The tectonic evolution of western Central Iran seen

Tectonophysics 651–652 (2015) 138–151

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The tectonic evolution of western Central Iran seen through detrital white mica Fariba Kargaranbafghi a,b,⁎, Franz Neubauer a,⁎, Johann Genser a a b

Department Geography and Geology, University of Salzburg, Salzburg, Austria Department of Geology, University of Yazd, Yazd, Iran

a r t i c l e

i n f o

Article history: Received 19 February 2014 Received in revised form 5 March 2015 Accepted 27 March 2015 Available online 10 April 2015 Keywords: Detrital mica Provenance Cimmerian orogeny 40 Ar/39Ar age dating Central Iran Source-sink

a b s t r a c t A first order survey of 40Ar/39Ar dating of detrital white mica from Jurassic to Pliocene sandstones has been carried out in order to reveal the tectonic evolution of blocks in Central Iran. The Central Iran block was believed to represent a stable Precambrian block. Our results indicate that: (1) Only a very small proportion of Precambrian but abundant Paleozoic and Mesozoic detrital white mica indicate the Phanerozoic, mostly Mesozoic age of metamorphic crust exposed in Central Iran. The oldest but scarce detrital white mica grains have ages ranging from 524 to 826 Ma heralding a Late Precambrian and Cambrian crystalline basement or cannibalism from older clastic successions. (2) Jurassic and Cretaceous sandstones from the west and east of the Chapedony fault yield different age spectra, with a dominance of Variscan ages (ca. 308–385 Ma) in the Biabanak unit west of the Chapedony fault compared to coeval sandstones from the block east of the Chapedony fault, where Variscan ages are subordinate and Cimmerian ages predominate. The micas from the Biabanak unit are most likely derived from the Variscan accretionary complex exposed in the Anarak–Jandaq areas further northwest. This result underlines the importance of a major block boundary identified as the Chapedony fault, which is in extension of a fault previously proposed. (3) Two stages of Cimmerian events are visible in our data set from Cretaceous and Paleogene sandstones, a cluster around 170 Ma and at ca. 205 Ma. These clusters suggest a two-stage Cimmerian evolution of the largely amphibolite-grade metamorphic Posht-e-Badam and Boneh Shurow complexes. (4) The youngest micas in Paleogene conglomerates have an age of ca. 100 Ma and are most likely derived from the base of the Posht-e-Badam complex. No record of the uplifted Eocene Chapedony metamorphic core complex has been found in Eocene and Pliocene clastic rocks. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The 40Ar/39Ar dating technique has been applied to detrital K-bearing minerals, most commonly white mica, and less frequently biotite (Aalto et al., 1998), K-feldspar (Copeland and Harrison, 1990) and amphibole (Cohen et al., 1995). Of these, white mica has proved the most versatile and has played a key role in unravelling palaeogeographies and the timescales involved in the sedimentary cycle in diverse geological settings (Kelley and Bluck, 1992; Renne et al., 1990). The success partly lies in the fact that white micas record the time at which the source block was experiencing temperatures of ca. 350–425 °C, and when in the sedimentary basin the eroded and deposited white micas are able to avoid complete or partial post-depositional setting. With this study, we demonstrate some important advantages as well as some limitations of the 40Ar/39Ar white mica method. The approach has been systematically applied to various stratigraphic levels of Jurassic to Neogene sedimentary basins of the Central Iran area (Figs. 1–3). Only few studies exist from Iran using the 40Ar/39Ar white mica system. Here, ⁎ Corresponding authors. E-mail address: [email protected] (F. Kargaranbafghi).

http://dx.doi.org/10.1016/j.tecto.2015.03.019 0040-1951/© 2015 Elsevier B.V. All rights reserved.

we compile the first-order results, which demonstrate the usefulness of the 40Ar/39Ar dating approach to reveal tectonic processes. Our results demonstrate the systematic change of source compositions from Jurassic to Neogene basins. The new data allow significant refinement of the Paleozoic, Mesozoic and Cenozoic tectonic evolution of western Central Iran, which is part of the Mesozoic Cimmerides orogen (Sengör, 1979; Wilmsen et al., 2009). Only one study on dating detrital minerals of Iran exists, which presents U–Pb ages of detrital zircon from Neoproterozoic to Cenozoic sandstones (Horton et al., 2008). Our study is aimed to complement this study.

