Joan Carles Melgarejo

Electromagnetic imaging of Variscan crustal structures in SW Iberia: the role of interconnected graphite

Abstract The western part of the Iberian Peninsula (Iberian Massif) is the best exposed fragment of the Variscan orogen in Europe. Its southern half was generated by an oblique collision between three continental terranes belonging to the margins of Laurassia (Avalonia) – the South Portuguese Zone (SPZ) – and Gondwana – the Ossa Morena Zone (OMZ) and the Central Iberian Zone (CIZ). The boundaries between them are considered to be suture zones. A 200 km long magnetotelluric profile across the three Variscan terranes was done in a NNE direction, approximately perpendicular to the main tectonic features. The results of two-dimensional inversion of the MT dataset reveal high-conductivity zones coinciding with the transitions SPZ/OMZ and OMZ/CIZ. These conductive bodies related to the sutures at depth were interpreted as graphite enrichments along shear planes formed due to the overall transpressive regime. A high-conductivity layer extending along the whole OMZ was found at a depth of 15–25 km, the top of which spatially correlates with a broad reflector detected by a recently acquired deep seismic reflection profile. The high conductivity was interpreted as caused by the Precambrian Serie Negra graphite-rich rocks. Carbon and oxygen X-ray mapping with electron microprobe on polished sections of Serie Negra samples from OMZ revealed the presence of interconnected graphite, which supports the hypothesis that graphite is determinant for the high conductivity. Two graphite types, which help to record the geological evolution, were identified: graphite accumulations in the schistosity surfaces produced by folding and metamorphism, and metallic films of graphite developed along late faults. The conductive layer shows blobs of higher conductivity suggesting macro-anisotropy. Additional mylonitisation and shearing produced by thrusting at depth can be the origin of these zones of enhanced conductivity, given that the detachment level is located within the Serie Negra. Several high-resistivity features were found in the upper crust, related to Devonian and Carboniferous successions and probably to some unexposed plutons in the SPZ and the Palaeozoic series of OMZ plus some granitic intrusions. In the CIZ, a high-resistivity zone extending to the whole crust is correlated with extensive late Variscan granite intrusions.

Primitive Cretaceous island-arc volcanic rocks in eastern Cuba: the Téneme Formation

The Teneme Formation is located in the Mayari-Cristal ophiolitic massif and represents one of the three Cretaceous volcanic Formations established in northeastern Cuba. Teneme volcanics are cut by small bodies of 89.70 ± 0.50 Ma quarz-diorite rocks (Rio Grande intrusive), and are overthrusted by serpentinized ultramafics. Teneme volcanic rocks are mainly basalts, basaltic andesites, andesites, and minor dacites, and their geochemical signature varies between low-Ti island arc tholeiites (IAT) with boninitic affinity (TiO2 < 0.4 %; high field strength elements << N-type MORB) and typical oceanic arc tholeiites (TiO2 = 0.5-0.8 %). Basaltic rocks exhibit low light REE/Yb ratios (La/Yb < 5), typical of intraoceanic arcs and are comparable to Maimon Formation in Dominican Republic (IAT, pre Albian) and Puerto Rican lavas of volcanic phase I (island arc tholeiites, Aptian to Early Albian). The mantle wedge signature of the Teneme Formation indicates a highly depleted MORB-type mantle source, without any contribution of E-MORB or OIB components. Our results suggest that Teneme volcanism represents a primitive oceanic island arc environment. If the Late Cretaceous age (Turonian or early Coniacian) proposed for Teneme Formation is correct, our results indicate that the Cretaceous volcanic rocks of eastern Cuba and the Dominican Republic are not segments of a single arc system, and that in Late Cretaceous (Albian-Campanian) Caribbean island arc development is not represented only by calc-alkaline (CA) volcanic rocks as has been suggested in previous works.

Niobium and rare earth minerals from the Virulundo carbonatite, Namibe, Angola

Abstract The Virulundo carbonatite in Angola is one of the largest in the world and contains pyrochlore as an accessory mineral in all of the carbonatite units (calciocarbonatites, ferrocarbonatites, carbonatite breccias and trachytoids). The primary magmatic pyrochlore is fluorine dominant and typically contains about equal molar quantities of Ca and Na at the A site. High-temperature hydrothermal processes have resulted in the pseudomorphic replacement of the primary pyrochlore by a second generation of pyrochlore with less F and Na. Low-temperature hydrothermal replacement of the first and second generation pyrochlore, associated with quartz-carbonate-fluorite vein formation in the carbonatite, has produced a third generation of pyrochlore, with a high Sr content. The Sr appears to have been released by low-temperature hydrothermal replacement of the primary magmatic carbonates. Finally, supergene alteration processes have produced late-stage carbonates, goethite, hollandite and rare earth element (REE) minerals (mainly synchysite-(Ce), britholite-(Ce), britholite-(La), cerite- (Ce)). Cerium separated from the other REEs in oxidizing conditions and Ce4+ was incorporated into a late generation of supergene pyrochlore, which is strongly enriched in Ba and strongly depleted in Ca and Na.

