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Central Iran Ophiolites and Igneous Rocks

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Ghodrat Torabi
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ABSTRACT The Eocene pluton intrudes the Paleozoic accretionary wedge in the Kal-e-Kafi area (NE of Anarak, Isfahan province, Central Iran). The results of the field studies, petrography evidences and chemical characteristics of minerals used to determine the nature and P-T condition of this thermal metamorphism. Metaperidotites units of this Paleotethys- related accretionary prism consist of metalherzolite, metaharzburgite and metadunite and are associated with the listwaenites and metasediments (schist and marble). Rock-forming minerals of the Kal-e-Kafi metaperidotites are olivine (forsterite and chrysolite, Mg#= 0.86-0.92 with CaO<0.02 wt%), orthopyroxene (enstatite, Mg#= 0.84-0.86), clinopyroxene (Mg#= 0.90-0.94, Al2O3 0.03-2.08 wt%), tremolite, tremolitic-hornblende and magnesio-hornblende, anthophyllite, serpentine (antigorite with Mg#= 0.82-0.95), talc, chlorite (clinochlore, Mg#= 0.94-0.95), chromian spinel (Cr#= 0.58-0.72), magnetite and plagioclase (Anorthite An0.91-0.95). All these minerals are metamorphic products and are not relicts of the primary igneous mineralogy, except the inner parts of Cr-spinels. The main textures of these rocks are porphyroblastic, granoblastic, nematoblastic, poikiloblastic, jack-straw and mesh texture. Chemical composition of the core of Cr-spinel crystals reveal that the protolith of the studied rocks were mantle peridotites which belong to the depleted and moderately depleted peridotite series. Mineral chemistry of clinopyroxenes (Cr2O3= 0-0.06 wt%, Al2O3= 0.03-2.08 wt%, average TiO2 value of 0.036 wt%, and trace element patterns in the normalization diagrams) indicate their metamorphic nature The field relationships (association of metaperidotites with Paleozoic schist and marbles), petrography evidences (the presence of metamorphic mineral assemblages of tremolite + olivine and olivine + orthopyroxene + anthophyllite), chemical characteristics of minerals (the average forsterite content of olivine neoblasts is ~90%; extensive variation in MnO content of olivines; the high value of Al2O3 in chlorite; low Cr2O3, Al2O3 and TiO2 contents of clinopyroxenes, and presence of anorthite plagioclase) and results of the thermobarometry calculations, show that the peridotites in the Kal-e-Kafi area have suffered a regional metamorphism in P-T condition of greenschist facies during the Paleozoic, intrusion of the Kal-e-Kafi pluton, caused a progressive contact metamorphism at 630 to 750 °C under a pressure less than 1 kbar (pyroxene hornfels facies) during the Eocene. Key words: Metaperidotite; Paleozoic; Kal-e-Kafi intrusion; Eocene; Regional and contact metamorphism; Anarak; Central Iran
Nargess Nosouhian
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Introduction In the northeastern part of the Isfahan province and 65 km northeast of the Anarak city (Kal-e-Kafi area), an I-type granitoid pluton cross cut the Paleozoic metamorphic rocks and Eocene volcanic rocks. In the contact of this granitoid body with sourrounding rock units, skarn and hornfels have been formed (Ahmadian, 2012; Ranjbar, 2010). The Kal-e-Kafi Eocene intrusive body presents a wide range of mineralogical and petrological compositions, from gabbro to alkali-feldspar granite. Presence of mafic to acidic rocks in this mostly-granitoid body indicates that fractional crystallisation has played an important role during magma evolution. The field and petrographical studies indicate the presence of anorthosite veins within the gabbro section. The mafic and basic parts of this pluton have not been studied yet. The mineralogy and chemistry of rock-forming minerals in the anorthosites and gabbros are the subject of this research study. Analytical Methods The mineralogical and petrographical studies have been done by using Olympus BH-2 polarizing