Compositional variations and tectonic settings of podiform chromitites and associated ultramafic rocks of the Neoproterozoic ophiolite at Wadi Al Hwanet, northwestern Saudi Arabia

Ahmed, Ahmed H.; Harbi, Hussein M.; Habtoor, Abdelmonem M.

doi:10.1016/j.jseaes.2012.05.002

Cited by 45 publications

(23 citation statements)

References 114 publications

(171 reference statements)

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“…Considering Cr# and Mg#, the Cedrolina chromite composition plots outside the fields of both podiform and stratiform chromitites, since they are characterized by high values of Cr# and low Mg# [33,34] (Figure 5B). They also differ from the Neoproterozoic podiform chromitites of Egypt and Saudi Arabia which are characterized by a large variation of Cr# and low Mg# [35][36][37][38]. Compared with the Cr-Al-Fe 3+ compositions of spinels from different metamorphic facies [39][40][41] (Figure 6A), as well as with spinel stability limits [42,43] ( Figure 6B), most of the Cedrolina chromite is comprised in the field of spinel metamorphosed in the upper greenschist facies, at temperatures ranging from 500 to 550 • C ( Figure 6B).…”

Section: Chromitite and Chromitementioning

confidence: 86%

“…They display a very restricted and highly refractory composition of Cr-spinel (high Cr#). Therefore, it differs from the Neoproterozoic ophiolitic chromitite of the Eastern Desert of Egypt and Saudi Arabia [35][36][37][38] that can be classified as Cr-and Al-rich. Regarding their TiO 2 content, few analyses of the Cedrolina chromitite plot in the field of stratiform chromitite (occurring in layered complexes).…”

Section: Discussionmentioning

confidence: 99%

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The Cedrolina Chromitite, Goiás State, Brazil: A Metamorphic Puzzle

et al. 2016

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Abstract:The Cedrolina chromitite body (Goiás-Brazil) is concordantly emplaced within talc-chlorite schists that correspond to the poly-metamorphic product of ultramafic rocks inserted in the Pilar de Goiás Greenstone Belt (Central Brazil). The chromite ore displays a nodular structure consisting of rounded and ellipsoidal orbs (up to 1.5 cm in size), often strongly deformed and fractured, immersed in a matrix of silicates (mainly chlorite and talc). Chromite is characterized by high Cr# (0.80-0.86), high Fe 2+ # (0.70-0.94), and low TiO 2 (av. = 0.18 wt %) consistent with variation trends of spinels from metamorphic rocks. The chromitite contains a large suite of accessory phases, but only irarsite and laurite are believed to be relicts of the original igneous assemblage, whereas most accessory minerals are thought to be related to hydrothermal fluids that emanated from a nearby felsic intrusion, metamorphism and weathering. Rutile is one of the most abundant accessory minerals described, showing an unusually high Cr 2 O 3 content (up to 39,200 ppm of Cr) and commonly forming large anhedral grains (>100 µm) that fill fractures (within chromite nodules and in the matrix) or contain micro-inclusions of chromite. Using a trace element geothermometer, the rutile crystallization temperature is estimated at 550-600 • C (at 0.4-0.6 GPa), which is in agreement with P and T conditions proposed for the regional greenschist to low amphibolite facies metamorphic peak of the area. Textural, morphological, and compositional evidence confirm that rutile did not crystallize at high temperatures simultaneously with the host chromitite, but as a secondary metamorphic mineral. Rutile may have been formed as a metamorphic overgrowth product following deformation and regional metamorphic events, filling fractures and incorporating chromite fragments. High Cr contents in rutile very likely are due to Cr remobilization from Cr-spinel during metamorphism and suggest that Ti was remobilized to form rutile. This would imply that the magmatic composition of chromite had originally higher Ti content, pointing to a stratiform origin. Another possible interpretation is that the Ti-enrichment was caused by external metasomatic fluids which lead to crystallization of rutile. If this was the case, the Cedrolina chromitites could be classified as podiform, possibly representing a sliver of tectonically dismembered Paleoproterozoic upper mantle. However, the strong metamorphic overprint that affected the studied chromitites makes it extremely difficult to establish which of the above processes were active, if not both (and to what extent), and, therefore, the chromitite's original geodynamic setting.

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Section: Chromitite and Chromitementioning

confidence: 86%

Section: Discussionmentioning

confidence: 99%

The Cedrolina Chromitite, Goiás State, Brazil: A Metamorphic Puzzle

et al. 2016

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“…Zhou et al 2005;Ahmed et al 2012). According to a number of studies Cr-spinels with Cr#<0.60 are commonly found in abyssal peridotites related to the lithospheric mantle emplaced near the ocean ridge, whereas those having higher values are found in peridotites produced in a supra-subduction zone (SSZ) environment (e.g.…”

Section: Primary Cr-spinel Compositionsmentioning

confidence: 99%

Composition and alteration of Cr-spinels from Milia and Pefki serpentinized mantle peridotites (Pindos Ophiolite Complex, Greece)

