Petrogenesis of layered pyroxenites from the Lherz,Freychinéde and Prades ultramafic bodies (Ariége,French Pyrénées)

The pyroxenite bands in spinel Iherzolites from three high-temperature peridotite massifs (Lherz, Freychinéde and Prades) in Ariège (French Pyrénées) have geochemical features including distributions of Mg, Ca, K, Ti, Zr and REE typical of cumulates derived by high P−T crystal segregation. However, these features of the whole-rocks are not reflected in the element distribution among the minerals which was partially modified during subsequent recrystallization. In addition, samples from the margins of the pyroxenite bands were affected by secondary processes such as metasomatism or subsolidus re-equilibration. Unlike pyroxenite xenoliths from basalts which are usually assumed to be derived from alkalic basaltic magmas, the pyroxenite layers are related to tholeiitic magmas similar to the parental liquids of the Triassic dolentes encountered throughout the Pyrénées. The data show that high P−T fractionation significantly affected the ascending tholeiitic magma and suggest that the continental tholeiites could have been derived from an upper mantle source with a chondrite shaped REE pattern.     

Electrical resistivity tomography to characterize a karstic Vauclusian spring: Fontaine d’Orbe (Pyrénées,France)

The study site lies on a karst system in the Pyrénées (France) that crosses the Urgonian limestones. Organic matter was dumped on the west bank of the River Vert d’Arette, which polluted a spring (the Fontaine d’Orbe) situated on the east bank. Electrical resistivity tomography was utilised in order to understand the geometry of this karst outlet. After tests with the pole-pole array, the ideal electrode spacing was found to be less than 3 m. A conduit, a little less than 10 m wide, was discovered, which proved to be the terminal conduit of the karst system. It was established that the conduit traverses both banks of the River Vert d’Arette, thus presenting a hydraulic link between the two sides. Moreover, this conduit, which is totally submerged, passes under the riverbed and goes upwards to the spring. In addition to the geophysical measurements, hydrogeological measurements and observations were made. Based on the characterisation of the geometry of the karst network’s terminal conduit, and the fact that is completely full of water and inclined, it is concluded that the Fontaine d’Orbe spring is of the Vauclusian variety.     

Abundant Fe–Ti oxide inclusions in olivine from the Panzhihua and Hongge layered intrusions, SW China: evidence for early saturation of Fe–Ti oxides in ferrobasaltic magma

Abundant Fe–Ti oxide inclusions in cumulus olivine (Fo_77–81) from the Panzhihua and Hongge intrusions, Emeishan large igneous province, SW China, document the first evidence for early crystallization of Fe–Ti oxides in ferrobasaltic systems in nature. The intrusions also contain significant stratiform Fe–Ti–V oxide ores. The oxide inclusions are sub-rounded or irregular, range from ∼5 to 50 μm in diameter, and are dominated by either titanomagnetite or ilmenite. The fact that the inclusions are either titanomagnetite- or ilmenite-dominant suggests that they are trapped crystals, instead of immiscible oxide melt, formed during growth of the host olivine. The absence of other silicate phases in the inclusion-bearing olivine is difficult to reconcile with a possible xenocrystic origin of the oxide inclusions. These oxide inclusions are thus interpreted to be cumulus minerals crystallized together and trapped in olivine from the same parental magma. In addition to Fe–Ti oxides, some inclusions contain amphibole + biotite ± fluorapatite that might have formed by reaction of trapped hydrous liquid with the host olivine. Numerical modeling of high-Ti Emeishan basalts using the MELTS program successfully simulates early crystallization of olivine (∼Fo₈₁) and Fe–Ti spinel in the presence of a moderate amount of H₂O (∼1.5 wt%) under pressure and fO₂ conditions generally pertinent to the Panzhihua and Hongge intrusions. The modal mineralogy of the oxide inclusions is in good agreement with the bulk compositions of the ore, as inferred from whole-rock data, in a given intrusion. This is consistent with the interpretation that the stratiform oxide ores in the intrusions formed by accumulation of Fe–Ti oxide crystals that appeared on the liquidus with olivine and clinopyroxene.     

