Chromite Pods Research Articles

Application of Satellite Remote Sensing, UAV-Geological Mapping, and Machine Learning Methods in the Exploration of Podiform Chromite Deposits

The irregular and sporadic occurrence of chromite pods in podiform chromite deposits (PCD), especially in mountainous terranes with rough topography, necessitates finding innovative methods for reconnaissance and prospecting. This research combines several remote sensing methods to discriminate the highly serpentinized peridotites hosting chromite pods from the other barren ultramafic and mafic cumulates. The case study is the area of the Sabzevar Ophiolite (NE Iran), which hosts several known chromite and other mineral deposits. The integration of satellite images [e.g., Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite sensor, Landsat series, and Sentinel-2] coupled with change detection, band rationing, and target detection algorithms [including the Spectral Angle Mapper (SAM)] were used to distinguish potential lithological units hosting chromites. Results have been verified by an initial on-field checking and compared with the high-resolution (GSD ~6 cm) orthomosaic images obtained by the processing of photographs taken from an Unmanned Aerial Vehicle (UAV) at a promising area of 35 km2. The combination of visual interpretation and supervised classification by machine learning methods [Support Vector Machine (SVM)] yielded the production of a geological map, in which the lithological units and structures are outlined, including the crust-mantle transition zone units, mafic cumulates, crosscutting dykes, and mantle sequences. The validation of the results was performed through a second phase, made up of field mapping, sampling, chemical analysis, and microscopic studies, leading to the discovery of new chromite occurrences and mineralized zones. All ultramafic units were classified into four groups based on the degree of serpentinization, represented by the intensity of their average spectral reflectance. Based on their presumed protolith, the highly serpentinized ultramafics and serpentinites were classified into two main categories (dunite or harzburgite). The serpentinite with probable dunitic protolith, discriminated for a peculiar Fe-rich Ni-bearing lateritic crust, is more productive for chromite prospecting. This is particularly true at the contact with mafic dykes, akin to some worldwide chromite deposits. The results of our work highlight the potential of multi-scale satellite and UAV-based remote sensing to find footprints of some chromite mineral deposits.

Feldspar microboudinage paleopiezometer and its applications to estimating differential stress magnitudes in the continental middle crust (examples from west Yunnan, China)

Feldspar, which is the most abundant crustal mineral, exhibits brittle behavior while quartz and mica deform plastically by dislocation creep, under the greenschist-facies conditions (300–450 °C). Pull-apart microboudinage, which formed by repeated cycles of tensile fractures, is a common microstructure observed in feldspar porphyroclasts embedded in the ductile matrix of quartz in granitic mylonites from the Red River, Chongshan and Gaoligong shear zones (Yunnan, China). These crustal-scale shear zones, which lie in the southeastern margin of the Tibet Plateau, have played a critical role in the southeastward lateral extrusion of the Indochina composite block in response to the Indo-Asian continental collision. Here we developed a feldspar microboudinage paleopiezometer. In each feldspar porphyroclast, the differential stresses estimated from microboudins progressively increases with sequential fracturing, depicting an evolution during exhumation of the mylonite upwards the brittle-ductile transition. The results from the feldspar microboudinage piezometer and the quartz recrystallized grain-size piezometer are largely consistent within the range of uncertainties. The microboudinage piezometer, combined with results of finite strain measurements, deformation temperature estimates, and thermochronological data, can yield the valuable information about rheological properties of naturally deformed rocks. It can also be directly used or adapted to a wide range of geological problems, for example, fractured pebbles in an unlithified matrix, chromite pods in serpentinized mantle rocks, and strong boudins in a soft material at any scale.

Suprasubduction-zone origin of the podiform chromitites of the Bir Tuluhah ophiolite, Saudi Arabia, during Neoproterozoic assembly of the Arabian Shield

