Batholith. The aureole presents lithologic zonation, with the inner garnet‐wollastonite zone indicating equilibrium with a water‐rich fluid at ∼525°C. Large garnet porphyroblasts feature cross‐cutting veins of secondary garnets that illustrate several stages of interface‐coupled dissolution‐reprecipitation processes facilitated by fluid infiltration. Diffusion modeling for Mg concentration profiles across vein‐host garnet yields a brief high‐temperature stage (25–150 kyr)—a duration still significantly overestimated considering factors that may accelerate diffusion in garnet, such as lattice parameters and hydration. Such short timescale aligns with conduction modeling of a cooling pluton, which, combined with the bulk‐rock mass balance analysis, indicates a local carbon flux comparable to fluxes at other modern tectonic settings. The metamorphic decarbonation, potentially episodic, in the continental arc may have been an important endogenic carbon source in the late Cretaceous.

The Late Cambrian to Neogene Evolution of the Khanom Core Complex (Peninsular Thailand)

Abstract The Khanom Core Complex in Peninsular Thailand is a part of the crystalline basement of Sundaland and plays a key role in our understanding of the evolution of Thailand and SE Asia. The complex comprises ortho- and paragneisses, schists, meta-volcanics, subordinate calcsilicate rocks, and postkinematic granitoids. New petrochronological data reveal that the sedimentation and metamorphism of the paragneiss precursors (Haad Nai Phlao complex, Khao Yoi paragneisses) occurred in the Late Cambrian at the latest. A syn- to postsedimentary andesitic intrusion/extrusion in the Haad Nai Phlao complex at 495 ± 10 Ma defines a minimum age for the former event(s). In the Early Ordovician (477 ± 7 Ma), the Haad Nai Phlao complex and the Khao Yoi paragneisses were intruded by the Khao Dat Fa granite. During the Indosinian orogenic events, the Laem Thong Yang (211 ± 2 Ma) and Haad Nai Phlao (210 ± 2 Ma) granitoid plutons were intruded. Immediately afterward (ca. 208–205 Ma), the first metamorphic overprinting of the Laem Thong Yang granite and the Haad Nai Phlao complex including the Khao Dat Fa granite occurred. A second metamorphic overprinting of all lithological units and the contemporaneous intrusion of the Khao Pret granite followed in the Late Cretaceous and Early Paleogene (ca. 80–68 Ma). The tectonic formation of the core complex took place in the Eocene (<42 Ma), followed by exhumation and regional cooling below ca. 450°C and the latest cooling to ca. 120°C in the Miocene (ca. 20 Ma). The evolutionary data show that the Khanom Core Complex is part of Sibumasu, and its Late Cretaceous-Neogene cooling pattern and exhumation history can be directly related to the northward drift of India.

Heflikite, ideally Ca2(Al2Sc)(Si2O7)(SiO4)O(OH), the first scandium epidote-supergroup mineral from Jordanów Śląski, Lower Silesia, Poland and from Heftetjern, Tørdal, Norway

