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Zircon U-Pb geochronology and emplacement history of intrusive rocks in the Ardestan section, central Iran. Geologica Acta: an international earth science journalvol. Along this belt, Eocene volcanics and some gabbroic to granitic bodies crop out. The main rock types of the studied intrusion are granite, granodiorite, and diorite. They have geochemical features typical of magnesian, calc-alkaline, metaluminous to slightly peraluminous granites and I-type intrusive rock that have a strong enrichment in Large-Ion Lithophile LIL elements e.

Description

Zircon Hf-isotope compositions show that during Cadomian magmatic arc activity, juvenile arc magmas interacted with reworked Archean crust to generate the Ediacaran-Cambrian igneous rocks.

Our document both inheritance of old zircons and the presence of zircons with juvenile atures in NW Iran, suggesting that the geotectonic setting for the Cadomian rocks was an Ediacaran iranian Collins MS dating magmatic arc and probably a neighboring back-arc basin. The occurrence of Ediacaran ophiolitic slices in NW Iran may provide evidence of back-arc basin opening at that time. The melts responsible for the formation of these rocks had an essentially juvenile ature with minor contamination by Archean to Cadomian middle-lower continental crust. Continuous convergence between Arabia and Iran was accompanied by the transition of SW Eurasia from a compressional to an extensional convergent plate margin in Eocene-Oligocene times, leading to orogenic collapse, core-complex formation, exhumation of Cadomian crust and a major increase in arc magmatism.

Collisional orogens, the mountain belts formed when continents collide, are among the most impressive manifestations of plate tectonics and are an ideal natural laboratory in which to study tectonic and magmatic processes. The central Iranian segment of the Caucasus-Bitlis-Zagros Orogen provides opportunities for a comprehensive understanding of the evolution of continental collision zones and their magmatic outputs.

Understanding of the ongoing collision and magmatic activity requires the recognition and dating of individual magmatic pulses, which are predominant in the Iranian crust. The central part of Iran has undergone a series of subduction events from Ediacaran to late Cenozoic, which have played major roles in the growth and modification of the crust.

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Simplified geological map of Iran-Turkey showing the distribution of Eocene magmatism, Paleozoic-Mesozoic ophiolites, Cadomian basement rocks and core complexes. Late Neoproterozoic-early Cambrian crustal evolution was influenced by Pan-African collisional events that formed the supercontinent Gondwana Powell et al. Much of this crust rifted away from Gondwana in early Paleozoic times Paleotethys opening and during the late Paleozoic Neotethys opening and was accreted to the southern flank of Eurasia.

Moreover, zircons in detrital sediments of the Tarim block, with a major peak in U-Pb ages at ca Ma, reflect the widespread Cadomian magmatism of northern Gondwana and may show a Gondwanan affinity for the Tarim block Ma et al.

Introduction

Cadomian crust now mostly dominates southern Eurasia west of Afghanistan Garfunkel,although this crust has been overprinted by younger deformation and igneous activity. The basement of Iran and Turkey includes abundant evidence of Cadomian crust about which we have much to learn. Cadomian exposures are also abundant in Iran; they are documented from regions in western Golpayegannorthwestern Khoy-Salmas, Zanjan-Takabnortheastern Torud, Taknarnorthern Lahijan granitesand central Iran Saghand Hassanzadeh et al. Understanding the age, distribution and especially derivation of late Neoproterozoic-Cambrian crustal rocks is a key to reconstructing the tectonic evolution of SW Asia during Neoproterozoic-early Paleozoic times.

Our understanding of the Cadomian crust is advancing rapidly, but remains incomplete, especially in Iran. Reliable geochronological and other isotopic data are not yet available for the meta-igneous rocks of most suspected Cadomian terranes in Iran. It is not yet clear if all the igneous rocks have late Neoproterozoic-early Cambrian ages or if older Neoproterozoic ages are also present. It is also unclear whether these older rocks have still older protoliths, and when real crustal growth might have occurred. The younger tectono-magmatic imprints that have affected these basement areas are also matters of ongoing debate.

Nearly all of the Cadomian terranes in Iran show evidence of late-stage magmatic intrusions during the Eocene-Miocene, which may be related to crustal extension during the main phase of Tethyan subduction beneath Iran in the Eocene or to Arabia-Eurasia collision in the Miocene Karagaranbafghi et al. The source and mechanism of this magmatism remains ambiguous as there are no detailed geochemical-isotopic studies, especially in situ Hf isotopes that can help us to understand the nature of the melts and to recognize if crustal growth or recycling were the main causes of the Eocene-Miocene magmatism in Iran.

