Massive Gabbros Research Articles

Hornblende gabbros of Wadi Az Zarib area, Central Eastern Desert, Egypt: opaque mineralogy, geochemistry, tectonic setting, and petrogenesis

A small intrusive fresh gabbroic mass intrudes the Neoproterozoic metasediments and Dokhan volcanics of Wadi Az Zarib area, Central Eastern Desert. It is composed of hornblende gabbros and leuco-hornblende gabbros. Their petrography, opaque mineralogy, and geochemistry are addressed to elucidate their tectonic setting and petrogenesis. They represent a subduction-related calc–alkaline magma that evolved in an island arc setting. In terms of maturity, the supposed arc represents an intermediate stage between continental arc and active continental margin. Thermobarometry and physical–chemical data of the parent magma as deduced from compositions of amphiboles, biotite, and plagioclase indicate crystallization temperatures of 931–825 °C at pressures of 6.16–4.01 kbar and H2Omelt of 6.4–5.2 wt%. Data, as presented, argue in favor of fractional crystallization mechanism to be accounted to the present suite to interpret the observed variations. The evolution of the suite from hornblende gabbros to leuco-hornblende gabbros was accompanied by decreasing of MgO, CaO, Cr, and Ni with simultaneous increasing of Al2O3, TiO2, Na2O, K2O, Ba, Rb, Sr, La, and Ce. Residuals calculated during mass balance fractional crystallization modeling suggest that brown and green hornblendes are the main fractionated phases which derived the melt composition towards the leuco-hornblende gabbros.

The Talazhin plagiodunite–troctolite–anorthosite–gabbro massif (East Sayan): petrogeochemistry and ore potential

The petrology and ore potential of the Talazhin massif located in northwestern East Sayan are studied. The internal structure of the intrusion, the petrographic composition of its rocks, and their metallogenic, petrostructural, and petrogeochemical features are considered. The probable temperature and chemical composition of the parental magma of the pluton were computed using the KOMAGMAT-3.52 program on the modeling of equilibrium crystallization. The obtained data indicate that the Talazhin massif is a rhythmically layered plagiodunite–troctolite– anorthosite–gabbro intrusion formed from low-Ti high-alumina olivine–basalt melt. It is promising for Cu–Ni–PGE mineralization.

Petrology of Lasail plutonic complex, northern Oman ophiolite, Oman: An example of arc-like magmatism associated with ophiolite detachment

Lasail plutonic complex (4.7×3.8km), as a typical example of late stage intrusive rocks, is located to the south of Wadi Jizi, and intrudes into the base of V1 volcanic rocks and sheeted dike complex. The Lasail plutonic complex consists of various rock types ranging from ultramafic cumulates to tonalite, and is associated with minor amounts of axis stage gabbro to quartz diorite. These rocks are classified into the following eleven rock types: massive gabbro 1, quartz diorite 1 (axis stage intrusive rocks), olivine websterite, olivine gabbronorite, gabbronorite, hornblende gabbronorite, leucogabbronorite (layered gabbros), massive gabbro 2, diorite, quartz diorite 2, and tonalite (late stage intrusive rocks). The layered gabbros are intruded by the massive gabbro 2, and often occur as large irregular blocks in the massive gabbro 2. The massive gabbro 2 intrudes into the layered gabbros, and sometimes grades into hornblende gabbronorite layer of the layered gabbros. In some places, anastomosing veins of the hornblende quartz gabbronorite injected into the gabbronorite, which continue to the layer of leucogabbronorite. These gabbroic rocks are intruded by small intrusions of diorite, quartz diorite 2, and tonalite. The quartz diorite 2 forms rather larger intrusive bodies that intrude into the gabbroic rocks in the higher level. The tonalite occurs as thin dike and sheet mainly in the layered gabbros. N-MORB normalized trace element patterns for the massive gabbro 2 are characterized by enrichment of LILEs relative to REEs, and resemble to ‘subduction component’ from the island arc tholeiite except a weak enrichment for middle to light REE and P. In contrast, the massive gabbro 1 (axis stage gabbro) shows rather flat pattern similar to MORB with no remarkable ‘subduction component’. From the examination of Sr and Nd isotopic signature, the Lasail plutonic rocks are characterized by higher ε-Sr values than those of MORB. This suggests that the primitive magma of Lasail complex was influenced by the rocks with higher ε-Sr values than those of MORB, e.g., the axis stage rocks interacted with seawater. These lines of evidence suggests that the massive gabbro 2 was formed by the partial melting of residual MORB mantle which is contaminated with slab melt derived from the axis stage rocks interacted with seawater. In addition, petrogenesis of felsic rocks in the Lasail complex can be explained by the partial melting model of pre-existing layered gabbro. U–Pb zircon ages analyzed by LA-ICPMS are 100±2 and 99±2Ma for late stage tonalite and 100±1Ma for axis stage quartz diorite. These ages are slightly older than the ages reported for felsic rocks in the Oman ophiolite (ca., 95Ma), and suggest that the conversion from ridge stage to detachment stage took place rapidly.

