M. Ebrahimi; M. Rafiei
Abstract
Eocene pyroclastic-volcanic rocks, ranging in composition from basic to intermediate and acidic rocks, are widely exposed at north of Zavieh, southwest of Karaj. The pyroclastic-volcanic sequence is composed of basalt, trachy-basalt, basaltic trachy-andesite, andesite, trachy-andesite, trachydacite, ...
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Eocene pyroclastic-volcanic rocks, ranging in composition from basic to intermediate and acidic rocks, are widely exposed at north of Zavieh, southwest of Karaj. The pyroclastic-volcanic sequence is composed of basalt, trachy-basalt, basaltic trachy-andesite, andesite, trachy-andesite, trachydacite, dacite and rhyolite along with various pyroclastic rocks including ignimbrite, welded tuff, crystal tuff, lithic tuff, lithic crystal tuff, ash tuff and lapillistone. The pyroclastic-volcanic sequence contains large amount of ignimbrite. Common textures found in the studied volcanic rocks are porphyritic, glomeroporphyritic, cumulophyric, amygdaloidal and trachytic. Rock fragments and broken crystals are usually found in the pyroclastic rocks. Eutaxitic texture is commonly observed in the ignimbrite. Flow banding is seen in the study rhyolitic rocks. Meanwhile, columnar structure is commonly observed in the lava flows and ignimbrites. Pyroxene, amphibole, biotite, plagioclase and quartz are the major minerals in the volcanic sequence along with K-feldspar, opaque minerals and altered olivine as minor minerals. Calcite, chlorite, epidote and sericite are also present as accessory minerals. According to geochemical data the studied volcanic rocks are mainly calc-alkaline, however few samples plot in the alkaline and shoshonitic fields in some diagrams. Negative anomalies of Nb and Ta, Pb positive anomaly, large ion lithophile elements enrichment, rare earth elements enrichments and depletion in high field strength elements all indicate that the parent magma of the studied rocks has originated from a metasomatized asthenospheric mantel over the Benioff-Waddatti zone. Common existence of sieve texture in plagioclase crystals, corroded crystals of plagioclase and pyroxene and dark rim of Fe-Ti oxides around biotite and amphibole crystals all indicate that a disequilibrium condition existed during crystallization which is most probably due to magma mixing. However, assimilation, decompression and rapid ascend of magma may also produce such textures.
M Ebrahimi; H Kouhestani; M.A.A Mokhtari; M Feizi
Abstract
The Aqkand area is located at 48 km north of Zanjan and is a part of the Tarom subzone within the Western Alborz-Azerbaijan structural zone. Acidic rocks exposed in this area consist of Oligocene rhyolite-rhyodacite, perlite, pitchstone and ignimbrite. Rhyolites and rhyodacites are exposed as domes and ...
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The Aqkand area is located at 48 km north of Zanjan and is a part of the Tarom subzone within the Western Alborz-Azerbaijan structural zone. Acidic rocks exposed in this area consist of Oligocene rhyolite-rhyodacite, perlite, pitchstone and ignimbrite. Rhyolites and rhyodacites are exposed as domes and lava flows. Perlites are located at the lower parts of the rhyolitic-rhyodacitic domes. Geochemical investigations indicate calc-alkaline nature along with magmatic arc affinity for the studied rocks. These rocks are characterized by LREE-rich patterns with high LREE/HREE ratio and distinctive negative Eu anomaly. Positive anomalies of Rb, Pb and Th on the spider diagrams suggest continental crust involvement in the genesis and evolution of the parental magma. Based on tectonic setting discrimination diagrams, Aqkand acidic rocks are plotted in the post-collision tectonic setting. In overall petrographic, petrologic and geochemical evidence demonstrate that during the final stages of the continental collision, the thickened sub-continental lithosphere in the Alborz was delaminated and replaced by hot asthenospheric mantle. Rapid upwelling of hot asthenosphere gave rise to the partial melting of sub-continental lithosphere which produced the Oligocene basaltic rocks of the study area. Emplacement of basaltic magma in the upper part of the continental crust led to the partial melting of the crust and generation of the acidic magmas. Rapid cooling of acidic lavas led to the generation of glassy rocks such as obsidian which are changed to perlite in contact with meteoric and warm waters.
L. Talebi; M. A. A. Mokhtari; M. Ebrahimi; H. Kouhestani
Abstract
Arpachay Pb-Zn-Cu mineralization occurrence is located in the Takab-Angouran-Takht-e-Soleyman metallogenic zone, 60 km north of Takab. Rock units exposed in the area consist of Oligo-Miocene volcanic, volcano-sedimentary and sedimentary units which are intruded by gabbroic and granodioritic plutons. ...