Abstract Heflikite, the first Sc-dominant epidote-supergroup mineral, was discovered in two occurrences. The holotype was found in a granitic pegmatite associated with rodingite-like calc-silicate rocks and metasomatised granitic bodies exposed in a serpentinite quarry at Jordanów Śląski near Sobótka, Lower Silesia, SW Poland. The cotype comes from the Heftetjern pegmatite, Tørdal region, Norway. The holotype is composed of (in wt.%): 35.69 SiO2, 0.22 TiO2, 21.98 Al2O3, 6.12 Sc2O3, 0.07 V2O3, 1.10 Fe2O3, 0.11 Y2O3, 1.55 La2O3, 4.05 Ce2O3, 0.31 Pr2O3, 1.53 Nd2O3, 0.40 Sm2O3, 0.11 EuO, 0.56 Gd2O3, 0.14 MnO, 3.56 FeO, 0.16 MgO, 19.16 CaO and 1.78 H2O(+)calc.; total 98.60. The cotype contains: 34.92 SiO2, 0.44 TiO2, 0.82 SnO2, 19.13 Al2O3, 4.79 Sc2O3, 1.96 Fe2O3, 2.55 La2O3, 7.39 Ce2O3, 0.48 Pr2O3, 0.67 Nd2O3, 0.12 EuO, 0.61 Gd2O3, 0.13 MnO, 5.97 FeO, 17.66 CaO and 1.73 H2O(+)calc.; total 99.37. The compositions correspond to the following empirical formulae: (Ca1.729Ce0.125La0.048Nd0.046Gd0.016Sm0.012Pr0.010Y0.005Eu2+0.003)Σ1.994[(Al2.182Sc0.449Fe3+0.070V3+0.005)Σ2.706(Fe2+0.251Mg0.020Mn0.010)Σ0.281Ti0.014]Σ3.001(Si3.006O11)O(OH) and (Ca1.644Ce0.235La0.082Nd0.021Gd0.018Pr0.015Eu2+0.004)Σ1.019[(Al1.958Sc0.362Fe3+0.128)Σ2.448(Fe2+0.434Mn0.009)Σ0.443(Ti0.029Sn0.029)Σ0.058]Σ2.949(Si3.033O11)O(OH), respectively, and to the ideal formula Ca2(Al2Sc)(Si2O7)(SiO4)O(OH). The crystal structure of the holotype was refined in the monoclinic system with an R1 index of 8.62%. The crystal-structure refinement indicates exclusively Si occupied T sites, Al occupied M1 and M2 sites, and a Ca occupied A1 site. The M3 site is filled predominantly by trivalent cations, mainly Sc3+, with divalent cations (mainly Fe2+) as minor occupants. The A2 site is filled mostly by Ca with minor amounts of rare earth elements (REE). The holotype heflikite crystallised from metasomatic fluids that infiltrated a contact between the granitic pegmatite and the surrounding rodingite-type calc-silicate rocks and serpentinites. The fluids that introduced Sc into the pegmatite could have been either hydrothermal or related to low-grade regional metamorphism that postdated the formation of the pegmatite. The cotype heflikite formed during the late-stage hydrothermal crystallisation of the Sc-enriched granitic pegmatite.

Geology and Petrography of the Basement Complex Rocks of Tsauni and Environs, North Central Nigeria

Detailed geological mapping and petrographic study of the rocks in Tsauni area and environs situated within Gwagwalada in Abuja Federal Capital Territory have been carried out to establish the geology relative to the rock types, texture, mineralogy, and mineralization. The area is part of the Basement Complex of North Central Nigeria and a newly discovered skarn environment with preserved basement metagranitoids of the migmatite-gneiss complex and lower grade metasedimentary schist belts. Major rock types identified include migmatite and various types of gneisses. Subordinate rocks include calc-silicate (skarn) rocks, ultramafic relicts and fine to medium grained granite all crosscut by younger Pan African granite, diorite, pegmatites, and quartzo-feldspathic veins. Two types of skarn; A and B were distinguished petrographically being the first mention of skarn occurrence in Nigeria. Petrographic study of selected rocks under plane and crossed polarized light alongside Back Scattered Electron (BSE) images from microprobe analysis showed the mineral suites to include quartz, feldspar (mostly plagioclase and subordinate alkali feldspar), biotite, minor amphibole, pyroxene, calcite and muscovite with accessory epidote, allanite, zircon, titanite, apatite and opaque minerals which selectively occur in both metagranitoids and skarn. Opaque minerals include magnetite, ilmenite± rutile, galena, pyrite sphalerite, and chalcopyrite. Structural features include foliation, joints, xenoliths and pendant (flame) textures, veins, and veinlets which are all aligned in the NE-SW, NW-SE, and minor N-S and E-W directions suggesting control from some deep-seated sources.