Melt percolation and reaction atop a plume: evidence from the poikiloblastic peridotite xenoliths from Borée (Massif Central, France)

Poikiloblastic harzburgite xenoliths (P-type) from Borée, France are characterised by large (>1 cm), essentially unstrained olivines and high equilibrium temperatures (>1200 °C). Mineralogical data, trace element abundances and Sr-Nd-O isotopes of the constituent minerals are consistent with formation as a result of melt percolation-reactions in a lherzolite precursor during lithospheric erosion by an upwelling plume. This petrogenetic model contrasts with previous models involving isochemical recrystallisation from a granular lherzolite precursor (G-type) or derivation as metacumulates from tholeiitic magmas. Numerical simulation of percolation reactions at the lithosphere-plume boundary using the plate model of Vernières et al. (1997) indicates that the different textured xenoliths may represent mantle from different levels in a percolation-reaction column. If correct then the P-type harzburgites resulted from pyroxene-dissolving and olivine-producing reactions at increasing melt fraction (>3%) at the lower part of column (base of the lithosphere), whereas the G-type lherzolites were located within the low-porosity domain (<0.1%) above a permeability barrier, and are formed through a melt-rock reaction at decreasing melt mass. Given the very low melt fraction, the REE fractionation in this zone is controlled by chromatographic effects coupled with source effects of reaction. The variations in porosity, melt/rock ratio and melt-rock reaction mechanism are believed to be responsible for the diversity of REE patterns and striking correlation between REE abundance and texture in Borée xenoliths. Received: 15 June 1997 / Accepted: 7 January 1998     

Exhumation cénozoı̈que des massifs du Canigou et de Mont-Louis (Pyrénées orientales,France)Cainozoic exhumation of the Canigou and Mont-Louis massifs (eastern Pyrenees,France)

The onset of exhumation of the Canigou massif, related to the reactivation of the Têt Fault during the Gulf of Lion opening, is dated at 26–27 Ma, using apatite fission track method and lasts until the Aquitanian, at a rate of $\text{0.29±0.07 }\text{mm}\text{yr}{}^{\mathrm{?1}}$. The subsequent evolution is necessarily followed by a slower rate of exhumation to account for total exhumation. At the hanging-wall of this fault, the granite of Mont-Louis records an older exhumation history, where the crossing of the 110 °C isotherm at 40±4 Ma is related to the Pyrenean thrusting. To cite this article: O. Maurel et al., C. R. Geoscience 334 (2002) 941–948.     

The origin of zircon and the significance of U–Pb ages in high-grade metamorphic rocks: a case study from the Variscan orogenic root (Vosges Mountains, NE France)

U–Pb zircon dating is combined with petrology, Zr-in-rutile thermometry and mineral equilibria modelling to discuss zircon petrogenesis and the age of metamorphism in three units of the Variscan Vosges Mountains (NE France). The monotonous gneiss unit shows results at 700–500?Ma, but no Variscan ages. The varied gneiss unit preserves ages between 600 and 460?Ma and a Variscan group at 340–335?Ma. Zircon analyses from the felsic granulite unit define a continuous array of ages between 500 and 340?Ma. In varied gneiss samples, zoned garnet includes kyanite and rutile and is surrounded by matrix sillimanite and cordierite. In a pseudosection, it points to peak conditions of?~16 kbar/850?°C followed by isothermal decompression to 8–10 kbar/820–860?°C. In felsic granulite samples, the assemblage K-feldspar–garnet–kyanite–Zr-rich rutile is replaced by sillimanite and Zr-poor rutile. Modelling these assemblages supports minimum conditions of?~13 kbar/925?°C, and a subsequent P–T decrease to 6.5–8.5 kbar/800–820?°C. The internal structure and chemistry of zircons, and modelling of zircon dissolution/growth along the inferred P–T paths are used to discuss the significance of the U–Pb ages. In the monotonous unit, inherited zircon ages of 700–500?Ma point to sedimentation during the Late Cambrian, while medium-grade metamorphism did not allow the formation of Variscan zircon domains. In both the varied gneiss and felsic granulite units, zircons with a blurred oscillatory-zoned pattern could reflect solid-state recrystallization of older grains during HT metamorphism, whereas zircons with a dark cathodoluminescence pattern are thought to derive from crystallization of an anatectic melt during cooling at middle pressure conditions. The present work proposes that U–Pb zircon ages of ca. 340?Ma probably reflect the end of a widespread HT metamorphic event at middle crustal level.     

