Chromitite and peridotite from Rayat, northeastern Iraq, as fragments of a Tethyan ophiolite
25
Citation
24
Reference
10
Related Paper
Citation Trend
Abstract:
Abstract Ophiolitic rocks (chromitites and serpentinized peridotites) were petrologically examined in detail for the first time from Rayat, in the Iraqi part of the Zagros thrust zone, an ophiolitic belt. Almost all the primary silicates have been altered out, but chromian spinel has survived from alteration and gives information about the primary petrological characteristics. The protolith of the serpentinite was clinopyroxene‐free harzburgite with chromian spinel of intermediate Cr# (= Cr/[Cr + Al] atomic ratio) of 0.5 to 0.6. The harzburgite with that signature is the most common in the mantle section of the Tethyan ophiolites such as the Oman ophiolite, and is the most suitable host for chromitite genesis. Except for one sample, which has Cr# = 0.6 for spinel, the Cr# of spinel is high, around 0.7, in chromitite. The variation in Cr# of spinel in chromitite observed here has been also reported in the Oman ophiolite. The peridotite with chromitite pods exposed at Rayat was derived from an ophiolite similar in petrological character to the Oman ophiolite, one of the typical Tethyan ophiolites (fragments of Tethyan oceanic lithosphere). This result is consistent with the previous interpretation based on geological analysis.Keywords:
Chromitite
Peridotite
Protolith
All six PGE have been recorded as concentrated at parts per million (ppm) in a number of ophiolite complexes. Occurrences of PGE concentrations in ophiolites are common in podiform chromitite where Os, Ir and Ru may be concentrated to give negative slope chondrite normalised patterns. If sulphur saturation of the magma occurred during chromite crystallisation then Pt and Pd + Rh will also be concentrated in the chromitite giving positive chondrite normalised patterns. PGE-rich ophiolite complexes are those where there has been sufficient mantle melting, in a subduction zone, to extract the PGE at a critical melting interval. Too much melting will dilute this melt with PGEbarren melt and too little melting will not extract the PGE. The Scottish Shetland ophiolite is an example in which all 6 PGE have been concentrated in base metal sulphide-bearing podiform chromitite. It is proposed that if all the PGE are concentrated in podiform chromitite then they crystallised from one PGE-rich melt that was close to sulphur saturation. Exploration for PGE within ophiolites is likely to be enhanced by growing evidence that there is a link between chromite composition and PGE concentration.
Chromitite
Chromite
Cite
Citations (14)
Chromitite
Chromite
Platinum group
Cite
Citations (80)
Chromitite
Peridotite
Massif
Cite
Citations (32)
Chromitite
Cite
Citations (7)
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.
Download Full Paper
Chromitite
Chromite
Peridotite
Ultramafic rock
Tectonite
Cite
Citations (2)
In central Iran; to the northwest of the Central Iranian micro-continental (CIM) block; the coloured melange is characterized by widespread ophiolite blocks. In the Nain area; these ophiolite blocks; known as Nain ophiolites; are mainly composed of mantle peridotites. Based on field and petrographic observations and on the chemistry of Cr-spinels; two types of chromitites have been recognized in the Nain ophiolites. The first type occurs as chromitite patches within the harzburgites. The Cr number [Cr# = 100*Cr/(Cr+Al)] of the Cr-spinels varies between 58 and 63 in chromitite patches and 39-40 in harzburgites. Calculated amount of Al2O3 and ratio of FeO/MgO in the parental melts of the chromitite patches indicate a boninitic melts for the source magmas of the Cr-spinels. The REE patterns for the studied harzburgites and lherzolites are similar to those from abyssal peridotites. These characteristics suggest a back-arc basin setting for the generation of the chromitite patches as well as of the harzburgites and lherzolites from the Nain ophiolites. The second type of chromitites occurs as lenses (chromitite pods) in dunites. The Cr number of Cr-spinels of these chromitites varies between 69 and 73. The Cr-spinels of these chromitites also show a boninitic nature for the parental melts. These characteristics of chromitite lenses show that they were generated at an arc setting. Therefore; intra-oceanic subduction in the Nain basin in Early Cretaceous can be proposed on the basis of these data on the origin of the chromitite lenses. Intra-oceanic rifting in a back-arc setting generated an oceanic back-arc basin in the Nain area in Late Cretaceous.; The chromitite patches and the studied harzburgites and lherzolites were likely formed in the mantle beneath the back-arc basin. On the whole; the field and petrographic observations as well as the degree of partial melting; calculated on the basis of the Cr#; suggest that the genesis of these rocks can be explained according to a mixed open-channel cumulate/rock-reaction model.
Chromitite
Chromite
Cite
Citations (18)
Ophiolites are conventionally regarded as fragments of former oceanic lithosphere. Mineralogical and field evidence indicates that peridotite of the Neyriz ophiolite was intruded at high temperature into folded crystalline limestones, forming skarns. This excludes the formation of the ophiolite at a mid-ocean ridge but is consistent with its origin by intrusion during continental rifting.
Peridotite
Obduction
Cite
Citations (17)
The northeast Indian ophiolite belt is exposed in parts of Nagaland and Manipur states. We present the results of an investigation into the petrology and constituent mineral chemistry of peridotite from the ophiolite belt of Phek District, Nagaland. Based on relict primary mineral compositions, the studied rocks are identified as abyssal peridotite. Spinel composition shows that the rocks have undergone low degree of partial melting (5 11%). Equilibration of the ophiolite mantle sequence peridotite was calculated at a temperature range of 850–1165oC, pressure ranging between 19 to 25 kbar, and an oxygen fugacity between 0.437 to 0.657 log units above the FMQ buffer, whereas for the xenolith peridotite the temperature was estimated to be between 850 to 1100 oC, pressure ranging from 12 to 17 kbar and oxygen fugacity between 0.577 to 0.706 log units above the FMQ buffer. Equilibrium condition suggests that the rocks were equilibrated in an upper mantle environment.
Peridotite
Mineral redox buffer
Fugacity
Cite
Citations (0)
Abstract Ophiolitic rocks (chromitites and serpentinized peridotites) were petrologically examined in detail for the first time from Rayat, in the Iraqi part of the Zagros thrust zone, an ophiolitic belt. Almost all the primary silicates have been altered out, but chromian spinel has survived from alteration and gives information about the primary petrological characteristics. The protolith of the serpentinite was clinopyroxene‐free harzburgite with chromian spinel of intermediate Cr# (= Cr/[Cr + Al] atomic ratio) of 0.5 to 0.6. The harzburgite with that signature is the most common in the mantle section of the Tethyan ophiolites such as the Oman ophiolite, and is the most suitable host for chromitite genesis. Except for one sample, which has Cr# = 0.6 for spinel, the Cr# of spinel is high, around 0.7, in chromitite. The variation in Cr# of spinel in chromitite observed here has been also reported in the Oman ophiolite. The peridotite with chromitite pods exposed at Rayat was derived from an ophiolite similar in petrological character to the Oman ophiolite, one of the typical Tethyan ophiolites (fragments of Tethyan oceanic lithosphere). This result is consistent with the previous interpretation based on geological analysis.
Chromitite
Peridotite
Protolith
Cite
Citations (25)