Geochronologically, there is no linkage between the Cadomian basement rocks and the younger Cenozoic magmatic pulses, but as most of the Cadomian terranes contain abundant Cenozoic magmatic intrusions, it is important to know if there is a Cadomian lower-crustal source iranian Collins MS dating the genesis of the younger magmatic rocks through partial melting or assimilation, or if the Cenozoic rocks have different magmatic sources.

From field observations it is difficult to distinguish any differences between the Cadomian gneissic rocks and Cenozoic meta-granitoids with gneissic appearance. However, combined with detailed field geology, U-Pb geochronology is a powerful tool with which to understand any ambiguities concerning the crustal architecture of the region in the sense of the relative abundances of the Cadomian versus younger plutons. In this study, we aim to understand the construction and evolution of the Iranian Late Neoproterozoic-Cambrian crust and the subsequent tectono-magmatic events affecting it, based on the ificance of new in situ zircon U-Pb ages as well as zircon Hf-O-trace element and bulk-rock Sr-Nd isotopic data from the Zanjan-Takab metamorphic complex in NW Iran Fig.

These data are then used to constrain the main periods of crustal growth in Iran. Zanjan-Takab is the best place for this study, because it is a core complex and therefore all the deep crustal rocks are readily accessible. This is also a place where the contacts between the Cadomian rocks and Cenozoic intrusives have been preserved at iranian Collins MS dating present-day erosion level.

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But how clear is the stratigraphic similarity of older rocks—Late Neoproterozoic-Cambrian—in Iran and Arabia? The eastern segment, adjacent to the Zagros orogen, should be more similar to the Iranian block, if the two were once ed together and then rifted apart. The eastern crystalline basement is unconformably overlain by a thick sequence of Cryogenian to Ediacaran volcano-sedimentary rocks of the Huqf Supergroup Allen, ; Bowring et al. Its seems the only correlative rocks in the Arabian Plate—SW of Zagros—are igneous boulders within the Hormuz salt domes, whose crystallization ages vary between ca Ma with older inheritance Faramarzi et al.

However, most parts of the Arabian continental crust are deeply buried under Paleozoic and Mesozoic sediments.

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Lower-crustal xenoliths, brought to the surface by Neogene basalts in NE Jordan North Arabiayield ages of to Ma, and suggest a fragment of Cadomian crust exists beneath the eastern basement Stern et al. The Ediacaran-Cambrian basement rocks of Iran Fig. There are several Eocene, Oligocene and Miocene core complexes, distributed throughout Iran and Turkey. The first phase of extension and core-complex formation started in Eocene time.

Cadomian basement in central Iran—the Saghand area—was exhumed during Eocene extension, followed by early Miocene erosion Kargaranbafghi et al. This complex is in fault contact with Miocene and younger volcano-sedimentary rocks and iranian Collins MS dating a core complex that formed in response to the Arabian-Iranian continental collision Stockli, ; Gilg et al. The Zanjan-Takab complex seems to be geochronologically similar to other Iranian basement exposures for which Cadomian ages Ma have been obtained Fig.

This complex consists of various types of meta-granite, ortho- and para-gneissic rocks, metabasites, amphibolites, calc-silicates, psammitic to pelitic schists, meta-ultramafic rocks and migmatites Fig. Undated meta-ultramafic rocks, meta-gabbros and metabasites may represent an ophiolite associated with the Prototethys Ocean Hajialioghli et al. Pelitic schists occur around Poshtuk village and south of Qozlu village Fig. Migmatites occur among the psammitic gneisses and amphibolites and are common NE of Takab, especially near Qare-Naz village.

Unmetamorphosed supracrustal units, including late Neoproterozoic dolomites, shales, sandstones, tuffs and rhyolites of the Qaradash, Bayandor and Soltanieh Formations, along with early Cambrian shales, sandstones, dolomites and limestones of the Zaigun and Barut Formations, are also common NW of Takab, where they are in fault contact with metamorphic rocks and associated granitoids Fig. Orthogneisses occur mostly around Moghanlu village and have Ma ages Fig. We collected representative samples from these three sections, and hereafter we will discuss the geology of these regions separately.

These are the best well-defined exposures. Schematic lithological columns showing the relationships between rock units in the target areas and zircon U-Pb ages obtained during this study. Undated granitoid intrusions are abundant in the Takab-Zanjan complex Fig. These plutonic rocks reflect magmatic activity in the Urumieh-Dokhtar belt.

