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Abstract

This ophiolite has experienced several metamorphic events since its formation. During Jurassic times, a regional metamorphism (M1) caused the formation of amphibolitic rocks from the oceanic basic rocks. These amphibolites were often covered by skarns formed during M2 metasomatism of carbonates. There are also evidences claiming a retrograde regional metamorphism (M3) that has affected both skarns and amphibolites. Additionally, some less highly metamorphosed sheeted dykes and pillow lavas occur in this ophiolite. Based on their geochemistry they were originally different from the amphibolitic rocks. Therefore, two phases of magma generation can be recognized: The first one, in Lower Jurassic, led to the formation of the basaltic protolith of the amphibolites. The second one, in Upper Cretaceous, caused the generation of sheeted dykes and pillow lavas.
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... In both ophiolitic mélanges, metaophiolitic rocks include metagabbros, metaplagiogranites, amphibolites, banded metacherts, and successions of marbles and schists (Sharkovski et al., 1984;Shirdashtzadeh et al., 2010). Shirdashtzadeh et al. (2010) and Torabi et al. (2011b) suggested that the protholiths of the amphibolitic rocks from both Nain and Ashin ophiolites were represented by diabasic dykes and basaltic pillow lavas both showing normal mid-ocean ridge affinity (N-MORB). ...
... In both ophiolitic mélanges, metaophiolitic rocks include metagabbros, metaplagiogranites, amphibolites, banded metacherts, and successions of marbles and schists (Sharkovski et al., 1984;Shirdashtzadeh et al., 2010). Shirdashtzadeh et al. (2010) and Torabi et al. (2011b) suggested that the protholiths of the amphibolitic rocks from both Nain and Ashin ophiolites were represented by diabasic dykes and basaltic pillow lavas both showing normal mid-ocean ridge affinity (N-MORB). However, Shirdashtzadeh et al. (2011) also proposed an IAT affinity for the amphibolites. ...
... According to these authors, metacherts and marbleschists successions can be regarded as the original sedimentary cover of the pillow lava basalts. Shirdashtzadeh et al. (2010) suggested that the metamorphic slices included within the Nain and Ashin mélanges represent relics a Jurassic oceanic crust that underwent a Late Cretaceous metamorphic event (Shafaii Moghadam et al., 2009). ...
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The Nain and Ashin ophiolites consist of Mesozoic mélange units that were emplaced in the Late Cretaceous onto the continental basement of the Central-East Iran microcontinent (CEIM). They largely consist of serpentinized peridotites slices; nonetheless, minor tectonic slices of sheeted dykes and pillow lavas - locally stratigraphically associated with radiolarian cherts - can be found in these ophiolitic mélanges. Based on their whole rock geochemistry and mineral chemistry, these rocks can be divided into two geochemical groups. The sheeted dykes and most of the pillow lavas show island arc tholeiitic (IAT) affinity, whereas a few pillow lavas from the Nain ophiolites show calc-alkaline (CA) affinity. Petrogenetic modeling based on trace elements composition indicates that both IAT and CA rocks derived from partial melting of depleted mantle sources that underwent enrichment in subduction-derived components prior to melting. Petrogenetic modeling shows that these components were represented by pure aqueous fluids, or sediment melts, or a combination of both, suggesting that the studied rocks were formed in an arc-forearc tectonic setting. Our new biostratigraphic data indicate this arc-forearc setting was active in the Early Cretaceous. Previous tectonic interpretations suggested that the Nain ophiolites formed, in a Late Cretaceous backarc basin located in the south of the CEIM (the so-called Nain-Baft basin). However, recent studies showed that the CEIM underwent a counter-clockwise rotation in the Cenozoic, which displaced the Nain and Ashin ophiolites in their present day position from an original northeastward location. This evidence combined with our new data and a comparison