New occurrence of potential phosphate resource in northeast Jordan

An exploration program aided by field investigation, exploration drilling, detailed sampling, lithological and petrological studies, geochemical investigation, and resource calculation leads to the discovery of a potential phosphate resource in northeast Jordan close to the international border with Saudi Arabia and Iraq. The studied phosphate is of the Middle Eocene age that belongs to the Wadi Shallala Formation. It is equivalent to the phosphate deposits recorded in the lower part of the Umm Wual Formation in the Turayf region of Saudi Arabia and the Eocene Ratga Formation in the Ethna phosphate deposit west of Iraq. The phosphorites in the region are broadly similar in mineralogical composition and geochemical affinities. X-ray diffraction indicates the presence of francolite with variable amounts of calcite and quartz. Most samples consist of phosphate clasts embedded in carbonate and silica matrix and cement. P₂O₅ content is up to 32.3 % with an average equal to 18.6 %. The impurity is caused by the presence of variable amounts of SiO₂ and CaO. The F% and F/P₂O₅ ratio in studied phosphates is lower compared with that in phosphates from Jordan and Saudi Arabia. The geological and geochemical results were integrated for resource estimation. Three high-grade phosphate layers with ≥23 % P₂O₅ were considered in the calculations. The phosphate resource is classified as an inferred resource. The total volume of the resource is about 649 million tons. The average P₂O₅ content is 24.57, and the stripping ratio is 1:5.8.     

Geochemical diagenetic trends during phosphorite formation – economic implications: The case of the Negev Campanian phosphorites,Southern Israel

Research on the Upper Campanian (Upper Cretaceous) Negev phosphorites (Mishash Formation), based on microprobe analyses, Fourier Transform Infrared spectroscopy, wet chemistry, microtextural (Scanning Electron Microscopy) studies and mineralogical analyses, together with quantified rates of sedimentation and P accumulation, enables the chemistry of these rocks to be better constrained across the Negev area and allows their suitability for the manufacture of P fertilizers to be better determined. Two phosphorite facies are differentiated: (i) a pristine phosphorite facies of low P content, more typical of basinal sections and (ii) a reworked, granular phosphorite facies commonly enriched in P, found predominantly near palaeo‐highs and forming most of the economic phosphates. The distribution of F/P₂O₅, CO₂/F, U(IV), Cd, Zn and other trace metals (Mo, Ni, Cr, Cu, V and Y), rare‐earth elements concentration, Ce and Eu anomalies and heavy rare earth elements enrichment, are controlled by these two facies. F/P₂O₅ in carbonate‐fluorapatite is much lower (0·090 to 0·107) in the pristine than in the reworked facies (0·107 to 0·120); in addition, the lower F/P₂O₅ in the pristine facies is coupled with: (i) higher Cd, Zn, Mo, Ni, Cr, Cu and V concentrations; (ii) a considerably reduced (< 10%) U(IV) fraction of total U; (iii) lower rare earth elements/P₂O₅ and Y/P₂O₅ ratios; (iv) less negative Ce and Eu anomalies and lower heavy rare earth elements (Lu/La) enrichment; (v) an increase in Fe‐rich smectites in the clay fraction; and (vi) presence of OH in the carbonate‐fluorapatite structure. Sedimentary reworking of previously formed pristine phosphate, together with its redeposition near structural highs in more oxic bottom conditions, results in considerable diagenetic changes in the chemistry of the phosphorites, making them more suitable for economic exploitation. The results presented here provide geochemical criteria for identifying pristine phosphate in other phosphorite sequences and may help to better locate phosphate strata chemically suitable for the phosphate industry elsewhere.     

Geochemical Characteristics of Apatite in Heavy REE‐rich Deep‐Sea Mud from Minami‐Torishima Area,Southeastern Japan

We have conducted geochemical and mineralogical investigations of the rare earth and yttrium (REY)‐rich mud from the Minami‐Torishima area in the Pacific in order to clarify the concentration of REY and their host‐phase in the mud. X‐ray diffraction analysis shows that the mud is mainly composed of phillipsite, fluorapatite, quartz, albite, illite and montmorillonite. Whole‐rock CaO, P₂O₅ and total REY contents of the mud are positively correlated. Relative abundance of apatite is also positively correlated to P₂O₅ and total REY contents. These correlations suggest that apatite is the main host of the P₂O₅ and REY in the mud. We make in situ compositional analyses of constituent minerals in the REY mud. The results show that the apatite is abundant in REY (9300–32,000 ppm) and is characterized by a negative Ce anomaly and enrichment in heavy rare‐earth elements. This abundance and composition of REY of the mud is similar those of fish debris apatites. In contrast, phillipsite is less abundant in REY (60–170 ppm). Therefore we conclude that the main REY host phase of the mud is apatite.     

