Calc-silicate Research Articles

Isotopic constraints on contamination processes in the Tonian Goiás Stratiform Complex

The Tonian Goiás Stratiform Complex (TGSC, Goiás, central Brazil), is one of the largest mafic-ultramafic layered complexes in the world, emplaced during the geotectonic events that led to the Gondwana accretion. In this study, we present trace elements and in-situ U/Pb-Lu-Hf analyses of zircons and 87Sr/86Sr ratios of plagioclases from anorthosites and gabbros of the TGSC.Although formed by three isolated bodies (Cana Brava, Niquelândia and Barro Alto), and characterized by a Lower and Upper Sequence (LS and US), our new U/Pb zircon data confirm recent geochemical, geochronological, and structural evidences that the TGSC has originated from a single intrusive body in the Neoproterozoic.New Hf and Sr isotope ratios construe a complex contamination history for the TGSC, with different geochemical signatures in the two sequences. The low Hf and high Sr isotope ratios of the Lower Sequence (εHf(t) from −4.2 down to −27.5; 87Sr/86Sr = 0.706605–0.729226), suggest the presence of a crustal component and are consistent with contamination from meta-pelitic and calc-silicate rocks found as xenoliths within the Sequence. The more radiogenic Hf isotope ratios and low Sr isotope composition of the Upper Sequence (εHf(t) from 11.3 down to −8.4; 87Sr/86Sr = 0.702368–0.702452), suggest a contamination from mantle-derived metabasalts in agreement with the occurrences of amphibolite xenoliths in the US stratigraphy. The differential contamination of the two sequences is explained by the intrusion of the TGSC in a stratified crust dominated by metasedimentary rocks in its deeper part and metavolcanics at shallower levels. Moreover, the differential thermal gradient in the two crystallizing sequences might have contributed to the preservation and recrystallization of inherited zircon grains in the US and total dissolution or magmatic overgrowth of the LS zircons via melt/rock reaction processes.

Деформаційний аналіз групи порід басейну Пур, Індія

Precambrian rocks exposed around Pur are a part of Pur-Banera belt that is stratigraphically placed within Aravalli Quartzite, Calc Silicate Gneiss, Quartzite and Garnetiferous Mica Supergroup. The different lithological units exposed in the area are Banded Magnetite Schist. These rocks have been subjected to multi-deformational episodes. In accordance with the modern system of multi-level deformation analysis proposed by Fedor Yakovlev in 2012, the results of the The work belongs to the scientific direction of tectonophysics, which was called “strain analysis of linear folding structures”. The present study involves analysis of strain in the rocks around Pur, Bhilwara District, Rajasthan (India). The deformation analysis at the level of folds (level three of seven) are presented in this work. The outcrop pattern is an isoclinal fold of D2 generation, whose axis plunges steeply NE’ly with an axial plane trending NNE-SSW. The quartzite band in this Pur Synform was sampled for the purpose of strain analysis. This section parallel to the YZ plane of finite strain ellipsoid was prepared (perpendicular to both Schistosity and the lineation). The accepted methods used to estimate strain are a) Fry method and b) “Isogon Rosette” method. The axial ratio of the strain ellipse and the orientation of the long axis of the strain ellipse were computed for every location. It was seen that the strain ratio is nearly uniform in magnitude. The orientation of the long axis of the strain ellipse (Y direction of the finite strain ellipsoid) is also constant in space and parallel to the axial trace of D2 Synform. From the analysis it was suggested a strong flattening following the buckling. The amount of this flattening strain was computed using the”Isogon rosette” method. We calculated reflects the flattening strain recorded in the rock with the very high constancy of the strain ratio and the orientation of the long axis of the strain ellipse irrespective of the methodology adopted. The results of performed researches bring tectonophysics database up-to-date providing an excellent opportunity for provenance analysis of the Pur-basin using the different types of undulose extinction of the quartzite for development of prospective mining projects around the area including iron deposits.