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Arpachay Pb-Zn-Cu mineralization occurrence is located in the Takab-Angouran-Takht-e-Soleyman metallogenic zone, 60 km north of Takab. Rock units exposed in the area consist of Oligo-Miocene volcanic, volcano-sedimentary and sedimentary units which are intruded by gabbroic and granodioritic plutons. Mineralization occurs as ore-bearing quartz veins hosted by gabbroic pluton in this area. The gabbro shows hetero-granoular texture consisting of plagioclase and clinopyroxene phynocrysts. Geochemical data indicate a calc-alkaline affinity for the gabbroic rocks, associated with active continental margin. These data demonstrate contamination of the parental magma with crustal materials. Based on microscopic studies, ore minerals include galena, sphalerite, chalcopyrite with minor pyrite, and quartz and calcite are present as gangue minerals in the Arpachy occurrence. The ore minerals show vein-veinlets, disseminated, breccia, open space filling, box-work, plumose, cockade and replacement textures. Hydrothermal alteration is restricted to the silicified, sericitic-argillitic and carbonatic altered parts of the ore zones., Alteration occurs as sericitization and calcitization-chloritization (propylitic alteration) of host gabbro. Four stages of mineralization can be distinguished at the Arpachy occurrence. These stages are progressed from brecciation and silicification of host rocks (stage 1) to quartz-chalcopyrite-pyrite‒cemented veins and breccias (stage 2), quartz-galena-sphalerite veins and veinlets (stage 4) and finally individual or sets of late barren quartz-carbonate veins and veinlets (stage 4) in the outer parts of the mineralization zones. REE pattern of the host rocks and the mineralized samples indicate that mineralized samples are depleted in REE. This signature may indicate high water/rock interaction at Arpachay. Geological, mineralogical and textural characteristics of Arpachay occurrence are comparable with intermediate-sulfidation style of epithermal base metal deposits.
GH Abbasi; M Ebrahimi; M Sharifi; N Shirdashtzadeh; J Ahmadian
Abstract
The studied volcanic rocks to the northeast of the Gavkhuni playa lake are composed of rhyolite, trachydacite, trachyandesite and pyroclastic rocks including lithic tuff and volcanic breccia. Rhyolitic rocks have flow structure and hyalophyric texture with glassy to cryptocrystalline ...
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The studied volcanic rocks to the northeast of the Gavkhuni playa lake are composed of rhyolite, trachydacite, trachyandesite and pyroclastic rocks including lithic tuff and volcanic breccia. Rhyolitic rocks have flow structure and hyalophyric texture with glassy to cryptocrystalline matrix in which quartz and feldspar crystal are present. The dominant texture in trachydacitic and trachyandesitic rocks in porphyritic texture in which plagioclase and rarely sanidine phynocrysts are set in a glassy to microcrystalline groundmass containing feldspar microlites. In trachydacites, quartz is present as small crystals and rarely as microphynocrysts. Plagioclase phynocrysts in trachydacitic and trachyandesitic rocks display sieve-texture. Amphibole and biotite are the mafic minerals of the volcanic rocks and they are partialy, at rims, or completely altered to Fe-Ti oxides. All of the studied rocks are high- K calc-alkaline in nature. According to the geochemical data, the volcanic rocks are depleted in high field strength elements such as Nb, Ti and Ta and enriched in large ion lithophile elements like Cs, K, Ba, Rb and Th which are characteristics of subduction related volcanic rocks. Nb negative anomaly is characteristic of continental rocks and indicates the continental crust involvement in the magmatic processes. The rocks are enriched in Cs, Rb, Ba and Pb indicating mantle metasomatism by fluids released from subducting oceanic lithosphere. Therefore, magma contamination and mantle metasomatism have affected the magma from which the volcanic rocks of the Gavkhuni playa lake have were generated. The calc-alkaline magmatism in the study area was associated with the closure of the Neotethyan Ocean.
F Mohamadi; M Ebrahimi; M.A.A Mokhtari
Abstract
The study area,~12 km to the southwest of the city Behabad, is a part of the Posht-e-Badam Block in Central Iran. Igneous rocks in the area occur as intrusive, sub-volcanic and volcanic bodies and exhibit a wide range of composition from felsic to mafic. The intrusive and sub- volcanic rocks include ...