Petrogenesis and Mineralization of Two‐Stage A‐Type Granites in Jiuyishan,South China: Constraints from Whole‐rock Geochemistry,Mineral Composition and Zircon U‐Pb‐Hf Isotopes

The Jiuyishan complex massif, located in the northern section of the Nanling region, is a combination of five plutons, namely, the Xuehuading, Jinjiling, Pangxiemu, Shaziling and Xishan plutons. Whole-rock geochemistry, mineral electron microprobe analysis, zircon U-Pb dating and Hf isotope analysis were carried out for the Jinjiling and Pangxiemu plutons. The zircon U-Pb dating yields weighted mean ages of 152.9±0.9 Ma for the Jinjiling pluton and 151.7±1.5 Ma for the Pangxiemu pluton, with a narrow gap between them. The Jinjiling and Pangxiemu plutons both have geochemical characteristics of high SiO2, Al2O3, Na2O, K2O and low TiO2, MgO, CaO, P2O5 contents, with intense depletionS in Sr, Ba, Ti, Eu and enrichments?in Ga, FeOT and HFSE, and these characteristics reflect an A-type affinity. From the Jinjiling to the Pangxiemu plutons, the mineral composition of mica changes from lepidomelane to zinnwaldite, with increases in F, Li2O and Rb2O contents. The mineral composition of zircon changes from low Zr/Hf to high Zr/Hf, with increasing HfO2, P2O5 and UO2+ThO2+Y2O3 contents. The mineral compositions of feldspar indicate that the Pangxiemu pluton contains more alkali feldspar than the Jinjiling pluton. The whole-rock geochemistry and mineral compositions reveal a higher degree of differentiation for the Pangxiemu pluton. The nearly uniform εHf(t) indicates the same source region for the two plutons: both were derived from partial melting of the lower crust, with small contributions of mantle materials. In addition, higher F, lower Nb/Ta and Zr/Hf ratios in the Pangxiemu Pluton suggest a closer relationship with the rare metal mineralization than for the Jinjiling pluton.     

Origin,age, and significance of deep‐seated granulite‐facies migmatites in the Barrow zones of Scotland,Cairn Leuchan,Glen Muick area

Petrological modelling of granulite‐facies mafic and semipelitic migmatites from Cairn Leuchan, northeast Scotland, has provided new constraints on the pressure (P) and temperature (T) conditions of high‐grade metamorphism in the type‐locality Barrow zones. Phase diagrams constructed in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂ system have constrained the P?T conditions of peak metamorphism in the Glen Muick region of the upper sillimanite zone (Sill+Kfs) to have been at least ～840°C at ～9 kbar (high‐P granulite facies). These conditions are ～120°C and ～3 kbar higher than those recorded by lower sillimanite zone (Sill+Ms) units located only a few kilometres away to the southeast at Glen Girnock, indicating the presence of a significant thermal and barometric high exposed within the Scottish Dalradian, and supporting previous suppositions of a potential tectonic break between the two regions. U–Pb zircon geochronology performed on these mafic migmatites produced ages of c. 540–470 Ma from grains with both igneous and metamorphic morphological characteristics. Their basaltic protoliths likely formed during a period of volcanism dated at c. 570 Ma, associated with passive‐margin extension prior to the onset of Iapetus Ocean closure, and high‐grade metamorphism and partial melting is interpreted to have taken place at c. 470 Ma, synchronous with sillimanite‐grade metamorphism recorded elsewhere in the Dalradian. These high‐grade Cairn Leuchan lithologies are interpreted as representing a fragment of Grampian Terrane lower crust that was exhumed via displacement along a steeply dipping tectonic discontinuity related to the Portsoy–Duchray Hill Lineament, and are not pre‐Caledonian Mesoproterozoic basement, as suggested by some previous studies. Veins within some mafic migmatites in the Cairn Leuchan area, composed almost entirely (>80%) of garnet, with minor quartz, plagioclase, amphibole, and clinopyroxene, are interconnected with leucosomes and are interpreted to represent former garnet‐bearing melt segregations that have been locally drained of almost all melt. Thus, mafic components of the lower crust, currently underlying relatively lower grade metasediments exposed to the southeast, may represent a potential source rock for widely documented, post‐orogenic felsic plutons, sills, and dykes that occur throughout the Grampian Terrane.     