High temperature gas–solid reactions in calc–silicate Cu–Au skarn formation; Ertsberg, Papua Province, Indonesia

The 2.7–3 Ma Ertsberg East Skarn System (Indonesia), adjacent to the giant Grasberg Porphyry Copper deposit, is part of the world’s largest system of Cu–Au skarn deposits. Published fluid inclusion and stable isotope data show that it formed through the flux of magma-derived fluid through contact metamorphosed carbonate rock sequences at temperatures well above 600° C and pressures of less than 50 MPa. Under these conditions, the fluid has very low density and the properties of a gas. Combining a range of micro-analytical techniques, high-resolution QEMSCAN mineral mapping and computer-assisted X-ray micro-tomography, an array of coupled gas–solid reactions may be identified that controlled reactive mass transfer through the ~ 1 km3 hydrothermal skarn system. Vacancy-driven mineral chemisorption reactions are identified as a new type of reactive transport process for high-temperature skarn alteration. These gas–solid reactions are maintained by the interaction of unsatisfied bonds on mineral surfaces and dipolar gas-phase reactants such as SO2 and HCl that are continuously supplied through open fractures and intergranular diffusion. Principal reactions are (a) incongruent dissolution of almandine-grossular to andradite and anorthite (an alteration mineral not previously recognized at Ertsberg), and (b) sulfation of anorthite to anhydrite. These sulfation reactions also generate reduced sulfur with consequent co-deposition of metal sulfides. Diopside undergoes similar reactions with deposition of Fe-enriched pyroxene in crypto-veins and vein selvedges. The loss of calcium from contact metamorphic garnet to form vein anhydrite necessarily results in Fe-enrichment of wallrock, and does not require Fe-addition from a vein fluid as is commonly assumed.

Multiphase formation of the Obří důl polymetallic skarn deposit, West Sudetes, Bohemian Massif: geochemistry and Re–Os dating of sulfide mineralization

The Obři důl Fe–Cu–As polymetallic sulfide skarn deposit is developed in a metamorphic series in the West Sudetes, Bohemian Massif. It consists of lenses of marble, calc–silicate rocks, and skarns. We studied the Gustav orebody, which is located few hundred meters away from the contact with a large, late-orogenic Variscan Krkonose–Jizera Plutonic Complex (KJPC) emplaced into shallow crust. Mineralogical and fluid inclusion study evidence indicates that the main sulfide stage, dominated by pyrrhotite, arsenopyrite, and chalcopyrite originated from aqueous hydrothermal fluids with salinity up to 8 wt% NaCl eq. with minimum homogenization temperatures ranging from 324 to 358 °C. These fluids mainly replaced carbonate-rich lithologies. Carbon, oxygen, and strontium isotope data in Ca-rich rocks imply total overprinting by channelized metasomatic fluid flow, which is most probably related to the intrusion of the KJPC, whereas δ34S values of sulfides argue for a magmatic source of sulfur. The Re–Os age of arsenopyrite overlaps published age data for the KJPC and suggests synchronous formation of the main sulfide mineralization and pluton emplacement.

Titanite-bearing calc-silicate rocks constrain timing, duration and magnitude of metamorphic CO2 degassing in the Himalayan belt

The pressure, temperature, and timing (P-T-t) conditions at which CO2 was produced during the Himalayan prograde metamorphism have been constrained, focusing on the most abundant calc-silicate rock type in the Himalaya. A detailed petrological modeling of a clinopyroxene+scapolite+K-feldspar+plagioclase+quartz±calcite calc-silicate rock allowed the identification and full characterization – for the first time – of different metamorphic reactions leading to the simultaneous growth of titanite and CO2 production. The results of thermometric determinations (Zr-in-Ttn thermometry) and U-Pb geochronological analyses suggest that, in the studied lithology, most titanite grains grew during two nearly consecutive episodes of titanite formation: a near-peak event at 730–740°C, 10kbar, 30–26Ma, and a peak event at 740–765°C, 10.5kbar, 25–20Ma. Both episodes of titanite growth are correlated with specific CO2-producing reactions and constrain the timing, duration and P-T conditions of the main CO2-producing events, as well as the amounts of CO2 produced (1.4–1.8wt% of CO2). A first-order extrapolation of such CO2 amounts to the orogen scale provides metamorphic CO2 fluxes ranging between 1.4 and 19.4Mt/yr; these values are of the same order of magnitude as the present-day CO2 fluxes degassed from spring waters located along the Main Central Thrust. We suggest that these metamorphic CO2 fluxes should be considered in any future attempts of estimating the global budget of non-volcanic carbon fluxes from the lithosphere.