2. 40Ar/39Ar single-grain dating of detrital white mica 40 Ar/39Ar dating of detrital white mica is a perfect tool (1) to demonstrate palaeogeographic relationships (e.g., Dallmeyer and Neubauer, 1994), (2) to constrain tectonic processes in the hinterland of sedimentary basins (Hodges et al., 2005; Kelley and Bluck, 1992); and (3) dynamics of sedimentary basins (Hodges et al., 2005; Najman et al., 2001). Dating is particularly useful when single grains are analysed, which avoids problems caused by mixing of different populations of

F. Kargaranbafghi et al. / Tectonophysics 651–652 (2015) 138–151

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Fig. 1. Simplified geological map of Iran; black dashed rectangles show the position of study area (Figs. 2, 3).

different age (Copeland and Harrison, 1990; Najman et al., 2001; Neubauer et al., 2007). The argon isotopic system of detrital white mica has been shown to be very resistant against mechanical and chemical weathering and sedimentary transport and is, therefore, very suitable for 40Ar/39Ar dating of detrital material (e.g. Clauer, 1981; Mitchell and Taka, 1984). In contrast, biotite is highly accessible to weathering and occurs only in minor amounts in clastic successions (see review in Rieser et al., 2005). Detrital white mica within clastic sediments originates from either pelitic metamorphic or plutonic source rocks, which were formed in the middle and lower levels of the continental crust. However, metamorphic source rocks like micaschist and gneiss predominate as only S-type granitoids comprise a significant proportion of white mica among plutonic rocks. Such S-type granitoids constitute a maximum of ca. 5–20% in present-day exposed plutonometamorphic basement, and its modal content of white mica is low (up to a maximum of ca. 5%) compared to micaschist (ca. 50–70%) and gneiss (ca. 5–20%). Consequently, white mica from metamorphic rocks should predominate, in average, the detrital mica population of sandstones. Furthermore, recycling of detrital white mica from older clastic successions could potentially contribute to the detrital white mica age populations although this effect was never investigated in detail. In low-grade metamorphic units, below metamorphic temperatures of ca. 400 to 450 °C, white mica is generally fine-grained (b200 μm) and

is increasingly coarse-grained above this temperature up to ca. 580–600 °C where white mica finally breaks down to K-feldspar and alumino-silicate (e.g., Spear, 1993). As isotopic data are generally obtained from grain sizes between 200 and 500 μm, the dated white mica originate generally from temperatures levels of 400–600 °C within the crust. In metamorphic units, white mica includes muscovite, paragonite and celadonite (or phengite) and mixtures between these end members (Rieder et al., 1998). According to experimental data, 40Ar/39Ar and K–Ar ages of white mica monitor the cooling of the crust of the hinterland below temperatures of 425 ± 25 °C (Harrison et al., 2009). In the past a range from about 350 to 450 °C was postulated as the approximate closure temperature of the argon isotopic system within white mica within regional metamorphic terrains (Hames and Bowring, 1994; Kirschner et al., 1996). The argon retention temperature is complicated by additional factors (Villa, 1998). These factors are, among others, duration of heating, cooling rate, grain size, deformation like kinking, and hydrothermal alteration. Usually, laser-probe single-grain 40Ar/39Ar dating using a highresolution gas mass spectrometer can yield total fusion age, which is equivalent to a conventional K–Ar age of the single grain. In appropriate cases, provided a relatively large grain size (200–500 μm), even a stepheating experiment can be performed, which allows the recognition of a thermal overprint on even a single grain. To detect a possible in-situ

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Fig. 2. Simplified geological map of western Central Iran showing the distribution of metamorphic basement complexes. Insert in lower left side shows tectonic framework of Central Iran. Modified after Ramezani and Tucker, 2003 and Bagheri and Stampfli, 2008.