Tracing the chemical evolution of primary pyrochlore from plutonic to volcanic carbonatites: the role of fluorine

Abstract Three Angolan carbonatites were selected to evaluate the change in composition of pyrochlores during magmatic evolution: the Tchivira carbonatites occur in a plutonic complex, the Bonga carbonatites represent hypabyssal carbonatites and the Catanda carbonatites are volcanic in origin. In Tchivira pyrochlore, zoning is poorly developed; fluorine is dominant at the Y site; chemical zoning may arise as a result of substitutions for Nb in the B site; and the rare earth element (REE), U, Th and large-ion lithophile element (LILE) contents are very low. Pyrochlores from Bonga show oscillatory zonation; the F and Na contents are lower than those in the pyrochlores from Tchivira; and as substitution of Na at the A site increases, the Th, U, REE contents and inferred vacancies also increase. Pyrochlores from Catanda display complex textures. They generally have a rounded corroded core, which is mantled by two or three later generations. The core composition is similar to the Bonga pyrochlores. The rims are enriched in Zr, Ta, Th, Ce and U, but depleted in F and Na. In pyrochlores from the Angolan carbonatites, the F and Na contents decrease from plutonic to volcanic settings and there is enrichment of Th, U and REE in the A site and Ta and Zr in the B site. Zoning may be explained by changes in the activity of F, due to the crystallization of fluorite or apatite in the plutonic and hypabyssal carbonatites, or to volatile exsolution in the volcanic carbonatites.

Structure solution from powder data of the phosphate hydrate tinticite

The crystal structure of the mineral tinticite has been solved by direct methods from integrated intensities of X-ray powder diffraction data and subsequently refined with the Rietveld technique. The sample used for the structure solution comes from the Gava-Bruguers area (20 km SW of Barcelona), which contains a large variety of phosphates, some of which were exploited in gallery mines during the ancient neolithic. Tinticite crystallizes in the triclinic space group P 1 with unit cell parameters a = 7.965(2) A, b = 9.999(2) A, c = 7.644(2) A, α = 103.94(2)°, β = 115.91(2)°, ω = 67.86(2)° and cell content Fe 3+ 5.34 (PO 4 ) 3.62 (VO 4 ) 0.38 (OH) 4 ·6.7 H 2 O; ρ exp = 2.94 g/cm 3 ; ρ cale = 2.88 g/cm 3 . The Rietveld refinement of the data set converged to R wp = 13.1 % and χ 2 = 3.3. Due to the complexity of the disorder in this structure, the refined structure model could only account for part of it. The octahedrally coordinated Fe 3+ ions form dreier single chains of general formula ∞ 1 [Fe 3 O 14 ] at y = 0 and trimers of type cis-[Fe 3 O 14 placed at y = 1/2. While the dreier single chains are linked to each other by fully occupied PO 4 groups yielding in this way predominantly ordered layers, the trimers arc partially disordered and connected to each other and to the ordered layers both by PO 4 groups and through H-bonds. The higher stability of the ordered layers is consistent with the observed platy nature of the microcrystals of tinticite.

The Catanda extrusive carbonatites (Kwanza Sul, Angola): an example of explosive carbonatitic volcanism

Carbonatite lavas and pyroclastic rocks are exposed in the volcanic graben of Catanda and represent the only known example of extrusive carbonatites in Angola. A new detailed geological map of the area is presented in this study as well as six different stratigraphic sections. Pyroclastic rocks, apparently unwelded, are dominant in the area and represented in all the stratigraphic columns. They form shallowly to moderately inclined layers, mostly devoid of internal structures, that range in thickness from several centimetres to metres. They are dominantly lapilli tuffs and minor tuffs occasionally comprising pelletal lapilli. Based on their different features and field relationships, at least five different pyroclastic lithofacies have been distinguished in the area. Carbonatitic lavas outcrop in the external parts of the Catanda graben, forming coherent layers interbedded with pyroclastic rocks. Calcite is the most common mineral in the lavas, but other accessory minerals such as fluorapatite, titaniferous magnetite, phlogopite, pyrochlore, baddeleyite, monticellite, perovskite, cuspidine and periclase have also been identified. At least four different types of lavas have been distinguished based on their mineral associations and textural features. This study reveals an overall abundance of pyroclastic material in comparison to lava flows in the Catanda area, suggesting that eruptive processes were dominated by explosive activity similar to what has been described in other carbonatite and kimberlite localities. The Catanda carbonatitic volcanism was associated with monogenetic volcanic edifices with tuff ring or maar morphologies, and at least seven possible eruptive centres have been identified in the area.