The ultramafic section of a dismembered ophiolite is exposed at Bir Tuluhah, in the north-central part of the Arabian Shield. It is penetratively serpentinized and locally carbonate-altered to talc‑carbonate and quartz‑carbonate rocks (listvenite) along shear zones and fault planes. Despite the high degree of mineral replacement, preserved mesh and bastite textures and fresh relics of primary Cr-spinel and olivine show that the protoliths were mainly harzburgite with minor dunite, with sparse massive chromitite bodies of various forms and sizes. Olivine inclusions in the chromitite lenses have higher forsterite content and NiO concentrations than fresh olivine relics in the host harzburgites and dunites, due to subsolidus re-equilibration. Cr-spinels in the chromitites have higher Cr# (0.74–0.82) than those hosted in dunite (0.72–0.76) or harzburgite (0.55–0.66). The scarce Cr-spinel crystals in harzburgite that have Cr# < 0.6 are interpreted to represent the population least affected by melt-rock interaction. The chromitite bodies are interpreted to have formed just below the contact between the oceanic crust and mantle sections (i.e., the petrologic Moho). The primary olivine (high Fo and Ni content) and Cr-spinel core compositions (high Cr# and low TiO2 content) of the Bir Tuluhah serpentinized peridotite are typical of modern supra-subduction zone (SSZ) fore-arc peridotites and consistent with crystallization from boninitic magma. The multistage petrogenesis leading to the chromitite bodies begins with moderate to high degrees of melt extraction from the protoliths of the serpentinized harzburgites, followed by reaction with melt compositions that evolved from tholeiite to boninite and left dunite residues. The massive Cr-rich chromitites in the Bir Tuluhah ophiolite are most probably the residues of such interaction between depleted harzburgite and ascending melts; mixtures of the reacted melts formed boninites, which became saturated with chrome-rich spinel and crystallized chromite pods before ascending past the Moho. We offer a novel thermodynamic model of this mixing and reaction process that quantifies the yield of Cr-spinel.

A Neoproterozoic hyper-extended margin associated with Rodinia's demise and Gondwana's build-up: The Araguaia Belt, central Brazil

The Araguaia Belt encloses a poorly constrained Pan-African (Brasiliano Cycle) continental suture marked by a series of (~750 Ma) ophiolitic units which, when properly characterized, could provide important informations on its geological history, closely linked with the Rodinia demise and further western Gondwana amalgamation. We present new bulk-rock and mineral major and trace element compositions for these ultramafic and mafic units. They mainly consist in fully serpentinized harzburgite, scarce dunite lenses and chromite pods, tectonically overlain by basaltic pillow lavas. Low Al2O3/SiO2 ratios (0.01 to 0.06), rather high MgO concentrations (42.28 to 45.29 wt%) and spinels' Cr# and Mg# ratios comprised between 0.36 and 0.51 and 0.59 and 0.72, respectively, indicate a depleted oceanic-like protolith. MORB-peridotite interactions are evidenced both by pyroxenite, olivine gabbro and diabase occurrences in the serpentinites and by high TiO2 (up to 0.42 wt%) contents in spinels from some Serra do Quatipuru serpentinites. These observations support that the Araguaia Belt ophiolitic bodies are the remnants of the upper mantle section of a MOR or subcontinental lithosphere. The serpentinites whole-rock REE content can be modeled as resulting from a dry partial melting involving 14 to 24% of melt extraction, coupled with refertilization by fertile melts, generated deeper in the mantle. Such an oceanic-like setting is also supported by the N-MORB signature of Serra do Tapa and Morro do Agostinho pillow lavas basalts. All together, these results tend to infirm the supra-subduction zone (SSZ) setting previously proposed for these ophiolitic units. Important LILE, B and Li enrichments in the serpentinites likely result from a metasomatic event involving sediments-derived fluids that occurred during the obduction of the units on the Amazonian Craton. Our results combined with (1) the apparent scarcity of igneous crustal rocks, (2) the proximal nature of the metasedimentary rocks hosting the ophiolitic units, and (3) the occurrences of Amazonian Craton fragments eastward of the ophiolitic bodies, allow us to propose that the Araguaia Belt comprises a fossil ocean-continent transition (OCT) accreted on the eastern border of the Amazonian Craton.

Morpho‐structural Features and Structural Classification of Chromite Pods in the Tropoje‐Has Ophiolite Massif, Albania

