Petrology
Azad karimi; ali ahmadi; Monireh Kheirkhah; Abdolreza Partabian
Abstract
The peridotites from Marivan-Kamyaran ophiolite suite (MKO), outer Zagros ophiolite belt, western Iran, are studied for the petrological and geochemical characteristics. Both mantle and crustal peridotites are present in the Marivan-Kamyaran ophiolitic sequence. Extensive compositional variations in ...
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The peridotites from Marivan-Kamyaran ophiolite suite (MKO), outer Zagros ophiolite belt, western Iran, are studied for the petrological and geochemical characteristics. Both mantle and crustal peridotites are present in the Marivan-Kamyaran ophiolitic sequence. Extensive compositional variations in MgO, Al2O3, Cr, Ni and Al2O3 / SiO2, combined with the abyssal (MOR) and supra-subduction zone (SSZ) nature of the peridotites, are indicative of variant degrees of depletion of the mantle peridotites. Various degrees of depletion are consistent with a wide range of partial melting percentages (10-15%) obtained from non-modal backward partial melting models. The enrichment in HFSE such as Hf, Ti, Ta and Th indicates melt-rock interaction in the mantle peridotites; while, the enrichment in U and LIL elements such as Cs, Rb, Pb is most probably related to the fluid-rock interaction. It is suggested here that MOR nature of a part of the mantle peridotites in the region has been changed to SSZ characteristics in later stages through processes such as melt-rock interaction. It is likely that, similar to other occurrences of outer Zagros ophiolite belt, mantle peridotites from MKO with MOR and SSZ characteristics had been originated in MOR setting, before their composition were modified in a SSZ environment.
A. R. Najafzadeh
Abstract
The Rudan ultramafic complex with Paleozoic age is located in Southern Kerman province and is composed mainly of massive harzburgite, small to medium sized lenticular dunite and subordinate very thin olivine websterite dykes. Granular and protogranular are the main textures in dunites and harzburgites, ...
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The Rudan ultramafic complex with Paleozoic age is located in Southern Kerman province and is composed mainly of massive harzburgite, small to medium sized lenticular dunite and subordinate very thin olivine websterite dykes. Granular and protogranular are the main textures in dunites and harzburgites, and orthopyroxene, olivine and to a lesser extent clinopyroxene and chromian spinel were found in the harzburgites and dunites. The forsterite content of olivine is slightly higher in dunites (Fo92.5) than those in harzburgites (Fo91). Detailed electron microprobe study reveals intermediate Cr# (0.33-0.56), relatively high Mg# (0.59-0.75) and very low TiO2 content (averaging 0.01 wt %) for chromian spinels in Rudan peridotites. The Fe3+# is very low, (s in the peridotites are very low (average 25 ppb) and are highly depleted in PGE contents relative to chondrites. The PGE spidergrams in dunites are unfractionated, and the PdN/IrN ratios averages 1.02, whereas the harzburgites show slightly positive slopes PGE spidergrams, together with a small positive Ru, Pd and Au anomalies, and their PdN/IrN ratio averages 2.75. Moreover, the PGE chondrite and primitive mantle normalized patterns of the harzburgites are relatively flat which are comparable to the depleted mantle peridotites indicating relatively high degree of partial melting (about 17-22%) of the mantle source. The mineral chemistry data and the noble metal geochemistry indicates that the harzburgites of the Rudan ultramafic complex are residual and dunites are replaced and tectonically have been generated in a fore-arc mantle region above a supra-subduction zone setting.
R Kohansal; M Ghorbani; S.M Pourmafi; M Khalatbari Jafari; J Omrani
Abstract
The Forumad ophiolite pillow lavas crop out at west of Sabzevar and at the periphery of north central Iran microcontinent. These lavas include Upper Cretaceous pelagic limestone with microfossils and are classified in two Late Turonian- Santonian and Coniacian- Late Campanian groups. These basaltic ...
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The Forumad ophiolite pillow lavas crop out at west of Sabzevar and at the periphery of north central Iran microcontinent. These lavas include Upper Cretaceous pelagic limestone with microfossils and are classified in two Late Turonian- Santonian and Coniacian- Late Campanian groups. These basaltic pillow lavas have tubular and flattened forms, with external (vitrified), medial (variolite) and internal (intergranuler) parts. Each part has its own texture with mineral composition of clinopyroxene ± plagioclase ± olivine. The medial part is characterized by formation of skeletal crystals of olivine and lesser extend of plagioclase, indicating high nucleation rate. The nature of Late Turonian- Santonian lavas is tholeiitic and Coniacian- Late Campanian lavas are tholeiitic and calc-alkaline. Despite of enrichment in LILE, relative depletion of HFSE, different enrichment in LREE and various linear trends of HREE which suggest the formation of pillow lavas in the arc environment and the heterogeneous mantle with varying degrees of partial melting, but low ratios of Th/ Yb, La/ Nb, Ce/ Nb and Ce/ Th indicate that they are more inclined to intermediate between IAB and MORB environments. Unlike Late Turonian – Santonian lavas, high ratios of La/Yb, Nb/ Yb in Late Coniacian – Late Campanian lavas represent mantle fertility and decrease of Nb/ Ta ratios as well as relative increase of Th exhibit the effect of partial melting of subducted slab in genesis of these lavas. The depth of partial melting was 40 to 70 km in the spinel lherzolite stability zone. It seems that the formation of Forumad ophiolitic pillow lavas is accrued in a marginal basin to a back arc basin of a supra-subduction zone.