Geochemical constraints on the origin of the rare metal mineralization in granite-pegmatite, evidence from three-kilometer scientific drilling core in the Jiajika Li deposit, eastern Tibetan Plateau

Although the metallogenic mechanism of the Jiajika super-large Li deposits in the eastern Tibetan Plateau has been proposed closely linked to development of the Majingzi granite pluton, the petrogenesis and evolutionary linkages between granite–pegmatites and Li mineralization still remain controversial. Here we investigated Li-Nd isotopes together with major and trace elemental data from a 3-km scientific drillcore and surface samples (including the Li-rich/poor pegmatites, the associated two-mica granite and the metasedimentary rocks) to reveal the magmatic evolution and Li-enrichment mechanism of the Jiajika deposit. In the drillcore, the Li-poor pegmatites overall exhibit low Li isotope values (δ7Li = −1.38 ‰ to +1.98 ‰, mean of +0.34 ‰) and low Li contents (67 to 45 ppm), while the Li-rich pegmatites show slightly higher δ7Li values (+0.9 ‰ to +1.1 ‰) and high Li contents (3078 to 6012 ppm). Additionally, the Li-poor pegmatites have εNd(t) values of −17.1 – −2.5 (mean of −10.34) and low Nd concentration (<10 ppm), similar to the two-mica granite (εNd(t)avg = −11.18 and Ndavg = 7.08 ppm), whereas the schists and calc-silicate rocks in the drillcore have relatively higher εNd(t) values (−11.9 to −2.4 with a mean of −7.3) and Nd contents (>20 ppm). These data, together with consistently low Nb/Ta、Zr/Hf of the deep-drilled pegmatites and two-mica granites, suggest that the Jiajika pegmatites are not directly derived from low degrees of partial melting of the Triassic metasediments (e.g., biotite shists or calc-silicate rocks), but more likely from extreme magmatic fractional crystallization of the Majingzi two-mica granite pluton. For the surface samples, the δ7Li values of the surface Li-poor pegmatite samples (−0.61 ‰ to +2.56 ‰ with a mean of +0.36 ‰) are slightly higher than that of the Li-rich pegmatites (−0.52 ‰ to −0.27 ‰ with a mean of −0.39 ‰), and the two-mica granites samples show relatively variation in δ7Li (−1.25 ‰ to +3.33 ‰ with a mean of +0.73 ‰). The surface granites and pegmatites are characterized by high A/CNK values (>1.1) and low K/Rb values (<200), and particularly, the degree of REE tetrad effect gradually increases from granites to pegmatites. We suggest that the Majingzi two-mica granites exposed by the Jiajika gneiss dome is analogous to the deeper granitic sheet implied by the 3 km deep drilling, which both are strongly peraluminous S-type granites with a highly fractionation. Based on the degree of evolution of the Li-rich, Li-poor pegmatites and the two-mica granites and other geochemical evidence, we used a Rayleigh fractionation model to reproduce Li isotope variations in different types of rocks around the Jiajika deposit, suggesting an extreme magmatic differentiation without significant fluid exsolution as the origin for Li enrichments in pegmatites.

Geochronology, geochemistry, and tectonic setting of the Neoproterozoic magmatic rocks in Pan-African basement, West Ethiopia