Acta Geologica Sinica - English EditionVolume 90, Issue s1 p. 234-234 IGCP-649 Workshop II: Probing the Oceanic Mantle: Troodos Ophiolite as a Case Study Morpho-structural Features and Structural Classification of Chromite Pods in the Tropoje-Has Ophiolite Massif, Albania Ibrahim MILUSHI, Corresponding Author Ibrahim MILUSHI ibrahimmilushi@yahoo.com Institute of Geosciences, Energy, Water and Environment, Tirana, AlbaniaCorresponding author: E-mail: ibrahimmilushi@yahoo.comSearch for more papers by this authorNezir MEKSHIQI, Nezir MEKSHIQI Institute of Geosciences, Energy, Water and Environment, Tirana, AlbaniaSearch for more papers by this author Ibrahim MILUSHI, Corresponding Author Ibrahim MILUSHI ibrahimmilushi@yahoo.com Institute of Geosciences, Energy, Water and Environment, Tirana, AlbaniaCorresponding author: E-mail: ibrahimmilushi@yahoo.comSearch for more papers by this authorNezir MEKSHIQI, Nezir MEKSHIQI Institute of Geosciences, Energy, Water and Environment, Tirana, AlbaniaSearch for more papers by this author First published: 31 October 2016 https://doi.org/10.1111/1755-6724.12992AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume90, Issues1Special Issue: The Seventh International Dyke Conference: Dyke Swarms: Keys to Paleogeographic Reconstruction IDC7, 18–20 August, BeijingOctober 2016Pages 234-234 RelatedInformation

Geophysical, Geochemical and Structural Features of Chromite Ore in Kuradawe Village/ Mawat Ophiolite Complex, Kurdistan Region, NE Iraq.

Small lensoidal bodies of massive and disseminated chromitites have been examined inassociation with ultramafic rocks of the Mawat Ophiolite Complex (MOC), Iraqi ZagrosThrust Zone, Northeastern Iraq. The Chromites ore have variable thickness enveloped bydunite and of, exhibiting transitional boundaries to harzburgitic host rocks.The primary Chromite composition exhibits high Cr varieties; the average Cr# ofChromite is 0.714, with &lt; 0.3 % TiO2 content, which may reflect the crystallization ofChromite from boninitic magma. The result of the gravity survey leads to lateraldelineation of the ore body and estimates an area of about 2800 m2. The thicknesses of different Chromite pods were estimated to be in the range of 10 – 35 m. Structural feature show that such ore bodies have internal structure making angle of 10–12 degrees with dunite wall and surrounding peridotite which indicates that Chromite ore subconcordant type.

Intrinsic geological model generation for chromite pods in the Sabzevar ophiolite complex, NE Iran

The Sabzevar ophiolite, with its colored mélange zone, is a highly disintegrated ophiolite complex located at the northern boundary of the central Iranian microcontinent. A large number of chromite pods occur in this area, which needs to be explored. In this study, a mathematical – geological genetic model is advanced as an exploratory tool that provides information for further exploration activity. A petrogenetic model of chromite ore was established on the basis of a geodata information database. This database consists of information from similar chromite mines from around the world. A detailed investigation of the geological, mineralogical and petrological characteristics of chromite pods in the Sabzevar region was conducted along with detailed petrological samplings, thin section studies and mineralogical analysis. In the next step, we developed a conceptual genetic model that defines areas with a high probability of the existence of chromite pods. The model was later refined using such parameters as a critical genetic factor (CGF) and critical reconnaissance criteria (CRC). Next, a linear function, which is a combination of these factors, provided promising regions as intrinsic geological units (IGU). Finally, a 3D model of lithological units depicting the IGU for chromite pods exploration is proposed.

Laterite Covered Mafic-ultramafic Rocks: Potential Target for Chromite Exploration – A Case Study from Southern Part of Tangarparha, Odisha

Abstract: Exposed chromite deposits in the Sukinda belt, Odisha, India, have already been identified and exploited; but a large part of the area is covered by laterite and remains unexplored. As a case study to establish the feasibility of chromite exploration under laterite rocks, an integrated ground-based gravity, magnetic and very low frequency (VLF) - electromagnetic study was performed over a laterite-covered area at Tangarparha within the belt. North of the present laterite-covered area, a quartzofeldspathic gneiss contains proved chromite pods within ultramafic complexes. The gneiss-laterite contact is depicted by a transition from low to high in both gravity and magnetic anomaly maps at the northern part of the present study area. High Bouguer and residual anomalies (&amp;gt; 10 mGal and &amp;gt; 1 mGal, respectively) within the laterite-covered area indicates the existence of a high density rock in the sub-surface. The 2D models of the residual gravity anomaly depict the presence of high density (&amp;gt; 3570 kg/m3) layer under the laterite cover. The 2D magnetic models mostly reveal shallow surface effects of laterite covers. However, along profiles P2 and P3 high magnetic susceptibilities are detected at depths ≥20 m, and are likely to be caused by sub-surface chromite mineralization, as the locations are coincident with gravity highs. High current densities in VLF profiles are also recorded at the same locations confirming the presence of conducting sub-surface layer. Thus, the zone with high density, magnetic susceptibility and conductivity is most likely to be a chromitite-bearing sub-surface layer. The targeted chromite ore is distributed in east-west direction in the form of discontinuous pods of variable vertical thickness and strike lengths at the central-eastern part of profiles P2 and P3. The present study demonstrates that integrated use of ground gravity, magnetic and VLF techniques can effectively identify the target chromite deposits even under lateritic cover.