of the chemical features of volcanic rocks from different ophiolites around the CEIM allow us to suggest that the Nain-Ashin volcanic rocks and dykes were formed in a volcanic arc that developed on the northern margin of the CEIM during the Early Cretaceous in association with the subduction, below the CEIM, of a Neo-Tethys oceanic branch that was existing between the CEIM and the southern margin of Eurasia. As a major conclusion of this paper, a new geodynamic model for the Cretaceous evolution of the CEIM and surrounding Neo-Tethyan oceanic basins is proposed. © 2019 China University of Geosciences (Beijing) and Peking University
... In the Yazd Block (Figure 1a), the Nain Mesozoic ophiolitic zone is a remnant of Neo-Tethys oceanic crust, occurring between Tertiary volcanic and sedimentary rocks to the north of Nain city (e.g., Davoudzadeh 1972) (Figure 1b). This ophiolitic mélange was formed along Nain-Baft fault, between the Sanandaj-Sirjan Zone and the CEIM, some time in late Lower Cretaceous-Palaeocene (e.g., Glennie 1992;Torabi 2004;Reichert 2007;Shirdashtzadeh et al. 2010Shirdashtzadeh et al. , 2011Shirdashtzadeh et al. , 2014. The Nain ophiolitic mélange (Figure 1c) is a mixture of various oceanic sedimentary, igneous and metamorphic lithologies, obducted onto the northwestern border of Yazd Block (e.g., Shirdashtzadeh et al. 2010Shirdashtzadeh et al. , 2014Shirdashtzadeh et al. , 2015Shirdashtzadeh et al. , 2020b. ...
... This ophiolitic mélange was formed along Nain-Baft fault, between the Sanandaj-Sirjan Zone and the CEIM, some time in late Lower Cretaceous-Palaeocene (e.g., Glennie 1992;Torabi 2004;Reichert 2007;Shirdashtzadeh et al. 2010Shirdashtzadeh et al. , 2011Shirdashtzadeh et al. , 2014. The Nain ophiolitic mélange (Figure 1c) is a mixture of various oceanic sedimentary, igneous and metamorphic lithologies, obducted onto the northwestern border of Yazd Block (e.g., Shirdashtzadeh et al. 2010Shirdashtzadeh et al. , 2014Shirdashtzadeh et al. , 2015Shirdashtzadeh et al. , 2020b. Several outcrops of ophiolitic M-type granitoids, including tonalitic dykes and plagiogranites, with contrasting age and origins are exposed along this ophiolitic zone. ...
... This is because its U-Pb zircon age (448 Ma, Ordovician) is much older than Mesozoic-Cenozoic ages (~ Early Jurassic to Eocene; e.g. Glennie 1992;Torabi 2004;Reichert 2007;Shirdashtzadeh et al. 2010Shirdashtzadeh et al. , 2011Shirdashtzadeh et al. , 2014Shirdashtzadeh et al. , 2020b suggested for subduction, closure and obduction of Neo-Tethys oceanic crust. Regarding the mélanges classification on the basis of processes and tectonic settings of their formation (Festa et al. 2010), then the Nain ophiolitic mélange is analogous to subduction and collision mélanges, generated by tectonic and gravitational processes during which exotic blocks are recycled from other previously formed mélanges. ...
Article
This study investigates relicts of some granitic Gondwanan basement unexpectedly outcropping in the northwest of Central-East Iranian Microcontinent (CEIM) and incorporated into an ophiolitic mélange. Based on petrographical (e.g. high modal content of muscovite (~10 vol.%), absence of hornblende, inherited zircons (>541 Ma)), geochemical (peraluminous and calc-alkaline S-type affinity, high silica, high ‘light rare earth element (LREE)/heavy rare earth element (HREE)’ ratios, negative Nb and Ti anomalies), and geochronological (magmatic zircon age ~448 Ma) results, it is an Ordovician anatectic granite formed from a sedimentary source during crustal thickening in a syn-collisional setting. It shows some signatures of metamorphic deformation (cataclastic fabric, quartz bulging recrystallization, and foliation) likely developed in the Devonian (~410 Ma). The U-Pb zircon ages from this granite are analogous to the other Ordovician collision-related magmatic events in the CEIM (Chahak to Airekan, Balvard). Our results confirm that Cadomian subduction and closure of the Proto-Tethys Ocean to the north of the Gondwana supercontinent resulted in crustal thickening during Ordovician collision-related magmatism and Devonian-Carboniferous regional metamorphism in the CEIM.