As most parts of the Zanjan-Takab complex have either been highly weathered and are covered by farmland or have been affected by intensive tectonics, the structural architecture of this complex has been obscured. A iranian Collins MS dating description is beyond the scope of this study, but to show better the relationships of the exposed units, we have drawn schematic lithological columns to show the approximate locations of the dated samples Fig. Orthogneisses are the predominant rocks in this area and show pristine intrusive relationships with their host rocks, which include amphibolites, meta-sedimentary gneisses and pelitic schists Fig.

Petrographically, they vary from granitic to granodioritic and even tonalitic, and contain enclaves of amphibolite, granitic gneiss, retrograded granulite, biotite schist and calc-schist Fig. Gneissic, fine-grained aplitic to coarse-grained dioritic-granitic dikes crosscut the orthogneisses Fig.

Late Ediacaran-Cambrian granitic and granodioritic gneisses and their enclaves from these outcrops were sampled for U-Pb geochronology and geochemistry. Cenozoic see U-Pb section fine-grained granitic orthogneisses and meta-granitoids are present near Alam-Kandi village. They are very similar in appearance to their older late Ediacaran-Cambrian equivalents U-Pb ages; see next sections. They do not have clear contacts, as a screen of biotite schist, muscovite-hematite schist, phyllite and thin marble layers is present between them.

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We sampled meta-tonalites from near Alam-Kandi village for U-Pb geochronology and geochemistry. Field photographs of the Zanjan-Takab metamorphic rocks. Paragneisses, amphibolites and biotite schists are the main rock units of this area Fig. Highly deformed amphibolites with a stretching lineation defined by amphibole and feldspar occur along with the paragneisses.

Meta-sedimentary gneisses and amphibolites show evidence of partial melting leading to the formation of anatexites and diatexites Fig. Late-stage granitic dikes crosscut amphibolites and paragneisses Fig 4f. Orthogneissic rocks including granodioritic to dioritic and even trondhjemitic gneisses seem to be intruded into metamorphic rocks, with either biotite-dominated weak foliation S-tectonites or amphibole-dominated stretching lineation S-L tectonites.

Granodioritic gneisses, paragneisses and amphibolites were selected from this area for further U-Pb geochronology and geochemistry. Migmatites are widespread around Qare Naz village. Amphibolites and paragneisses with minor metapelitic schists, retrograde granulites and migmatitic gneisses are also common.

Orthogneisses are less abundant than psammitic gneisses. Paragneisses are interlayered with amphibolites, marbles and calc-schists. Leucosomes are millimeters to tens of centimeters long and vary from weakly to highly foliated Fig. Melanosomes are restites composed of massive and gneissic amphibolite, rich in amphibole, plagioclase and epidote with minor quartz.

Migmatites show schlieren, nebulitic, ptygmatic, stromatic, raft- and vein-like structures. These migmatites show evidence of Oligocene Ma partial melting Moghadam et al. Late-stage granitoid dikes crosscut gneissic amphibolites and paragneisses. Coarse-grained and flaser-like gabbroic-dioritic gneisses are common near Qara Dash village.

They have porphyroclastic textures with well-developed ultra-mylonitic shear zones Fig. Amphibole and feldspar augen are abundant. These rocks have intrusive contacts with slightly metamorphosed, garnet-bearing leucogranites. Less deformed Cadomian diorites with abundant small, late-stage Cenozoic granodioritic-dioritic lenses and granitic dikes are common near Qara Dash village. We sampled orthogneisses, paragneisses, leucosomes, granulites, dioritic to gabbroic gneisses and late-stage Iranian Collins MS dating dikes for U-Pb geochronology and geochemistry. Orthogneisses are petrographically granodioritic -tonalitic to granitic gneiss.

Granitic gneisses contain large K-feldspar grains av.

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Granodioritic -tonalitic gneisses contain large zoned plagioclase crystals av. Epidote overgrowths on allanite are common. Enclaves in orthogneisses include a range from biotite schists to granitic gneisses. Metamorphosed aplitic to granitic dikes with perthitic K-feldspar, quartz ribbons and minor biotite and plagioclase crosscut the orthogneisses.

Granulitic enclaves contain deformed quartz, plagioclase, orthopyroxene, clinopyroxene, garnet, amphibole, rutile and titanite. Meta-tonalites show slight deformation with large plagioclase, K-feldspar, quartz, hornblende, biotite, allanite and titanite. The older gneissic rocks plot predominantly in the field of granite to granodiorite and even monzodiorite whereas younger, late-stage intrusive rocks tend to plot in the granodiorite-tonalite fields.

Data for Iranian Cadomian rocks are from Moghadam et al.