Tremolite–calcite veins in the footwall of the Simplon Fault,Antigorio Valley,Lepontine Alps (Italy)

The lowermost units of the nappe pile of the Lepontine Alps crop out in the Antigorio valley in the footwall of the Simplon Fault. The whole orthogneiss section of the Antigorio Unit is exposed on both sides of the valley, sandwiched between the Mesozoic metasedimentary sequences of the Baceno unit below and the Tèggiolo unit above. The petrography and mineral composition of tremolite–calcite veins occurring in dolomite marble in both metasedimentary sequences were investigated. Tremolite–calcite (with lesser talc and minor phlogopite) veins have rhythmic banded texture. Banding is due to cyclic differences in modal abundances and fabric of tremolite and calcite. These veins are very similar to those occurring in dolomite rafts within the Bergell granite and it is inferred that they formed by the same “fracture-reaction-seal” mechanism. Veins formed by reaction of a silica-rich aqueous fluid with the host dolomite marble along fractures. According to thermo-barometric calculations, based on electron microprobe analyses, reaction occurred at temperatures between 450 and 490°C and minimum pressure of 2–3 kbar. Such temperature conditions occurred in this footwall region of the Simplon Fault Zone around 15 Ma, during exhumation and cooling of the nappe pile and a transition to brittle behaviour. Aqueous, silica-rich fluids concentrated along fractures, forming tremolite–calcite veins in the dolomite marbles and quartz veins in the orthogneiss.     

Nb–Ta–(Ti–Sn) oxide mineral chemistry as tracer of rare-element granitic pegmatite fractionation in the Borborema Province,Northeastern Brazil

The Borborema Pegmatitic Province (BPP), northeastern Brazil, is historically important for tantalum mining and also famous for top-quality specimens of exotic Nb–Ta oxides and, more recently, for the production of gem quality, turquoise blue, ‘Paraíba Elbaite.’ With more than 750 registered mineralized rare-element granitic pegmatites, the BPP extends over an area of about 75 by 150 km in the eastern part of the Neoproterozoic Seridó Belt. The Late Cambrian pegmatites are mostly hosted by a sequence of Neoproterozoic cordierite–sillimanite biotite schists of the Seridó Formation and quartzites and metaconglomerates of the Equador Formation. The trace-element ratios in feldspar and micas allow to classify most pegmatites as belonging to the beryl–columbite phosphate subtype. Electron microprobe analyses (EMPA) of columbite, tapiolite, niobian–tantalian rutile, ixiolite and wodginite group minerals from 28 pegmatites in the BPP are used to evaluate the effectiveness of Nb–Ta oxide chemistry as a possible exploration tool, to trace the degree of pegmatite fractionation and to classify the pegmatites. The columbite group mineral composition allows to establish a compositional trend from manganoan ferrocolumbite to manganocolumbite and on to manganotantalite. This trend is typical of complex spodumene- and/or lepidolite-subtype pegmatites. It clearly contrasts with another trend, from ferrocolumbite through ferrotantalite to ferrowodginite and ferrotapiolite compositions, typical of pegmatites of the beryl–columbite phosphate subtype. Large scatter and anomalous trends in zoned crystals partially overlap and conceal the two main evolution patterns. This indicates that a large representative data set of heavy mineral concentrate samples, collected systematically along cross-sections, would be necessary to predict the metallogenetic potential of individual pegmatites. Other mineral species, e.g. garnets and/or tourmaline, with a more regular distribution than Nb–Ta oxides, would be more appropriate and less expensive for routine exploration purposes. The currently available Nb–Ta oxide chemistry data suggest the potential for highly fractionated Ta–Li–Cs pegmatites in the BPP, so far undiscovered, and encourages further, more detailed research.     

Fe–Ti–Au–U Mineralization of the Ozerninskii Ore Cluster (West Trans-Baikal Region,Russia)

Doklady Earth Sciences - Comprehensive mineralogical and geochemical studies of gold–brannerite nuggets have been carried out in alluvial deposits of Kamennyi Creek. Its source is located...     