Fluorophlogopite-bearing and carbonate metamorphosed xenoliths from theCampanian Ignimbrite (Fiano, southern Italy): crystal chemical, geochemical and volcanological insights

AbstractFluorine-, boron- and magnesium-rich metamorphosed xenoliths occur in the Campanian Ignimbrite deposits at Fiano (southern Italy), at ∼50 km northeast of the sourced volcanic area. These rocks originated from Mesozoic limestones of the Campanian Apennines, embedded in a fluid flow. The Fiano xenoliths studied consist of ten fluorophlogopite-bearing calc-silicate rocks and five carbonate xenoliths, characterized by combining mineralogical analyses with whole-rock and stable isotope data. The micaceous xenoliths are composed of abundant idiomorphic fluorophlogopite, widespread fluorite, F-rich chondrodite, fluoborite, diopside, Fe(Mg)-oxides, calcite, humite, K-bearing fluoro-richterite and grossular. Of the five mica-free xenoliths, two are calcite marbles, containing subordinate fluorite and hematite, and three are weakly metamorphosed carbonates, composed only of calcite. The crystal structure and composition of fluorophlogopite approach that of the end-member. The Fiano xenoliths are enriched in trace elements with respect to the primary limestones. Comparisons between the rare-earth element (REE) patterns of the Fiano xenoliths and those of both Campanian Ignimbrite and Somma-Vesuvius marble and carbonate xenoliths showthat the Fiano pattern overlaps that of Somma-Vesuvius marble and carbonate xenoliths, and reproduces the trend of Campanian Ignimbrite rocks. Values of δ13C and δ18O depict the same trend of depletion in the heavy isotopes observed in the Somma-Vesuvius nodules, and is related to thermometamorphism. Trace-element distribution, paragenesis, stable isotope geochemistry and data modelling point to infiltration of steam enriched in F, B,Mg and As into carbonate rocks at a temperature of ∼300–450°C during the emplacement of the Campanian Ignimbrite.

Proterozoic evolution of part of the Embu Complex, eastern São Paulo state, SE Brazil

This paper presents detrital zircon ages obtained in rocks of the Embu Complex, southeastern São Paulo State, Brazil. The Embu Complex encompasses a Paleoproterozoic basement represented by migmatitic hornblende and biotite orthogneisses covered by (kyanite)-(sillimanite)-(garnet) bearing biotite-muscovite schists and paragneisses with decametric intercalations of quartzites and calcsilicate rocks. In the studied area this metasedimentary sequence is intruded by the porphyritic Serra do Quebra Cangalhagranite. Through field and microstructural studies, four ductile deformational phases wereidentified. Metamorphic events related to the Brasiliano Orogeny that affected the studied rocks were dominantly under medium temperature and pressure conditions, from greenschist to middle amphibolite facies. Detrital zircon crystals from a ∼10 m thick quartzite layer were analyzed by LA-ICP-MS resulting in a wide range of ages between 2100 and 600 Ma that fall mainly in four groups: the first group between 2.1 and 1.6 Ga, with apex at 1.7 Ga; two less expressive Mesoproterozoic groups with values between 1.6 and 1.2 Ga; and a fourth group with values between 1.2 and 0.6 Ga. Considering the geochronological data, the sedimentation of the basin began after 852 ± 40 Ma (the youngest igneous grain) and finished before ∼786 Ma (metamorphic rim). The age of the intrusive Serra do Quebra Cangalha granite (∼680 Ma) is consistent with this minimum age. The opening of the basin could be related to the break-up of Rodinia, which resulted in several small continents, among them the Paranapanema and São Francisco paleocontinents. Comparing these data with similar provenance data from the Apiaí terrane, itseems probable that the Embu Complex was physically connected with it during most of their evolution.

P-T evolution of metapelites from the Bajgan complex in the Makran accretionary prism, south eastern Iran

The Bajgan Complex, one of the basement constituents of the arc massif in Iranian Makran forms a rugged, deeply incised terrain. The complex consists of pelitic schists with minor psammitic and basic schists, calc silicate rocks, amphibolites, marbles, metavolcanosediments, mafic and felsic intrusives as well as ultramafic rocks. Metapelitic rocks show an amphibolite facies regional metamorphism and contain garnet, biotite, white mica, quartz, albite±rutile±apatite. Thermobarometry of garnet schist yields pressure of more than 9kbar and temperatures between 560 and 675°C. The geothermal gradient obtained for the peak of regional metamorphism is 19°C/km, corresponding to a depth of ca. 31km. Replacement of garnet by chlorite and epidote suggest greenschist facies metamorphism due to a decrease in temperature and pressure through exhumation and retrograde metamorphism (370–450°C and 3–6kbar). The metapelitic rocks followed a ‘clockwise’ P–T path during metamorphism, consistent with thermal decline following tectonic thickening. The formation of medium-pressure metamorphic rocks is related to presence of active subduction of the Neotethys Oceanic lithosphere beneath Eurasia in the Makran.