Review of platinum-group element distribution and mineralogy in chromitite ores from southern Iran

The platinum-group element (PGE) potential of four chromite mining districts in southern and south-eastern Iran was investigated using geochemical and mineralogical methods. A total of 20 mines and prospects were sampled in the Neyriz ophiolite, the Abdasht and Sikhuran complexes in the Esfandagheh district, and the Faryab district. Chromitite and dunite were analysed for major, trace element and PGE geochemistry, and the platinum-group element mineralogy (PGM). In all mining districts, chromites analysed from mantle tectonite, probable transition zone and cumulate zone chromitites are refractory, low in TiO2 (<0.3wt.%) as well as Cr- and Mg-rich, with Cr# [100∗Cr/(Cr+Al)] ranging from 70 to 81 and Mg# [100∗Mg/(Mg+Fe2+)] ranging from 40 to 83. Associated olivine is highly magnesian (Mg# 89–98) and Ni-rich (0.1–1.0wt.% NiO).PGE concentrations range from <100 to >5000ppb, with median values around 200ppb in all complexes investigated. The 187Os/188Os isotope ratios of chromitite indicate chondritic to slightly suprachondritic initial Os. Mantle-normalised Pt/Ir ratios are generally low (<0.2), but are up to 0.5 in small chromite pods in the harzburgitic mantle section of the Neyriz ophiolite. PGM, mainly laurite–erlichmanite, mostly occur as small (<1 to 30μm) inclusions in chromite. They are associated with hydrous silicates (calcic amphibole), base metal sulphides (Ni- and Cu-sulphides) and other PGM, such as cuproiridsite, Ni–Ir sulphides, and Os–Ir alloys. PGMs interstitial to chromite are represented by irarsite, hollingworthite, Ru–Os–Ir alloys, tolovkite, RhNiAs, Pt–Fe alloy, probable Ru-rich oxides, and others.In the Sikhuran complex, pods of sulphide-bearing chromitite (0.2–1.2% sulphides) occur in a dunite zone interpreted as an ultramafic cumulate zone. Chromites in chromitite are characterised by high Cr# (75–81) and intermediate Mg# numbers (45–65); in associated dunite, chromites (Cr#, 74–80) are more Fe-rich (Mg#, 29–50). Chromitite and dunite carry up to 5.2ppm total PGE and are characterised by high normalised Pt/Ir (up to 5), Pt/Pd<1, and radiogenic Os (moderately suprachondritic initial Os). The interstitial sulphides are dominated by pentlandite that carries on average 0.45% Co, 25ppm Os, 11ppm Ir and 8ppm Rh. Palladium and Pt concentrations in pentlandite are below the detection limit of the Laser Ablation–ICP-MS technique. Pentlandite is associated with rare grains of Pd- and Pt-minerals such as stibiopalladinite and Pt–oxide, and with Cu-rich phases.Large degrees of partial melting in a supra-subduction zone geotectonic setting, probably in a two-stage melting process, produced boninitic melts that crystallised refractory, Cr–Mg rich and Ti-poor chromite. In some cases, sulphide saturation was achieved during and after chromite formation. Saturation during chromite crystallisation is evidenced by Ni–Cu-rich sulphide droplets preserved in some chromitites. Sulphur saturation after chromite crystallisation governed the formation of Ni-rich sulphide liquids that exsolved monosulphide solid solution, which later crystallised to form PGE-bearing pentlandite. Platinum and Pd did not partition into pentlandite but crystallised as Pt- and Pd-rich PGM associated with the pentlandite.

