Chromian andradite and olivine-chromite relations in a chromitite layer from the Jijal Complex, northwestern Pakistan
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Abstract:
Green chromian andradite occurs associated with chrysotile in a chromitite layer within the Jijal Complex (northern Pakistan). The garnet contains, on average, 10.1 wt. % Cr 2 O 3 (range 9.2-11.6%) and has a formula Ca (sub 3.04) (Cr (sub 0.67) Fe (sub 3+) (sub 1.29) Al (sub 0.02) )Si (sub 2.98) O 12 . The garnet formed during retrograde greenschist-facies metamorphism of the Jijal Complex.--Modified journal abstract.Keywords:
Andradite
Chromitite
Chromite
Greenschist
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The texture, mineralogy and composition of chromite in the upper chromitite of the Muskox intrusion, in the Northwest Territories, have been studied in two 0.5-meter sections of drill core. The principal rock-type is an orthopyroxenite that contains cumulus olivine, orthopyroxene and chromite, and the intercumulus minerals clinopyroxene and plagioclase. The minor minerals ilmenite and biotite are found, together with a number of accessory minerals, in pockets that are interpreted as sites of late intercumulus melt. The chromitite seam is up to 10 cm thick and contains chromite with a narrow range in composition: 0.64 2 ) = -9.1. The disseminated chromite in the orthopyroxenite shows a much greater range in composition, and increases in Fe (super 2+) /(Fe (super 2+) +Mg), Fe (super 3+) /(Fe (super 3+) +Al+Cr), Ti and Ni with stratigraphic height above the massive chromitite. The chromite in the Muskox chromitite is significantly higher in Fe (super 3+) , Ti and Fe (super 2+) /(Fe (super 2+) +Mg) than chromite in the Bushveld, Stillwater and Great Dyke chromitites; furthermore, the Muskox chromitites formed much higher in the stratigraphic section of the layered series than in these other intrusions. The Muskox chromitites are considered to have formed late in the magmatic history of the intrusion as a result of mixing of a fractionated magma with a more primitive magma and a component due to wall-rock assimilation.
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Chromite
Layered intrusion
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Chromite
Chromitite
Dharwar Craton
Ultramafic rock
Greenstone belt
Layered intrusion
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A second occurrence of chrome‐rich clinopyroxene has been discovered as inclusions in orthopyroxene in orthopyroxenite from Maowu, the Dabie Mountains, Central China. The average formula for chrome‐rich clinopyroxene can be expressed as (Na 0.39 Ca 0.54 ) 0.93 (Mg 0.57 Fe 2+ 0.06 Fe 3+ 0.01 Cr 0.24 Al 0.15 ) 1.03 Si 2.02 O 6 , with a maximum amount of kosmochlor component of 28.52 mol%. The unit cell parameters obtained from a single‐crystal are a = 9.614 Å, b = 8.800 Å, c = 5.240 Å, β = 106.59°, space group C2 / c . The indices of refraction are α = 1.697, β = 1.704, γ = 1.726. Chrome‐rich clinopyroxene, which coexists with chromite, chromian rutile and chromian pyrope, crystallized at a temperature of 1025 °C and very high pressure, and therefore represents a mantle relic. Together with the appearance of low‐pressure inclusion mineral assemblage and the estimation of physical–chemical conditions for matrix minerals, the Maowu eclogite–ultramafic complex is considered to be formed during ultrahigh‐pressure metamorphism from the mantle‐derived protolith.
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Petrogenesis
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Coesite
Peridotite
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Omphacite
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The Khanozai Ophiolite is an important fragment of Zhob valley ophiolite belt. This contains thick serpentinized mantle peridotite which consists of ultramafic tectonite and transition zone. Ultramafic tectonite comprises of largely harzburgite with subordinate dunite and lherzolite whereas transition zone is dominantly made up of dunite with subordinate harzburgite. Peridotite contains thick chromitite bodies. The chromitite of Khanozai Ophiolite is found as pods, lenses and layered shapes. These chromitites bodies occur in massive, disseminated and nodular forms. The chromitite for Khanozai Ophiolite is classified as high-Cr chromitite based on Cr# (aver. 0.72). Chromite grains in chromitite are largely uniform in composition but few grains show ferrit-chromite alteration along veins and grain margins. High Cr#, low TiO2 and chromite composition of high-Cr indicated that high-Cr chromite ore deposits in the mantle portion have been crystallized from parental melt of boninitic composition. Geochemical composition of chromite spinels in chromitites and peridotite suggests that the chromitite and peridotite of Khanozai Ophiolite was formed in suprasubduction zone environment.
