Gabbro

... Available online 12 March 2002. Abstract. The present contribution shows the results of tests on geological materials for determination of the rare-earth elements La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, as well as Y and Sc. ...

The Kletno deposit in the Śnieżnik Massif (Central Sudetes, SW Poland), mined for Fe, U, Ag, Cu, fluorite, and marble through the ages, developed at the contact of marbles and orthogneiss. Here, we present a new Fe-Ti-V-ore (containing up to 14.07 wt.% Fe, 2.05 wt.% Ti, and 2055 ppm V in bulk rock) and ornamental- to gem-quality talc prospect at the southwest margin of this deposit. This newly documented Fe-Ti-V mineralization is hosted in hornblendites, dolomite veins, and chlorite schists, which, along with talc, envelopes a tectonic slice of serpentinite. Hornblendites are interpreted as metamorphosed ferrogabbros, derived from the same mafic melts as adjacent barren metagabbros. The oxygen and carbon isotope compositions of metabasites and dolomite veins (amphibole δ18O values = 8.8-9.3‰; carbonate δ18O values = 12.8-16.0‰, and δ13C values = −8.3‰ to −7.2‰), in combination with those of the country marbles (carbonate δ18O and δ13C values = 23.2‰ and +0.1‰, respectively), suggest that mineralization-bearing hornblendites formed due to interaction of the mafic magma with CO2 released during the decarbonation of the sediments. The CO2-bearing fluid interaction with gabbros likely caused carbonation of the gabbros and formation of the dolomite veins, whereas talc formed due to Si-rich fluids, possibly derived from a mafic intrusion, interaction with serpentinite, or due to the metasomatism of the serpentinite-gabbro assemblage. Moreover, fluids leaching Fe and Ti from the adjacent sediments can mix with the mafic magma causing enrichment of the magma in Fe and Ti. Consequently, the mineralization-bearing ferrogabbros became even more enriched in Fe and Ti, which can be linked with the formation of Fe-Ti-V ore bodies.

The Mesoarchean Nuasahi chromite deposits of the Singhbhum Craton in eastern India consist of a lower chromite-bearing ultramafic unit and an upper magnetite-bearing gabbroic unit. The ultramafic unit is a ∼5 km long and ∼400 m wide linear belt trending NNW-SSE with a general north-easterly dip. The chromitite ore bodies are hosted in the dunite that is flanked by the orthopyroxenite. The rocks of the ultramafic unit including the chromitite crystallized from a primitive boninitic magma, whereas the gabbro unit formed from an evolved boninitic magma. A shear zone (10–75 m wide) is present at the upper contact of the ultramafic unit. This shear zone consists of a breccia comprising millimeter- to meter-sized fragments of chromitite and serpentinized rocks of the ultramafic unit enclosed in a pegmatitic and hybridized gabbroic matrix. The shear zone was formed late synkinematically with respect to the main gabbroic intrusion and intruded by a hydrous mafic magma comagmatic with the evolved boninitic magma that formed the gabbro unit. Both sulfide-free and sulfide-bearing zones with platinum group element (PGE) enrichment are present in the breccia zone. The PGE mineralogy in sulfide-rich assemblages is dominated by minerals containing Pd, Pt, Sb, Bi, Te, S, and/or As. Samples from the gabbro unit and the breccia zone have total PGE concentrations ranging from 3 to 116 ppb and 258 to 24,100 ppb, respectively. The sulfide-rich assemblages of the breccia zone are Pd-rich and have Pd/Ir ratios of 13–1,750 and Pd/Pt ratios of 1–73. The PGE-enriched sulfide-bearing assemblages of the breccia zone are characterized by (1) extensive development of secondary hydrous minerals in the altered parts of fragments and in the matrix of the breccia, (2) coarsening of grain size in the altered parts of the chromitite fragments, and (3) extensive alteration of primary chromite to more Fe-rich chromite with inclusions of chlorite, rutile, ilmenite, magnetite, chalcopyrite, and PGE-bearing chalcogenides. Unaltered parts of the massive chromitite fragments from the breccia zone show PGE ratios (Pd/Ir = 2.5) similar to massive chromitite (Pd/Ir = 0.4–6.6) of the ultramafic unit. The Ir-group PGE (IPGE: Ir, Os, Ru) of the sulfide-rich breccia assemblages were contributed from the ultramafic–chromitite breccia. Samples of the gabbro unit have fractionated primitive mantle-normalized patterns, IPGE depletion (Pd/Ir = 24–1,227) and Ni-depletion due to early removal of olivine and chromite from the primitive boninitic magma that formed the ultramafic unit. Samples of the gabbro and the breccia zone have negative Nb, Th, Zr, and Hf anomalies, indicating derivation from a depleted mantle source. The Cu/Pd ratios of the PGE-mineralized samples of the breccia zone (2.0 × 103–3.2 × 103) are lower than mantle (6.2 × 103) suggesting that the parental boninitic magma (Archean high-Mg lava: Cu/Pd ratio ∼1.3 × 103; komatiite: Cu/Pd ratio ∼8 × 103) was sulfur-undersaturated. Samples of the ultramafic unit, gabbro and the mineralized breccia zone, have a narrow range of incompatible trace element ratios indicating a cogenetic relationship. The ultramafic rocks and the gabbros have relatively constant subchondritic Nb/Ta ratios (ultramafic rocks: Nb/Ta = 4.1–8.8; gabbro unit: Nb/Ta = 11.5–13.2), whereas samples of the breccia zone are characterized by highly variable Nb/Ta ratios (Nb/Ta = 2.5–16.6) and show evidence of metasomatism. The enrichment of light rare earth element and mobile incompatible elements in the mineralized samples provides supporting evidence for metasomatism. The interaction of the ultramafic fragments with the evolved fluid-rich mafic magma was key to the formation of the PGE mineralization in the Nuasahi massif.