B Taghipour; M Nejadhadad; M.A Makizadeh; M Noghreian; P Rahimi
Abstract
Dunite and serpentinized harzburgite from northwest of the Neyriz ophiolites are host rocks of some podiform chromite deposits in Cheshmeh-Bid area. These chromite deposits occurred as aligned massive lenses in various sizes. The most important ore textures are massive, nodular, layered and disseminated ...
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Dunite and serpentinized harzburgite from northwest of the Neyriz ophiolites are host rocks of some podiform chromite deposits in Cheshmeh-Bid area. These chromite deposits occurred as aligned massive lenses in various sizes. The most important ore textures are massive, nodular, layered and disseminated grains. These textures are goodevidences of crystals settling crystals during the process of magma cooling in the chamber. The mineralogical studies revealed that the pyroxene melting tookplace in the peridotite host rock and olivine is recrystallized in dunite envelope. Mineral chemistry of chromite indicates high Cr number(72-77), high Mg number(69-62) and low TiO2 content(0.04-0.14 %) in chromite samples. These amounts are comparable with those chromitites, which areformed from the high partial melting rate of mantle. The medium amount of PGE determined about 212ppb, which is much lessthan the Tang-e-Hana chromite (1556ppb) deposits. The trace element content of chromitite samples and dunite envelope like Ga, V, Zn, Co, Ni and Mn are between 17-24, 466-842, 852-1048, 22-84, 115-171, and 852-1220 ppm respectively. The U shape of REY pattern indicates the interaction of high volatile and REE bearing melt(boninitic source of parent magma) with host rock. Therefore,the Cheshmeh-Bid chromitite resulted likely from the intrusion of boninitic magma ofprimary mantle with high partial melting in the peridotite host rock. The geotectonic setting is above supra-subduction zone of island arc.
A Shaker Ardakani; M Arvin; B Oberhänsli
Abstract
The Lower Paleocene felsic plutonic rocks in the N- NE of Ghaleh-Ganj, are located on the western side of the Makran accretionary prism and the Jazmurian Depression and the east side of the Jiroft fault. They are parts of the intrusives in the Ganj complex, which itself is a part of the Jazmurian ophiolitic ...
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The Lower Paleocene felsic plutonic rocks in the N- NE of Ghaleh-Ganj, are located on the western side of the Makran accretionary prism and the Jazmurian Depression and the east side of the Jiroft fault. They are parts of the intrusives in the Ganj complex, which itself is a part of the Jazmurian ophiolitic belt or Inner Makran. Petrographically, the intrusives are quartz diorite, plagiogranite, tonalite, granodiorite, monzogranite and albite granite in composition with mainly equigranular to inequigranular textures and subordinate textures such as micrographic, granophyric, poikilitic and zoning. Mineralogically, the plutonic rocks consist of plagioclase, quartz, orthoclase, microcline, amphibole, biotite and opaques. The plagioclases show normal, reverse and oscillatory zoning and based on their mineral chemistry studies, are mainly albite to oligoclase in composition. Amphiboles are calcic type and magnesiohornblende in composition with affinity to actinolite. The geochemical studies show that the felsic plutonic rocks are trondhjemite to tonalite in composition and belonging to a sub-alkaline magmatic series with calc-alkaline nature, and have characteristics of peraluminous granitoids. Using trace element discrimination diagrams and ocean ridge granites-normalized multi-elements diagram clearly show that the Ganj felsic plutonic massifs are plotted in arc-related granites field with affinity towards ocean ridge granites field. These characteristics along with Nb-Ta negative anomalies are known as evidences for formation of Ganj felsic plutonic rocks in a supra-subduction zone environment.
A.R Najafzadeh; H Ahmadipour
Abstract
The Abdasht complex is one of the major ultramafic complexes in south-east Iran (Esfandagheh area). This complex is composed mainly of dunite, harzburgite, lherzolite, chromitite, and subordinate wehrlite. The detailed electron microprobe study revealed very high Cr# (48-86), Mg# (26-56) and very low ...
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The Abdasht complex is one of the major ultramafic complexes in south-east Iran (Esfandagheh area). This complex is composed mainly of dunite, harzburgite, lherzolite, chromitite, and subordinate wehrlite. The detailed electron microprobe study revealed very high Cr# (48-86), Mg# (26-56) and very low TiO2 content (averaging 0.07 wt %) for chromian spinels in peridotites. The Fe3+# is very low (<0.08 wt%) in the chromian spinel of peridotites which reflects crystallization under the low oxygen fugacities. The harzburgite, dunite and lherzolite samples are highly depleted in PGE contents relative to the chondrites. The PdN/IrN ratios in dunites are unfractionated, averaging 0.72, whereas the harzburgites and lherzolites show slightly positive slopes PGE spidergrams, together with a small positive Ru anomaly, and their PdN/IrN ratio averages 2.4 and 2.3, respectively. Moreover, the PGE chondrite and primitive mantle normalized patterns of harzburgite, dunite and lherzolite of this complex are relatively flat which are comparable to the highly depleted mantle peridotites and indicating a high degree of partial melting (about 20-25%) of the mantle source. The mineral chemistry data and PGE geochemistry of Abdasht peridotites indicate that the Abdasht ultramafic complex was generated from an arc-related magma with boninitic affinity above a supra-subduction zone setting.