The West Ethiopian plateau lies in the transition zone between the Arabian-Nubian shield and the Mozambique belt of the Pan-African orogen, underlain by Precambrian to Tertiary rocks in the Gimbi-Nejo area. The Precambrian basement consists mainly of intrusive-meta-intrusive and meta-sedimentary rocks with ice-rafting deposits. Based on the field survey, unique material records, deformation features, magmatism, and metamorphism indicate that the Gimbi-Nejo area likely underwent four tectonic stages during the Neoproterozoic era.The sedimentary formation of 980 ± 3.9 Ma rift valleys is represented by meta-clastic rocks, calc-silicate rocks, meta-basic rocks bearing marble in Chochi, and the Kata domain. Geochemically, meta-peridotite and meta-gabbros show tholeiitic features, while meta-granitoids are medium-K calc-alkaline peraluminous to metaluminous A-type (Ce/Nb 2.1–11.9; Y/Nb 1.2–8; Yb/Ta 2.6–14). Meta-gabbro samples with immobile HFSE (Th, Ta, Hf) and REE (La, Sm, Yb) fall in the within-plate alkaline basalt, oceanic island basalt (OIB), and continental rift basalt field. Meta-granitoid samples also fall in the within-plate granite field. The lithospheric subduction stage magmatic arc, composed of gabbros and granitoids, was emplaced around 827 ± 3.2 Ma. The granitoids include calcic tonalite and trondhjemite showing an adakitic signature with high Sr/Y (14.2–48.2) ratios. Gabbro samples fall in the island arc basalt and continental arc basalt field. Meta-peridotites and meta-gabbros to gabbros originated from the partial melting of garnet-spinel lherzolite to spinel lherzolite (<10 %) and garnet lherzolite to garnet-spinel lherzolite (<30 %) mantle source, respectively. The continental collisional area contains thrust folds and faults trending in a north–south direction. The ca. 797 ± 3.7 Ma calc-alkalic granitoid rocks along the suture zone are high-K peraluminous I-type granitoids (ASI < 1.1). The strike-slippage area is represented by sinistral ductile shearing deformation. The alkali-calcic granitoid rocks associated with shearing and decollements are low to high-K calc-alkaline peraluminous-metaluminous I-S type granitoids (ASI = 0.95–1.17). The U-Pb age of alkali-calcic granitoid is 564 ± 3.4 Ma. Syn-collisional calc-alkalic and late-post orogenic alkali-calcic granitoid samples both fall within the category of within the plate granite.The εHf (t) values of meta-granitoid (+9.4 to + 15.5), calcic granitoid (+7.6 to + 13.2), calc-alkalic granitoid (+7.4 to + 15.5), and alkali-calcic granitoid (+7.11 to + 17.12) suggest that these Neoproterozoic granitoids have been derived from a Meso-Neoproterozoic juvenile crust that formed through depleted mantle source. Depleted mantle magma likely ascended to the juvenile crust and remained there for a certain period (mean time = 122 Ma, 257 Ma, 248 Ma, and 256 Ma, respectively).

Exploring hindered decarbonation in contact metamorphism: A glimpse into marble aureoles in Southern Tibet

Fluid-driven contact metamorphism in the Gangdese Arc, resulting from Late Cretaceous intrusions, led to a diverse range of aureoles in carbonate sequences in southern Tibet. These include extensive carbon loss in calc-silicate rocks with minor calcite remnants and minimal carbon loss in predominantly pure marble formations. Many studies connecting contact metamorphism and decarbonation flux focus on metacarbonate aureoles comprising calc-silicate assemblages, while marble formations represent a less prominent endmember. To investigate the factors determining decarbonation extent, we analyzed the thermobarometric, geochemical, and isotopic signatures of two representative marble aureoles in southern Tibet. Our findings suggest that isobarically exsolved magmatic fluid from a neighboring ∼64 Ma granodiorite pluton led to limited fluid-rock interaction in the marble aureole. Based on the petrologic observations, we present a conceptual model to elucidate this process. The heating and fluid infiltration were brief, likely due to short-lived intrusion events. Consequently, a thin ductile layer was formed adjacent to the pluton, while the majority of the country rock remained brittle. The lithostatic fluid in the ductile layer likely escaped through structurally weak zones, creating veins along the boundary between the pluton and marble. The fluids traversed the brittle-ductile transition experienced a sudden pressure drop from lithostatic in the ductile rock to hydrostatic in the brittle regime. The boiling fluid escaped by brecciating the brittle country rock and resulted in brecciation and stockwork of veins. In both cases, most fluid escaped through pathways, preventing infiltration into the country rock and decarbonation reactions. In contrast, a longer-lived heat source would have produced more extensive ductile country rocks, allowing for widespread fluid infiltration and calc-silicate formation. Thus, the thermal history of the intrusion and the rheology of the country rock significantly influence the extent of decarbonation in contact aureoles.