New Fossil Procercopidae (Hemiptera, Cicadomorpha) from the Early Cretaceous of Northeastern China

A new fossil species,Anthoscytina macula sp.nov.,of the family Procercopidae is described from the Early Cretaceous Yixian Formation in Huangbanjigou,Chaomidian Village,Beipiao City,Liaoning Province,China.A key to the species of the genus Anthoscytina Hong,1983 is provided.The diagnosis of the genus is revised.     

Carbonate-rich melt infiltration in peridotite xenoliths from the Eurasian–North American modern plate boundary (Chersky Range,Yakutia)

A suite of mainly spinel peridotite and subordinate pyroxenite xenoliths and megacrysts were studied in detail, enabling us to characterize upper mantle conditions and processes beneath the modern North American–Eurasian continental plate boundary. The samples were collected from 37-Ma-old basanites cropping out in the Main Collision Belt of the Chersky Range, Yakutia Republic (Russian Far East). The spinel lherzolites reflect a mantle sequence, equilibrated at temperatures of 890–1,025 °C at pressures of 1.1–2 GPa, with melt extraction estimated to be around 2–6 %. The spinel harzburgites are characterized by lower P–T equilibration conditions and estimated melt extraction up to 12 %. Minor cryptic metasomatic processes are recorded in the clinopyroxene trace elements, revealing that percolating hydrous fluid-rich melts and basaltic melts affected the peridotites. One of the lherzolites preserves a unique melt droplet with primary dolomite in perfect phase contact with Na-rich aluminosilicate glass and sodalite. On the basis of the well-constrained P–T frame of the xenolith suite, as well as the rigorously documented melt extraction and metasomatic history of this upper mantle section, we discuss how a carbonated silicate melt infiltrated the lherzolite at depth and differentiated into an immiscible carbonate and silicate liquid shortly before the xenolith was transported to the surface by the host basalt. Decreasing temperatures triggered crystallization of primary dolomite from the carbonate melt fraction and sodalite as well as quenched glass from the Na-rich aluminosilicate melt fraction. Rapid entrainment and transport to the Earth’s surface prevented decarbonatization processes as well as reaction phenomena with the host lherzolite, preserving this exceptional snapshot of upper mantle carbonatization and liquid immiscibility.     

Fluid immiscibility and gold deposition in the Birimian quartz veins of the Angovia deposit (Yaouré, Ivory Coast)

The Paleoproterozoic terranes (Birimian) of West Africa are well known to host numerous economic gold mineralizations. The Angovia gold mineralization is located in a brecciated and mylonitic zone within the Birimian greenstones. The sulfide–gold mineralization is mainly represented by gold associated with pyrite and chalcopyrite. A fluid inclusion study undertaken on mineralized quartz veins revealed the presence of aqueous-carbonic (CO₂–H₂O) fluids, the association of carbonic (CO₂) and early aqueous fluids, followed by later aqueous (H₂O-salt) and finally nitrogen-rich fluids. Entrapment of the initial homogeneous aqueous-carbonic fluids prior to fluid immiscibility depicts the evolution of the P–T conditions during the exhumation of the terranes after the peak of green-schist metamorphism. The CO₂ rich-fluid occurs especially in gold-bearing quartz, and are considered as the main evidence of the ore-forming process in the gold-bearing quartz veins. It is considered as a product of immiscibility of the CO₂–H₂O parent. The volatile fraction of carbonic and aqueous-carbonic fluid inclusions is dominated by CO₂, containing minor amounts of N₂, even smaller amounts of CH₄ and sporadically, H₂S. The aqueous-carbonic fluids have moderate salinity (3–10 wt.% eq. NaCl). Late aqueous and N₂ – (CH₄–CO₂) fluids are considered as later, unrelated to the main ore stage, and were trapped during the cooling of the hydrothermal system from 300 to 200 °C.The immiscibility has been favored by a strong pressure drop, the main trapping P–T conditions being 320–370 °C and 105–135 MPa. The mineralizing process is likely related to the immiscibility event, which was probably favored by the release of the fluid pressure after fracturing along the main shear zones. The ore process is likely to have occurred along the main shear zones or related secondary structures affected by cycling of the fluid pressure and quartz sealing–fracturing processes. The superimposed process can also explain the relative complexity of the quartz textures and fluid inclusion microfractures, and the rather wide range in the density of both parent fluid and CO₂-dominated fluid.     