UHT metamorphism on Seram,eastern Indonesia: reaction microstructures and P–T evolution of spinel‐bearing garnet–sillimanite granulites from the Kobipoto Complex

The island of Seram, part of the northern limb of the Banda Arc in eastern Indonesia, exposes an extensive Mio‐Pliocene granulite facies migmatite complex (the Kobipoto Complex) comprising voluminous leucosome‐rich diatexites and scarcer Al–Fe‐rich residual granulites. The migmatites are intimately associated with ultramafic rocks of predominantly lherzolitic composition that were exhumed by substantial lithospheric extension beneath low‐angle detachment faults; heat supplied by the lherzolites was evidently a major driver for the granulite facies metamorphism and accompanying anatexis. Residual garnet–sillimanite granulites sampled from the Kobipoto Mountains, central Seram, contain scarce garnet‐hosted inclusions of hercynite spinel (~1.5 wt% ZnO) + quartz (± ilmenite) in direct grain‐boundary contact – an assemblage potentially indicative of metamorphism under ultrahigh‐temperature (UHT) conditions. thermocalc ‘Average P–T’ reactions and melanosome‐specific thermocalc , T–M_O, and P–T pseudosections in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCKFMASHTO) chemical system, supported by Ti‐in‐garnet thermobarometry, are permissive of the rock having experienced a clockwise P–T path peaking at 925 °C and 9 kbar – thus narrowly reaching UHT conditions – before undergoing near‐isothermal decompression to ~750 °C and ~4 kbar. Spinel + quartz assemblages are interpreted to have formed at or just after the metamorphic peak from localized reactions between sillimanite, ilmenite and surrounding garnet. Further decompression of the rock resulted in the formation of complex reaction microstructures comprising cordierite ± plagioclase coronae around garnet, and symplectic intergrowths of cordierite + spinel + ilmenite around sillimanite. Small grains of sapphirine + corundum developed subsequently within spinel by localized quartz‐absent reactions. The post‐peak evolution of the granulites may be related to previously published U–Pb zircon and ⁴⁰Ar/³⁹Ar ages of c. 16 Ma, further substantiating the claim for the Kobipoto Complex granulites having recorded Earth's youngest‐identified episode of UHT metamorphism, albeit at slightly lower temperature and higher pressure than previously inferred. The Kobipoto Complex granulites demonstrate how UHT conditions may be achieved in the ‘modern’ Earth by extreme lithospheric extension, which, in this instance, was driven by slab rollback of the Banda Arc.     

The effect of Mn on mineral stability in metapelites revisited: new a–x relations for manganese‐bearing minerals

The a–x relations recently presented in White et al. ( 2014 , Journal of Metamorphic Geology, 32, 261–286) are extended to include MnO. This provides a set of internally consistent a–x relations for metapelitic rocks in the MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂ (MnNCKFMASHTO) system. The mixing parameters for the Mn‐bearing minerals were estimated using the micro‐? approach of Powell et al. ( 2014 , Journal of Metamorphic Geology, 32, 245–260). Then the Mn‐end‐member thermodynamic properties were calibrated using a database of co‐existing minerals involving literature data from rocks and from experiments on natural materials. Mn‐end‐members were calibrated for orthopyroxene, cordierite, staurolite, chloritoid, chlorite, biotite, ilmenite and hematite, assuming known properties for the garnet end‐member spessartine. The addition of MnO to phase diagram calculations results in a marked expansion of the stability of garnet‐bearing assemblages. At greenschist facies conditions garnet stability is extended down temperature. At amphibolite facies conditions, the garnet‐in boundary shifts to lower pressure. While the addition of MnO greatly influences the stability of garnet, it has relatively little effect on the stability of other common metapelitic minerals, with the resultant diagrams being topologically very similar to those calculated without MnO. Furthermore, the addition of MnO in the amounts measured in most metapelites has only a small effect on the mode of garnet, with calculated garnet modes remaining smaller than 1% in the P–T range outside its predicted Mn‐free P–T range.     

Petrogenesis of Late Cretaceous Jiangla’angzong I-Type Granite in Central Lhasa Terrane, Tibet, China: Constraints from Whole-Rock Geochemistry, Zircon U-Pb Geochronology, and Sr-Nd-Pb-Hf Isotopes