Thermal influence of sills on the chemostratigraphy and mineralogy of carbonate rocks from the Irati formation, Paraná basin

Carbonate rocks and shales are prominent lithologies in atypical petroleum systems. In these settings, the thermal effects of sills play a crucial role leading to mineralogical and geochemical alteration in the host rocks and in the maturation process of the associated organic matter. While extensive research has been conducted on shales within atypical systems, the understanding of the petrophysical and geochemical properties of carbonate rocks remains uncertain in numerous basins. In this study, we selected the Irati Formation (Paraná Basin) to investigate the thermal effects caused by diabase sills on the chemostratigraphic and lithologic features of carbonate rocks. We conducted mineralogical analyses, examined oxygen and carbon stable isotopes, and analyzed inorganic geochemical data from two distinct sections of the formation: one with diabase sills intruding the sequence and the other without identified intrusions in the area. Widespread recrystallization of minerals is observed throughout the studied rock wall, resulting in the formation of marbles and calc-silicate rocks, where dolomite is mainly replaced by calcite. The carbonate bank, located 5–7 m below the sill, is marked by brucite and chrysotile, while the rocks up to 1 m below and above the intrusion show the presence of tremolite, diopside, and antigorite. Substantial variations in elemental data are observed, including a significant increase in SiO2 and K2O below the sill, as well as elevated MgO levels within the carbonate bank. Additionally, stable isotopic signatures of oxygen and carbon display a negative shift influenced by decarbonation and intense fluid flow controlling the reactions. The contact aureole resulting from the intrusion is identified at least 7 m above and below the sill based on mineralogical and inorganic geochemical data.

Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway)

Graphite formation in the deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten–Vesterålen Complex, northern Norway. Graphite schist is hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsilicate rocks and amphibolite. The schist has major graphite (&lt;modality 39%), quartz, plagioclase, pyroxenes, biotite (Mg# = 0.67–0.91; Ti &lt; 0.66 a.p.f.u.) and K-feldspar/perthite. Pyroxene is orthopyroxene (En69–74) and/or clinopyroxene (En33–53Fs1–14Wo44–53); graphite occurs in assemblage with metamorphic orthopyroxene. Phase diagram modelling (plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage) constrains pressure-temperature conditions of 810–835 °C and 0.73–0.77 GPa; Zr-in-rutile thermometry 726–854 °C. COH fluids stabilise graphite and orthopyroxene; the high Mg#-ratio of biotite and pyroxenes, and apatite Cl &lt; 2 a.p.f.u., indicate the importance of fluids during metamorphism. Stable isotopic δ13Cgraphite in the graphite schist is −38 to −17‰; δ13Ccalcite of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for δ13Ccalcite = −8.7‰ to −9.5‰ and heavier values for δ13Cgraphite = −11.5‰ to −8.9‰. δ18Ocalcite for marble shows lighter values, ranging from −15.4‰ to −7.5‰. We interpret the graphite origin as organic carbon accumulated in sediments, while isotopic exchange between graphite and calcite reflects metamorphic and hydrothermal re-equilibration.

How is carbonate crust digested by magma?