microscope in the mineralogy laboratory of the University of Isfahan. EPMA and LA-ICP-MS analyses were used to obtain chemical characteristics of rock-forming minerals. Major-elements composition of minerals were performed by JEOL JXA-8800, WDS microprobe electron analyzer with accelerator voltage of 15 kV, current of 15 nmA, diameter of 3 μm, and a counting time of 40 seconds at the Kanazawa University of Japan. Natural and synthetic minerals and compounds were used as standards. The ZAF program was used for data correction. Trace element values of plagioclases and clinopyroxenes were analyzed by LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) using an ArF 193 nm Excimer Laser coupled to an Agilent 7500S at the Earth Science Department of the Kanazawa University, Japan. The diameter of the analyzed points was 110 µm at 10 Hz with energy density of 8 J/cm2 per pulse. Mineral abbreviations in tables and photomicrographs are adopted from Whitney and Evans (2010). Results and Discussion The Eocene Kal-e-Kafi pluton includes a wide range of rocks from gabbro to alkali-feldspar granite, which points to an extensive magmatic differntiation. Field relationships indicate presence of at least 4 magmatic phases, and gabbro is the first and oldest phase. The most predominant rock unit in the Kal-e-Kafi intrusive body is granitoid. However, in the northern parts, the gabbro and anorthosite present substantial exposures. The anorthosites and gabbros are associated with each other in the field. Anorthositic veins with up to 15 cm thickness cut the gabbro. Gabbro is composed of bytownite and anorthite plagioclase (An= 84 – 94 %; some of them have been altered to bytownite, andesine and oligoclase), clinopyroxene (diopside, Mg#= 0.75), orthoclase (Or0.88), apatite, magnetite, and prehnite. Anorthosite rock-forming minerals are anorthite plagioclase (An= 89 – 95 %; some anorthite plagioclase have been altered to bytownite and labradorite), sphene and zircon. The main texture of these rocks are granular, intergranular and poikilitic. Field studies suggest that anorthosites are associated with gabbros which have filled the fractures of gabbros. Very simmilar petrography and chemical composition of plagioclases in the anorthosites and gabbros possibly reveal their cogenetic nature. It seems that the primary magma in the magma chamber, first crystallized the clinopyroxene and plagioclase, which caused formation of gabbros. In the next stage, by occurrence of a tectonic activity, the gabbros have broken and the remaining magma which was rich in plagioclase components, crystallized the anorthosites in the fractures. This reveals that the anorthosites of the study area are the plagioclase rich part of the primary basic magma which have formed the gabbros. According to the field relationships, it is generally believed that anorthosites are differentiates of gabbroic magmas. The studied anorthosite veins and gabbros of the Kal-e-Kafi area are consanguineous. These anorthosites are perhaps generated by the process of collection of plagioclase crystals from a gabbroic magma under the action of gravity and tectonic activity (filter pressing). Pyroxene is one of the common minerals. The chemical composition of this mineral provides valuable information about the nature of magma, H2O content, Oxygen fugacity, type of magmatic series, tectonic setting, as well as temperature and pressure of crystallisation (Schweitzer et al., 1979; Leterrier et al., 1982; D’Antonio and Kristensen, 2005). Chemistry of clinopyroxens within the gabbros of the Kal-e-Kafi area shows that the parental magma belongs to the sub-alkaline and calc-alkaline magmatic series and these rocks are similar to those of volcanic arcs. The time and place of formation of these plutonic rocks possibly indicate that they are formed by subduction of the Central-East Iranian Microcontinent (CEIM) – confining oceanic crust beneath the CEIM. Acknowledgments The authors thank the University of Isfahan for financial supports.