Geoheritage in Morocco: The Neoproterozoic Ophiolite of Bou Azzer (Central Anti-Atlas)

The Bou Azzer ophiolitic complex (Anti-Atlas, southern Morocco) marks the suture of the Pan-African orogenic belt. This fragment of old oceanic crust (697 ± 8 Ma) obducted onto the continental margin of the West African Craton during a collisional event that occurred between 650 and 580 Ma. Very well exposed desert outcrops display serpentinites associated with chromite pods and clinopyroxenolites, ultrabasic and basic cumulates, quartz diorite, a sheeted-dyke complex, basaltic pillow lavas, and red cherts, i.e., the set of lithologies composing an ophiolitic sequence. Other geological units cropping out in the area are: (1) the underlying continental margin of the West African Craton; (2) a volcanic arc next to the oceanic crust represented by the ophiolite; (3) overlying unconformable volcanic and sedimentary rocks. The unusual character of this Neoproterozoic magmatic and tectonic geoheritage site, together with the excellent quality of the outcrops and the relatively easy access to the area, make this complex attractive from both a scientific and a geotouristic perspective. Its declaration as a geoheritage site will serve as a stimulus for the preservation of this singular area.

An application of exploratory data analysis (EDA) as a robust non-parametric technique for geochemical mapping in a semi-arid climate

Exploratory data analysis (EDA) techniques based on the boxplot and robust-class selection were applied to the analysis of single-element stream sediment data in the Collo area (N-E Algeria). The area is characterised by many factors that affect data variability: variation of lithology, combined existence of permanent and ephemeral streams, flash floods, rugged terrain, and climate. The boxplot proved to be very useful in capturing the empirical data distribution, the skewness, and in defining outliers. No data transformation was needed prior to the analysis of single element distributions (Cr, Pb, Zn, Cu, As, and Fe) as is the case in classical statistics. Geochemical mapping of these elements was based on resistant class selection as defined by the boxplot. Results showed the close spatial correlation of outlier data for Cr with a plagioclase-lherzolite intrusion and known chromite pods. The geochemical maps of Pb, Zn, Cu and As concentrations showed an association of these elements coincident with known base-metal sulphides and arsenopyrite mineralisation, delineating a northeastern anomaly spreading well over known mineralisation. The choice of the robust classes based on the boxplot also showed the close spatial distribution of Fe with known hematite and magnetite mineralization. EDA techniques proved to be very useful in delineating known mineralisation in the Collo area where stream sediment data is subject to variability owing to many factors (geologic, physiographic and climatic). EDA proved to be a simple and very useful tool in analysing single-element geochemical data. EDA could be used in similar terrains as an alternative to the classical methods where a normality precondition is needed prior to analysis and where class selection may be affected by the presence of “wild” data.

Petrology of dunite/harzburgite with decimeter-scale stratification in a drill core from the Tari-Misaka ultramafic complex, southwestern Japan.

A drill core from Tari-Misaka ultramafic complex, southwestern Japan, shows a one meter interval in which a decimeter scale stratification of dunite/harzburgite is well preserved. This part was petrologicaly examined in detail. Special attention was focused on the morphology and chemistry of chromian spinel. Chromian spinel is more abundant, more euhedral and higher in Ti and Fe3+ contents, and slightly lower in Cr/(Cr+Al) ratio in dunite than in harzburgite. Harzburgite becomes orthopyroxene-poor, and its chromian spinel tends to be similar to dunite spinel near (usually within 5 cm) the lithological between dunite. It was presumed that the dunite was formed as veins as a result of a reaction between harzburgite and an exotic melt: olivine crystals was formed by a reaction between hartzbergite and the melt. The petrological and chemical features of this part of the drill is basically similar to those in dunite with large chromitite pods, and harzburgite widely distributed in the Wakamatsu mine area. It is suggested that the dunites within harzburgite have the same origin irrespective of their dimension and the presence of chromite pods or not. The lower Cr# of spinel in the drill-core rocks than those in widely distributed dunite is possibly due to the difference in amount of orthopyroxene reacted with the melt. A large amount of orthopyroxene reacted and turbulency of melt current may be indispensable for the formation of podiform chromitite. The large melt conduit represented by dunite of semi-regional distribution in the Wakamatsu mine area possibly fulfilled the all of the conditions for the formation through this process of chromitite pods.

Oceanic ridge off-axis deep structure in the Mansah region (Sumail massif, Oman ophiolite)

Large scale structural mapping of the Oman ophiolite indicates that the Mansah area (Sumail massif) was a ridge off-axis region at the time the ophiolite was detached. This paper presents a detailed structural mapping of the region. We show that, as opposed to other off-axis areas, it contains plunging lineations, correlated with a thick Moho transition zone and chromite pods, indicative of a mantle diapir. However this diapir has a discontinuous structure, it is bounded by shear zones and types of diabases that are not found elsewhere in Oman; it also has a broken crust, strongly affected by hydrothermal alteration. This suggests that Mansah fossilized an off-axis diapir intruding a cooling lithosphere. It may be a good candidate to be the root of an off-axis seamount such as those found in the East Pacific, and may bring new insights into this particular volcanism which we are only beginning to explore.