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Chromitite
Chromite
Peridotite
Ultramafic rock
Tectonite
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Chromitite
Chromite
Ultramafic rock
Peridotite
Lithology
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Chromitite
Chromite
Phlogopite
Diopside
Enstatite
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Serpentinites (massive and schistose) and listvenite occur as tectonic sheets and lenses within a calcareous metasedimentary mélange of the Tulu Dimtu, western Ethiopia. The massive serpentinite contains high-magnesian metamorphic olivine (forsterite [fo] ~96 mol%) and rare relict primary mantle olivine (Fo90–93). Both massive and schistose serpentinites contain zoned chromian spinel; the cores with the ferritchromite rims preserve a pristine Cr/(Cr+Al) atomic ratio (Cr# = 0.79–0.87), suggesting a highly depleted residual mantle peridotite, likely formed in a suprasubduction zone setting. Listvenite associated with serpentinites of smaller ultramafic lenses also contain relict chromian spinel having identical Cr# to those observed in serpentinites. However, the relict chromian spinel in listvenite has significantly higher Mg/(Mg+Fe2+) atomic ratios. This suggests that a nearly complete metasomatic replacement of ultramafic rocks by magnesite, talc, and quartz to prevent Mg–Fe2+ redistribution between relict chromian spinel and the host, that is, listvenite formation, took place prior to re-equilibration between chromian spinel and the surrounding mafic minerals in serpentinites. Considering together with the regional geological context, low-temperature CO2-rich hydrothermal fluids would have infiltrated into ultramafic rocks from host calcareous sedimentary rocks at a shallow level of accretionary prism before a continental collision to form the East African Orogen (EAO).
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Ultramafic rock
Chromite
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Peridotite
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Different types of chromitite, including texturally massive, disseminated and banded varieties, are present in the Kop ophiolitic ultramafic rocks, which extend from Erzincan to Erzurum in northeastern Turkey. The chemical composition of the chromian spinel in these deposits is relatively homogeneous, with 0.64 2 O 3 and FeO/MgO values in the range 0.71–1.12) from which the Kop chromitites precipitated is very similar to picritic basalt and boninite, in distinct contrast to MORB-type magma. Total contents of platinum-group elements (PGE) in different deposits range from 87 to 520 ppb, with an average of 199 ppb. Values normalized to chondrite (CI) show negatively sloping PGE patterns with slight positive Ru and Pd anomalies. Primary solid inclusions of silicates consist of olivine, amphibole, phlogopite and rare clinopyroxene, randomly distributed within the chromitites. Chlorite and a few grains of garnet were also identified as secondary phases. Mostly IPGE-dominated single and composite inclusions of sulfides and alloys are the main PGM phases in chromian spinel. Some PPGE phases, which are uncommon for ophiolitic chromitites, also were found as inclusions in unaltered chromian spinel. Stabilization of PGE alloys and laurite during the early stage of PGM crystallization in the Kop chromitites is explained by the relatively high temperature (T) and low fugacity of sulfur f (S 2 ) of the melt during the precipitation of the chromian spinel. Low-temperature in situ alteration and modification of PGM by loss of S and addition of base metals led to the transformation of laurite to alloys or S-depleted laurite in some samples of chromitite. The calculated composition of the parental melts, the high-Cr content (Cr# > 0.60) and low values of Ti in the chromian spinel in the presence of primary hydrous silicate inclusions suggest that the chromian spinel crystallized from a hydrous melt, enriched in Na and K, formed as a result of high degree of partial melting of upper mantle or of peridotite–melt interaction, probably in a suprasubduction setting.
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Rare-earth element
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Compositions of chromium-bearing garnet with little grossular content belong to the uvarovite–andradite solid-solution series. They are extremely rare in nature. Here, we report the occurrence of chromian andradite with up to 50 mol.% uvarovite component from hydrothermally altered podiform chromitite of the Rutland ophiolite, Andaman Island, India. The mantle tectonites of this ophiolite are characterized by a broad spectrum of peridotitic rocks ranging from depleted harzburgite to clinopyroxene-bearing harzburgite that hosts impersistently developed podiform chromite. Chromian garnet is concentrated at the margins of serpentine-rich veins that dissect the chromitite pods. The mineral assemblages are suggestive of low-temperature hydrothermal alteration of the host peridotite. The formation of chlorite and tremolite is simultaneous, and linked to the process of magnesiochromite oxidation. Garnet formed by reaction between tremolite and chromian spinel, which, during its growth, engulfed the tremolite needles.
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