Ghodrat Torabi
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The Lower Oligocene Kal-e-kafi (East of Anarak, Central Iran) lamprophyres occur as stocks and dikes, which cross-cut the Eocene volcanic and Cretaceous sedimentary rocks. The predominant minerals of these lamprophyres are hornblende (magnesiohastingsite) and clinopyroxene (diopside) phenocrysts set in a fine- to medium-grained matrix of the same minerals plus plagioclase (labradorite to bytownite), sanidine, apatite, and magnetite. Secondary minerals are chlorite, magnetite, calcite, and epidote. Petrography, mineral chemistry, and whole rock compositions classify these rocks as calc-alkaline lamprophyre, in general, and spessartite in particular. These samples have intermediate compositions (SiO2 ~ 58 wt %). The chondrite- normalized REE patterns and primitive mantle-normalized multi-element spider diagram of Kal-ekafi lamprophyres are remarkably parallel and suggest that these dikes and stocks were derived from the same parental magma and underwent similar melt extraction. These rocks are enriched in alkalies, large-ion lithophile elements (e.g., Rb, Ba, K), and light rare-earth elements (e.g., La, Ce), and exhibit moderate to high fractionation in LREE patterns, with an average La/Lu ratio of 112. The large amount of hydrous fluids coming from the subducted slab rather than sediments caused to the enrichment and metasomatism of subcontinental lithospheric mantle source. Crustal contamination and assimilation of host rocks also played role in the genesis of these lamprophyres. Geochemical characteristics of the studied rocks suggest that parental magma have been derived from partial melting of a metasomatized amphibole-bearing spinel lherzolite of lithospheric mantle, which was previously modified by dehydration of a subducting slab. Subduction of oceanic crust around the Central-East Iranian Microcontinent (CEIM) is the most reasonable mechanism to explain enrichment in volatiles of the mantle, and the lamprophyric magmatism of the Kal-e-kafi area in Lower Oligocene times. Several tectonomagmatic discrimination diagrams indicate that the Kal-e-kafi lamprophyres occurred during postcollisional period of lithospheric extension.
Nargess Shirdashtzadeh
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[PUBLISHED IN: NEW FINDINGS IN APPLIED GEOLOGY] ***** The study of petrological findings and geochemical data of mafic and ultramafic rocks and related petrographic units in the ophiolites can be an introduction to environmental geology studies, agriculture and natural resources. Regarding the urban and rural areas of Nain to Ashin in the vicinity of ophiolitic zone (Central Iran), geochemical investigations of mafic and ultramafic rocks of these ophiolites have very important applications in the field of environmental geology of these areas. Therefore, available geochemical data of basic rocks (pillow lavas and basalts), metabasic rocks (amphibolites) and metamorphosed and altered peridotites are considered based on environmental factors. For examples, in the peridotites which are one of the most abundant rock units in these ophiolites, the enrichment factor (Ef) for Ni is extremely high, for Cr is very high, and for Co is high. In addition, the geo-accumulation index or Igeo for Ni (~4) and Ni (>5) in peridotites is heavily to extremely high. therefore, high Ef and Igeo factors of the studied heavy metals (e.g., chrome, nickel, cobalt, arsenic, vanadium) and asbestose minerals (hornblende, tremolite, talc and chrysotile) in the mafic and ultramafic rocks of Ophiolites can be known as some dangerous environmental pollutants. Thus, investigatation