Geochemical features and geodynamic setting of formation of the Lukinda dunite–troctolite–gabbro massif (southeastern framing of the Siberian Platform)

The Lukinda dunite–troctolite–gabbro massif in the Selenga–Stanovoy superterrane on the southeastern framing of the Siberian Platform was earlier considered Precambrian. The performed 40Ar/39Ar dating of the massif plagioclase yielded an Early Permian age (285 ± 7.5 Ma). The main specific petrochemical features of the intrusion rocks during their crystallization differentiation are an increase in SiO2 and CaO contents and a decrease in FeOtot content, with TiO2 content remaining low and showing minor variations. A specific geochemical feature of the Lukinda massif ultrabasite–basites is a slight domination of LREE over HREE, with (La/Yb)N= 1.0–8.2. The depletion of the massif rocks in LILE (except for Sr and Ba), REE, and HFSE suggests that the massif formed on an active continental margin.

Major elements hydrochemistry and groundwater quality of Wadi Fatimah, West Central Arabian Shield, Saudi Arabia

Wadi Fatimah is the largest NE-SW trending low topographic area in the west central Arabian Shield, Saudi Arabia. It is bounded by in the NW side by the faulted and folded volcano-sedimentary succession of Fatima Formation and in the SE side by the metamorphic rocks of Jeddah Group. Some massive gabbros and granites and foliated amphibolites are also present along both sides and in the upstream of Wadi Fatimah. The groundwater of Wadi Fatimah is present in the shallow Quaternary alluvial deposits and also in the underlying fractured Precambrian bedrocks. The description of the chemical composition of the groundwater of Wadi Fatimah includes the variations of the groundwater salinity, the major and trace constituents as well as the groundwater character. According to the chemical analysis and the trilinear and durov diagrams, the prevalent chemical character is calcium chloride followed by sodium chloride water types. All the sub-basins of Wadi Fatimah are characterized by a relatively low to medium saline water. Highly mineralized water zone marked the lower part of the wadi, e.g., Al Jumum-Bahrah area, where the groundwater salinity reached up to 26,500 μS/cm with an average of about 9,500 μS/cm. Major and trace elements analyses of the groundwater samples from the different tributaries of Wadi Fatimah revealed its suitability for irrigation purposes.

Pavlovskyite Ca8(SiO4)2(Si3O10): A new mineral of altered silicate-carbonate xenoliths from the two Russian type localities, Birkhin massif, Baikal Lake area and Upper Chegem caldera, North Caucasus

The new mineral pavlovskyite Ca8(SiO4)2(Si3O10) forms rims together with dellaite Ca6(Si2O7)(SiO4)(OH)2 around galuskinite Ca7(SiO4)3CO3 veins cutting calcio-olivine skarns in the Birkhin gabbro massif. In addition, skeletal pavlovskyite occurs in cuspidine zones of altered carbonate xenoliths in the ignimbrites of the Upper Chegem caldera (North Caucasus). The synthetic analog of pavlovskyite has been synthesized before and is known from cement-like materials. Isotypic to pavlovskyite is the synthetic germanate analog Ca8(GeO4)2(Ge3O10). The crystal structure of pavlovskyite, space group Pbcn, a = 5.0851(1), b = 11.4165(3), c = 28.6408(8) Å, V = 1662.71(7) Å3, Z = 4, has been refined from X-ray single-crystal data to R1 = 3.87%. The new colorless mineral has a Mohs hardness of 6-6.5, biaxial (-), α = 1.656(2), β = 1.658(2), γ = 1.660(2) (589 nm), 2V (meas) = 80(5)°, 2V (calc) = 89.9°, medium dispersion: r > v, optical orientation: X = b, Y = c, Z = a.