The Jiangla'angzong granite in the northern part of the Central Lhasa Terrane is composed of syenogranite and adamellite. LA-ICP-MS zircon U-Pb analyses suggest that syenogranite has a weighted mean ~(206) Pb/~(238) U age of 86±1 Ma(mean square weighted deviation=0.37), which is in accordance with the muscovite Ar-Ar age(85±1 Ma) of Cu-Au ore-bearing skarns and the zircon U-Pb age(84±1 Ma) of adamellite. This suggests that the Jiangla'angzong magmatism and Cu–Au mineralization events took place during the Late Cretaceous. The granite contains hornblende, biotite, and pyroxene, and does not contain Al-bearing minerals, such as muscovite, cordierite, and garnet. It has high contents of SiO_2(65.10–70.91 wt%), K_2O(3.44–5.17 wt%), and total K_2O+Na_2O(7.13–8.15 wt%), and moderate contents of A_(12)O_3(14.14–16.45 wt%) and CaO(2.33–4.11 wt%), with a Reitman index(σ43) of 2.18 to 2.33, and A/CNK values of 0.88 to 1.02. The P_2O_5 contents show a negative correlation with SiO_2, whereas Pb contents show a positive correlation with SiO_2. Th and Y contents are relatively low and show a negative correlation with the Rb contents. These characteristics suggest that the Jiangla'angzong granite is a high K calc–alkaline metaluminous I–type granite. It is enriched in light rare earth elements(LREE) and large ion lithofile elements(LILE), and depleted in heavy rare earth elements(HREE) and high field strength elements(HFSE), with LREE/HREE ratios of 11.7 to 18.1. The granite has negative Eu anomalies of 0.58 to 0.94 without obvious Ce anomalies(δCe=1.00–1.04). The relatively low initial 87 Sr/86 Sr ratios of 0.7106 to 0.7179, positive εHf(t) values of 1.0 to 4.1, and two-stage Hf model ages(TDM2) ranging from 889 Ma to 1082 Ma, These geochemical features indicate that the granite derived from a juvenile crust. The(~(143) Nd/~(144) Nd)_t values from the Jiangla'angzong granite range from 0.5121 to 0.5123, its εNd(t) values range from-10.17 to-6.10, its(~(206) Pb/~(204) Pb)_t values range from 18.683 to 18.746, its(~(207) Pb/~(204) Pb)_t values range from 15.695 to 15.700, and its(~(208) Pb/~(204) Pb)_t values range from 39.012 to 39.071. These data indicate that the granite was formed by melting of the upper crust with the addition of some mantle materials. We propose that the Jiangla'angzong granite was formed during the postcollision extension of the Qiangtang and Lhasa terranes.     

Pressure–temperature evolution of blueschist facies rocks from Sifnos, Greece, and implications for the exhumation of high-pressure rocks in the Central Aegean

The blueschist and greenschist units on the island of Sifnos, Cyclades were affected by Eocene high‐pressure (HP) metamorphism. Using conventional geothermobarometry, the HP peak metamorphic stage was determined at 550–600 °C and 20 kbar, close to the blueschist and the eclogite facies transition. The retrograde P–T paths are inferred with phase diagrams. Pseudosections based on a quantitative petrogenetic grid in the model system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O reveal coeval decompression and cooling for both the blueschist and the greenschist unit. The conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10–12 °C km^?1 for both units. The retrograde overprint can be assigned to low‐pressure blueschist to HP greenschist facies conditions. This result cannot be reconciled with the (prograde) Barrovian‐type event, which affected parts of the Cyclades during the Oligocene to Miocene. Instead, the retrograde overprint is interpreted in terms of exhumation, directly after the HP stage, without a separate metamorphic event. Constraints on the exhumation mechanism are given by decompression‐cooling paths, which can be explained by exhumation in a fore‐arc setting during on‐going subduction and associated crustal shortening. Back‐arc extension is only responsible for the final stage of exhumation of the HP units.     

Phase equilibria modelling of blueschist and eclogite from the Sanbagawa metamorphic belt of southwest Japan reveals along‐strike consistency in tectonothermal architecture

The Sanbagawa metamorphic belt of southwest Japan is one of the type localities of subduction‐related high‐P metamorphism. However, variable pressure–temperature (P–T) paths and metabasic assemblages have been reported for eclogite units in the region, leading to uncertainty about the subduction zone paleo‐thermal structure and associated tectonometamorphic conditions. To analyse this variation, phase equilibria modelling was applied to the three main high‐P metabasic rock types documented in the region – glaucophane eclogite, barroisite eclogite and garnet blueschist – with modelling performed over a range of P, T, bulk rock H₂O and bulk rock ferric iron conditions using thermocalc . All samples are calculated to share a common steep prograde P–T path to similar peak conditions of ～16–20 kbar and 560–610 °C. The results establish that regional assemblage variation is systematic, with the alternation in peak amphibole phase due to peak conditions overlapping the glaucophane–barroisite solvus, and bulk composition effects stabilizing blueschist v. eclogite facies assemblages at similar P–T conditions. Furthermore, the results reveal that a steep prograde P–T path is common to all eclogite units in the Sanbagawa belt, indicating that metamorphic conditions were consistent along strike. All localities are compatible with predictions made by a ridge approach model, which attributes eclogite facies metamorphism and exhumation of the Sanbagawa belt to the approach of a spreading ridge.     

Simultaneous Determination of Major and Trace Elements in Fused Volcanic Rock Powders Using a Hermetic Vessel Heater and LA‐ICP‐MS

Fused glass prepared without the addition of a flux is generally more homogeneous than a pressed powder pellet and thus ideal for analysis of bulk samples by LA‐ICP‐MS. In this work, a new glass‐making method using a boron nitride crucible was developed to prepare homogenous glass samples from silicate rock powder. The apparatus consisted of a small boron nitride vessel with net volume of about 34 mm³ and two molybdenum strips. Applying the summed metal oxide normalisation technique, both major and trace element contents in the fused glass were measured by LA‐ICP‐MS. Analyses of five geochemical reference materials (spanning the compositional range basalt–andesite–rhyolite) indicated that the measured SiO₂, Al₂O₃ and P₂O₅ contents matched the preferred values to within 5%, and the other major elements generally matched the preferred values to within 8%. Except for the transition metals, the measured trace element contents generally matched the preferred values to within 10%. Compared with the iridium heater method developed by Stoll et al. (2008), element volatilisation during high‐temperature melting was effectively suppressed in our method, but metal segregation caused by reduction of BN may cause loss of Cr, Ni and Cu. Although analysis with a large spot size has the advantage of improving counting statistics, matrix effects induced by mass loading of the ICP may hamper the accurate determination of some elements.     