Host rock geochemistry, texture and chemical composition of magnetite in iron ore in the Neoarchaean Nyong unit in southern Cameroon

ABSTRACTA combination of petrography, whole-rock geochemistry, geochronology and compositional variation in magnetite is employed in this study to elucidate the nature and origin of enigmatic magnetite mineralisation hosted within gneissic rocks in the Nyong Unit in southern Cameroon. The mineralisation occurs as magnetite-bearing calc–silicate gneisses. The host rock mineral assemblage comprises quartz–plagioclase–biotite–amphibole–chlorite–clinopyroxene–garnet that provides evidence of medium-grade metamorphism and retrograde alteration. Textural and chemical analyses of the magnetite grains show variable textural and trace element chemical characteristics attributed to metamorphic-hydrothermal overprint and weathering. Magnetite occurs as disseminations and stringers commonly intergrown with amphiboles. It is also observed to show vermiforms wrapped around quartz and clinopyroxenes within a biotite–chlorite- plagioclase groundmass. Massive-granoblastic magnetite is rare and mainly observed within vein-like domains. On backscattered secondary electron images the magnetite grains are anhedral, with minor spinel exsolution lamellae. Electron microprobe analysis on magnetite suggests both a hydrothermal skarn and banded iron formation (BIF) affinity. The lack of negative Ce anomalies excludes a Proterozoic BIF setting, but it is in agreement with Archaean BIF. Sensitive high resolution ion microprobe U-Pb isotope data on zircon in the magnetite gneiss define an identical Wetherill concordia and Tera–Wasserburg Neoarchaean age of 2699 ± 7 Ma (1σ; MSWD (mean square weighted deviation) = 1.3; n = 13), and Pan-African disturbance at about 500 ± 200 Ma. The Neoarchaean age is in accordance with the known onset of BIF deposition at the northern edge of the Congo Craton and therefore constrains the maximum age of formation of the Nyong magnetite gneisses.

Corrigendum to: “Metamorphic CO2 Production in Collisional Orogens: Petrological Constraints from Phase Diagram Modeling of Himalayan, Scapolite-bearing, Calc-silicate Rocks in the NKC(F)MAS(T)-HC system”

Corrigendum to: “Metamorphic CO2 Production in Collisional Orogens: Petrological Constraints from Phase Diagram Modeling of Himalayan, Scapolite-bearing, Calc-silicate Rocks in the NKC(F)MAS(T)-HC system”

Age and Origin of the Tsavorite and Tanzanite Mineralizing Fluids In the Neoproterozoic Mozambique Metamorphic Belt

Abstract The genetic model previously proposed for tsavorite- (and tanzanite-) bearing mineralization hosted in the Neoproterozoic Metamorphic Mozambique Belt (stretching from Kenya through Tanzania to Madagascar) is refined on the basis of new Sm-Nd age determinations and detailed Sr-O-S isotope and fluid-inclusion studies. The deposits are hosted within meta-sedimentary series composed of quartzites, graphitic gneisses, calc-silicate rocks intercalated with meta-evaporites, and marbles. Tsavorite occurs either in nodules (also called “boudins”) oriented parallel to the metamorphic foliation in all of the deposits in the metamorphic belt or in quartz veins and lenses located at the hinges of anticlinal folds (Lelatema fold belt and Ruangwa deposits, Tanzania). Gem tanzanite occurs in pockets and lenses in the Lelatema fold belt of northern Tanzania. The Sm-Nd isotopic data for tsavorites and tanzanites hosted in quartz veins and lenses from Merelani demonstrate that they formed at 600 Ma, during the retrograde metamorphic episode associated with the East African Orogeny. The tsavorites hosted in nodules do not provide reliable ages: their sedimentary protoliths had heterogeneous compositions and their Sm-Nd system was not completely rehomogenized, even at the local scale, by the fluid-absent metamorphic recrystallization. The initial 87 Sr/ 86 Sr isotopic ratios of calcite from marble and tanzanites from Merelani fit with the strontium isotopic composition of Neoproterozoic marine carbonates. Seawater sediment deposition in the Mozambique Ocean took place around 720 Ma. The quartz-zoisite O-isotopic thermometer indicates a temperature of formation for zoisite between 385 and 448 °C. The sulfur isotopic composition of pyrite (between –7.8 and –1.3‰ V-CDT) associated with tsavorite in the Lelatema fold belt deposits suggests the contribution of reduced marine sulfate. The sulfur in pyrite in the marbles was likely derived from bacterial sulfate reduction which produced H 2 S. Fluid inclusion data from tsavorite and tanzanite samples from the Merelani mine indicate the presence of a dominant H 2 S-S 8 ±(CH 4 )±(N 2 )±(H 2 O)-bearing fluid. In the deposits in Kenya and Madagascar, the replacement of sulfate by tsavorite in the nodules and the boron isotopic composition of tourmaline associated with tsavorite are strong arguments in favor of the participation of evaporites in garnet formation.