A new metallogenic model of the Panzhihua giant V–Ti–iron oxide deposit (Emeishan Large Igneous Province) based on high-Mg olivine-bearing wehrlite and new field evidence

The Panzhihua layered intrusion hosts a giant V–Ti–iron oxide deposit with ore reserves estimated at 1333 Mt. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon dating of comagmatic anorthosite yields a crystallization age of 259.77 ± 0.79 million years, coeval with the Emeishan flood basalts. Recently, we identified a small wehrlite dike in microgabbroic rocks and marbles. The wehrlite consists of high-Mg olivine phenocrysts with up to 90.44 wt.% Fo. Incompatible element-normalized patterns between bulk wehrlite and clinopyroxenes in gabbro suggest that they are cogenetic. The Panzhihua parental magma is estimated to have been picritic (∼10 wt.% FeO and ∼16 wt.% MgO), produced by partial fusion of garnet peridotite. Much of the melting occurred in garnet-facies mantle at an initial melting temperature of about 1530°C and pressure of ∼3.4 GPa, suggesting involvement of a mantle plume. The degree of partial melting was rather modest and could have been generated by plume–lithosphere interaction or ascending plume-derived melting contaminated by lithospheric mantle. Field relationships show sharp contacts between the massive ores and gabbro, between wehrlite and fine-grained gabbro, and between disseminated ores and gabbro. Considering the entire intrusion, which is locally cut by dikes or veins of anorthosite, together with the occurrence of a breccia made up of gabbro clasts cemented by disseminated ores, we suggest that different types of magmas were generated by liquid differentiation in a deeper-level chamber. This differentiation could have resulted from double-diffusive convection cells, with melt later intruding into a higher-level chamber, rather than by crystal settling or in situ growth on the floor of the intrusion. However, rhythmic layering produced by in situ crystallization only occurs in the middle of the Panzhihua intrusion and was caused by periodic fluctuation in water pressure.

Spectral characterization and ASTER-based lithological mapping of an ophiolite complex: A case study from Neyriz ophiolite, SW Iran

The Neyriz ophiolite occurs along the Zagros suture zone in SW Iran, and is part of a 3000-km obduction belt thrusting over the edge of the Arabian continent during the late Cretaceous. This complex typically consists of altered dunites and peridotites, layered and massive gabbros, sheeted dykes and pillow lavas, and a thick sequence of radiolarites. Reflectance and emittance spectra of Neyriz ophiolite rock samples were measured in the laboratory and their spectra were used as endmembers in a spectral feature fitting (SFF) algorithm. Laboratory spectral reflectance measurements of field samples showed that in the visible through shortwave infrared (VNIR–SWIR) wavelength region the ultramafic and gabbroic rocks are characterized by ferrous-iron and Fe, Mg OH spectral features, and the pillow lavas and radiolarites are characterized by spectral features of ferric-iron and Al OH. The laboratory spectral emittance spectra also revealed a wide wavelength range of Si O spectral features for the ophiolite rock units. After continuum removal of the spectra, the SFF classification method was applied to the VNIR + SWIR 9-band stack, and to the 11-band data set of SWIR and TIR data sets of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor, using field spectra as training sets for evaluating the potential of these data sets in discriminating ophiolite rock units. Output results were compared with the geological map of the area and field observations, and were assessed by the use of confusion matrices. The assessment showed, in terms of kappa coefficient, that the SFF classification method with continuum removal applied to the SWIR data achieved excellent results, which were distinctively better than those obtained using VNIR + SWIR data and TIR data alone.

New ophiolite occurrences in Sudan and constraint on the western boundary of the Nubian Shield: Petrographical and geochemical evidence

Important mafic–ultramafic masses have been located for the first time in the intersection area between the Keraf Shear Zone and the Nakasib Suture Zone of the Nubian Shield. The masses, comprising most of the members of the ophiolite suite, are Sotrebab and Qurun complexes east of the Nile, and Fadllab complex west of the Nile. The new mafic–ultramafic masses are located on the same trend of the ophiolitic masses decorating the Nakasib Suture. A typical complete ophiolite sequence has not been observed in these complexes, nevertheless, the mafic–ultramafic rocks comprise basal unit of serpentinite and talc chlorite schists overlain by a thick cumulate facies of peridotites, pyroxenites and layered gabbros overlain by basaltic pillow lavas with dolerite dykes and screens of massive gabbros. Associated with pillow lavas are thin layers of carbonates and chert. The best section of cumulate mafic–ultramafic units has been observed in Jebel Qurun and El Fadlab complexes, comprising peridotites, pyroxenites and layered gabbros. Dolerite dykes and screens of massive gabbros have been observed with basaltic pillow lava sections in Wadi Dar Tawaiy. The basal ultramafic units of the complexes have been fully or partly retrograded to chlorite magnetite schist and talc to talc-carbonate rocks (listowenites), especially in the Jebel Qurun and Sotrebab complexes. Petrographically, the gabbros (layered and massive) and the basaltic pillow lavas show mineral assemblages of epidote amphibolite facies. The mafic members from the three complexes show a clear tholeiitic trend and oceanic floor affinity. The pillow lavas plot in the field of oceanic floor basalt, namely in the back arc field. Primitive mantle normalized spider diagram of the pillow lavas reveals a closer correspondence to Enrich-Mid-Oceanic Ridge Basalt (E-MORB) type, which is confirmed by the flat chondrite normalized Rare Earth Elements (REE) pattern. Field, petrographical and geochemical evidence supports ophiolitic origin of the three complexes. The newly discovered ophiolitic complexes mark the western continuation of the Nakasib Suture Zone.