Diagenetic trends of fluorine concentration in Negev phosphorites, Israel: implications for carbonate fluorapatite composition during phosphogenesis

An electron probe and chemical study of bulk phosphorite samples and separated constituents from various Negev deposits was carried out together with XRD, FTIR spectroscopy and textural analysis. The results allow a better understanding of the distribution of fluorine in these Upper Cretaceous phosphorite sequences and shed light on variations in the composition of the carbonate fluorapatite (CFA) phase during phosphogenesis. Two facies are recognized: (1) a pristine, microbially generated phosphorite facies; (2) a recycled, peloidal and biodetrital facies. Fluorine distribution in the Negev phosphorites is facies controlled: F/P₂O₅ is much lower in the pristine facies (0·090–0·107) than in the recycled facies (0·107–0·120). In addition, F/P₂O₅ varies considerably between the various constituents of the phosphate fraction; F‐poor francolites (F/P₂O₅ as low as 0·080) co‐exist with F‐rich francolites (F/P₂O₅ as high as 0·135) in the same phosphorite bulk sample. A lower F/P₂O₅ in francolite is associated with higher Cd and Zn concentrations in the phosphorite, an increase in Fe‐rich smectites in the clay fraction and the presence of structural OH in the francolite. The lower F/P₂O₅ ratios in the pristine facies are attributed to high organic deposition rates during the formation of these matted sediments, leading to rapid burial of the in situ‐forming CFA. This is possibly coupled with diffusion of F from sea water into bottom sediments being hampered by microbial mat coatings. These conditions resulted in O₂‐depleted porefluids, inducing the precipitation of Cd‐rich Zn sulphides and the formation of Fe‐rich smectites. F‐enrichment probably takes place when the earlier formed F‐poor ‘primary’ CFA is relocated close to the sea floor and bathed with interstitial sea water solutions of higher F concentrations. Oxidation and removal of the sulphide‐bound Cd and Zn apparently occurred together with enrichment in F of the francolite. Combining chemical data with XRD and FTIR results suggests a multistage growth for the Negev phosphate constituents in shifting formational sites and porefluids of varying F concentrations. This multiphase growth is reflected in the patchy distribution of F in the Negev constituents and might explain the inverse correlation between mean CO₂/F and F/P₂O₅ ratios of the analysed phosphorites in the two facies. It also suggests that CFA (or an amorphous precursor) initially formed with some OH groups in the apatite structure, which were subsequently substituted by F ions in recycled francolite through re‐equilibration with porefluids of higher F concentrations.     

The tectono‐metamorphic evolution of the very low‐grade hangingwall constrains two‐stage gneiss dome formation in the Montagne Noire (Southern France)

The Montagne Noire in the southernmost French Massif Central is made of an ENE‐elongated gneiss dome flanked by Palaeozoic sedimentary rocks. The tectonic evolution of the gneiss dome has generated controversy for more than half a century. As a result, a multitude of models have been proposed that invoke various tectonic regimes and exhumation mechanisms. Most of these models are based on data from the gneiss dome itself. Here, new constraints on the dome evolution are provided based on a combination of very low‐grade petrology, K–Ar geochronology, field mapping and structural analysis of the Palaeozoic western Mont Peyroux and Faugères units, which constitute part of the southern hangingwall of the dome. It is shown that southward‐directed Variscan nappe‐thrusting (D₁) and a related medium‐P metamorphism (M₁) are only preserved in the area furthest away from the gneiss dome. The regionally dominant pervasive tectono‐metamorphic event D₂/M₂ largely transposes D₁ structures, comprises a higher metamorphic thermal gradient than M₁ (transition low‐P and medium‐P metamorphic facies series) and affected the rocks between c. 309 and 300 Ma, post‐dating D₁/M₁ by more than 20 Ma. D₂‐related fabrics are refolded by D₃, which in its turn, is followed by dextral‐normal shearing along the basal shear zone of both units at c. 297 Ma. In the western Mont Peyroux and Faugères units, D₂/M₂ is largely synchronous with shearing along the southern dome margin between c. 311 and 303 Ma, facilitating the emplacement of the gneiss dome into the upper crust. D₂/M₂ also overlaps in time with granitic magmatism and migmatization in the Zone Axiale between c. 314 and 306 Ma, and a related low‐P/high‐T metamorphism at c. 308 Ma. The shearing that accompanied the exhumation of the dome therefore was synchronous with a peak in temperature expressed by migmatization and intrusion of melts within the dome, and also with the peak of metamorphism in the hangingwall. Both, the intensity of D₂ fabrics and the M₂ metamorphic grade within the hangingwall, decrease away from the gneiss dome, with grades ranging from the anchizone–epizone boundary to the diagenetic zone. The related zonation of the pre‐D₃ metamorphic field gradients paralleled the dome. These observations indicate that D₂/M₂ is controlled by the exhumation of the Zone Axiale, and suggest a coherent kinematic between the different crustal levels at some time during D₂/M₂. Based on integration of these findings with regional geological constraints, a two‐stage exhumation of the gneiss dome is proposed: during a first stage between c. 316 and 300 Ma dome emplacement into the upper crust was controlled by dextral shear zones arranged in a pull‐apart‐like geometry. The second stage from 300 Ma onwards was characterized by northeast to northward extension, with exhumation accommodated by north‐dipping detachments and hangingwall basin formation along the northeastern dome margin.     