Petrogenesis of the Xuanwoling mafic–ultramafic intrusion in the northeastern Tarim Block(Northwest China)

Beishan Terrane, located in the northeast of the Tarim Block, in northwest China, has developed a 500-km long and 100-km wide belt of Permian mafic–ultramafic intrusions One of these mafic–ultramafic intrusions, the Xuanwoling Intrusion, is composed of dunite, troctolite, olivine gabbros and gabbros, with cumulate texture and rhythmic layering The crystallization sequence is olivine ? spinel ? plagio clase ? pyroxene, indicating that the crystallization pressure is lower than 0.5–0.8 GPa and that the intrusion has undergone variable degrees of crustal contamination, increasing from dunite to gabbros. The olivines found in the Xuanwoling Intrusion have high Fo values(up to 90), suggesting a primary magma with a high composition of mg. It is likely that this high-mg magma was produced at extremely high temperatures(1,330–1,350 °C), and as a result, Nd–Sr isotopic compositions similar to oceanic island basalts are found in the Xuanwoling Intrusion, which we propose arose from the mantle plume.     

Mineralogy and geochemistry of fine-grained Dahab stream sediments,Southeastern Sinai,Egypt: emphasis on the intergrowths of Fe–Ti oxides

Acta Geochimica - Mineralogical and geochemical characteristics of stream sediments collected from 30 stations in the Dahab-Wadi Kid area (southeastern Sinai, Egypt) are presented. The studied...     

Origin of nelsonite and Fe–Ti oxides ore of the Damiao anorthosite complex,NE China: Evidence from trace element geochemistry of apatite,plagioclase, magnetite and ilmenite

Nelsonite and Fe–Ti oxides ore are common in Proterozoic massif-type anorthosites and layered intrusions. Their geneses have long been controversial, with existing hypotheses including liquid immiscibility between Si-rich and Fe–Ti–P-rich melts and gravitational fractionation among apatite, magnetite, ilmenite and silicates. In this paper, we report detailed field geology and mineral geochemical studies of the nelsonite and Fe–Ti oxides ore from the Damiao anorthosite complex, NE China. Geological observations indicate that the nelsonite and Fe–Ti oxides ore occur as irregularly inclined stratiform-like or lensoid or veins, and are in sharp contact with the anorthosite and gabbronorite. The widespread veins and lenses structure of the Damiao nelsonite and Fe–Ti oxides ore in the anorthosite indicates their immiscibility-derived origin. The apatite in the nelsonite and gabbronorite shows evolution trends different from that in the gabbronorite in the diagrams of Sr versus REEs and Eu/Eu*, suggesting that petrogenesis of the nelsonite and gabbronorite is different from the gabbronorite. Compared with the gabbronorite, the nelsonite and Fe–Ti oxides ore have magnetite high in Cr, plagioclase high in Sr and low in An, and apatite high in Sr, low in REEs with negative Eu anomaly. The evidence permits us to propose that the Damiao Fe–Ti oxides ore/nelsonite and gabbronorite were derived from different parental magmas. The gabbronorite was formed by solidification of the interstitial ferrodioritic magma in the anorthosite, which was the residual magma after extensive plagioclase and pyroxene crystallization and was carried upward by the plagioclase crystal mesh. In contrast, the Fe–Ti oxides ore and nelsonites and mangerite were produced by crystallization of the Fe–Ti–P-rich and SiO₂-rich magmas, respectively, due to the liquid immiscibility that occurred when the highly evolved ferrodioritic magma mixed with newly replenished magmas. The variation from Fe–Ti oxides ore to nelsonite and gabbro-nelsonite upwards (as apatite content increases with height) in the steeply inclined Fe–Ti oxides orebodies suggest that gravity fractionation may have played important roles during the crystallization of the Fe–Ti–P-rich magma.     