Magma’s volatile budget depends on deep magmatic sources, the degree of differentiation and degassing conditions, and volatile input from the assimilated crust. It is, therefore, important to know the exact mechanism by which the crust is assimilated into magma to understand volatile budgets and eruption behavior. To explore reactions between carbonate, calc–silicate, or skarn xenoliths and basaltic andesite magma, we studied ten calc–silicate xenoliths from the 1994, 1998, 2006, and 2010 Merapi eruptions and four sediment samples of local Javanese carbonate crust. An in situ electron probe microanalysis of the 1994–2010 calc–silicate xenolith minerals and glasses suggests that calcite is a minor and metastable mineral phase in association with wollastonite. In addition, carbonate melts quenched to calcic glasses (32 ± 7 wt% SiO2; 38 ± 3 wt% CaO), similar to experimental glasses produced by crust–melt interaction experiments. Thermodynamic modeling using rhyolite MELTS (version 1.2.0) predicts the production of highly silicic (up to ∼84 wt% of SiO2) and CaO-rich (up to ∼25 wt%) melts during partial melting of calc–silicate material. The observed mechanism of calc–silicate xenolith assimilation is the generation of highly silicic (77 ± 4 wt% of SiO2) melts in association with idiomorphic diopside [Wo49En29; 57 ± 3 Mg# = Mg/(Mg+Fe2+)] and other calcic pyroxenes (Wo54-82 En2-21; 16–43 Mg#) due to partial melting of xenolith and incongruent dissolution reactions. We hypothesize that the rate-limiting process is the subsequent mixing of the produced crustal melts with representative resident andesitic melt (average 65 wt% SiO2) through chemical diffusion, which explains major and volatile (Cl) element contents in the Merapi glass products. In addition to high Sr contents and radiogenic 87Sr/86Sr and elevated CO2, Ba, Co, Cr, Cu, V, Zn, and Zr contents in the magmatic minerals and associated glasses, the recrystallized and residual metamorphic sphene, quartz, garnet, and apatite predicted by rhyolite-MELTS or Magma Chamber Simulator modeling during wallrock melting and residual metastable calcite and wollastonite are important tracers of calc–silicate crust assimilation. The disequilibrium process of calc–silicate crustal assimilation can, thus, be well predicted by dissolution experiments and thermodynamic modeling using rhyolite-MELTS or Magma Chamber Simulator. The rate of calc–silicate crustal assimilation is still unconstrained without adequate high-temperature kinetic time-series experiments. We predict that the crustal assimilation rate is controlled by the Si–Al diffusion and associated convection in the hydrous silicate magma.

Physical properties and crystal structure of near end-member oxy-dravite from the Beluga occurrence, Nunavut territory, Canada

AbstractOxy-dravite, ideally Na(Al2Mg)(Al5Mg)(Si6O18)(BO3)3(OH)3(O), was found in a composition near its ideal end-member at the Beluga occurrence, Nunavut territory, Canada. It occurs in retrograde albite–muscovite–corundum–calcite domains in a calc-silicate rock. This uncommon oxy-dravite occurs as dark brown, equant to short-prismatic, idiomorphic crystals with vitreous lustre and up to ca. 4 × 3 cm in size. The oxy-dravite is optically uniaxial (–), with ω = 1.6453(5) and ɛ = 1.6074(18); its calculated density is 3.069 g.cm–3 with a compatibility index of 0.016. The Beluga oxy-dravite has trigonal symmetry, space group R3m with a = 15.9121(2) Å, c = 7.1788(10) Å, V = 1574.12(5) Å3 and Z = 3. The crystal structure was refined to R1 = 1.45 using 1613 unique reflections. The empirical crystal-chemical formula is X(Na0.88Ca0.08□0.03K0.01)Y(Al1.49Mg1.31Fe0.15Ti0.04Zn0.01)Z(Al5.42Mg0.58)T(Si5.84Al0.16O18)B(BO3)3V(OH2.95O0.05)W(O0.84OH0.01F0.15).Oxy-dravite in nature commonly occurs in a solid solution with foitite, schorl and oxy-schorl. At the Beluga occurrence, its minor contents of Al, vacancy [□], and Ca are most likely compensated by (□Al)(NaR2+)–1 and (CaMg)(NaAl)–1 exchanges of the oxy-magnesio-foitite and magnesio-lucchesiite components. The Beluga occurrence of oxy-dravite is characterised by an Mg-rich environment related to a metamorphic overprint of the original sedimentary sequence. This sequence of marine dolomitic argillaceous marl was influenced by (B,Cl)-rich fluids, probably proximally-derived from mineral breakdown reactions in the calc-silicate during the retrograde stage of metamorphism. The locality is a rare example of a tourmaline + corundum assemblage.