of the volume of such elements penetrated into the surface and underground waters and soils in the villages and cities at the foot of these ophiolites could be a theme for ongoing studies in environmental geology of these areas. بررسی یافته‌های سنگ‌شناسی و داده‌های زمین‌شیمبایی سنگ‌های مافیک و الترامافیک و واحدهای سنگ‌شناسی مرتبط با آنها در افیولیت‌ها، مقدمه‌ای برای بررسی‌های زمین‌شناسی زیست‌محیطی، کشاورزی و منابع طبیعی است. با توجه به قرارگیری مناطق شهری و روستایی نایین تا عشین در نزدیکی مناطق افیولیتی (شمال‌غرب خردقاره مرکز – شرق ایران)، بررسی زمین‌شیمی واحدهای مافیک و الترامافیک این افیولیت‌ها از دیدگاه زمین‌شناسی زیست‌محیطی اهمیت بالایی دارد. برای این منظور، داده‌های زمین‌شیمیایی موجود برای سنگ‌های بازیک (گدازه‌های بالشی و دایک‌های دیابازی)، متابازیک (آمفیبولیت‌ها)، پریدوتیت‌ها ی دگرگون و دگرسان‌شدة این دو افیولیت از دیدگاه فاکتورهای زیست‌محیطی بررسی شدند. برای نمونه، در پریدوتیت‌ها که از فراوان‌ترین گروه‌های سنگی این افیولیت‌ها به‌شمار می‌روند، مقدار غنی‌شدگی (Ef) برای عنصر نیکل غنی‌شدگی بسیار بسیار بالا، برای کروم غنی‌شدگی بسیار بالا و برای کبالت غنی‌شدگی بالا را نشان می‌دهد. همچنین، مقدار شاخص تجمع زمین (Igeo) برای عنصرهای کروم (4 Igeo ~ ) و نیکل (5Igeo > ) در پریدوتیت‌ها به شدت بسیار بالاست. ازاین‌رو، مقدار بالای فاکتورهای Ef و Igeo فلزات سنگین بررسی‌شده (مانند: کروم، نیکل، کبالت، آرسنیک و وانادیم) و کانی‌های آزبستوزی (مانند هورنبلند، ترمولیت، تالک و کریزوتیل) در سنگ‌های مافیک و الترامافیک افیولیت‌ها می‌توانند از آلاینده‌های خطرناک زیست‌محیطی در این افیولیت‌ها به‌شمار بروند. بنابراین، بررسی میزان ورود این عنصرهای به آب‌های زیرزمینی و زنجیره غذایی ساکنین روستاها و شهرهای دامنه مناطق افیولیتی می‌تواند موضوع بررسی‌های زیست‌محیطی در این مناطق باشد.
Ghodrat Torabi
added 2 research items
Detrital chromian spinels from adjacent sediment (recent sands and a Tertiary sandstone) were used to obtain a general view of the lithological and petrological characteristics of the Nain ophiolitic mélange, Iran. They display a wide chemical range in terms of Cr# (= atomic ratio), from 0.10 to 0.92. Except for some grains (11% of the total), the spinels show low TiO2 contents (0.25 wt%, up to 1.26 wt%) are relatively low in Mg# (= atomic ratio), from 0.15 to 0.65, and were possibly derived from mantle peridotite that reacted with impregnating melt (e.g., dunite or plagioclase-bearing peridotites). The high-Cr# (0.55–0.92) spinels, which are dominant in the Tertiary sandstone, are chemically homogenous and clearly different from high-Cr# altered spinels (ferritchromite and Cr-magnetite), which are formed by low-T alteration, and are low in Mg# and high in Fe3+. The higher abundance of the high-Cr# spinels in the Tertiary sandstone than in the recent sands indicates higher abundance of refractory lithologies in the “Paleo-Nain ophiolite” than the present one. The refractory lithologies for the source of the high-Cr# spinels have not been reported from the Nain mélange, which implies that the refractory dunite and harzburgite were effectively sheared to provide the mélange matrix and then eroded during later stages of emplacement. This indicates uplift and intermittent protrusion of a vertical slice of heterogeneous upper mantle in the Nain since the late Cretaceous. This is in good agreement with the geological situation of the Nain mélange located on a fossil transcurrent fault (Nain-Baft fault zone), which was involved in the opening of the Nain-Baft basin along an active margin of the central-east Iranian microplate (the Sanandaj-Sirjan block). The initial presence of the high-Cr# spinels implies an origin from a spreading center above a subduction zone (e.g., back-arc basin) for the Nain mélange.