Origin Of Scapolite-Hosted Sapphire (Corundum) Near Kimmirut, Baffin Island, Nunavut, Canada

Gem-quality corundum (sapphire) occurs in scapolite-rich calc-silicate rock hosted in marble of the Lake Harbour Group near Kimmirut, southern Baffin Island. A deposit of blue and colorless gem corundum (Beluga occurrence) is compared to a similar calc-silicate pod generally lacking corundum but containing nepheline (Bowhead occurrence) and located 170 m to the SSW. Corundum formation was made possible by three equally important sequential metamorphic reactions: (1) formation of nepheline, diopside, and K-feldspar (inferred) at granulite facies peak metamorphic conditions; (2) partial retrograde replacement of the peak assemblage by phlogopite, oligoclase, calcite, and scapolite (Me-50-Me-67) as a result of CO2-, H2O-, Cl-, F-bearing fluid influx at 1782.5 +/- 3.7 Ma (P-T 720 degrees C, 6.2 kbar); and (3) retrograde breakdown of scapolite + nepheline (with CO2-and H2O-bearing fluid) to form albite, muscovite, corundum, and calcite. Late, low-temperature zeolite mineralization is common in corundum-bearing zones. Based on thermodynamic models, the corundum-forming reaction only occurs in a 100 degrees C window with an upper limit determined by scapolite-nepheline stability, and a lower limit determined by the formation of Al-silicate rather than corundum. The protolith is inferred to be dolomitic argillaceous marl with no evidence to suggest the initial presence of evaporites. The enrichment of trace metals V and Cr, and the depletion of Co, Ni, and Mn, suggest reducing diagenetic conditions in the initial sediment. Beluga calc-silicate rock is strongly depleted in REE. Oxy-dravite delta B-11 (+3.9 +/- 0.7%) is consistent with a marine boron source. The oxygen isotope composition of corundum (delta O-18(VSMOW) = 16.4 +/- 0.1%) is comparable to that of corundum in marble or desilicated pegmatite associated with marble.

Ni-(Cu-PGE) deposits in Finland – Geology and exploration potential

Several types of nickel sulphide deposits occur in Finland. Some of them are also important sources of Cu, Co and PGE. The main deposit types are related to Archaean and Palaeoproterozoic mafic to ultramafic rocks, Palaeoproterozoic metamorphosed black shales of the Talvivaara type, or the ultramafic and metasomatic calc-silicate rocks of the ophiolitic Outokumpu association. Several komatiite-hosted Ni-Cu-PGE deposits have been discovered in the Archaean and Paleoproterozoic greenstone belts in northern and eastern Finland, but only one has been briefly mined, in 1989. Many 2.44Ga Palaeoproterozoic mafic-ultramafic layered intrusions in Finland host contact-type PGE-Ni-Cu deposits and significant reef-type PGE deposits; typically the Ni resources are of low grade but large tonnage. The mafic-ultramafic magmatism at ca. 2.06Ga produced significant Ni-Cu-PGE resources in the Central Lapland greenstone belt, including the Kevitsa and Sakatti deposits. The Svecofennian domain in central and southern Finland contains hundreds of small, ca. 1.88Ga, mafic-ultramafic orogenic intrusions; many of these host nickel mineralization. Nickel exploration potential was evaluated in general, based on identification of critical factors within a mineral system framework and a quantitative assessment of the potential occurrence of undiscovered nickel resources. The MgO content of the parental magma was estimated for 28 deposits: 6 komatiite-hosted occurrences and 22 Svecofennian orogenic intrusions. The estimated MgO content of the parental magmas for the selected deposits varies from around 7% to 15% in basaltic to komatiitic basalt systems and up to 30% in more magnesian komatiitic systems; the Ni/Cu value varies mostly between 0.7 and 12, correlating in general positively with the MgO content of the magma. Exceptionally high or low Ni/Cu values in some deposits may result from, for example, copper loss during post-magmatic processes (serpentinization and/or metamorphism) or part of the sulphide ore (zoned by the Ni/Cu value) still being unknown. Recent advances in understanding of komatiites in Finland indicates high exploration potential for nickel within the Paleoproterozoic komatiitic formations, which typically are also significantly enriched in Cu and PGE. However, the quantitative assessment of undiscovered nickel resources in Finland suggests that the contact-type Ni-Cu-PGE deposits hosted by 2.44Ga mafic-ultramafic layered intrusions in northern Finland have the greatest potential for hosting undiscovered resources among the assessed nickel deposit types; these occur within restricted areas, resulting in a high deposit density. Finally, considerable nickel resources might exist in undiscovered Talvivaara-, Kevitsa- or Sakatti-type deposits. These deposit types should be assessed as soon as more geological information becomes available.