thermal overprint, the step-wise heating of small multi-grain concentrates using ca. 10 to 20 grains is also applied. This allows the detection of small-percentage argon loss. Note that the loss of radiogenic argon could result from two possible causes: (1) A younger overprint on old micas in the crystalline complex at depth prior to exhumation, and (2) in-situ, post-depositional overprint in the sedimentary unit when the metamorphic overprint reaches a minimum of ca. 425 °C (Harrison et al., 2009). As erosion and sediment transport are generally rapid processes, the lag time is controlled mainly by exhumation rates in the source region. When upper, less metamorphic, sectors of the metamorphic/orogenic wedge reaches the surface, erosion of metamorphic minerals gives an age signal in the associated sedimentary sequence. Particularly during development of a compressional sedimentary basin (e.g., peripheral foreland basin or accretionary wedge), the lag time changes from high values at the base to low values at the top. 3. Geological setting The geology and especially the tectonic style of Iran are highly influenced by the development and history of the Tethyan region. The Tethyan region, which includes the Iranian Plate and the adjacent areas, underwent three major evolutionary stages. The first stage was the closing of the Paleotethys and rifting of the Neotethys from early Permian

to late Triassic times (Zanchi et al., 2009). With the second stage, the subduction process of the Neotethys and the collision of the Indian Plate with the Eurasian Plate from the Jurassic to the Paleogene began. The third and last stage is associated with the collision between the Arabian plate and the Eurasian plate from Paleogene to the present (Agard et al., 2011; Shufeng et al., 2002). The central Iranian segment separated from the Arabian plate along the line of the present High Zagros Zone (Agard et al., 2011; Alsharhan et al., 2001; Moix et al., 2008). The result of this process was the opening of the Neotethys. The closure of Neotethys started with the Late Cretaceous and proceeded into Cenozoic times. The Central Iranian Terrane is located northeast of the Zagros–Makran Neotethyan suture and its sub-parallel Cenozoic magmatic arc, between the convergent Arabian and Eurasian plates. Thus, due to the collision setting, continuous continental deformation processes affect Central Iran. Central Iran comprises three major crustal domains: the Lut Block, Tabas Block, and the Yazd Block (Fig. 1). The area of the Central Iranian terrane is surrounded and limited by faults and fold-and-thrust belts and Upper Cretaceous to Lower Eocene ophiolite and ophiolitic melange (Davoudzadeh, 1997). In Mesozoic and Tertiary times tectonic activities affected particularly Central Iran. These movements were accompanied by folding, uplift processes, metamorphism, and magmatism (Khalili, 1997). Over a long period, the plutono-metamorphic basement of western Central Iran was considered as Precambrian in age with Paleozoic, Jurassic, Cretaceous, Eocene and Neogene sedimentary successions as cover


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Fig. 3. Detailed geological map of the Saghand area (part of the Yazd block) in western Central Iran with samples locations. Msg: Mean age of 40Ar/39Ar single-grain stepwise experiments performed on detrital white mica from Central Iran. Msgt: Mean age of 40Ar/39Ar total fusion experiments performed on detrital white mica from Central Iran. Western, Central and Eastern Lithotectonic domains are according to Ramezani and Tucker (2003). These units are separated by major regional faults.

on the basement. However, new age dating have shown that this basement is younger (Bagheri and Stampfli, 2008; Ramezani and Tucker, 2003), and the Paleotethyan suture was identified recently to running across western Central Iran (Bagheri and Stampfli, 2008; Ramezani and Tucker, 2003; Zanchi et al., 2009 and references therein).

Compilation of available maps and published literature allows us to distinguish the following major basement units in western Central Iran largely from west to east (Figs. 2–4): (1) The Kharanagh block includes a metamorphic basement, intruded by the Zarrin granite, and the Jurassic Kharanagh sandstones, a molasse-type deposit (Fig. 3). (2) The “Variscan

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Fig. 4. Graphic scheme showing tectonostratigraphy of the Saghand area in western Central Iran using data of Ramezani and Tucker (2003) and Bagheri and Stampfli (2008).