REE AND SR-PB-ND ISOTOPE GEOCHEMISTRY OF TIBETAN PERIDOTITES: IMPLICATIONS FOR MELTING PROCESSES

It is ubiquitously accepted that mantle peridotite are residue after melting and are formed after extraction of variable amount of MORB type melt (Prinzhofer and Allegre, 1985; Jonhson et al., 1990; Snow et al., 1994). For this reason, they are a key in understanding melting in the mantle and melt extraction processes. They also provide information about mantle source heterogeneity and constraints on temporal evolution of the mantle. Nevertheless petrogenetic studies of ophiolite complexes generally concentrate on the magmatic oceanic crust. Trace elements and isotopic studies of highly depleted peridotite are rare partly due to their extensive alteration and weathering. Moreover Radiogenic and REE are found in very low concentration in mantle peridotites meaning that it is difficult to obtain high quality data. The Tibetan ophiolites provide a unique opportunity to study mantle processes because they are fresh (<10% serpentinisation) and well exposed. We have used ID ICP-MS and NdO techniques to measure REE and Nd isotopes in these highly depleted peridotites. We present major, trace element and Nd isotope data of 15 harzburgites from Luobusa ophiolite massif, Tibet. The aim of this study is to obtain a better understanding of the mantle processes that lead to the geochemical characteristics of these residues and to the formation of the oceanic crust. The Luobusa peridotite massif is situated in the Yarlung Zangbo Suture Zone and is assumed to have the same formation and emplacement ages as the Xigaze ophiolite (respectively 110 and 50 Ma). The massif is 43 km by 3-5 km. It has a mantle sequence mainly composed of diopside-bearing harzburgites with abundant dunite lenses and podiform chromitites that are residual after extraction of tholeitic (MORB type) magmas (Zhou et al., 1996). Remains of the oceanic crust (cumulates rocks, pillow lava, marine sediments) are present in a melange zone tectonically overlying the mantle peridotites (Zhou et al., 1996). The major element compositions of Luobusa peridotites show a large range of variation: CaO and MgO contents vary between 0.6 and 2 wt% and 42.1 and 45.1 wt% respectively. There is an excellent correlation between major element contents (e.g., Al2O3 or CaO) and HREE. Similar relationships have been reported elsewhere and are consistent with HREE depletion being associated with the loss of Aland Ca-rich phases during melting. Luobusa peridotites have REE patterns with subparallel HREE and constant MREE depletion. LREE are significantly more variable. Some samples are highly depleted in LREE, with minimum values of La at 0.004 x chondrite. Other have a “spoon-shaped” REE patterns, with a marked inflection at Sm. Their (Ce/Sm)N ratios varies between 0.2 and 0.9 and reflected different degree of LREE enrichment. The (143Nd/144Nd)i ratios vary between 0.5120 and 0.5130 and Sm/Nd from 1.1 to 3. (143Nd/144Nd)i also correlates positively with (Ce/Sm)N. The degree of LREE depletion in the peridotites is controlled by stratigraphic position: shallow peridotites being characterised by the highest (Ce/Sm)N and the highest depletion in HREE . Together with the “spoonshaped” REE patterns, these data imply that either melt was trapped in the residua during the extraction process or that the residua have been subsequently LREE enriched by melt addition. Similar observations have been made for other peridotites massif (Voykar and Trinity) and several possible mechanisms have been put forward to explain the geochemical data. The high degree of depletion of the Tibetan peridotites implies that they have recorded a multistage melting event. The samples with LREE depleted and high Nd ratios probably represent the residue after MORB type melt extraction from a previous depleted source. The LREE enrichment can be explained by 2 alternatives models: 1)- Melt/Residue interaction in a chromatographic column (Navon and Stolper, 1987; Bodinier et al., 1990 and others) or 2)- Crustal contamination either in a supra-subduction environment or during emplacement of the ophiolite (Sharma and Wasserburg, 1996; Gruau et al., 1998). The large range in Nd isotopic composition and the presence of samples showing less radiogenic Nd ratios associated with the highest degree of LREE enrichment tend towards the second alternative: a second melting event in a supra-subduction zone. This idea is also supported by the composition of the chromite pods present in the Luobusa massif that show boninite affinity (Zhou et al., 1996). This preliminary study show that the LREE enrichment is not as extensive as reported for other massif and tend to suggest that the geochemical characteristics are mantle melting related feature and not due to secondary alteration by infiltrating groundwater. The detailed REE and Sr/Pb/Nd isotope analysis of minerals are underway and will constrain the nature and the provenance of the melt involved and the succession of processes that occurred in the melting column and the geodynamic environment in which the massif was formed.