This study reports the composition of the oceanic crust from the 16.5°N region of the Mid-Atlantic Ridge, a spreading ridge segment characterized by a complex detachment fault system and three main oceanic core complexes (southern, central, and northern OCCs). Lithologies recovered from the core complexes include both greenschist facies and weathered pillow basalt, diabase, peridotite, and gabbro, while only weathered and fresh pillow basalt was dredged from the rift valley floor. The gabbros are compositionally bimodal, with the magmatic crust in the region formed by scattered intrusions of chemically primitive plutonic rocks (i.e., dunites and troctolites), associated with evolved oxide-bearing gabbros. We use thermodynamic models to infer that this distribution is expected in regions where small gabbroic bodies are intruded into mantle peridotites. The occurrence of ephemeral magma chambers located in the lithospheric mantle enables large proportions of the melt to be erupted after relatively low degrees of fractionation. A large proportion of the dredged gabbros reveal evidence for deformation at high-temperature conditions. In particular, chemical changes in response to deformation and the occurrence of very high-temperature ultramylonites (>1000 °C) suggest that the deformation related to the oceanic detachment commenced at near-solidus conditions. This event was likely associated with the expulsion of interstitial, evolved magmas from the crystal mush, a mechanism that enhanced the formation of disconnected oxide-gabbro seams or layers often associated with crystal-plastic fabrics in the host gabbros. This granulite-grade event was soon followed by hydrothermal alteration revealed by the formation of amphibole-rich veins at high-temperature conditions (~900 °C).

The Puy Beaunit maar presents a large variety of mantle xenoliths (spinel peridotites, pyroxenites and layered rocks). A detailed study of the textures and mineral equilibria shows the unusual character of this occurrence and the local complexity of the upper mantle beneath the French Massif Central. Ultramafic nodules have a metamorphic, magmatic or pyrometamorphic origin; they display different stages of

Although the structure of mantle plume-derived oceanic plateaus has recently been assessed using remote geophysical techniques combined with petrological modelling, it is nevertheless desirable to test whether these (theoretical) rock types (dunites, gabbros and basalts) actually exist, and to establish their geochemical nature. Oceanic plateaus may have initially formed above or near sea level during a short vigorous pulse, and thereafter commonly subside to abyssal depths as the lithosphere cools, thus making sampling of their deeper levels extremely difficult. However, the Cretaceous-age Colombian-Caribbean oceanic plateau was partially accreted against the South American continent so making the imbricated segments available for study. During the process of plateau accretion and imbrication it is predominantly the basaltic layers which are obducted, but parts of the sequence down to layered and banded gabbros with associated pyroxenites and dunites (sometimes foliated) can be exposed where the imbricate thrusting brings up deeper levels. Most of the upper crustal sequence in western Colombia is composed of basaltic pillowed and massive flows and sills that are chemically uniform and `undepleted' relative to normal mid-ocean ridge basalts. Komatiites and (more abundant) picrites are found at intervals, and appear to occur near the base of the sequence. In these zones both `depleted' and moderately `enriched' basalt and komatiite compositions occur, and may result from dynamic partial melting and mixing processes associated with the high-temperature part of the plume. It is possible to integrate these compositional characteristics into a general model for oceanic plateau structure where the rate of magma supply is in excess of that which can be accommodated by normal spreading processes, thus leading to extrusion of flows and the emplacement of sills and high-level magma chambers (but relatively few dykes). The dense and chemically heterogeneous ultramafic magmas intrude the base of the pile and undergo fractionation to form ultramafic cumulates, whereas the well-mixed basaltic magmas are erupted to form the homogeneous plateau basalts. Based on these observations a possible structure of the Caribbean-Colombian plateau is proposed which is compatible with geophysical models for other less well exposed oceanic plateaus.