Exhumation of a migmatite complex along a transpressive shear zone: inferences from the Variscan Juzbado–Penalva do Castelo Shear Zone (Central Iberian Zone)

High-grade metamorphic rocks associated with S-type granites are recorded in the Central Iberian Zone, Iberian Variscides. Though most of these occur as inferred metamorphic core complexes affiliated with detachment faults, others, such as the Figueira de Castelo Rodrigo–Lumbrales Anatectic Complex, crop out between low-grade metamorphic rocks separated by steeply-dipping strike-slip shear zones, such as the Juzbado–Penalva do Castelo Shear Zone. Our structural analysis has been able to constrain two major deformation stages during the Variscan D 3 : (a) D 3a ductile deformation event, with clear sinistral kinematic criteria; and (b) D 3b thrusting ductile–brittle deformation event. The petrological investigation confirmed the jump in metamorphic grade between the host rocks and the anatectic complex. P–T estimates on calc-silicate rocks interlayered with the metapelites of the anatectic complex provided minimum metamorphic peak conditions of T = 761 ± 50°C and P = 5.0 ± 1.0 kbar. However, petrological modelling results show that P–T conditions must have exceeded T > 800°C. Both structural and geothermobarometric data support a two-step model for the exhumation of the Anatectic Complex, including a 5 – 8 km vertical exhumation along a 65 – 100 km horizontal displacement due to a simple shear-dominated transpression mechanism during the Variscan D 3 events. Supplementary material: The petrology data, mineral chemistry analyses of the calc-silicate units and EPMA analytical conditions, and the P–T modelling methodology can be found at https://doi.org/10.6084/m9.figshare.c.3785648

U-Pb zircon ages of the Wildhorse gneiss, Pioneer Mountains, south-central Idaho, and tectonic implications

The gneiss complex of Wildhorse Creek (Wildhorse gneiss) forms the central component of the lowest structural plate in the Pioneer metamorphic core complex of south-central Idaho. The oldest rock in the complex is a felsic ortho­gneiss, with Neoarchean U-Pb magmatic zircon ages of 2.60–2.67 Ga. The ortho­gneiss overlaps in age and is interpreted to be part of the Grouse Creek block of the Albion Mountains to the south. This Archean metagranitoid is structurally interleaved with paragneiss containing quartzite and calc-silicate rock. Structurally below the orthogneiss, some quartzites have multiple concordant populations of detrital-zircon grains as young as ca. 1700 Ma, while others have no zircon grains younger than ca. 2500 Ma. Structurally above the Archean gneiss is a heterogeneous paragneiss that contains calc-silicate and quartzitic rocks with detrital zircons as young as ca. 1460 Ma. Amphibolite in this unit contains zircons dated at ca. 1850 Ma, indicating that this rock can be no older than that and implying considerable structural complexity. The upper part of the Wildhorse gneiss contains metaquartzites bearing zircons as young as ca. 1400 Ma. The protolith of this paragneiss is interpreted as the southernmost exposures of the Lemhi subbasin of the Mesoproterozoic Belt Supergroup. The upper Wildhorse gneiss includes ca. 695 Ma intrusive orthogneiss that is coeval with Neoproterozoic rift-related volcanic or intrusive rocks near Poca­tello, House Mountain, and Edwardsburg, Idaho. This Cryogenian meta-intrusive rock is the likely source of the 650–710 Ma detrital-zircon population in the Big Lost River that drains the core complex. Initial eHf values from 675 Ma zircons are between 3.4 and –2.4, suggesting the granitoids had a mixed source in both continental crust and juvenile mantle.