The western part of the Central-East Iranian microcontinent (CEIM) hosts the Anarak, Jandaq, Bayazeh and Posht-e-Badam ophiolites of Paleozoic age. The Bayazeh ophiolite is situated in the Eastern margin of the Yazd Block and is exposed along the Bayazeh Fault. This ophiolite consists of serpentinized peridotites, metagabbro, metamorphosed ultrabasic dykes, metapicrite, serpentinite and metalistwaenite which are covered by Late Paleozoic schist and marble. Mineral association of the metapicrites in the Bayazeh Ophiolite is olivine (completely altered to serpentine), clinopyroxene (diopside), phlogopite, apatite, prehnite, amphibole (tremolite, actinolite and tremolitic hornblende), chlorite (clinochlore, penninite and diabantite), ilmenite and magnetite. Matrix glass is significantly devitrified and chlorite is present throughout the matrix. Clinopyroxene and phlogopite occur as relicts of the primary igneous mineralogy. Petrography and trace element composition of clinopyroxenes indicate near-simultaneous crystallization of clinopyroxene and plagioclase from the magma. The ultramafic character of the Bayazeh metapicrites is shown by their high MgO (25.8 to 28.0 wt%) and low SiO2 (37.5 to 39.4 wt%) contents. They are characterized by high Mg# (80.61 to 81.60), Ni (975 to 1020 ppm) and Cr (1300 to 1431 ppm) contents, suggesting that this melt closely approached the composition of a primitive mantle-derived melt. High-field strength element (HFSE) enrichment, high Mg# and Ni values, enrichment in light rare earth elements (LREE) (e.g. [La/Yb]CN=11.65-12.31), associated with a large variation of large ion lithophile element (LILE) concentrations indicate metasomatic enrichment of an asthenospheric mantle source with a subduction-related components prior to melting. Geochemical characteristics show that the metapicrites were generated by partial melting of a metasomatized asthenospheric amphibole-bearing spinel lherzolite. The presence of phlogopite as a primary hydrous mineral together with high LILE/HFSE ratios (e.g. Sr/Sm = 63.01-104.86) and a negative Ti anomaly reveal the role of previously subducted oceanic crust. Subduction of the Paleo-Tethys from the Early to the Late Paleozoic is the cause of volatile enrichment and mantle metasomatism. Involvement of hydrous fluids related to Paleo-Tethys subduction, enabled the peridotite source to melt despite the absence of abnormal thermal condition.
Nargess Shirdashtzadeh
added 2 research items
Mesozoic ophiolte melange of Nain is located in Central East Iranian microcontinent. Field studies and petrography indicate presence of three types of granitoids (plagiogranites, high-K granites, and tonalites) with different origins in ophiolitic mélange of Nain. Three granitoid types are different in mineralogy and are including of: (1) Plagiogranites (containing Qtz+Pl+Amp±Prh±Chl) (Mineral abbrevations from [1]); (2) high-K granites (including of Qtz+Pl+Or+Ms+small magmatic Grt (Alm) grains); (3) tonalites that are seen as intruded dikes in the amphibolitic rocks, containing Qtz+Pl+Amp+ small metamorphic Grt (Alm-Sps) grains ±Prh.
One of the rock units of Nain ophiolitic mélange is high-K granitoid. Their garnet is almandine in composition. According to petrography, garnets are igneous type and have originated from melting the high Ca and Al sediments. Application of Garnet-Muscovite thermometer for these rocks indicates an average temperature of 528 oC.
Ghodrat Torabi
added 29 research items
The Naein ophiolite is the most complete ophiolitic exposure in Cental Iran and considered as a remnant of the Mesozoic Central East Iranian microcontinent (CEIM) confining oceanic crust. In the northeastern part of this ophiolite (Darreh Deh area) within the mantle peridotites, a few hundred meters below the top of the Moho transition zone (MTZ), the hornblendites are present as dykes (former cracks and joints) from a few millimeters to nearly 50 cm wide. They have sharp boundaries with the surrounding mantle harzburgites and dunites. These hornblendites are pale green and coarse-grained in hand specimen and composed of magnesio-hornblende (Mg# = 0.93), chlorite (penninite and clinochlore, Mg# = 0.95), Cr-spinel (chromite, Cr# = 0.67 and Mg# = 0.55), tremolite, calcite and dolomite. Tremolites were formed by retrograde metamorphism of hornblendes. Calcite and dolomite occur as late-stage veins. Very high amount of primary hydrous phases (~94 vol % hornblende and chlorite), as well as peculiar mineralogical and chemical characteristics of the Naein ophiolite mantle hornblendites, do not match a magmatic origin. They are possibly products of the reaction between mantle peridotites and seawater-originated supercritical fluids, rich in silicate components. The presence of primary hydrous phases (hornblende and chlorite) may reveal high activity of H2O in the involved solution. The chemical composition of chromite in the hornblendites is near to the average chromite composition from the surrounding harzburgite and dunite. This suggests that the main source of Cr should be chromites of nearby peridotites, which were totally or partly dissolved by hydrothermal fluids. The positive anomaly of Eu in the chondrite-normalized REE patterns of hornblendes, high modal abundance of Ca-rich hornblende, as well as presence of calcite and dolomite, point to seawater ingression through the gabbros in to the uppermost mantle peridotites. The higher value of MgO than CaO, presence of high-Cr chromite and Cr-enrichment of hornblendes and chlorites indicate a higher contribution of peridotites rather than gabbros to the chemical characteristics of the involved fluids. This study shows that circulation of possibly seawater-derived high temperature hydrous fluids in the upper mantle can leach and provide necessary elements to form hornblendite in joints and cracks of the uppermost mantle.