accretionary” complex is exposed in Anarak and Jandaq areas (Fig. 2) comprises the Morghab schist unit with various schists, the Jandaq high-temperature metamorphic belt with, e.g., staurolite- and sillimanite-bearing schists, amphibolites, and the Arusan ophiolite (Bagheri and Stampfli, 2008); (3) the Biabanak unit with Jurassic and Cretaceous unmetamorphic rocks; (4) the Eocene Chapedony metamorphic core complex with migmatites intruded by Eocene diorite and granite; (5) the overlying Eocene volcanosedimentary succession with conglomerate, andesite, and marls; (6) the Posht-e-Badam metamorphic complex with gneisses and schists, which is overlain by metamorphosed Permian marbles and unmetamorphic Cretaceous red sandstones and limestones; (7) the Boneh Shurow metamorphic complex with gneisses, amphibolites and schists, which are intruded by Triassic granites (Kargaranbafghi et al., 2011, 2012a,b; Masoodi et al., 2013); (8) the Cambrian Tashk Formation with low-grade metamorphic volcanic and metasedimentary rocks (Masoodi et al., 2013). Locally, in the Polo valley within Polo Mts. north of Posht-e-Badam (Fig. 3), equivalents of the Boneh Shurow complex locally coined as Polo metamorphic complex (respectively as Polo shear zone) are structurally overlain by low-grade metamorphic Raetoliassic rocks, mainly phyllites. Their low-grade metamorphism must be younger than Liassic and proves, therefore, a Mid-Late Mesozoic age of low-grade metamorphism. For the Chapedony metamorphic unit of Central Iran, Ramezani and Tucker (2003) recently found Eocene U–Pb zircon ages of gneisses. Ages include: the weighted average 207Pb/206Pb age from a porphyroblastic gneiss sample of the Kalut-e-Chapedony is 52.4 ± 0.9 Ma that represents a good estimate for the age of zircon crystallization in the rock. U–Pb zircon ages of biotite gneiss from Neybaz Mt. define a discordia with a lower intercept at 46.8 ± 2.5 Ma interpreted to date timing of migmatite formation. Kargaranbafghi et al. (2007) and Kargaranbafghi et al. (2012b) found the Eocene 40Ar/39Ar mineral ages ranging between 48 and 41 Ma for all samples from the Chapedony complex. The findings of Ramezani and Tucker (2003) were taken by Kargaranbafghi et al. (2007, 2012a,b) and Verdel et al. (2007) to explain the cooling history. Together, they found the Chapedony metamorphic core complex cooled quite fast from initial migmatite metamorphic conditions to ca. 250 °C between 49 and 41 Ma. (U–Th)/He apatite ages range from 43 to 33 Ma (Kargaranbafghi et al., 2012a).

The hangingwall units, the Posht-e-Badam and Boneh Shurow complexes, include Late Precambrian–Cambrian magmatic gneisses and Triassic granites as well as abundant metamorphic rocks (Kargaranbafghi et al., 2012a,b; Masoodi et al., 2013) mainly found Cimmerian ages of metamorphism (see below). 4. Previous geochronology From basement rocks of the Saghand area, previous geochronology brought evidence for Late Neoproterozoic to Cambrian magmatic and Cimmerian (Late Triassic/Early Jurassic) magmatic and metamorphic events, mainly by U–Pb zircon dating (Bagheri and Stampfli, 2008; Ramezani and Tucker, 2003; Verdel et al., 2007). Only a few older K– Ar and new 40Ar/39Ar white mica data exist and these data are compiled in Table 1. These include: Ages between 333 and 320 Ma from the Jandaq, Anarak and Kabudan areas and 163–156 Ma from the Jandaq area (Bagheri and Stampfli, 2008). Kargaranbafghi et al. (2007) and Kargaranbafghi et al. (2012b) reported 40Ar/39Ar white mica ages: Mylonites from the Neybaz–Chatak shear zone at the hangingwall boundary of the Chapedony metamorphic core complex yielded ages at 43 Ma and 55 Ma, the latter with a significant younger overprint. The Posht-e-Badam complex is overprinted by two stages of metamorphism. White mica ages at 203 and 181 Ma are related to cooling after medium-grade metamorphism respectively shearing. These ages are variably overprinted by low-grade metamorphism also affecting Jurassic metasediments in the Polo Mountains. Consequently, white mica ages are younger, and plateau ages at 140 and 90 Ma have been found. Hornblende and biotite ages are at 335 and 120 Ma (Kargaranbafghi et al., 2012b). 40Ar/39Ar biotite and K-feldspar ages range from 231 Ma to 182 Ma equivalent to the Early (or Eo-)Cimmerian event (Masoodi et al., 2013). This event was related to slow post-metamorphic cooling started from late Early Jurassic and continued until Middle Jurassic times. 40Ar/39Ar muscovite ages are at ca. 169 Ma and are thus equivalent to the Mid-Cimmerian event. Late Jurassic–Early Cretaceous continuation of collision is characterized by reactivation and/or formation of new dextral shear zones, which cross-cut the normal shear zones. 40 Ar/39Ar muscovite and K-feldspar indicate an age of 166 Ma and 116 Ma for this stage.