Occurrence of népouite in the serpentinite of the Manipur ophiolite belt,Northeastern India: Implication for melt-rock interaction in a supra-subduction zone

Serpentinization is pervasive in the ultramafic rocks of Manipur ophiolite belt (MOB), Northeastern India. Electron microprobe data of a serpentinite from the Ukhrul-Nungbi sector of MOB shows Ni-rich serpentine mineral (NiO = 33.4-33.9 wt %, SiO₂= 37.55-38.96 wt %, MgO= 14.83-16.89 wt %). The composition and X-ray diffraction pattern characterize this Ni-rich serpentine mineral as népouite which is suggested to be a hydrothermal alteration product of NiO-rich olivine in a fore-arc peridotite. The genesis of this NiO-rich olivine is attributed to the melt-rock interaction in a supra-subduction zone setting.     

New cranial remains of titanosaurian sauropod dinosaurs from the Late Cretaceous of Fox-Amphoux-Métisson (Var,SE France)

The Fox-Amphoux Syncline of the Aix Basin (SE France) has yielded a diverse Late Cretaceous vertebrate assemblage, including several taxa of dinosaurs. Here, we report on cranial material of titanosaurian sauropods, which consist of a partial braincase, two fragmentary skull roofs, and nine teeth, from the Métisson locality (Var Department). The braincase differs from those found previously in Europe (i.e., Ampelosaurus atacis, Lirainosaurus astibiae, and an unnamed juvenile skull from Romania) as well as from other titanosaurian braincases in having a groove that extends along the ventral surface of the occipital condyle neck (this feature may be autapomorphic). One of the fragmentary skull roofs may belong to the same taxon, whereas the other suggests the presence of a second titanosaur at Métisson very close to Ampelosaurus. Two dental morphologies are present in the sample; one of them includes teeth of different sizes. We suggest that this could be accounted for by age differences within a single taxon, or be due to different positions in the tooth row. The presence of a new, still unnamed titanosaurian taxon in the Ibero-Armorican Island supports previous works indicating a high titanosaurian diversity during the Campanian-Maastrichtian in southern Europe.     

Petrogenesis of ultramafic rocks and olivine-rich troctolites from the East Taiwan Ophiolite in the Lichi mélange

Mineralogy and Petrology - We examine ultramafic and olivine-rich troctolite blocks of the East Taiwan Ophiolite (ETO) in the Lichi Mélange. Although ultramafic rocks are extensively...     

New calibration data for the Fe–Ti oxide thermo-oxybarometers from experiments in the Fe–Ti–O system at 1 bar, 1,000–1,300°C and a large range of oxygen fugacities

The current formulations of the Fe–Ti oxide thermobarometer (titanomagnetite–ilmenite_ss) fail to reproduce experimental results, in particular at the high temperatures that are relevant for basaltic assemblages. With the aim of improving the experimental basis of the calibration in the Fe–Ti–O system, we have synthesised assemblages of titanomagnetite–ilmenite_ss (Tmt–Ilm_ss), ilmenite_ss–pseudobrookite_ss (Ilm_ss–Psb_ss) and single-phase samples under a wide range of fO₂ (fixed with CO/CO₂ mixtures or by solid oxygen buffers) in sub-solidus conditions (1,000–1,300°C) at 1 bar. Runs lasted 24 h at 1,300°C and up to 240 h at 1,000°C and were terminated by quenching in water. All run products are polycrystalline, roughly equigranular aggregates, with grain sizes of 10–50 m. They were examined and analysed with the SEM and EMP. Tmt compositions are broadly in accordance with the current models at moderate fO₂, but significantly richer in Ti at low fO₂ and high T, due to cationic vacancies. Ilm_ss compositions depart from the predicted values practically at all fO₂ and T conditions, which is related to unsatisfactory thermodynamic models for the rhombohedral oxide. For Ilm_ss–Psb_ss assemblages the best agreement between our data and current calculations is at 1,000°C and moderately high fO₂. Otherwise, experimental and calculated data strongly disagree. The experimental data set on the three Fe–Ti oxide solid solutions presented here is intended to support new versions of both the titanomagnetite–ilmenite_ss thermo-oxybarometer and the ilmenite_ss–pseudobrookite_ss oxybarometer.