Petrogenesis of incipient charnockite in the Ikalamavony sub-domain, south-central Madagascar: New insights from phase equilibrium modeling

Incipient charnockites representing granulite formation on a mesoscopic scale occur in the Ambodin Ifandana area of Ikalamavony sub-domain in south-central Madagascar. Here we report new petrological data from these rocks, and discuss the process of granulite formation on the basis of petrography, mineral equilibrium modeling, and fluid inclusion studies. The incipient charnockites occur as brownish patches, lenses, and layers characterized by an assemblage of biotite+orthopyroxene+K-feldspar+plagioclase+quartz+magnetite+ilmenite within host orthopyroxene-free biotite gneiss with an assemblage of biotite+K-feldspar+plagioclase+quartz+magnetite+ilmenite. Lenses and layers of calc-silicate rock (clinopyroxene+garnet+plagioclase+quartz+titanite+calcite) are typically associated with the charnockite. Coarse-grained charnockite occurs along the contact between the layered charnockite and calc-silicate rock. The application of mineral equilibrium modeling on the mineral assemblages in charnockite and biotite gneiss employing the NCKFMASHTO system as well as fluid inclusion study on coarse-grained charnockite defines a P–T range of 8.5–10.5kbar and 880–900°C, which is nearly consistent with the inferred P–T condition of the Ikalamavony sub-domain (8.0–10.5kbar and 820–880°C). The result of T versus H2O activity (a(H2O)) modeling demonstrates that orthopyroxene-bearing assemblage in charnockite is stable under relatively low a(H2O) condition of 0.42–0.43, which is consistent with the popular models of incipient-charnockite formation related to the lowering of water activity and stabilization of orthopyroxene through dehydration of biotite. The occurrence of calc-silicate rocks adjacent to the charnockite suggests that the CO2-bearing fluid that caused dehydration and incipient-charnockite formation might have been derived through decarbonation of calc-silicate rocks during the initial stage of decompression slightly after the peak metamorphism. The calc-silicate rocks might have also behaved as a cap rock that trapped CO2 infiltrated from an external source. ‘CO2-rich fluid ponds’ formed beneath calc-silicate layers could have enhanced dehydration of biotite to orthopyroxene, and produced layers of coarse-grained charnockite adjacent to calc-silicate layers.

Lithostratigraphy of the Mesoproterozoic Jannelsepan Formation

The Jannelsepan Formation forms part of the 1.30 to 1.24 Ga Areachap Group. It is a pre-tectonic calc-alkaline volcano-sedimentary unit in the 1.2 to 1.0 Ga Namaqua Sector of the Namaqua-Natal Metamorphic Province. The main rock types are amphibolites with subordinate clinopyroxene or biotite. Although some primary volcanic textures have been identified, primary features are generally poorly preserved. Minor interlayered units include felsic metavolcanic rocks, pelitic schists and calc-silicate rocks which may represent primary marls. Field and geochemical evidence suggests that the Jannelsepan Formation represents metamorphosed basaltic to intermediate lavas and associated sedimentary rocks which formed in a subduction-related arc setting.

Lithostratigraphy of the Mesoproterozoic Bethesda Formation

The metamorphic Bethesda Formation forms part of the Areachap Group, a volcano-sedimentary unit affected by the 1.2 to 1.0 Ga tectonothermal event in the Namaqua Sector of the Mesoproterozoic Namaqua-Natal Metamorphic Province. It comprises mainly pale brown biotite- and muscovite-rich metapelitic and quartzitic schist and gneiss with subordinate amphibolite, quartzite and calc-silicate rocks. It is commonly migmatitic with pegmatitic leucosomes and veins, deeply weathered and generally poorly exposed. The extrusion of volcanic rocks in the Areachap Group is constrained between 1.30 and 1.24 Ga, whereas the Bethesda Formation is younger than 1224 ± 20 Ma based on detrital zircon dating. The regional foliation of the Areachap Group generally strikes northwesterly, reflecting isoclinal F 2 folding which obscures contact relationships. Upper amphibolite facies metamorphism in the Bethesda Formation at about 18 km depth is dated at 1205 ± 16 Ma. The close association of the Bethesda Formation with the calc-alkaline metavolcanic Jannelsepan Formation, with which it is in contact, suggests that they both formed in a volcanic arc setting shortly before the onset of the Namaqua orogeny. The Areachap Group as earlier defined included the Sprigg, Van Wykspan, Rateldraai, Bethesda and Jannelsepan Formations in the Upington area. Lithological correspondence between the Bethesda Formation and the quartzo-feldspathic Van Wykspan and contact-metamorphic Rateldraai Formations suggests that these formations should be included in the Bethesda Formation, as implemented here. The conglomeratic Sprigg Formation might be younger than the other formations of the Areachap Group, but more data on detrital zircons is needed to evaluate this. For practical reasons the original name Bethesda Formation for this mappable unit is retained, although there is a complete lack of primary characteristics, way-up and base or top criteria.