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Table 1 Previous 40Ar/39Ar and K–Ar geochronology of various tectonic units in Central Iran. Rock

Method and dated mineral

Age and error (Ma)

Interpretation

Author(s)

Chapedony complex, mylonitic granite Posht-e-Badam complex, phyllite Chapedony complex, granitic dyke Posht-e-Badam complex, silicate marble Boneh Shurow, Raetoliassic phyllite Posht-e-Badam complex, silicate marble Posht-e-Badam complex, heavily retrogressed gneiss Posht-e-Badam complex, phyllite Jandaq (Rashid kuh), micaschist Jandaq (Chah Rizab), micaschist Jandaq, Chah Rizab, micaschist Anarak (Chah Kharboze), garnet micaschist Anarak (Palhavand), mica gneiss Anarak (Morghab), muscovite schist NW Kuh-e-Chamgou metarhyolite NE Kuh-e-Gorwan plagiogranite Kuh-e-Surk syenite Kuh-e-Mobaraki granite

40

43.1 ± 0.4 93.6 ± 0.8 42.8 ± 0.3 220.5 ± 0.5 140.8 ± 0.3 45.5 ± 0.3 55.4 ± 0.3 180.9 ± 0.7 166.0 ± 1.5 323.2 ± 2.3 325.1 ± 1.8 333.9 ± 1.9 322.5 ± 1.7 319.0 ± 1.6 170.0 ± 11 187.0 ± 10 128.0 ± 5 140.0 ± 5

Shear zone activity Cooling through ca. 400 °C Shear zone activity Cooling through ca. 400 °C Metamorphism Shear zone activity Overprint by shear zone activity Cooling through ca. 400 °C Cooling through ca. 400 °C Cooling through ca. 400 °C Cooling through ca. 400 °C Cooling through ca. 400 °C Cooling through ca. 400 °C Cooling through ca. 400 °C Intrusion Intrusion Intrusion Intrusion

Kargaranbafghi et al. (2007) Kargaranbafghi et al. (2007) Kargaranbafghi et al. (2012b) Kargaranbafghi et al. (2012b) Kargaranbafghi et al. (2012b) Kargaranbafghi et al. (2012b) Kargaranbafghi et al. (2012b) Kargaranbafghi et al. (2012b) Bagheri and Stampfli (2008) Bagheri and Stampfli (2008) Bagheri and Stampfli (2008) Bagheri and Stampfli (2008) Bagheri and Stampfli (2008) Bagheri and Stampfli (2008) Aistov et al. (1984) Aistov et al. (1984) Aistov et al. (1984) Aistov et al. (1984)

Ar/39Ar white mica 40 Ar/39Ar white mica 40 Ar/39Ar white mica 40 Ar/39Ar white mica 40 Ar/39Ar white mica 40 Ar/39Ar white mica 40 Ar/39Ar white mica 40 Ar/39Ar white mica 40 Ar/39Ar white mica 40 Ar/39Ar white mica 40 Ar/39Ar white mica 40 Ar/39Ar white mica 40 Ar/39Ar white mica 40 Ar/39Ar white mica K–Ar K–Ar K–Ar K–Ar

