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Geochemistry of platinum-group elements in the podiform chromitites and associated peridotites of the Nain ophiolites,Central Iran:Implications for geotectonic setting

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Acta Geochimica 2021年第5期40卷 752-772页

作者： Batoul Taghipour Somayeh Shabani Alireza K Somarin Department of Earth Sciences Faculty of SciencesShiraz UniversityShirazIran Department of Geology Brandon UniversityBrandonMBCanada

The Nain ophiolite complex with an extent of[600 km2 is a part of the Central Iranian ophiolite,which is related to the opening and subsequent closure of the Neo-Tethys *** and serpentinized harzburgite in the Nain area host podiform chromitites that occur as three(eastern Hajhossein,western Hajhossein,and Soheil Pakuh)schlieren-type tabular and aligned massive lenses with various *** most common chromitite ore textures are massive,nodular,disseminated,and banded,reflecting crystal settling *** Cr#[Cr/(Cr+Al)]ranges from 0.43 to 0.81(average 0.63).The Mg#[Mg/(Mg+Fe2+)]varies from 0.25 to 0.78(average0.62).The Nain ophiolite and hosted chromitite are generally characterized by high Cr#,reflecting crystallization from a very hot boninite magma in a MORB *** high Cr#in the Nain chromitite also indicates a high degree of melting(15%–35%)of the depleted *** average total PGE content in the ophiolitic host rock(harzburgite and dunite)and chromite are 107 and 221 ppb,*** Nain ophiolite and chromitite have high IPGE/PPGE and negative Pt*(Pt/Pt*=0.6)anomaly,which is a characteristic of high Cr#*** U-shaped REE pattern of dunite host rock suggests the interaction of depleted mantle peridotite with boninitic *** data suggest that the Nain chromitites are related to the boninitic magma emplacement in a suprasubduction zone.

关键词： Nain chromitite Platinum group elements Ushaped REE pattern Boninitic magma supra-subduction zone

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Petrology and geochemistry of mafic and ultramafic cumulate rocks from the eastern part of the Sabzevar ophiolite(NE Iran): Implications for their petrogenesis and tectonic setting

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Geoscience Frontiers 2020年第6期11卷 2347-2364页

作者： Fatemeh Rahmani Mohamad Ali Mackizadeh Moussa Noghreyan Claudio Marchesi Carlos J.Garrido Department of Geology Faculty of SciencesUniversity of IsfahanP.O.Box:81746-73441IsfahanIran Instituto Andaluz de Ciencias de La Tierra(IACT) CSIC-Universidad de GranadaAvenida de Las Palmeras 418100ArmillaGranadaSpain Departamento de Mineralogía y Petrología Facultad de CienciasUniversidad de GranadaAvenida Fuentenueva s/n18002GranadaSpain

The Late Cretaceous Sabzevar ophiolite represents one of the largest and most complete fragments of Tethyan oceanic lithosphere in the NE *** is mainly composed of serpentinized mantle peridotites slices;nonetheless,minor tectonic slices of all crustal sequence constituents are observed in this *** crustal sequence contains a well-developed ultramafic and mafic cumulates section,comprising plagioclase-bearing wehrlite,olivine clinopyroxenite,olivine gabbronorite,gabbronorite,amphibole gabbronorite and quartz gabbronorite with adcumulate,mesocumulate,heteradcumulate and orthocumulate *** crystallization order for these rocks is olivine±chromian spinel→clinopyroxene→plagioclase→orthopyroxene→*** presence of primary magmatic amphiboles in the cumulate rocks shows that the parent magma evolved under hydrous ***,the studied rock units are characterized by low TiO2(0.18-0.57 wt.%),P2 O5(<0.05 wt.%),K2 O(0.01-0.51 wt.%)and total alkali contents(0.12-3.04 wt.%).They indicate fractionated trends in the chondrite-normalized rare earth element(REE)plots and multi-element diagrams(spider diagrams).The general trend of the spider diagrams exhibit slight enrichment in large ion lithophile elements(LILEs)relative to high field strength elements(HFSEs)and positive anomalies in Sr,Pb and Eu and negative anomalies in Zr and Nb relative to the adjacent *** REE plots of these rocks display increasing trend from La to Sm,positive Eu anomaly(Eu/Eu*=1.06-1.54)and an almost flat pattern from medium REE(MREE)to heavy REE(HREE)region[(Gd/Yb)N=1-1.17].Moreover,clinopyroxenes from the cumulate rocks have low REE contents and show marked depletion in light REE(LREE)compared to MREE and HREE[(La/Sm)N=0.10-0.27 and(La/Yb)N=0.08-0.22].The composition of calculated melts in equilibrium with the clinopyroxenes from less evolved cumulate samples are closely similar to island arc tholeiitic(IAT)*** mineralogy,geochemical features

关键词： Petrology Geochemistry Cumulate rocks Sabzevar ophiolite supra-subduction zone Iran

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Genesis and Tectonic Implications of the Kabr El-Bonaya Ultramafic Rocks,Sinai Peninsula,Egypt:Constraints from Mineralogical and Geochemical Characteristics

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Acta Geologica Sinica(English Edition) 2021年第2期95卷 393-418页

作者： Moustafa Mohammed MOGAHED Geology Department Benha University-EgyptBenhaEgypt

The Kabr El-Bonaya mafic–ultramafic intrusion is exposed along the southeastern border of the Sinai Peninsula and the northernmost segment of the Arabian-Nubian Shield(ANS).It occurs as an elliptical intrusive body that is located along the major NE–SW trending fracture zones that prevail in the Kid metamorphic *** ultramafic rocks in the complex comprise ultramafic cumulates of peridotites(dunite,harzburgite and wehrlite)and *** rocks are generally unmetamorphosed and have intrusive contacts with the country *** chemistry and whole-rock chemical compositions of these ultramafic rocks are mostly consistent with those of residual mantle peridotites from refractory suprasubduction tectonic *** on the variations of the major elements,the studied ultramafic rocks are consistent with those of a supra-subduction zone mantle,as it seems to have melted at 1–2 GPa and 1300–1450℃.Linear variations of Al2O3,Ca O,V and Ni with Mg O,coupled with incompatible and rare-earth-element depletion and mineral compositions,suggest prior events of partial melting in both wehrlites and *** LREE enrichment in the harzburgite,as well as the development of Cr-rich spinel,is consistent with a history of melt–peridotite *** calculated(Sm/Yb)N variations for the studied peridotites indicate a general increase in the addition of fluids with an increasing degree of melting from the wehrlite(~13–15 wt%of fluid)in the source,after initial spinel peridotite melting to the harzburgite(~20–25 wt%of fluid)in the same source,which is contrary to normal abyssal *** estimated equilibration temperature ranges from 1214 to 1321℃for the studied wehrlites and from 1297 to 1374℃for *** Mg-rich nature of the analysed olivines from the studied ultramafic rocks(Fo=81.41 to 91.77)reflect their primary composition and are similar to olivines in Alaskan-type ultramafic *** Fo content of the analyzed olivines decrease sli

关键词： cumulate ultramafic rocks Alaskan-type rocks partial melting hydrous melting conditions supra-subduction zone Kabr El-Bonaya South Sinai Arabian-Nubian Shield

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