Erratum to: Metamorphic CO2 Production in Collisional Orogens: Petrological Constraints from Phase Diagram Modeling of Himalayan, Scapolite-bearing, Calc-silicate Rocks in the NKC(F)MAS(T)-HC system

Erratum to: Metamorphic CO2 Production in Collisional Orogens: Petrological Constraints from Phase Diagram Modeling of Himalayan, Scapolite-bearing, Calc-silicate Rocks in the NKC(F)MAS(T)-HC system

The High-Grade Mo-Re Merlin Deposit, Cloncurry District, Australia: Paragenesis and Geochronology of Hydrothermal Alteration and Ore Formation

The Merlin deposit, located in the Cloncurry district, Mt. Isa Inlier, is the world’s highest-grade Mo-Re deposit. Although the district is renowned for iron oxide copper-gold (IOCG)–type mineralization, Merlin and other Mo-rich occurrences in the district cannot be classified as IOCG-type deposits. The Merlin deposit is hosted by the Kuridala Group (1690–1650 Ma), which comprises interbedded phyllite and carbonaceous slate, calcsilicate rocks, and silicified metasiltstone. The Mount Dore Granite, part of the Williams-Naraku batholith (ca. 1540–1490 Ma), is locally thrust over the metasedimentary rock package. An early Cu-Pb-Zn sulphide stage (Mount Dore deposit) mostly overlies, with only partial spatial overlap, the younger Mo-Re mineralization (Merlin deposit). Cu-Pb-Zn is mainly hosted by carbonaceous slate as infill of hydrothermal coast-supported carbonate breccias; the younger Mo-Re mineralization (Merlin deposit) occurs as molybdenite infill of hydrothermal matrix-supported breccias along or near the contact between carbonaceous slate and calc-silicate rocks, and locally cuts the Cu-Pb-Zn mineralization. Based on detailed petrography and radiometric age dating (U-Pb zircon and titanite ages, Re-Os molybdenite ages), we define four main stages of hydrothermal alteration. Stage 1 represents the earliest hydrothermal stage, dated here by U-Pb titanite geochronology at 1551 ± 12 Ma, and is characterized by Na-(Ca) alteration with the formation of hematite-dusted albite + amphibole + carbonates + titanite + apatite + quartz ± magnetite ± epidote. Stage 2 is characterized by the formation of K-feldspar + tourmaline + carbonates + quartz ± apatite and Cu-Pb-Zn mineralization with chalcopyrite + sphalerite + pyrite ± galena. Stage 3 is dated at 1535 ± 6 Ma (Re-Os molybdenite) and consists of the main Mo-Re event that formed intensely mineralised, matrix-supported breccias accompanied by K-feldspar ± chlorite alteration. Stage 4, dated here at 1521 ± 3 Ma (Re-Os molybdenite), is of an age similar to that of the emplacement of the Mount Dore Granite (1517 ± 7 Ma; U-Pb zircon), and is interpreted as either a remobilization event or a later addition of Mo-Re in veins and disseminations that extends several meters below the Mo-Re breccias, with formation of chlorite + K-feldspar + apatite ± rutile. The Mount Dore Granite was thrust above the metasedimentary package at ca. 1500 Ma, which coincided with the waning of the hydrothermal activity in the Cloncurry district. This event is interpreted to be manifested at the Merlin deposit as minor, millimeter-scale Mo-Re vein formation. Collectively, our data indicate that the Merlin and the Mount Dore mineralization formed via a complex and protracted hydrothermal history that includes a time period (ca. 1560–1540) that has previously been regarded as unprospective for ore deposit formation in the region.