Blueschist‐facies metapelites from the Malpica–Tui Unit (NW Iberian Massif): phase equilibria modelling and H2O and Fe2O3 influence in high‐pressure assemblages

The Malpica–Tui Unit (Galicia, NW Spain) records eclogite‐ and blueschist‐facies metamorphism during the onset of the Variscan orogeny in Europe. Petrological analysis involving pseudosections calculated using thermocalc shows that the Upper Sheet of this unit, the Ceán Schists, recorded a three‐stage metamorphic evolution involving (i) Early subduction‐related medium‐pressure/low‐temperature metamorphism (M₁) constrained at ～350–380 °C, 12–14 kbar, which is only recorded in the basal part (lower metapelites, LM) of the Ceán Schists. (ii) Subduction‐related blueschist facies prograde metamorphism (M₂) going from ～19 kbar, 420 °C to 21 kbar, 460 °C in the LM, and from 16 kbar 430 °C to 21–22 kbar, 520 °C in the structurally upper metapelites (UM). (iii) Exhumation‐related metamorphism (M₃) is characterized by a decompression to 8–10 kbar, 470–490 °C in the LM. This decompression is also recorded in the UM, but it was not possible to estimate precise P–T conditions. The calculations indicate that (i) the prograde evolution in subduction zones may occur in fluid‐undersaturated conditions due to the crystallization of lawsonite, even in metapelitic rocks. This significantly influences phase equilibria and hence the P–T estimates. (ii) The proportion of ferric iron also has a strong influence on phase equilibria, even in metapelites. However, the analysed values of Fe₂O₃ may not reflect the oxidation state during the main metamorphic evolution and are probably easily modified by superficial alteration even in apparently fresh samples. The use of P–T–X(Fe₂O₃) pseudosections together with petrographic observations is then necessary to estimate the real oxidation state of the rocks and correctly evaluate the P–T conditions.     

Fluid Inclusions and Hydrogen,Oxygen, Sulfur Isotopes of Nuri Cu‐W‐Mo Deposit in the Southern Gangdese,Tibet