Melt–Peridotite Reactions and Fluid Metasomatism in the Upper Mantle, Revealed from the Geochemistry of Peridotite and Gabbro from the Horoman Peridotite Massif, Japan

An investigation of the petrology and geochemistry of peridotites and gabbros in the Horoman massif, Hokkaido, Japan was undertaken to constrain geochemical processes in the upper mantle. Two types of sample were studied: one type comprises peridotites and gabbros forming thin layers varying from a few millimeters to centimeters in scale (thin-layer peridotites and gabbros); the other comprises thick layers (>1 m scale; massive peridotites and gabbros). There is no clear trace element evidence for metasomatism in the thin-layer peridotites. Instead, they have melt–rock reaction textures interpreted in terms of the formation of secondary pyroxene at the expense of primary porphyroclastic olivine and dissolution of primary porphyroclastic pyroxene to form secondary olivine. The thin-layer gabbros also exhibit no metasomatic effects; they have incompatible element depleted trace element characteristics and mid-ocean ridge basalt (MORB)-like isotopic signatures consistent with the presence of a new type of gabbro that previously has not been described from the Horoman Massif. The whole-rock chemistry of the thin-layer peridotites and thin-layer gabbros can be explained by melt–peridotite reactions between isotopically highly depleted MORB mantle (represented by the thin-layer peridotites) and melt with geochemical affinity to Pacific MORB (represented by the thin-layer gabbros). Sm–Nd and Lu–Hf isotope systematics suggest that these reactions might have occurred at ∼300 Ma. Some of the plagioclase lherzolites and all of the spinel lherzolites and harzburgites within the massive peridotites show enrichment in incompatible trace elements and more radiogenic Hf–Nd–Pb isotopic compositions than the incompatible-element depleted thin-layer peridotites. The analyzed massive gabbros are interpreted as subduction-related magmas formed in a MORB-source mantle wedge, which have subsequently interacted with a fluid or melt in the Hidaka subduction zone. Hf–Nd–Pb isotope systematics reveal that this interaction may have occurred at an age younger than ∼50 Ma. Melt- and fluid-related processes occurring in the upper mantle are systematically identified from the samples of the Horoman Massif based on petrography, major and trace element, and Sr–Nd–Pb–Hf isotope geochemistry. These processes occurred in different tectonic settings such as the convecting oceanic mantle and supra-subduction zone mantle wedge and have variably modified the original chemistry of residual mantle protolith, formed by partial melting of a depleted MORB source mantle at ∼1 Ga.

Sm-Nd and Rb-Sr ages of gabbroids, granitoids, and titanomagnetite ores from layered intrusions of the Kusa-Kopan Complex (South Urals)

732 A band of layered gabbro intrusions belonging to the Kusa–Kopan Complex is confined to the zone of the Zyuratkul riftogenic fault in the eastern part of the Bashkir Meganticlinorium, where it consists of several massifs (Matkal, Kopan, Medvedevo, Kusa) with the Ryabinovka and Gubenskii granitoid massifs located in the hanging wall (Fig. 1). The gabbro massifs enclose synonymous ilmenite–titanomagnetite deposits of the Urals in the form of stratified series, vein, and lenticu� lar lodes of continuous and disseminated ores, which alternate with gabbroids. These deposits were discov� ered in the 1930s, and since that time their geological structure and composition of ores and host rocks have been studied by many researchers. The primary atten� tion was paid to the most debatable factors responsible for strong lateral variations in the composition of rocks and ores within the complex, represented by horn� blende gabbro and gneiss–granites in the Kusa and Gubenskii massifs and ilmenite–magnetite ores in the Kusa and Medvedevo deposits in the north and by gab� bronorites (Matkal and Kopan massifs), porphyroid granophyric granites (Ryabinovka Massif), and titano� magnetite ores in the south. Some researchers explain such differences in the mineral composition by the influence of superposed metamorphic processes on rocks and ores of the Kusa Massif [1, 2], which occurred substantially after the formation of intru� sions. Our petrological studies performed in recent years revealed that the formation depth of granitoids, gab� broids, and enclosed ores increases along this band in the northerly direction from 3–4 km in the south to 20–30 km in the north. Accordingly, the P–T param� eters of rock and ore formation vary in the same direc� tion: = 0.5–3.0 kbar and T = 900–1100°C in the south (Kopan and Matkal gabbro massifs and associ� ated deposits, Ryabinovka granite massif) and P = 6– 9 kbar and T = 600–900°C in the north (Kusa and Medvedevo gabbro and Gubenskii granite massifs,