... In addition, pillow lavas are associated with sheeted basaltic flows and radiolarian chert. NOM is considered to have witnessed several magmatic and metamorphic events related to multiple phases of magma generation during lower Jurassic to upper Cretaceous time 20 . ...
... NOM experienced variable degrees of alteration and fragmentation, owing to shear movements 18,20 . The present study reveals that at the base of Nain ophiolite, a sheared (fault-controlled) tectonized peridotite ( Figure 2 a) is well-developed, which is sometimes found to be intruded by coarse-grained pegmatitic gabbroic dykes (Figure 2 b). ...
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The present study documents detailed mineral chemistry perspective of Nain ophiolite mélange (NOM) of Central Iran with an aim to deciphering the mineral systematics and understanding geothermobarometric equilibration. The NOM covers ~600 km2 and is located at the northwest margin of Central Iranian Microcontinental block. NOM is represented by a sheared, tectonized and serpentinized peridotite intruded by coarse-grained pegmatitic gabbroic dykes, layered gabbro, sheeted dolerite dykes (with typical rodingite alteration) and pillow basalts. Plagioclase in pillow basalt is albitic and indicates its spilitic affinity, while pyroxene is typically quad pyroxene (augite to diopside). Amphiboles belong to calcic group and range from actinolite to magnesio hornblende. Ilmenite is the characteristic opaque phase. Clinopyroxene thermometry records a temperature span of 1100– 1300°C, while amphibole thermometry records 979– 1145°C. Two-feldspar thermometry also records a similar thermometric range. Amphibole barometry shows higher pressure of equilibration for mantle pegmatite in general and a very low equilibration pressure for sheeted dyke. Pyroxene compositions typically indicate a calc-alkaline basaltic (orogenic) parentage. NOM signifies lherzolite ophiolite type in a chromite-free environment and it is analogous to an idealized ophiolite succession, but has been emplaced in the form of discrete tectonic mélange.
... In addition, pillow lavas are associated with sheeted basaltic flows and radiolarian chert. NOM is considered to have witnessed several magmatic and metamorphic events related to multiple phases of magma generation during lower Jurassic to upper Cretaceous time 20 . ...
... NOM experienced variable degrees of alteration and fragmentation, owing to shear movements 18,20 . The present study reveals that at the base of Nain ophiolite, a sheared (fault-controlled) tectonized peridotite ( Figure 2 a) is well-developed, which is sometimes found to be intruded by coarse-grained pegmatitic gabbroic dykes (Figure 2 b). ...
... Presence of Paleo-Tethys and Neo-Tethys related ophiolitic suits in Iran point to the spreading and closure of oceanic crusts during Paleozoic and Mesozoic eras [26,34,35,[38][39][40][41]. Plagiogranites are one of the important rock units of Iranian ophioites. ...
... The final closure of this oceanic crust and obduction has occurred in Paleocene to Eocene. Nearly the same geological history of two various magmatic phases in an ophiolite is reported by Shirdashtzadeh et al. [34] about of Naein (Nain) ophiolite, which is situated at the south of the Ashin ophiolite (Fig. 1). In the Naein ophiolite, two Early Jurassic and Cretaceous magmatic phases recognized. ...