The Nuri Cu‐W‐Mo deposit is located in the southern subzone of the Cenozoic Gangdese Cu‐Mo metallogenic belt. The intrusive rocks exposed in the Nuri ore district consist of quartz diorite, granodiorite, monzogranite, granite porphyry, quartz diorite porphyrite and granodiorite porphyry, all of which intrude in the Cretaceous strata of the Bima Group. Owing to the intense metasomatism and hydrothermal alteration, carbonate rocks of the Bima Group form stratiform skarn and hornfels. The mineralization at the Nuri deposit is dominated by skarn, quartz vein and porphyry type. Ore minerals are chalcopyrite, pyrite, molybdenite, scheelite, bornite and tetrahedrite, etc. The oxidized orebodies contain malachite and covellite on the surface. The mineralization of the Nuri deposit is divided into skarn stage, retrograde stage, oxide stage, quartz‐polymetallic sulfide stage and quartz‐carbonate stage. Detailed petrographic observation on the fluid inclusions in garnet, scheelite and quartz from the different stages shows that there are four types of primary fluid inclusions: two‐phase aqueous inclusions, daughter mineral‐bearing multiphase inclusions, CO₂‐rich inclusions and single‐phase inclusions. The homogenization temperature of the fluid inclusions are 280°C–386°C (skarn stage), 200°C–340°C (oxide stage), 140°C–375°C (quartz‐polymetallic sulfide stage) and 160°C–280°C (quartz‐carbonate stage), showing a temperature decreasing trend from the skarn stage to the quartz‐carbonate stage. The salinity of the corresponding stages are 2.9%–49.7 wt% (NaCl) equiv., 2.1%–7.2 wt% (NaCl) equiv._, 2.6%–55.8 wt% (NaCl) equiv. and 1.2%–15.3 wt% (NaCl) equiv., respectively. The analyses of CO₂‐rich inclusions suggest that the ore‐forming pressures are 22.1 M Pa–50.4 M Pa, corresponding to the depth of 0.9 km–2.2 km. The Laser Raman spectrum of the inclusions shows the fluid compositions are dominated in H₂O, with some CO₂ and very little CH₄, N₂, etc. δD values of garnet are between ?114.4‰ and ?108.7‰ and δ¹⁸O_H2O between 5.9‰ and 6.7‰; δD of scheelite range from ?103.2‰ to ?101.29‰ and δ¹⁸O_H2O values between 2.17‰ and 4.09‰; δD of quartz between ?110.2‰ and ?92.5‰ and δ¹⁸O_H2O between ?3.5‰ and 4.3‰. The results indicate that the fluid came from a deep magmatic hydrothermal system, and the proportion of meteoric water increased during the migration of original fluid. The δ³⁴S values of sulfides, concentrated in a rage between ?0.32‰ to 2.5‰, show that the sulfur has a homogeneous source with characteristics of magmatic sulfur. The characters of fluid inclusions, combined with hydrogen‐oxygen and sulfur isotopes data, show that the ore‐forming fluids of the Nuri deposit formed by a relatively high temperature, high salinity fluid originated from magma, which mixed with low temperature, low salinity meteoric water during the evolution. The fluid flow through wall carbonate rocks resulted in the formation of layered skarn and generated CO₂ or other gases. During the reaction, the ore‐forming fluid boiled and produced fractures when the pressure exceeded the overburden pressure. Themeteoric water mixed with the ore‐forming fluid along the fractures. The boiling changed the pressure and temperature, oxygen fugacity, physical and chemical conditions of the whole mineralization system. The escape of CO₂ from the fluid by boiling resulted in scheelite precipitation. The fluid mixing and boiling reduced the solubility of metal sulfides and led the precipitation of chalcopyrite, molybdenite, pyrite and other sulfide.     

An Eocene/Oligocene blueschist‐/greenschist facies P–T loop from the Cycladic Blueschist Unit on Naxos Island,Greece: Deformation‐related re‐equilibration vs. thermal relaxation

Geothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist/greenschist facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure–temperature (P–T) method of thermocalc coupled with detailed textural work, we separate an early blueschist facies event at 576 ± 16 to 619 ± 32°C and 15.5 ± 0.5 to 16.3 ± 0.9 kbar from a subsequent greenschist facies overprint at 384 ± 30°C and 3.8 ± 1.1 kbar. Multi‐mineral Rb–Sr isochron dating yields crystallization ages for near peak‐pressure blueschist facies assemblages between 40.5 ± 1.0 and 38.3 ± 0.5 Ma. The greenschist facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist facies reworking, obtained from disequilibrium patterns, cluster near c. 32 Ma, with one sample showing rejuvenation at c. 27 Ma. We conclude that the high‐P rocks from south Naxos were exhumed to upper mid‐crustal levels in the late Eocene and early Oligocene at rates of 7.4 ± 4.6 km/Ma, completing a full blueschist‐/greenschist facies metamorphic cycle soon after subduction within c. 8 Ma. The greenschist facies overprint of the blueschist facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid‐controlled metamorphic re‐equilibration, and is unrelated to the strong high‐T metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.     

Geochemical Constraints on the Origin of Banded Iron Formation‐Hosted Iron Ore from the Archaean Ntem Complex (Congo Craton) in the Meyomessi Area,Southern Cameroon

Banded iron formations (BIFs) are the most significant source of iron in the world. In this study, we report petrographic and geochemical data of the BIF from the Meyomessi area in the Ntem Complex, southern Cameroon, and discuss their genesis and the iron enrichment process. Field investigations and petrography have revealed that the studied BIF samples are hard; compact; weakly weathered; and composed of magnetite, subordinate quartz, and geothite. The geochemical composition of the whole rock reveals that iron and silica represent more than 98 wt% of the average composition, whereas Al₂O₃, TiO₂, and high‐field strength elements (HFSE) contents are very low, similar to detritus‐free marine chemical precipitates. The total iron (TFe) contents range from 48.71 to 65.32 wt % (average of 53.29 wt %) and, together with the low concentrations of deleterious elements (0.19 wt % P on average), are consistent with medium‐grade iron ores by global standards. This interpretation is confirmed by the SiO₂/Fe₂O_3total versus (MgO + CaO + MnO)/Fe₂O_3total discrimination plot in which most of the Meyomessi BIF samples fall in the field of medium‐grade siliceous ore. Only one sample (MGT94) plots in the high‐grade magnetite–geothite ore domain. The high Fe/Ti (376.36), Fe/Al (99.90), and Si/Al (29.26) ratios of the sample are consistent with significant hydrothermal components. The rare earth elements (REE) contents of the studied BIF samples are very low (∑REE: 0.81–1.47 ppm), and the Post‐Archaean Australian Shale (PAAS)‐normalized patterns display weak positive Eu anomalies (Eu/Eu*: 1.15–1.33), suggesting a syngenetic low‐temperature hydrothermal solutions, similar to other BIF worldwide. However, the Meyomessi BIFs show high Fe contents when compared to the other BIFs. This indicates an epigenetic mineralization process affected the Meyomessi BIF. From the above results and based on the field and analytical data, we propose that the genetic model of iron ores at the Meyomessi area involves two stages of the enrichment process, hypogene enrichment of BIF protore by metamorphic and magmatic fluids followed by supergene alteration as indicated by the presence of goethite in the rocks.     