Estimation of the emplacement depth of a magmatic intrusive body based on the data for distribution of isogrades in the surrounding metamorphic zoning (Model approximation)

A method of model calculations for the approximate estimation of the emplacement depth of magmatic intrusive body based on data for the distribution of isotherms in the near-intrusive area is proposed. Isotherms are maximums of respective temperatures reached when exocontact rocks are heated at a different time; some of them represent isogrades. The calculations require knowing the form of the intrusive body, the magma composition and initial temperature, the regional geothermal gradient in the Earth’s crust during the magma intrusion, the thermal and physical properties of rocks and melt, and the distribution of isogrades/isotherms in the enclosing rocks. The probable depth of emplacement of the Kharlovo gabbro massif in the northwestern foothill belt of Altai is estimated as an example. It was determined to be 7–8 km. The estimates of depth that are obtained in such a way cannot be absolutely accurate, since they are a result of calculations in a simplified model statement.

Petrographic contrast between ilmenite- and magnetite-series gabbroids in the Ryoke and San-in belts, southwestern Japan Arc

To determine the petrographic contrasts between ilmenite- and magnetite-series magmas, we assessed the rock descriptions, modal compositions, and whole-rock chemical compositions for ilmenite- and magnetite-series gabbroic masses in the Ryoke and San-in belts and focused on the primitive phases of the two plutonic belts. We obtained the following results: (1) The San-in gabbroids contain variable modal amounts of magnetite, up to 5.8%, whereas no magnetite was detected in the Ryoke gabbroids. The outcrop measurements of magnetic susceptibility exhibited good positive correlation with the magnetite contents of the outcrop samples. (2) As compared to the San-in gabbroids, the Ryoke gabbroids are generally less abundant in alteration minerals such as chlorite and fibrous actinolite and more abundant in olivine- and orthopyroxene-bearing phases. (3) The rock texture and whole-rock chemical compositions indicate that the Ryoke gabbroids have more cumulative characteristics than the San-in gabbroids. The compositional differences between them can be mainly explained by the differences between their degrees of crystal accumulation. (4) We were unable to detect any significant differences between the N-MORB normalized patterns of trace-elements and REEs in the Ryoke and San-in gabbroids. Features (2) and (3) may be responsible for the differences between the emplacement depths of the Ryoke and San-in gabbroic masses, although these features are independent of redox states. Feature (4) suggests that the two gabbroids have a common petrogenetic background. Based on these results, we conclude that the contamination of Ryoke gabbroic magma by sediments is insignificant, having little or no effect on redox states. It is more likely that the divergent redox states of the two gabbroic magmas are primarily attributable to the differences in the volatile components involved in the fO2 buffer reactions.

Mantle Melting, Melt Transport, and Delivery Beneath a Slow-Spreading Ridge: The Paleo-MAR from 23 15'N to 23 45'N