Melt impregnated plagioclase lherzolites from the Nain mélange, central Iran, contain pyroxenes enriched and chemically zoned with Sr. Pyroxenes from the lherzolite and the clinopyroxenite seams, which have been precipitated from the impregnating melt, show similar trace element geochemical characteristics, including a similar Sr anomaly. The associated plagioclase, precipitated from the impregnating melt, has been selectively altered to isotropic saussurite. Strontium concentration increases in the pyroxenes from the core to the rim and toward crosscutting saussurite trails in orthopyroxene porphyroclast cores. The highest Sr content (up to 10.8 ppm in clinopyroxene and 3.8 ppm in orthopyroxene) is found in the finer pyroxenes surrounded by thicker saussurite layers. The Sr enrichment within pyroxenes is neither caused by metasomatism nor modified by fluids involved in hydrothermal alteration because pyroxenes are extremely depleted in fluid-mobile and light rare earth elements. Also, Sr enrichment cannot be related to the melt impregnation, since the Sr supply from the impregnating melt was consumed by plagioclase crystallization. The Sr enrichment in the pyroxene postdates the melt impregnation and is due to the relatively high-temperature (375 °C-850 °C) of saussuritization, that is, the breakdown of plagioclase. Plagioclase decomposition has released appreciable amounts of Sr to enrich adjacent pyroxenes. Saussurite shows significantly lower Sr contents than the plagioclase. Sr enrichment in peridotite pyroxenes, which is ascribed primarily to the metasomatism of slab-derived fluids, should be treated carefully, particularly when altered plagioclase is present.
The Oligocene alkaline basalts of Toveireh area (southwest of Jandaq, Central Iran) exhibit northwest–southeast to west–east exposure in northwest of the central-east Iranian microcontinent (CEIM). These basalts are composed of olivine (Fo70–90), clinopyroxene (diopside, augite), plagioclase (labradorite), spinel, and titanomagnetite as primary minerals and serpentine and zeolite as secondary ones. They are enriched in alkalis, TiO2 and light rare earth elements (La/Yb = 9.64–12.68) and are characterized by enrichment in large ion lithophile elements (Cs, Rb, Ba) and high field strength elements (Nb, Ta). The geochemical features of the rocks suggest that the Toveireh alkaline basalts are derived from a moderate degree partial melting (10–20%) of a previously enriched garnet lherzolite of asthenospheric mantle. Subduction of the CEIM confining oceanic crust from the Triassic to Eocene is the reason of mantle enrichment. The studied basalts contain mafic-ultramafic and aluminous granulitic xenoliths. The rock-forming minerals of the mafic-ultramafic xenoliths are Cr-free/poor spinel, olivine, Al-rich pyroxene, and feldspar. The aluminous granulitic xenoliths consist of an assemblage of hercynitic spinel + plagioclase (andesine–labradorite) ± corundum ± sillimanite. They show interstitial texture, which is consistent with granulite facies. They are enriched in high field strength elements (Ti, Nb and Ta), light rare earth elements (La/Yb = 37–193) and exhibit a positive Eu anomaly. These granulitic xenoliths may be Al-saturated but Si-undersaturated feldspar bearing restitic materials of the lower crust. The Oligocene Toveireh basaltic magma passed and entrained these xenoliths from the lower crust to the surface.