Genesis of the Xuebaoding W–Sn–Be Crystal Deposits in Southwest China: Evidence from Fluid Inclusions,Stable Isotopes and Ore Elements

The Xuebaoding crystal deposit, located in northern Longmenshan, Sichuan Province, China, is well known for producing coarse‐grained crystals of scheelite, beryl, cassiterite, fluorite and other minerals. The orebody occurs between the Pankou and Pukouling granites, and a typical ore vein is divided into three parts: muscovite and beryl within granite (Part I); beryl, cassiterite and muscovite in the host transition from granite to marble (Part II); and the main mineralization part, an assemblage of beryl, cassiterite, scheelite, fluorite, apatite and needle‐like tourmaline within marble (Part III). No evidence of crosscutting or overlapping of these ore veins by others suggests that the orebody was formed by single fluid activity. The contents of Be, W, Sn, Li, Cs, Rb, B, and F in the Pankou and Pukouling granites are similar to those of the granites that host Nanling W–Sn deposits. The calculated isotopic compositions of beryl, scheelite and cassiterite (δD, ?69.3‰ to ?107.2‰ and δ¹⁸O_H2O, 8.2‰ to 15.0‰) indicate that the ore‐forming fluids were mainly composed of magmatic water with minor meteoric water and CO₂ derived from decarbonation of marble. Primary fluid inclusions are CO₂? CH₄+ H₂O ± CO₂ (vapor), with or without clathrates and halites. We estimate the fluid trapping condition at T = 220 to 360°C and P > 0.9 kbar. Fluid inclusions are rich in H₂O, F^‐ and Cl^‐. Evidence for fluid‐phase immiscibility during mineralization includes variable L/V ratios in the inclusions and inclusions containing different phase proportions. Fluid immiscibility may have been induced by the pressure released by extension joints, thereby facilitating the mineralization found in Part III. Based on the geochemical data, geological occurrence, and fluid inclusion studies, we hypothesize that the coarse‐grained crystals were formed by: (i) the high content of ore elements and volatile elements such as F in ore‐forming fluids; (ii) occurrence of fluid immiscibility and Ca‐bearing minerals after wall rock transition from granite to marble making the ore elements deposit completely; (iii) pure host marble as host rock without impure elements such as Fe; and (iv) sufficient space in ore veins to allow growth.     

Subduction and Collision of the Jinsha River Paleo-Tethys: Constraints from Zircon U-Pb Dating and Geochemistry of the Ludian Batholith in the Jiangda–Deqen–Weixi Continental Margin Arc

The Jiangda–Deqen–Weixi continental margin arc(DWCA) developed along the base of the Changdu–Simao Block and was formed as a result of the subduction of the Jinsha River Ocean Slab and the subsequent collision. The Ludian batholith is located in the southern part of the DWCA and is the largest batholith in northwest Yunnan. Granite samples from the Ludian batholith yield an early Middle Permian age of 271.0 ± 2.8 Ma. The geochemical data of the early Middle Permian granitoids show high Si2 O, low P2 O5 and MgO contents that belong to calc-alkaline series and peraluminous I-type rocks. Their εHf(t) values range from-5.01 to +0.58, indicating that they were formed by hybrid magmas related to the subduction of the Jinsha River Tethys Ocean. The monzonite and monzogranite samples yield Late Permian ages of 250.6 ± 1.8 Ma and 252.1 ± 1.3 Ma, respectively. The Late Permian granitoids are high-K calc alkaline and shoshonite series metaluminous I-type rocks. Their εHf(t) values range from-4.12 to-1.68 and from-7.88 to-6.64, respectively. The mixing of crustal and mantle melts formed the parental magma of the Late Permian granitoids. This study, combined with previous work, demonstrates the process from subduction to collision of the Jinsha River Paleo-Tethys Ocean.     

Ultrahigh‐temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long‐lived high‐T orogen

We report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930°C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. Eu anomalies in monazite reflect changes in fO₂ relative to quartz–fayalite–magnetite related to the growth and breakdown of spinel. The ratio Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (<29 ± 8 Ma) suggests that a thin mantle lithosphere (<80 km) or advective heating may have also been important in the formation of this high‐T, low‐P terrane.