Kane Megamullion, an oceanic core complex near the Mid-Atlantic Ridge (MAR) abutting the Kane Transform, exposes nearly the full plutonic foundation of the MARK paleo-ridge segment. This provides the first opportunity for a detailed look at the patterns of mantle melting, melt transport and delivery at a slow-spreading ridge. The Kane lower crust and mantle section is heterogeneous, as a result of focused mantle melt flow to different points beneath the ridge segment in time and space, over an ∼300–400 kyr time scale. The association of residual mantle peridotite, dunite and troctolite with a large ∼1 km+ thick gabbro section at the Adam Dome Magmatic Center in the southern third of the complex probably represents the crust–mantle transition. This provides direct evidence for local melt accumulation in the shallow mantle near the base of the crust as a result of dilation accompanying corner flow beneath the ridge. Dunite and troctolite with high-Mg Cpx represent melt–rock reaction with the mantle, and suggest that this should be taken into account in modeling the evolution of mid-ocean ridge basalt (MORB). Despite early precipitation of high-Mg Cpx, wehrlites similar to those in many ophiolites were not found. Peridotite modes from the main core complex and transform wall define a depletion trend coincident with that for the SW Indian Ridge projecting toward East Pacific Rise mantle exposed at Hess Deep. The average Kane transform peridotite is a lherzolite with 5·2% Cpx, whereas that from the main core complex is a harzburgite with only 3·5% Cpx. As the area corresponds to a regional bathymetric low, and the crust is apparently thin, it is likely that most residual mantle along the MAR is significantly more depleted. Thus, harzburgitic and lherzolitic ophiolite subtypes cannot be simply interpreted as slow- and fast-spreading ridges respectively. The mantle peridotites are consistent with a transform edge effect caused by juxtaposition of old cold lithosphere against upwelling mantle at the ridge–transform intersection. This effect is far more local, confined to within 10 km of the transform slip zone, and far smaller than previously thought, corresponding to ∼8% as opposed to 12·5% melting of a pyrolitic mantle away from the transform. Excluding the transform, the overall degree of melting over 3 Myr indicated by the peridotites is uniform, ranging from ∼11·3 to 13·8%. Large variations in composition for a single dredge or ROV dive, however, reflect local melt transport through the shallow mantle. This produced variable extents of melt–rock reaction, dunite formation, and melt impregnation. At least three styles of late mantle metasomatism are present. Small amounts of plagioclase with elevated sodium and titanium and alumina-depletion in pyroxene relative to residual spinel peridotites represent impregnation by a MORB-like melt. Highly variable alumina depletion in pyroxene rims in spinel peridotite probably represents cryptic metasomatism by small volumes of late transient silica-rich melts meandering through the shallow mantle. Direct evidence for such melts is seen in orthopyroxenite veins. Finally, a late hydrous fluid may be required to explain anomalous pyroxene sodium enrichment in spinel peridotites. The discontinuous thin lower crust exposed at Kane Megamullion contrasts with the >700 km2 1·5 km+ thick Atlantis Bank gabbro massif at the SW Indian Ridge (SWIR), clearly showing more robust magmatism at the latter. However, the SWIR spreading rate is 54% of the MAR rate, the offset of the Atlantis II Fracture Zone is 46% greater and Na8 of the spatially associated basalts 16% greater—all of which predict precisely the opposite. At the same time, the average compositions of Kane and Atlantis II transform peridotites are nearly identical. This is best explained by a more fertile parent mantle beneath the SWIR and demonstrates that crustal thickness predicted by simply inverting MORB compositions can be significantly in error.

Bol’shakovskii gabbro massif as a fragment of the Southern Urals zone of Early Carboniferous rift

In this paper new data on the absolute age and geochemistry of rocks of the Bol’shakovskii massif, situated in the central part of the Aramil-Sukhteli zone of the Southern Urals, are given. The obtained values are evidence for its Visean age. By the geological-petrographic and petro- and geochemical features, the rocks of the Bol’shakovskii complex differ sharply from ophiolite-type gabbroids, although they reveal a substantial similarity with the gabbro-granite formation of the Magnitogorsk megazone. The Bol’shakovskii massif is situated in the northern branch of the South Urals zone of Early Carboniferous riftogenesis; and its formation is most probably associated with magmatism events during the rift regime in the died_out island arc.

Geological mapping strategy using visible near‐infrared–shortwave infrared hyperspectral remote sensing: Application to the Oman ophiolite (Sumail Massif)