The U–Pb study of detrital zircon from Cambrian to Cenozoic ages yield “Variscan” ages (two peaks, at 325 and 370 Ma) and ages 213 ± 5.8 Ma constraining the age of the older Cimmerian event (Horton et al., 2008). 5. Sample strategy and materials In the following, we describe the samples in ascending stratigraphic order. GPS-coordinates of samples and their main constituents and fabrics are given in Table 2. Samples of Jurassic molasse-type sandstones were collected to study the effects of the Cimmerian orogeny. Samples were collected in the Kharanagh area (sample FK-156) from a thick succession of molasse-type sandstones and from a similar sandstone succession exposed South of Posht-e-Badam (sample FK-176). The reason to study these samples is to study the nature of the Cimmerian overprint on various basement complexes of western Central Iran. Three samples from a Cretaceous sandstone were collected from a platform-type sandstone/carbonate succession exposed NW Saghand (samples FK-218B and FK-182) and SW Saghand (sample FK-288). One sample was collected from a Cretaceous sandstone West of the Chapedony fault (sample FK-154). The reason to study the Cretaceous samples is to study possible changes in the sedimentary basin during development of post-Cimmerian basins in western sectors of Central Iran. Five samples were collected from sandstones and conglomerate intercalated within the Palaeogene Kerman Conglomerate. Samples FK33 and FK-34 are from locations SW Posht-e-Badam, sample FK-35 from an exposure West of Posht-e-Badam and samples FK-229A and FK-229B are conglomerates from North of Posht-e-Badam. Sample FK229A is from the Kerman Conglomerate from below an unconformity. Sample FK-229B is a Pliocene conglomerate above the unconformity. The purpose in studying samples from the Kerman Conglomerate is to determine possible changes during development of the halfgraben during rapid exhumation of the Eocene Chapedony metamorphic core complex. A few Eocene, Miocene and Pliocene sandstone samples (FK-164, FK-175, FK-229B and FK-153) were also collected although these are mostly poor in mica, and mostly show a rather local hinterland and source–sink relationships. Variable grain sizes have chosen for singlegrain 40Ar/39Ar analysis. In most cases, a grain size of 160–500 μm has been selected. 6. 40Ar/39Ar analytical techniques 40 Ar/39Ar analytical techniques largely follow descriptions given in Handler et al. (2004) with modifications used for single-grain

dating in Rieser et al. (2006). Preparation of the samples before and after irradiation, 40Ar/39Ar analyses, and age calculations was carried out at the ARGONAUT Laboratory of the Department Geography and Geology at the University of Salzburg. Mineral concentrates are packed in aluminium-foil and loaded in quartz vials. For calculation of the J-values, flux-monitors are placed between each 4–5 unknown samples, which yield a distance of ca. 5 mm between adjacent fluxmonitors. The sealed quartz vials are irradiated in the MTA KFKI reactor (Budapest, Hungary) for 16 h. Correction factors for interfering isotopes were calculated from 10 analyses of two Ca-glass samples and 22 analyses of two pure K-glass samples, and are: 36Ar/37Ar(Ca) = 0.00026025, 39Ar/37Ar(Ca) = 0.00065014, and 40Ar/39Ar(K) = 0.015466. Variation in the flux of neutrons was monitored with DRA1 sanidine standard for which a 40Ar/39Ar plateau age of 25.03 ± 0.05 Ma has been reported (Wijbrans et al., 1995). After irradiation the minerals are unpacked from the quartz vials and the aluminiumfoil packets, and hand-picked into 1 mm diameter holes of the oneway Al-sample holders. 40 Ar/39Ar analyses are carried out using a UHV Ar-extraction line equipped with a combined MERCHANTEK™ UV/IR laser system, and a VG-ISOTECH™ NG3600 mass spectrometer. Heating analyses until fusion of single white mica grains are performed using a defocused (~ 1.5 mm diameter) 25 W CO2-IR laser operating in Tem00 mode at wavelengths between 10.57 and 10.63 μm. The laser is controlled from a PC, and the position of the laser on the sample is monitored on the computer screen via a video camera in the optical axis of the laser beam through a double-vacuum window on the sample chamber. Gas clean-up is performed using one hot and one cold Zr–Al SAES getter. Gas admittance and pumping of the mass spectrometer and the Ar-extraction line are computer controlled using pneumatic valves. The NG3600 is a 18 cm radius 60° extended geometry instrument, equipped with a bright Nier-type source operated at 4.5 kV. Measurements are performed on an axial electron multiplier in static mode, peak-jumping and stability of the magnet is controlled by a Hall-probe. For each increment the intensities of 36 Ar, 37Ar, 38Ar, 39Ar, and 40Ar are measured, the baseline readings on mass 35.5 are automatically subtracted. Intensities of the peaks are back-extrapolated over 16 measured intensities to the time of gas admittance either by a straight line or a curved fit, depending on intensity and type of pattern of the evolving gas. Intensities are corrected for system blanks, background, post-irradiation decay of 37 Ar, and interfering isotopes. Isotopic ratios, ages and errors for individual steps are calculated following suggestions by McDougall and Harrison (1999) using decay factors reported by Steiger and Jäger (1977). Definition and calculation of plateau ages have been carried out using ISOPLOT/EX (Ludwig, 2001).