Lors de la 2eme campagne de restauration architecturale de Tell Beydar (avril-mai 2004), le demontage d’un mur ancien du temple C a permis la decouverte d’un fragment de bol en pierre, d’apparence inhabituelle. Le prelevement d’un tres petit echantillon a permis, deux ans plus tard, d’identifier le materiau utilise : un (meta) gabbro, dont l’origine est a situer dans le Desert Oriental egyptien. Un examen comparatif de l’objet permet de dater sa manufacture vraisemblablement de l’epoque protodynastique (dynastie I-II), meme si la morphologie assez simple de ce recipient ne permet pas d’exclure totalement une datation plus ancienne (dynastie 0) ou plus recente (dynastie III-IV). Le contexte chronologique etant date de la deuxieme moitie du XXVe siecle av. n. e. (Beydar IIIb, phase 3 de l’acropole Early Jezirah IIIb), le fragment de vase est donc manifestement plus ancien. Inclus dans la masse des briques crues constituant un mur du Temple C, sa position est secondaire. La fragmentation de la piece rend l'hypothese d’un depot de fondation tres peu credible. Il s’agit, a ce jour, du seul fragment d’objet manufacture egyptien date du Bronze ancien decouvert a l’est de l’Euphrate, en Jezireh syrienne ou, de maniere plus large, en Haute Mesopotamie. Cet article presente cet objet et esquisse les raisons de sa presence a Tell Baydar.

The Kletno deposit in the Śnieżnik Massif (Central Sudetes, SW Poland), mined for Fe, U, Ag, Cu, fluorite, and marble through the ages, developed at the contact of marbles and orthogneiss. Here, we present a new Fe-Ti-V-ore (containing up to 14.07 wt.% Fe, 2.05 wt.% Ti, and 2055 ppm V in bulk rock) and ornamental- to gem-quality talc prospect at the southwest margin of this deposit. This newly documented Fe-Ti-V mineralization is hosted in hornblendites, dolomite veins, and chlorite schists, which, along with talc, envelopes a tectonic slice of serpentinite. Hornblendites are interpreted as metamorphosed ferrogabbros, derived from the same mafic melts as adjacent barren metagabbros. The oxygen and carbon isotope compositions of metabasites and dolomite veins (amphibole δ18O values = 8.8–9.3‰; carbonate δ18O values = 12.8–16.0‰, and δ13C values = −8.3‰ to −7.2‰), in combination with those of the country marbles (carbonate δ18O and δ13C values = 23.2‰ and +0.1‰, respectively), suggest...

The Troodos complex, Cyprus, provides an opportunity to study the structural configuration along a fossil intersection of a
spreading axis and a transform fault. We complement studies at Troodos that have reconstructed the brittle deformation of the upper crust by new paleomagnetic data from the gabbro suite. The gabbro suite is exposed at the extinct spreading axis continuing the Solea graben toward the intersection with the fossil Arakapas oceanic transform. This is a unique exposure of deep crustal rocks formed at both an inside-corner and an outside-corner of a ridge-transform intersection. Remanence directions from gabbros (23 sites) were used as indicators for rigid body rotation. The spatial distribution of rotation axes allow recognition of three regions to which deformation is
partitioned: 1) a western region (outside corner) that experienced primarily tilt about horizontal axis 2) a central region with minor rotation and, 3) an eastern area (inside corner) where vertical axis rotations are dominant. The absence of significant rotation in the 6 km-wide central domain together with its location between the inside- and the outside corner uncover the root of a fossil axial volcanic zone, a zone sufficiently hot so the upper crust can decouple from the substrate. Clockwise rotation in the gabbro increases from the axial zone eastward, similar to that in the overlying dikes, indicating coupling of the lower crust with the brittle upper oceanic crust. The transition from the decoupled layers of sheeted dikes and gabbro in the axial zone to the dikes–gabbro coupling in the inside corner is in keeping with deepening of the brittle–ductile transition from the dike–gabbro boundary into the lower crust away from the axial zone. Our conclusions are consistent with one of the previous reconstructions in which the Solea spreading axis was orthogonal to the Arakapas transform fault, and with recent studies of the present-day lower oceanic crust. However, the newly inferred surface trace of the Solea spreading axis is further to the east, probably reflecting the tilt of axial upper crust rotated blocks.