An airborne hyperspectral survey of the Oman ophiolite (Sumail Massif) has been conducted using the HyMap airborne imaging spectrometer with associated field measurements (GER 3700). An ASD FieldSpec3 spectrometer was also used in order to constrain the spectral signatures of the principal lithologies cropping out in the surveyed area. Our objective was to identify and map the various igneous lithologies by a direct comparison at high spectral resolution between field and airborne spectra despite strong variations in outcropping conditions such as (1) lighting, (2) surface roughness geometry, (3) blocks coated with red/brown patina and exfoliation products, or (4) deep hydrothermal weathering. On the basis of spectral signatures, we are able to distinguish three end‐members of olivine‐orthopyroxene bearing assemblages in the mantle sequence: (1) harzburgites, (2) dunites, and (3) a harzburgite with interstitial carbonate. Because plagioclase is spectrally featureless in the wavelength range studied it cannot be detected. In the crustal sequence, we therefore identified four end‐members with variable abundance of clinopyroxene: (1) massive gabbros, (2) amphibolized (upper) gabbros associated with intrusive dykes, (3) wehrlite with high serpentine content, and (4) gabbronorite (a lithology not previously recognized in the studied area). With the exception of wehrlite, spectra of olivine‐rich end‐members display characteristic Mg‐OH narrow absorption features caused by their high serpentine content. We take advantage of this observation to split the data into two subsets, corresponding to the mantle and crustal sequences, respectively. Pixels of an image often correspond to heterogeneous areas in the field and a direct comparison between airborne and in situ spectra is not straightforward. However, comparing spectra of pixels associated with the most homogeneous areas in the field with the spectra acquired in situ at the same location, we found a systematic change both in mean intensity and overall spectral shape. Dividing each spectrum by its low‐pass trend removes the effects caused by surface light scattering associated with each scale of analysis and results in an exceptional match between field and airborne spectra. However, the albedo information is lost and as a consequence, rock types only characterized by albedo change cannot be discriminated. A spectrum of a mixture of powdered minerals is usually seen as a linear combination of mineral spectra proportional to their abundance. However, this is no longer the case when minerals occur in complex arrangements in rock types. We thus develop a synthetic spectral library of all possible combinations of rock types covering the surface area of a pixel and use a simple distance calculation to identify the best match between each pixel and modeled spectra. This procedure allows the determination of the fractional cover of each rock type in a given pixel and to establish maps for each spectral end‐member. The final product is a geological map, derived from the combination of end‐member fractional cover maps, and is broadly consistent with the existing geological maps. Beyond this general agreement which demonstrates the potential of this new approach for geological mapping, imaging spectrometry allows (1) to map in detail the outline of the Moho north of Maqsad and (2) to identify a new crustal sequence enriched in silica south of Muqzah, revealing the presence of orthopyroxene, the nature and distribution of which are of relevance to the petrological and tectonic understanding of the Oman ophiolite evolution.

Petrogenesis of the Refahiye Ophiolite and its Tectonic Significance for Neotethyan Ophiolites Along the İzmir-Ankara-Erzincan Suture Zone

The Refahiye ophiolite, situated near Erzincan in the eastern part of the İzmir-Ankara-Erzincan Suture Zone (İAESZ), is one of the best exposures of oceanic lithosphere in the northern branch of the Neotethyan Ocean. The ophiolite, mainly thrust southwards, transported on an ophiolitic mélange, over the Early Triassic-Campanian Munzur Limestone of the Anatolide-Tauride Platform, was also emplaced by north-directed backthrusting onto the Pontides in the Late Cretaceous. It displays an almost complete ophiolite series unconformably overlain by Tertiary volcano-sedimentary units. The mantle peridotites are dominated by harzburgites, with dunite bands and lenses locally cut by isolated diabase dykes, whereas the mafic-ultramafic cumulates of the mantle-crust transitional zone consist of dunites, wehrlites, pyroxenites and gabbros. The diabase dykes of the sheeted-dyke complex show a magmatic boundary with the underlying massive gabbros which are in places crosscut by plagiogranite dykes. The basic and leucocratic rocks exhibit greenschist facies assemblages of ocean floor metamorphism. The spilitic pillow basalts and cover sediments (radiolarites, pelagic limestones) of the upper units of the oceanic crust are preserved as megablocks in the ophiolitic mélange. Petrological features of the ophiolitic rocks show that the ultramafic, mafic and leucocratic rocks belong to a co-magmatic differentiated tholeiite series. The crystallization order is olivine-clinopyroxene (locally with orthopyroxene)-plagioclase. The gabbros and diabases are tholeiitic (Nb/Y= 0.05-0.5). Whereas the LIL elements (K, Sr, Rb, Ba), with the exception of Th, show variable scatter because of ocean floor hydrothermal alteration, the HFSE (Nb, Ti, Zr, Y) and the LREE (La, Ce, Nd) have been depleted relative to N-MORB. The ratios of the selected trace elements (Ti/V, Zr/Y, Th/Y, Ti/Zr) and the tectonomagmatic discrimination diagrams for Ti-poor gabbros and dykes suggest that the Refahiye ophiolite has island arc tholeiitic (IAT) and possibly boninitic affinities. The mantle peridotites have very low REE concentrations, showing U-shaped REE patterns. The isotropic gabbros and plagiogranites exhibit high REE concentrations with positive and negative Eu anomalies respectively, patterns typical of SSZ magmatism. The ultramafic-mafic and leucocratic rocks of the Refahiye ophiolite were formed in the earliest stages of island arc development in a suprasubductional, forearc tectonic setting in the northern branch of the Neotethyan Ocean, similar to most of the Cretaceous Eastern Mediterranean ophiolites.