Article
The Ashin ophiolite is situated in the western part of Central Iran and presents two stages of Jurassic and Cretaceous spreading. The Ashin ophiolite represents fragments of the Neo-Tethys oceanic lithosphere. Plagiogranite intrusions of this ophiolite have good exposures. Plagiogranites of Cretaceous are more fresh than the metamorphosed samples of Jurassic. The main minerals of plagiogranites from the Ashin ophiolite are plagioclase, quartz and amphibole. Plagiogranites of the Jurassic have tholeitic nature with higher amounts of amphibole, \({\text{F}}{{{\text{e}}}_{2}}{\text{O}}_{3}^{*},\) TiO2, Co and lower values of Mg#, Th and Sr than the Cretaceous calc-alkaline plagiogranites. The chondrite-normalized REE patterns of these plagiogranites are characterized by higher values of REEs and negative Eu anomalies for the Jurassic samples and low values of REEs and positive Eu anomalies for the Cretaceous ones. Very low values of HREEs in the Cretaceous plagiogranites indicates a non-peridotitic source rock. We suggest that the Jurassic plagiogranites are formed by fractional crystallization of a low-K tholeitic magma; and the adakitic Cretaceous plagiogranites are formed by partial melting of an amphibolite in the subducting slab. Geochemical criteria of the Ashin plagiogranites indicate changing the Ashin ophiolite tectonic setting from a mid-ocean ridge system in the Jurassic to a supra-subduction zone in the Cretaceous.
... Finally, CEIM located between Arabia (Gondwana) and Turan plates (Laurasia) by the Cenozoic closure of Neo-Tethys and Alpine-Himalayan orogeny. The suture zones are defined by ophiolitic outcrops from Baft, Naein, Ashin and Jandaq (e.g., Shafaii Moghadam and Stern, 2014;Shirdashtzadeh et al., 2010Shirdashtzadeh et al., , 2014etc.). ...
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In the center of Iran, Central-East Iranian Microcontinent (CEIM) was a part of Gondwana supercontinent in pre-Palaeozoic during Pan-African Orogeny. It is a zone of several tectonomagmatic and metamorphic episodes from Neoproterozoic to earliest Palaeozoic. In the north of CEIM, Airekan granite is a relic of Paleozoic magmatism in northern Gondwanaland. It is potentially a significance pluton that preserved the magmatic/metamorphic evolution of the active continental margin of the vanished Ocean of Proto-Tethys. This pluton is characterized by SiO2> 70 wt%, A/CNK>1, Rb >~160 ppm, Y <50 ppm, Th <30 ppm, Th/Ta >5. The δ18O value of quartz (average ~11.86 ‰; n=8), calculated δ18O value of whole rock (average ~10.75 ‰), absence of hornblende, presence of biotite, muscovite and inherited zircon, higher content of orthoclase, and microgranular granitic enclaves are all consistent with it being a continental collision-related peraluminous S-type granite. Th+U versus 206Pb/238U ratios of zircons correlate with decreasing crystallization temperatures related to the Cambrian-Ordovician magmatic events preserved in the inherited and magmatic zircons, toward their Devonian metamorphic overgrowth occurred via Caledonian Orogeny. It is probably formed by mica-dehydration melting at ~ 690-820 °C/ 10-15 kbar, and it is geochronologically and geochemically comparable with other Gondwanan collision-related granitic plutons (along north of Africa, Turkey, Iran to Himalaya).
... The subduction of the main branch of the Neo-Tethyan oceanic crust beneath central Iran began in the Triassic and finally closed in the Eocene by continental collision Torabi, 2009 Nazemei et al., 2018). In the second stage, during the Late Triassic, the CEIM was encircled by the eastern branch of the Neo-Tethys, which is supported by the presence of ophiolitic rocks around the CEIM (Shirdashtzadeh and Torabi, 2009) (Figure 1). Magmatism related to the subduction of the eastern branch of the Neo-Tethyan oceanic crust occurs throughout the CEIM. ...