Kapsiotis¹

2014

Geologica Carpathica

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The Pindos Ophiolite rocks include variably serpentinized peridotites derived from a harzburgitic and subordinately dunitic mantle. In the serpentinized matrix of these rocks pseudomorphic (mesh, bastite) and non-pseudomorphic (interpenetrating, type-2 hourglass) textures were recognized. Chromian spinel (Cr-spinel) is anhedral to subhedral and often replaced by a porous opaque phase. Chemistry data show that Cr-spinel cores retain their original composition, having Cr#[Cr/(Cr + Al)] that ranges between 0.45 and 0.73, and Mg#[Mg/(Mg + Fe2+)] that varies between 0.52 and 0.65, accompanied by low content in TiO2 ( < 0.11wt. %). The relatively wide variation of their Cr# values reflects that the studied peridotites were produced by variable degrees of melting. It is likely that the Pindos peridotites represent mantle residues originally formed in a mid-ocean ridge (MOR) environment, which were subsequently entrapped as part of a mantle wedge above a supra-subduction zone (SSZ) regime. Cr-spinel adjacent to clinochlore systematically displays limited compositional and textural zoning along grain boundaries and fractures. However, the degree of peridotite serpentinization does not correlate with the abundance of zoning effects in accessory Cr-spinel. Thus, Cr-spinel zoning is thought to represent a secondary feature obtained during the metamorphic evolution of the host peridotites. Core to rim compositional trends are expressed by MgO and Al2O3 impoverishment, mainly compensated by Cr2O3 and FeO increases. Such chemical trends are produced as a result of Cr-spinel re-equilibration with the surrounding serpentine, and their subsequent replacement by ferrian (Fe3+-rich) chromite and clinochlore, respectively, during a brief, fluid assisted, greenschist facies metamorphism episode (T > 300 °C). The limited occurrence of ferrian chromite with high Fe3+# values suggests that elevated oxidizing conditions were prevalent only on a local scale during Cr-spinel alteration

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“…However, the general association of chromitite with dunite implies the involvement of melt-rock interaction processes in the formation of podiform chromitites (e.g. Uysal, 2008;Ahmed et al, 2012;Kapsiotis, 2013a;Moghadam et al, 2013;Rajabzadeh et al, 2013), intra-arc and back-arc basins (e.g. According to this aspect dunite is formed after effective interaction between harzburgite and continuous injections of exotic ultramafic/mafic melts, which result in the extraction of the pyroxene constituent into the melts, forming high permeability dunite bodies within the shallow sub-oceanic lithospheric mantle (e.g.…”

Section: Introductionmentioning

confidence: 99%

Alteration of chromitites from the Voidolakkos and Xerolivado mines, Vourinos ophiolite complex, Greece: implications for deformation-induced metamorphism

Kapsiotis

2014

Geol. J.

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Chromitites, associated with upper mantle spinel peridotites from the Voidolakkos and Xerolivado districts, located in the Vourinos ophiolite complex, northwestern continental Greece, were re‐investigated with respect to their structural and textural mode of occurrence, as well as their compositional signatures. They include variably deformed banded and podiform chromitite bodies made up of massive, semi‐massive, nodular, anti‐nodular, schlieren and disseminated chromian spinel. Chromitites have suffered intense shearing that was more severe in all but disseminated textured ore. Deformation has partly produced elongation of chromian spinel nodules and widespread protocataclastic zones within chromitites. The examined deposits are composed of magnesiochromite that shows a quite restricted range of Cr# [Cr/(Cr + Al)] values varying between 0.76 and 0.83, whereas Mg# [Mg/(Mg + Fe2+)] ranges from 0.55 to 0.67 accompanied by relatively low content in TiO2 (<0.15 wt.% on average). Compositional data indicate that these high‐Cr chromitite bodies crystallized from melts of boninitic affinities that have been compositionally modified after reaction with depleted harzburgite, followed by interaction with relatively undifferentiated low SiO2 melts within an intertwined system of dunite channels in the mantle wedge below the fore‐arc region of a supra‐subduction zone (SSZ). Magnesiochromite displays limited textural modification, being scarcely transformed to an opaque spinel phase along grain boundaries and fracture walls. The opaque spinel phase is characterized by elevated Cr# (0.76–0.97), relatively low Mg# (0.33–0.63) and low Fe3+# (≤0.14) and corresponds to ferrian chromite. Microscopic studies revealed that ferrian chromite is paragenetically associated with clinochlore even in unaltered chromitite specimens and the degree of serpentinization does not correlate with the frequency and abundance of alteration effects on magnesiochromite. Therefore, it is supported that regional metamorphism prior to serpentinization was responsible for the formation of the ferrian chromite–clinochlore association. In addition, magnesiochromite alteration was systematically recorded only in variably sheared chromitites displaying protocataclastic brecciation, thus it can be claimed that metamorphism was mainly governed by deformation mechanisms, which took place during the transition from ductile to semi‐brittle conditions. Ferrian chromite can be locally erratically enriched in MnO and ZnO, which is attributed to a Mn‐ and Zn‐bearing, slightly acid post‐magmatic fluid that invaded the chromitites across weakness zones. Overall, the data suggest that after magnesiochromite equilibration with the intergranular olivine, both phases were partly replaced by ferrian chromite and clinochlore, respectively, during a brief, fluid assisted episode of retrogade metamorphism that took place within a temperature interval between 700 and 300 °C, before ocean‐floor alteration. Copyright © 2014 John Wiley & Sons, Ltd.

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Compositional variations and tectonic settings of podiform chromitites and associated ultramafic rocks of the Neoproterozoic ophiolite at Wadi Al Hwanet, northwestern Saudi Arabia

Cited by 45 publications

References 114 publications

The Cedrolina Chromitite, Goiás State, Brazil: A Metamorphic Puzzle

The Cedrolina Chromitite, Goiás State, Brazil: A Metamorphic Puzzle

Composition and alteration of Cr-spinels from Milia and Pefki serpentinized mantle peridotites (Pindos Ophiolite Complex, Greece)

Alteration of chromitites from the Voidolakkos and Xerolivado mines, Vourinos ophiolite complex, Greece: implications for deformation-induced metamorphism

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