Hydration due to high-T brittle failure within in situ oceanic crust, 30°N Mid-Atlantic Ridge

Analysis of an in situ fault zone within the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge) provides clues to the relevant deformation mechanisms and their temporal evolution within oceanic crust. IODP EXP304/305 drilled a succession of gabbroic lithologies to a final depth of 1415 m below the sea floor (mbsf), with very high recovery rates of up to 100% (generally ∼ 80%). We identified an intra-crustal fault zone between 720 and 780 mbsf in a section of massive gabbro, olivine gabbro, oxide gabbro units, and minor diabase intrusions. Of particular interest is the section between 744 and 750 mbsf, which unfortunately was marked by low recovery rates (17%). Electrical borehole-wall images show a 1-m-thick zone of east-dipping fractures within this interval, which is otherwise dominated by N–S dipping structures. Despite the high fracture density in this section, the hole walls are smooth, with rare breakouts, suggesting that the low recovery rate was due to a change in lithology rather than well conditions. The recovered rocks include ultracataclasite and possibly incohesive fault gouge that formed in the upper amphibolite regime, with mostly amphibole infill. Logging data suggest that the gabbroic rocks in this interval are rich in hydrous phases, consistent with increased amounts of amphibole found in the core. Equilibration temperature conditions of about 640 °C were obtained for plagioclase clasts and aluminous actinolite, assuming a pressure of 200 MPa. The permeability of the fault zone is in the range of 10 − 19 to 10 − 17 m 2. Although the permeability appears to be high within the fault zone relative to other parts of the section, it is no higher than that in typical lower crustal material. As a consequence, because brittle failure occurred at high temperatures, the fault zone was subsequently completely sealed by hydrous minerals, thereby preventing further fluid circulation and preserving water in the crust.

Geochemical Comparison of Ultramafic-Mafic Cumulate Rocks from the Central Anatolian Ophiolites, Turkey

Ophiolites, formed during closure of the Neo-Tethyan ocean, are widespread in Turkey and have been extensively studied. In addition to ophiolites showing recognizable stratigraphic sequences, numerous massive and layered gabbroic masses occur as isolated outcrops in the Central Anatolian Crystalline Complex. An example from the Akcakent region ranges from gabbro to diorite; another, from the Kustepe area, is characterized by dunite, wehrlite, troctolite, olivine gabbro, clinopyroxene gabbro, clinopyroxene hornblende gabbro, uralite gabbro, and fine-grained gabbro. Samples from both regions display adcumulate and orthocumulate textures and show low-K and subalkaline characters; they are enriched in some LILE (e.g., Sr, Rb, K, Ba) and depleted in HFSE (e.g., Ta, Nb, Zr, Ti) relative to N-MORB. Petrological data, negative Ta, Nb, Zr, Ti anomalies, and ratios of elements such as Ba/Nb and La/Nb indicate a suprasubduction setting for the central Anatolian ultramafic-mafic rocks, suggesting that both rock types may have formed in a backarc setting.

Melt–rock reaction in the lower oceanic crust and its implications for the genesis of mid-ocean ridge basalt

Primitive cumulates from a 2–3 Ma old gabbro massif exposed in the Kane Megamullion (23°N, Mid-Atlantic Ridge) contain abundant clinopyroxene with high Mg# (86–91). Such magnesian clinopyroxenes have hitherto been taken to signify crystallization at elevated pressures. Kane clinopyroxenes, however, are dominantly oikocrysts that overgrow olivine and plagioclase, indicating crystallization occurred at low pressure. The oikocrysts have textures and compositions indicative of disequilibrium processes. First, many of the oikocrysts enclose resorbed plagioclase with lower anorthite contents than plagioclase in the host rock, and olivine is notably absent as a chadacryst despite being abundant in the host rock. Second, the oikocryst minor element compositions are inconsistent with equilibrium growth from a MORB melt. These data indicate that high-Mg# clinopyroxene in the Kane gabbros formed as a result of reaction between primitive cumulates and migrating melt in the lower oceanic crust, with clinopyroxene and secondary plagioclase growing at the expense of olivine and primary plagioclase. Thus high-Mg clinopyroxene does not result from high-pressure crystallization as has been inferred previously. Assimilation–fractional crystallization modeling indicates that melts undergoing such reactions are enriched in Al 2O 3 and MgO and depleted in CaO and SiO 2. This effect is similar to that expected for fractional crystallization of MORB at elevated pressures, and reacted melts yield higher calculated pressures than starting melts. This suggests that the CaO–Al 2O 3–MgO–SiO 2 relationships of MORB may result from melt–rock reaction, and that calculated pressures of MORB fractionation are overestimated as a result. Melt–rock reaction in the lower oceanic crust may thus account for both lines of evidence for high-pressure fractionation of MORB.