... These ophiolites are highly dismembered and sliced, and characterized as tecc tonic melanges. The spreading of this ocean comm menced in Triassic and terminated in the Eocene ( Shirdashtzadeh et al., 2010;Torabi, 2009aTorabi, , 2009cTorabi, , 2010. Therefore, the former subduction of the Ashin oceanic crust and dehydration or melting of the subb ducted slab is the best candidate for the mechanism of upper mantle metasomatism and shoshonitic volcann ism. ...
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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.
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The amphiboles are the most complex group of rock-forming minerals, exhibiting wide chemical variation and a bewildering variety of parageneses. They are common constituents across the complete range of igneous rocks. In sedimentary rocks, amphiboles occur both as detrital and authigenic phases. In metamorphic rocks, amphiboles are important constituents from very low grade to high grade and over a wide variety of rock compositions. This study summarizes the existing body of knowledge on the crystallography and crystal chemistry of the amphiboles. It is shown that amphiboles belong to five principal structure-types, with space groups C2/m, P2/a, P2//1/m, Pnma, and Pnmn, but the C2/m and Pnma structures are by far the most common. The M(4) site is of major importance in amphibole crystal chemistry. Spectroscopic analysis data a presented and analyzed, and cation distributions in amphiboles, factors affecting cation ordering, oxidation-dehydroxylation characteristics, electrical, magnetic, and elastic properties, and deformation behavior in amphiboles are discussed in detail.
Chapter
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ABSTRACT Volcanic arc basalts are all characterized by a selective enrichment in incompatible elements of low ionic potential, a feature thought to be due to the input of aqueous fluids from subducted oceanic crust into their mantle source regions. Island are basalts are additionally characterized by low abundances [for a given degree of fractional crystallization) of incompatible elements of high ionic potential, as feature for which high degrees ot'melting, stability of rninor residual oxide phases, and remelting of depleted mantle are all possible explanations. Calc-alkaline basalts and shoshonites are additionally characterised by enrichment of Th, P and the light REE in addition to elements of low ionic potential, a feature for which one popular explanation is th contamination of their mantle source regions by a melt derived from subducted sediments. By careful selection of variables, discrimination diagrams can be drawn which highlight these various characteristics and therefore enable volcanic arc basalts to he recognized in cases where geological evidence is ambiguous. Plots of Y against Cr, K[Yb, Ce/Yb, or Th/Yb against Ta/Yb, and Ce/Sr against Cr are all particularly successful and can be modelled in terms of vectors representing different petrogenctic processes. An additional plot of Ti/Y against Nb/Y is useful for identifying 'anomalous' volcanic arc settings such as Grenada and parts of the Aleutian arc. Intermediate and acid rocks from volcanic are settings can also be recognized using a simple plot of Ti against Zr. The lavas from the Oman ophiolite complex provide a good test of the application of these techniques. The results indicate that the complex was made up of back-arc oceanic crust intruded by the products of volcanic arc magmatism.
Chapter
Now that the characteristics of modern active and inactive continental margins are being studied, it is appropriate to see whether or not they can be identified in supposed ancient continental margins that have been involved in later crogeny. If so, then the nature of these margins can be determined and the results applied to the study of the pre-orogenic evolution of the region. The present paper is an attempt to apply this procedure to the southern part of the Tethyan orogenic belt, from the Mediterranean Sea eastward to Indonesia.