Formation of podiform chromitites by melt/rock interaction in the upper mantle

High-Cr and high-Al podiform chromitites are derived from melts formed by high and low degrees of partial melting in the upper mantle, respectively. The close association of the chromitites, regardless of compositions, with dunites and harzburgites can be explained by interaction between melts and harzburgites or depleted lherzolites in the upper mantle. Continuous reaction between melt and wallrock modifies the compositions of the magmas towards the precipitation of chromite alone under upper mantle conditions. The modified magmas would have higher silica and lower magnesia and would be extracted into the crust or erupted, leaving chromite pods in situ with dunite rinds that grade to harzburgites or lherzolites.

On the formation mode of the Guleman chromite deposits (Turkey)

Kefdag and Soridag chromite pods occur in upper mantle residual peridotites, which consist of harzburgite and dunites. The peridotites represent the residual of multistage, depleted upper-mantle peridotites. The chromitite bodies were formed during the uprising of chromium-rich picritic melts, through the residual upper mantle diapir, along the magma conduits. Chromitite grains were deposited in the caves of the magma conduits under the control of the convection currents.

Ophiolites in Northeast and East Africa: implications for Proterozoic crustal growth

The presence of ophiolite complexes in NE and E Africa has been documented using Landsat, field and geochemical studies. The present work identifies five major ophiolitic sutures in NE Africa, while plate reconstruction of Africa and Madagascar suggests a possible sixth ophiolite belt to the east. The ophiolites are considered to be remnants of supra-subduction zones and back-arc basins. The ophiolites are dismembered, and their mode of occurrence varies widely resulting in different structural relationships. In Western Ethiopia, the Yubdo complex is formed of harzburgite which grades into a cumulate sequence of ultramafic and gabbroic rocks and metabasalts. In Kenya, the Baragoi complex is formed of tectonized harzburgite with dunite and chromite pods, a cumulate sequence of ultramafic and gabbroic units and a dyke unit. Trace element data from the Baragoi complex show a transitional mid-ocean ridge basaltic to island arc tholeiitic affinity, and the presence of boninites suggests a supra-subduction setting, while data from the Adola-Moyale belt (S. Ethiopia-NE Kenya) indicate an island-arc and MORB geochemistry, which developed in a back-arc tectonic setting. In Sudan, the Ingessana complex which has an island arc tholeiitic tectonic affinity indicates development in a supra-subduction setting, while the trace element suggest that the Sol Hamed complex developed in a back-arc basin. These ophiolite belts represent sutures marking the position of island arcs extending to those in Saudi Arabia on a pre-Red Sea drift reconstruction and further south to Mozambique. The terrane delimited by a proposed Ingessana-Port Sudan suture and by the Adola-Moyale ophiolite belts as defined in this study encompasses an area in which the crust was derived from oceanic and island arc material. Relative age reconstruction across the Arabian-Nubian Shield in East Africa broadly indicates eastward younging of the ophiolite belts. Regional geological, tectonic and geochemical studies suggest rifting at c. 1200 Ma and subsequent convergence led to the development of intra-oceanic arcs and associated marginal basins in the north and narrow basins within the sialic basement gneisses further south in Kenya and Tanzania. This was followed by continent-continent collision which led to accretion of island arcs by gentle collision from the northeast in Saudi Arabia and severe crustal shortening in S Sudan, Kenya and SE Ethiopia as compared to Saudi Arabia, NE Sudan and N and W Ethiopia owing to oblique collision from the southeast.

Chromite pods in a lherzolite massif (Collo, Algeria): Evidence of oceanic-type mantle rocks along the West Mediterranean Alpine Belt

The high-temperature lherzolite massifs of Ronda, southern Spain, and Beni Bousera, northern Morocco, are ascribed by various authors to sub-continental mantle diapirs emplaced within a granulite metamorphic aureole in continental rifts along transcurrent lithospheric faults. The Collo plagioclase-lherzolite massif, with associated kinzigite and related anatectic rocks, belongs to the same type of geodynamic context. About 17 chromitite bodies are found in the Collo lherzolite massif: their structural, textural and geochemical characteristics are those of chromitite pods that are typical of ophiolite complexes, i.e. of an oceanic lithosphere. The Collo massif is an atypical high-temperature lherzolite massif displaying both continental and oceanic characteristics. It could correspond to a mantle intrusion in a transition zone from continental- to oceanic-rift type.