Capabilities of through-the-substrate microdiffraction: application of Patterson-function direct methods to synchrotron data from polished thin sections

Some theoretical and practical aspects of the application of transmission microdiffraction (µXRD) to thin sections (≤30 µm thickness) of samples fixed or deposited on substrates are discussed. The principal characteristic of this technique is that the analysed micro-sized region of the thin section is illuminated through the substrate (tts-µXRD). Fields that can benefit from this are mineralogy, petrology and materials sciences since they often require in situ lateral studies to follow the evolution of crystalline phases or to determine new crystal structures in the case of phase transitions. The capability of tts-µXRD for performing structural studies with synchrotron radiation is shown by two examples. The first example is a test case in which tts-µXRD intensity data of pure aerinite are processed using Patterson-function direct methods to directly solve the crystal structure. In the second example, tts-µXRD is used to study the transformation of laumonite into a new aluminosilicate for which a crystal structure model is proposed.

Crystal-structure refinement of Fe3+-rich aerinite from synchrotron powder diffraction and Mössbauer data

Pale-blue fibres of an Fe 3+ -rich variety of aerinite from Tartareu (Catalunya, Spain) were studied by synchrotron powder diffraction. Crystal data and composition are: a = b = 16.9161(1), c = 5.2289(1) A, V = 1296 A 3 , space group P 3 c 1, D calc = 2.46 g cm 3 ; (Ca 4.31 , Na 0.07 ) (Al 5.25 , Si 0.43 , Mg 0.33 ) (Fe 1.36 , 3+ Fe 0.64 2.5+ ) (Fe 1.2 3+ , Mg 0.5 , Al 0.3 ) [Si 12 O 36 (OH) 12 ] [(CO 3 ) 0.75 (SO 3 ) 0.25 (H 2 O) 11.1 ], Z = 1. The oxidation states of Fe in these fibres were determined from Mossbauer spectroscopy. The model of the structure was refined with the Rietveld method to the residual value R wp = 0.036 (χ 2 = 1.73). The unit cell of aerinite contains two similar basic building units (columns) formed by three pyroxene chains pointing inwards producing two different cation sites (M1a,b) at the centres of the resulting face-sharing octahedra. The composition of M1a is 68 % Fe 3+ and 32 % mixed-valence iron (Fe 2.5+ ) and the composition of M1b is 60 % Fe 3+ , 25 % Mg 2+ and 15 % Al 3+ . The presence of mixed-valence iron provides an explanation for the observed blue colour of aerinite. According to the electron-microprobe analysis, 0.43 Si 4+ can not be hosted in the pyroxene chains and most probably occupy M2 positions. However, due to the small amount that it represents, i.e. 6 % of the M2 sites, these positions have been refined as Al 3+ assuming an octahedral coordination (final M2–O average bond length of 1.92(6) A). The large “channel” includes 0.75 CO 3 2− and 0.25 sulphur atoms, probably as sulphite groups, stacked along one threefold axis. Inspection of a polished thin section of the same Tartareu specimen from which the studied aerinite fibres were taken shows the presence of an additional deeper blue phase forming a stripe of approximately 0.2–0.3 mm thickness between the pale-blue aerinite and the laumontite substrate. The electron-microprobe analysis of this new phase indicates that it is related to aerinite but Si-richer and Fe- and Ca-poorer. Both phases also display a different optical behaviour under transmitted polarised light.

Comments on the paper "Ochreous laterite: a nickel ore from Punta Gorda, Cuba" by Oliveira et al.