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The study area,~12 km to the southwest of the city Behabad, is a part of the Posht-e-Badam Block in Central Iran. Igneous rocks in the area occur as intrusive, sub-volcanic and volcanic bodies and exhibit a wide range of composition from felsic to mafic. The intrusive and sub- volcanic rocks include the relatively large Homijan granitoid, Ferdows granitoid, gabbro- diorite stocks and Kuh-Siah sub- volcanic rhyolitic dome. The Homijan granitoid is composed of a shallow-level intrusion in the center to rhyolitic lavas and tuffs in the margins. The whole assemblage is covered by dolomites of the Rizu series, with no thermal metamorphism in the covering rocks. Homijan granitoid displays porphyritic, porphyroid and graphic textures composed of coarse plagioclase, alkali feldspar and quartz in a fine- grained quartz- feldspatic matrix; the marginal rhyolitic lavas have porphyritic and spherolitic textures with quartz and alkali feldspar phenocrysts. Rhyolitic tuffs have porphyroclastic texture. Ferdows granite has hetero-granular, graphic and perthitic texture composed of quartz, orthoclase and plagioclase. Kuh-Siah rhyolites have porphyric, felsophyry and felsitic textures with small quartz and alkali feldspar phenocrysts. Geochemical studies demonstrate that Homijan and Ferdows granitiods and the marginal rhyolites of the Homijan, as well as the Kuh-Siah rhyolitic dome have high- K calc- alkaline to shoshonitic nature and can be classified as S-type peraluminous granitoids with some tendency to I-type granitoids. Based on the spider diagrams, all rocks have similar trend which is indicative for their genetic relation. These diagrams indicate enriched LILEs (Rb, K, Th and Pb) along with negative anomalies of HFSEs (Nb and Ti). Chondrite normalized REE patterns demonstrate LREEs-enriched patterns with high ratios of LREE/HREE.The positive and negative anomalies of the mentioned elements in the studied rocks probably are related to lower partial melting degrees of a metasomatized mantle along with crustal contamination of the magma. Based on field investigation, petrographic studies and geochemistry, and using the granitoid discrimination tectonic setting diagrams, it seems that Homijan granitiods and related felsic rocks formed in a post- collisional setting within the Posht-e-Badam Block.
Z Bikdeli; M Ebrahimi; Gh Nabatian; M.A.A Nabatian
Abstract
Chehrabad Cu deposit is located in the Zanjan province and in the northwest corner of Central Iran. Rock units exposed in this area belong to the Cenozoic era (especially Miocene period) and include the Lower Red Formation, Qom Formation, Upper Red Formation and Quaternary deposits. In the study area, ...
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Chehrabad Cu deposit is located in the Zanjan province and in the northwest corner of Central Iran. Rock units exposed in this area belong to the Cenozoic era (especially Miocene period) and include the Lower Red Formation, Qom Formation, Upper Red Formation and Quaternary deposits. In the study area, the Upper Red Formation which hosts the mineralization consists of alternating beds of marl, siltstone, oxidized red sandstone and light gray sandstone. Mineralization has occurred within the gray sandstones of bleached zone containing organic materials. In Chehrabad, copper deposit mineralization has been recognized in five horizons. Each horizon is composed of three zones including reduced zone with mineralization, bleached zone and oxidized red zone. The dominant texture in the Chehrabad Cu deposit is replacement texture which is accompanied with other textures such as disseminated, solution seams, lens like and interparticle cement texture. Lithology (sandstone host rock), sulfide mineralogy (e.g. pyrite, chalcocite, bornite and chalcopyrite), structure and texture, stratigraphy as well as factors controlling the mineralization (presence of plant remains performing as environment reductant and also the permeability in the host rock) suggest that this deposit is similar to the red-bed type sedimentary Cu deposits which is formed during the early to late diagenesis.
Chehrabad Cu deposit is located in the Zanjan province and in the northwest corner of Central Iran. Rock units exposed in this area belong to the Cenozoic era (especially Miocene period) and include the Lower Red Formation, Qom Formation, Upper Red Formation and Quaternary deposits. In the study area, the Upper Red Formation which hosts the mineralization consists of alternating beds of marl, siltstone, oxidized red sandstone and light gray sandstone. Mineralization has occurred within the gray sandstones of bleached zone containing organic materials. In Chehrabad, copper deposit mineralization has been recognized in five horizons. Each horizon is composed of three zones including reduced zone with mineralization, bleached zone and oxidized red zone. The dominant texture in the Chehrabad Cu deposit is replacement texture which is accompanied with other textures such as disseminated, solution seams, lens like and interparticle cement texture. Lithology (sandstone host rock), sulfide mineralogy (e.g. pyrite, chalcocite, bornite and chalcopyrite), structure and texture, stratigraphy as well as factors controlling the mineralization (presence of plant remains performing as environment reductant and also the permeability in the host rock) suggest that this deposit is similar to the red-bed type sedimentary Cu deposits which is formed during the early to late diagenesis.