An Ediacaran mafic intrusion of south Jordan is a distinctive appinitic igneous rock with a possibly unique texture, characterized by spherical clots up to 40 mm in diameter composed of amphibole cores from which plagioclase euhedra radiate; we call it the Rosetta Gabbro. It is exposed as a small (ca. 750 m 2) outcrop in the Neoproterozoic basement of south Jordan. A second outcrop of otherwise similar gabbro is located about 400 m to the north of the Rosetta Gabbro, but it lacks the distinctive texture. The Rosetta Gabbro could represent a magma pipe. It intrudes the Aqaba Complex (~600 Ma) granitoids and metasediments of the Janub Metamorphic Complex (633-617 Ma). The gabbro is an Ol-to QZ tholeiite with the following chemical characteristics: SiO 2 = 46.2-47.8 wt.%; Al 2 O 3 = 16.4-17.7 wt.%, TiO 2 = 1.70-2.82 wt.%, Na 2 O = 1.27-2.83 wt.%. K 2 O = 0.82-1.63 wt.%; Mg# 58-63; Σ REE = 70-117 ppm; La/Yb ~ 6 to 8; and Eu/Eu* = 1.05-1.2. The investigated gabbro has the geochemical features of a continental flood tholeiitic basalt emplaced in a within-plate tectonic setting. Two varieties of amphiboles are found: 1) large, 3-5 mm, brown ferri-titanian-tschermakite (K 0.09 Na 0.28)(Na 0.20 Ca 1.80)(Mn 0.04 Fe 3+ 1.1 Mg 2.34 Fe 2+ 0.90 Ti 0.29 Al 0.22)(Al 1.85 Si 6.15)O 22 (OH) 1.95 of the calcic amphi-bole group which is riddled with opaques; and 2) acicular yellowish-light green ferrian-magnesiohornblende (K 0.04 Na 0.153)(Ca 1.755 Na 0.245) (Fe 3+ 0.66 Mn 0.01 Fe 2+ 1.01 Mg 3.03 Ti 0.06 Al 0.22)(Al 1.03 Si 6.97)O 22 (OH) 1.95. Scattered flakes of phlogopite also occur. Tabular radiating plagioclase (An 64-79) are complexly twinned, with broad lamellae that show no zoning. Laser-ablation ICP-MS analyses of amphibole and plagioclase reveal considerable variation in trace element abundance, in spite of more subtle major element variations except for TiO 2 in amphibole. The REE in the amphibole shows an order of magnitude variation with a concave-downward pattern and a positive Eu anomaly Eu/Eu* = 0.6-2, though far less pronounced compared to the Eu/Eu* = 5-45 of plagioclase. The 3D dandelion-like texture of the rosettas is broadly similar to "Chrysanthemum Stone", which is a diagenetic growth in sedimentary rock, but we can find no description of similar textures in igneous rocks. The formation of the rosettas is thought to reflect loss of magmatic water resulting in supersaturation of plagioclase, which grew rapidly around amphibole and may have floated in the magma. This implies magmatic evolution in shallow (10 to 12 km deep) crust where temperatures were nevertheless in the range of ca. 750 to 900 °C.

Orthopyroxenes and clinopyroxenes in metabasites in contact with the massif-type Ana-Sira anorthosite yield the following equilibrium temperatures using various geothermometers: 840°C (Wells 1977); 8000 C (Wood & Banna 1973), 650"C (Lindsley & Andersen 1983). Gamet-cordierite-plagioclase-sillimanite­ quartz assemblages in neighbouring metapelites give values around 600°C and 3 ± 0.5 kb by means of several geothermobarometers. Armoured relics of symplectitic quartz + spinel in gamet suggest that equilibrium was attained during decreasing temperature. Fayalite + quartz and Ca-poor + Ca-rich clinopyroxene assemblages in charnockitic rocks of late tectonic intrusions yield 800-850"C (Lindsley & Andersen 1983) and 6-7.5 kb for the upper part of the Bjerkreim-Sokndal lopolith and a minimum pressure of 5-5.5 kb for the Hidra massif (Boblen & Boettcher 1981). When interpreted in terms of the regional geological evolution, the conditions measured reflect two successive stages of a ...

... Indian Ridge Mathilde Cann t, Catherine M vel and Debra Stakes LRA 127S-GDR "Gen se el Evolution des Domaines Oc aniques", UBO, 6 ... 1979; Malcolm, 1981; Ito and Anderson, 1983; Honnorez et al., 1984; Stakes and Vanko, 1986; M vel, 1987, 1988): in contrast with the ...

The Integrated Ocean Drilling Program’s (IODP) Expeditions 309 and 312 successfully completed the first sampling of an intact section of upper oceanic crust, through lavas and the sheeted dikes into the uppermost gabbros. Hole 1256D, which was initiated on the Ocean Drilling Program’s (ODP) Leg 206, now penetrates to…