Structural mapping in the Oman ophiolites: Mantle diapirism along an oceanic ridge

Structures induced in peridotites by high-temperature plastic flow at the oceanic spreading centre source have been mapped in the Oman ophiolite. They can be reset to the geometry of the spreading system by rotating the boundary between these peridotites and the layered gabbros (the Moho) to the horizontal and by equating the trend of the sheeted dyke complex with that of the ridge. A number of typical asthenospheric flow patterns thus emerge which are more specifically described in a companion paper ∗ ∗ Ceuleneer et al. (this issue). . The most spectacular is a structure characterized by divergence of flow planes and flow lines from the vertical to the horizontal above and around mantle diapirs 10 × 15 km in diameter. Such diapirs are also recognized by traces of an intense magmatic activity (magmatic impregnation, and an abundance of gabbro dykes and intrusions and chromite pods). The general structure of the 500 km long Oman palaeo-ridge is dominated by these diapirs which are spaced approximately every 50 km along strike. In the preliminary picture which is revealed by this study, no other mantle structure typical of a ridge segmentation is documented with the possible exception of a transform structure within the Wadi Tayin massif. Evidence converges to indicate that Oman ophiolites correspond to a fast-spreading ridge.

Pge in the 3.5 jamestown ophiolite complex, Barberton greenstone belt, with implications for pge distribution in simatic lithosphere

The 3·5 Ga mafic-ultramafic rocks of the Barberton greenstone belt, South Africa, form a pseudostratigraphy comparable to that of Phanerozoic ophiolites. This Archaean complex, referred to here as the Jamestown Ophiolite Complex, consists of high temperature tectonometamorphic peridotites overlain by an intrusive-extrusive igneous section, which in turn is capped by a chert-shale sequence. Elevated PGE concentrations with distinctly different chondrite normalized patterns have been found at three pseudostratigraphic levels of this ophiolite. From the top down, high PGE values are first encountered in an undeformed chromitite, interlayered with meta-pyroxenites. The PGE abundances and normalized patterns are similar to those reported from PGE mineralized zones in large mafic layered complexes. Chromitemagnetite pods occur stratigraphically below the chromitite. Their PGE patterns resemble those reported from chromite pods in Phanerozoic ophiolites and those of the overlying chromitite. The host rocks of the pods (deformed talc-carbonate rocks) are interpreted as a mixture of deformed high-temperature cumulates and depleted upper mantle residue, within a ductile shear zone. In all the above occurrences, the PGE were concentrated by igneous upper mantle-crustal fractionation processes of mantle derived liquids. The third type of PGE concentration is found in an unusual, sulphur-poor NiFe-silicate-spinel pod (called the Bon Accord-[BA]-deposit) and an associated hematite-magnetite-chromite (HMC) pod, both of which occur in coarse-grained meta-dunites at the lowest levels in the ophiolite. These host rocks are interpreted as high-temperature tectonites (strongly depleted in REE); they probably represent depleted upper mantle residue from which the overlying rocks were derived by partial melting. BA can be divided into two groups. Group A has pat, chondritelike PGE patterns. Group B has a lower concentration of all the PGE, but the relative depletion is most severe for Os and Ir (which are the most refractory); thus the patterns have a positive slope. Both BA groups have negative Pt anomalies in their PGE patterns (more pronounced for Group A), while the HMC-pod, which is depleted in the other PGE, relative to BA, has a strong positive Pt anomaly. It is difficult to explain these fractionations in terms of known upper mantle-crustal processes, especially in the absence of sulphur. The PGE differentiation of BA is interpreted as an indication of chemical fractionation which started in the lower mantle. We conclude that the body represents a metal-silicate sample derived from an almost chondrite-like lower mantle where it resided as a left-over from inefficient core formation. This siderophile-enriched mantle fractionated during ascent and some of its residue was incorporated into the lower portion of the lithosphere during formation of the Archaean oceanic crust. Tectonic and chemical data suggest that the thick and old ‘lithospheric keel’ beneath the South African craton has a large component of Archaean residual sima and is therefore likely to be relatively enriched in PGE.