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The Nain ophiolite is a highly dismembered ophiolite complex cropping out at the north of the Nain town to the west of central Iran. The igneous rocks of this complex consist of both mantle and crustal suites and include serpentinized peridotites, peridotites, harzburgites associated with dunite and lherzolite, pegmatitic and isotropic gabbros, plagiogranites, sheeted dikes and pillow lavas. Several pyroxenite, wehrlite and rodingite dikes are present in the ultrabasic rocks. The sheeted dikes include subalkaline basalts, basaltic andesites and andesites. Their magma was of sub-alkaline (low potassium tholeiite) type and they are chemically similar to island arc tholeiitic basalts. The N-MORB-normalized incompatible elements for the sheeted dike samples indicate depletion in most of the high field strength elements (HFSE). The concentrations of the large ion lithophile elements (LILE) in these rocks are all greater than those in the N-MORB. Significant chemical characteristics of the these rocks are the positive anomaly for Th and negative anomaly of Nb that are considered to represent a subduction zone component. The chondrite-normalized rare earth element (REE) patterns of these rocks show HREE enrichment and LREE depletion [(LaN/SmN)ave = 0.63]. Their geochmistry also shows that the primary melt derived from high degrees of partial melting of a mantle source previously depleted with respect to the source of mid-ocean ridge basalts, and were subsequently enriched by aqueous fluids driven off subducted oceanic lithosphere in an arcbasin setting. We conclude that the Nain ophiolite is a supra-subduction zone type ophiolite.
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The ophiolites of the Alpine folded region of Iran are examined as an indication of the extent of ancient oceanic realms bordered by ancient continental margins. They are grouped into four geographically and geologically distinct zones, differing from each other in composition, structure, and age. The possibility of these four zones marking former continental margins is then checked against the background of the general structural evolution of Iran. It is concluded that during Paleozoic time Iran was an extension of the Arabian platform, and thus a part of Gondwanaland, possibly bordered by a “Paleo-Tethys” in the north, along the present northern foot of the Alborz Range. Closing of the “Paleo-Tethys,” short of a possible modern relict in the South Caspian depression, may have been related to Hercynian orogenic processes in the ScythoTuranian plate to the north and was completed by Liassic time. A rift in the Arabian-Iranian platform along the “Main Zagros Thrust line” in the early Mesozoic or late Paleozoic was followed by the formation of a “Neo-Tethys” in the south, possibly interrelated and simultaneous with the closing of the “Paleo-Tethys” in the north. Further breakup of Iran led to the formation of several branch troughs of the “Neo-Tethys” and temporary isolation of a “Central-and-East Iranian Microcontinent” in the late Mesozoic. Closing of the “Neo-Tethys” in the early Maestrichtian was followed by reintegration of the “microcontinent” and folding of central and north Iran during the Paleocene paroxysm of the Alpine orogeny.
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Heat capacities (5-380 K) have been measured by adiabatic calorimetry for five highly disordered alkali feldspars (Ab99Or1, Ab85Or15, Ab55Or45, Ab25Or75 and Ab1Or99). The thermodynamic and mineralogical implications of the results are discussed. The new data are also combined with recent data for plagioclases in order to derive a revised expression for the two-feldspar thermometer. T calculated from the revised expression tend to be higher than previous calculations.-J.A.Z.
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The revised quasi-crystalline model was used to calculate phase relations in the granite (An-Ab-Or-Q-(H2O)) system and bounding binary and ternary subsystems for P and water contents of relevance to felsic magma crystallization. The predicted phase relations have been compared with available H2O-saturated experimental phase equilibrium data on the subsystems Ab-Or-H2O, An-Ab-H2O and An-Or-H2O and indicate good agreement. Comparisons of experimental data against calculated phase relations for the boundary subsystems Ab-Or-Q-H2O, Ab-An-Q-H2O, Ab-Or-An-H2O and An-Or-Q-H2O also indicate favourable agreement, particularly for the An-poor regions. Use of the model in a predictive capacity to calculate H2O-undersaturated phase relations for the granite system and relevant subsystems, indicates that the effect of decreasing P or increasing H2O content is the differential contraction of the Q + L field and concomitant shift of the feldspar/quartz cotectics towards more Q-rich melt compositions. Although H2O- saturated phase relations also indicate a shift of the feldspar/quartz cotectics toward the Q apex with decreasing P, the magnitude of this shift is less than for H2O-undersaturated conditions.-J.M.H.