The paper, “Ochreous laterite: a nickel ore from PuntaGorda, Cuba,” by Oliveira et al. (2001) presents acharacterization of oxidized nickel ore from the PuntaGorda deposit in the Moa district (eastern Cuba). Theauthors assume that the nickel ore derives from an ophioliticassociation (serpentinized ultramafic rocks, olivine gabbros,and plagiogranites) strongly affected by lateritization. Theynote that ‘no extensive study has so far been published aboutthe Ni mineralization in regard to the mineralogical andchemical nature of the ore’ and reach the conclusion thatnickel is associated with goethite and cobalt with manga-nese minerals. The authors find two ore types, Ni-rich(1.56–2.06%) and Ni-poor (0.24–0.96%), and infer that thetwo types are related to differences in the parent rocks.Oliveira et al. contend that the Ni-rich ore derives fromperidotites containing Ni-bearing orthopyroxenes and thatthe Ni-poor ore forms from peridotites containing Al-clinopyroxene depleted in Ni. Despite their interestingattempt, we believe these points require some criticalcomments that point to possible alternative interpretations.First, the Moa-Baracoa ophiolitic massif consists of alower unit of mantle tectonites made up of serpentinizedharzburgite with minor dunite, ‘impregnated peridotite’(plagioclase- and clinopyroxene-bearing peridotite), wehr-lite, and pyroxenite and a crustal sequence composed oflayered gabbro (olivine gabbro and gabbronorite) anddiscordant pillowed basalts and sediments of the Quivija´nFormation (Guild, 1947; Wadge et al., 1984; Ri´os andCobiella, 1984; Fonseca et al., 1985; Iturralde-Vinent, 1989,1996, 1998; Keer et al., 1999; Proenza et al., 1999a,b).Towards the top of the mantle tectonites, the harzburgitecontains increasing amounts of dunite, gabbro sills, andchromitite, which form elongated pseudotabular bodiesparallel to the foliation of the host harzburgite, as well asdiscordant dikes of gabbro and pegmatitic gabbro (Fig. 1).This upper zone has been considered as forming part of theMoho Transition Zone (MTZ) (Proenza et al., 1999a,b).Second, Oliveira et al. (2001) ignore many publishedstudies on the geological, mineralogical, and chemicalcharacteristics of the Cuban lateritic nickel deposits(Kudelasek et al., 1967; Vera, 1979; Formell and Oro,1980; Vershinin et al., 1984; Quintana-Puchol, 1985;Ostroumov et al., 1985, 1987; Cordeiro et al., 1987;Rojas-Puro´n et al., 1993; Rojas-Puro´n and Carballo, 1993;Capote et al., 1993; Almaguer and Zamarzry, 1993;Rojas-Puro ´n and Beyris, 1994; Rojas-Puron and Orozco,1995; Almaguer, 1995, 1996; Lavaut, 1998). These studiescontain extensive results about the Ni mineralization in theMoa district and conclude that Ni in ochreous laterite isassociated with goethite and Co with manganese minerals(asbolanes).Third, the differences in the mineralogical and chemicalcomposition of the Ni-rich and Ni-poor ores cannot beexplained by the presence of Ni-bearing orthopyroxene orNi-depleted, Al-rich clinopyroxene in the parent rock, asOliveira et al. (2001) conclude, because the Moa peridotitesare made up of only harzburgites and dunites (Proenza et al.,1999a,b), neither of which contains significant clinopyrox-ene. The average forsterite and NiO content of olivine in theharzburgites is 91.0 and 0.39 wt%, respectively, whereas indunites, these values are 91.5 and 0.38 wt% (Table 1). TheNi content of orthopyroxene is very low (NiO , 0.1 wt%),usually below the detection limit of the electron microprobe(Table 1). However, clinopyroxene-bearing harzburgites arevery scarce in the whole Moa-Baracoa massif. Therefore,the Ni-rich ore can only derive from harzburgite and dunite

Ilmenite as a recorder of kimberlite history from mantle to surface: examples from Indian kimberlites

Five compositional-textural types of ilmenite can be distinguished in nine kimberlites from the Eastern Dharwar craton of southern India. These ilmenite generations record different processes in kimberlite history, from mantle to surface. A first generation of Mg-rich ilmenite (type 1) was produced by metasomatic processes in the mantle before the emplacement of the kimberlite. It is found as xenolithic polycrystalline ilmenite aggregates as well as megacrysts and macrocrysts. All of these ilmenite forms may disaggregate within the kimberlite. Due to the interaction with low-viscosity kimberlitic magma replacement of pre-existing type 1 ilmenite by a succeeding generation of geikielite (type 2) along grain boundaries and cracks occurs. Another generation of Mg-rich ilmenite maybe produced by exsolution processes (type 3 ilmenite). Although the identity of the host mineral is unclear due to extensive alteration and possibility includes enstatite. Type 4 Mn-rich ilmenite is produced before the crystallization of groundmass perovskite and ulvöspinel. It usually mantles ilmenite and other Ti-rich minerals. Type 5 Mn-rich ilmenite is produced after the crystallization of the groundmass minerals and replaces them. The contents of Cr and Nb in type 2, 4 and 5 ilmenites are highly dependent on the composition of the replaced minerals, they may not be a good argument in exploration. The highest Mg contents are recorded in metasomatic ilmenite that is produced during kimberlite emplacement, and cannot be associated with diamond formation. The higher Mn contents are linked to magmatic processes and also late processes clearly produced after the crystallization of the kimberlite groundmass, and therefore ilmenite with high Mn contents cannot be considered as a reliable diamond indicator mineral (DIM) and kimberlite indicator mineral (KIM).