Scandio-winchite, ideally ☐(NaCa)(Mg4Sc)(Si8O22)(OH)2: The first Sc-dominant amphibole-supergroup mineral from Jordanów Śląski, Lower Silesia, southwestern Poland

Abstract Scandio-winchite, the first natural Sc-dominant amphibole-supergroup mineral, has been discovered in a granitic pegmatite that crops out in close association with rodingite-like calc-silicate rocks and metasomatized granitic bodies in a serpentinite quarry at a Jordanów Śląski village near Sobótka, ~30 km south of Wrocław, Lower Silesia, SW Poland. It occurs as an isolated subhedral crystal, with the size of ~20 × 8 μm in planar section, and as three polycrystalline aggregates, up to 50 μm across, composed of needle-shaped crystals dominated by {110}. It is present within chlorite aggregates that supposedly represent remnants of partly recrystallized xenoliths of the blackwall chlorite schists and is in quartz-feldspar portions of the pegmatite adjoining such xenolithic assemblages. Owing to the scarcity of the material and the exceptionally small size of the crystals, the color, streak, and optical properties could not be measured. By analogy with other amphiboles, scandio-winchite has a vitreous luster, brittle tenacity, and a Mohs hardness of ~5½. The mineral shows an uneven fracture and {110} perfect cleavage, with an angle of ~56° between cleavage planes. The density calculated from the empirical formula and refined unit-cell parameters is 3.026 g/cm3. The holotype crystal is composed of (in wt%): 55.88 SiO2, 0.11 TiO2, 0.53 Al2O3, 9.22 Sc2O3, 0.44 MnO, 8.89 FeO, 12.77 MgO, 5.71 CaO, 4.12 Na2O, 0.17 K2O, and 2.09 H2Ocalc(+); total 99.93. The composition normalized on the basis of 22 O2– + 2 (OH)– ions corresponds to the empirical formula A(☐0.966K0.031Na0.003)Σ1B(Na1.132 Ca0.868)Σ2C(Mg2.704Fe1.0552+Mn0.053Sc1.140Al0.023Ti0.012)Σ4.987T(Si7.935Al0.065)Σ8.000O22(OH)2, simplified formula (☐,K)(Na,Ca)2[(Mg,Fe)4Sc](Si8O22)(OH)2, and the ideal formula ☐(NaCa)(Mg4Sc)(Si8O22)(OH)2. The crystal structure was refined in the monoclinic system, space-group symmetry C2/m, with R1 index of 6.57%. Its unit-cell parameters are: a = 9.864(2) Å, b = 18.163(3) Å, c = 5.3053(16) Å, β = 104.41(3)°, V = 920.6(4) Å3; Z = 2, and the a:b:c ratio is 0.5431:1:0.2921. The crystal-structure refinement indicates almost exclusively Si-occupied T sites, the M4 sites occupied by nearly equal amounts of Na and Ca, M1 and M3 sites by divalent Mg + Fe cations, and M2 filled in equal proportions by divalent cations and Sc. These results, along with the dominant vacancy at the A site, univocally indicate that the mineral corresponds to a M2Sc-analog of winchite. Scandio-winchite is most likely a secondary phase of metasomatic origin related to the evolution of the country rocks and partial alteration of the blackwall chlorite schists xenolith induced by the pegmatitic melt and associated fluids.

Linking sedimentary provenance, climate and tectonics in the Neoproterozoic Seridó Belt, Borborema Province (NE Brazil)

The Seridó Belt, at the extreme northeast of Brazil, is one of the several Neoproterozoic mobile belts related to the Brasiliano/Pan-African Orogeny in Western Gondwana. The lithostratigraphy of the Seridó Belt comprises basal quartzites and metaconglomerates (Equador Formation) overlain by hornblende-bearing paragneisses, calc-silicate rocks and marbles (Jucurutu Formation) and biotite-schists (Seridó Formation). We investigated the sedimentary provenance of these rocks based on an integration of compiled whole-rock geochemistry, Sm-Nd isotopes, U-Pb detrital zircon ages and a new set of whole-rock trace elements analyses. Equador Formation quartzites have major and trace elements patterns that resemble cratonic sandstones, with a typical felsic provenance consistent with the isotopic composition of plutonic Archean to Paleoproterozoic basement rocks. Geochemical proxies suggest that these rocks were deposited in fresh waters under anoxic conditions and moderate to intense weathering conditions (CIA = 58–79). A shift in provenance led to an increase in intermediate-mafic sources during the deposition of the Jucurutu and Seridó formations. Trace elements from rock types in these units are nearly identical, yet they differ on the contents of some high field-strength elements (e.g., Sc, V and Ni), which are higher in the Seridó Formation than average pelites. Geochemical proxies show similar depositional environments, with brackish to marine conditions for the Jucurutu and Seridó formations under anoxic to euxinic conditions. Weathering conditions are consistently lower in the Jucurutu Formation (CIA = 58–66) in comparison with the Seridó Formation (CIA = 60–88). Compiled U-Pb data suggest maximum depositional ages of 633 ± 11 Ma and 633 ± 6 Ma for Jucurutu and Seridó formations, respectively, suggesting deposition following the ca. 645–635 Ma Marinoan glaciation. We compared the geochemistry of the Seridó Formation schists with currently exposed basement rocks and showed a strong affinity with Neoproterozoic arc-related intermediate rocks, which is in agreement with their slightly radiogenic signature (average εNd(630Ma) = –1.68). When comparing with other pelitic sequences on the western Borborema Province, most of them fit the same sources and weathering conditions of the Seridó Formation schists and Jucurutu Formation paragneisses. We propose that the Seridó Belt represents an intracontinental rift basin (Equador Formation) that was overlain by fine-grained magmatic arc-related sediments (Seridó and Jucurutu formations) during the post-Marinoan deglaciation and sea level rise. Similar pelitic deposits in western Borborema Province suggest a large marine-influenced continental depositional system that was mostly sourced from magmatic arcs of the West Gondwana Orogen.

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.

Methane in Carbonate Melt Inclusions in the Rock-Forming Minerals of Calc-Silicate Rocks of the Kokchetav Massif

Secondary inclusions of the carbonate melt in garnets and clinopyroxenes from ultrahigh-pressure calc-silicate rocks were studied. Secondary carbonate inclusions are located in the healed cracks confined to large (100 µm–3 mm) primary carbonate inclusions in garnet. In some cases, the healed cracks form plates that intersect both garnet and inclusions of potassium-bearing clinopyroxene in garnet. The high content of K2O (0.64 wt %) in the healed cracks in clinopyroxene inclusions suggests that their formation and healing took place during the stage of ultrahigh pressures. Interpretation of the Raman spectra of the secondary inclusions of the carbonate melt in garnet and clinopyroxene confirmed the presence of the following phases: methane, graphite, calcite, dolomite, muscovite, and phlogopite. The data obtained allowed us to recognize the stage of ultrahigh pressure metamorphism in the calc–silicate rocks of the Kokchetav massif, during which methane accumulation in the carbonate melt took place, for the first time.

METHANE IN CARBONATE MELT INCLUSIONS FROM THE ROCK-FORMING MINERALS OF CALC-SILICATE ROCKS FROM THE KOKCHETAV MASSIF

Secondary inclusions of carbonate melt were studied in garnets and clinopyroxenes from ultrahigh pressure calc-silicate rocks. Secondary inclusions are located in healed cracks associated with large (100 µm – 3 mm) carbonate inclusions in garnet. In some cases, healed cracks intersect inclusions of potassium-bearing clinopyroxene in garnet. Since the amount of K2O in healed cracks in clinopyroxene-inclusions reaches 0.64 wt. %, it was concluded that their formation and healing occurred at the ultrahigh pressure metamorphic stage. Raman spectra interpretation of secondary carbonate melt inclusions in garnet and clinopyroxene confirmed the presence of the following phases: methane, graphite, calcite, dolomite, muscovite, and phlogopite. The obtained results imply for the ultrahigh-pressure metamorphic stage in the evolution of calc-silicate rocks from the Kokchetav massif, corresponding to the methane accumulation in the carbonate melt.