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Table 2 Location, stratigraphic age and description of samples collected for 40Ar/39Ar dating. Sample no.

Site

GPS coordinates

Stratigraphic age

Rock name

Cement/matrix

Main clasts

Subordinate clasts and remarks

Deformation features

FK-156

Kharanagh

N32°21.1487′ E54°40.2056′

Jurassic

Arkosic quartz wacke

Pressure solution cleavage; few layers with small rigid opaque grains with quartz fibres within strain shadow

Carbonate

Quartz

Marly sandstone

Carbonate, quartz

FK-154

W Chapedony

Upper Cretaceous

Hybrid sandstone with mud

Carbonate, phyllosilicates Carbonate

Feldspar, white mica, chlorite White mica

Pressure solution; replacement of carbonate by fine-grained carbonate grains

NW Saghand

Jurassic? or Cretaceous? Lower Cretaceous Cretaceous

Arkosic sandstone

FK-218B

N33°02.1382′ E55°32.5606′ N32°29.940′ E55°12.624′ N32°36.6779′ E55°12.8172′ N32°48.7020′ E55°09.8320′

Quartz sandstone

FK-288

S Posht-e-Badam SW Saghand

Quartz, plagioclase, K-feldspar Quartz, carbonate

White mica, sericite

FK-176

Fine-grained phyllosilicates and quartz Carbonate

Recrystallization of carbonate cement

FK-182

NW Saghand

Carbonate

Calcite, quartz

SW Posht-e-Badam

Cretaceous?, Permian? Paleogene

Carbonate wacke

FK-33

N32°35.6332′ E55°11.4220′ N32°53.722′ E55°29.369′

White mica; no metamorphic rock clasts Opaque, white mica

Lithic hybrid arenite

Carbonate

Calcite, quartz

FK-34

SW Posht-e-Badam

N32°53.722′ E55°29.369′

Paleogene

Carbonatic quartz arenite

Carbonate

Carbonate, quartz

FK-35

W Posht-e-Badam

N33°03.972′ E55°00.508′

Paleogene

Lithic arenite–carbonate

Carbonate

Quartz, calcite, feldspar

FK-229A N Posht-e-Badam FK-164 W Chatak

N33°03.8684′ E55°33.5017′ N32°58.1341′ E55°16.9885′

Paleogene

Conglomerate

Carbonate

Quartz, feldspar

Pliocene

Arkose

Carbonate

FK-175

Saddle S Post-e-Badam W Chapedony

N32°53.3567′ E55°30.0773′ N32°50.6867′ E55°12.1199′

Miocene

Arkosic sandstone

Carbonate

Pliocene

Lithic arenite

Calcite

Quartz, feldspar, deformed quartz, carbonate Quartz, feldspar, calcite Feldspar, calcite, biotite

N Posht-e-Badam

N33°03.8684′ E55°33.5017′

Pliocene

Conglomerate

FK-229B

Many lithic metamorphic clasts like phyllite and quartz phyllite Corals(?) fossils, phyllite and quartz–phyllite Lithic metamorphic clasts like phyllite and quartz phyllite

Fossil, with mica, zircon, hornblende Strongly deformed quartz grains Opaque mineral, lithic micro-crystalline volcanics

Pressure solution cleavage


FK-153

Carbonate, quartz

White mica, chlorite


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7. Results

7.2. Single-grain step-heating experiments

Representative microphotographs of all samples are shown in Fig. 5. Detailed analytical results are given in Supplementary information. The single-grain step-heating results are graphically presented in Figs. 6 and 7. For graphic representation of single-grain total fusion results (Fig. 8) we use the approach of Sircombe (2004). A summary of results is presented in Fig. 3.

7.2.1. Biabanak block (W of Chapedony fault) Sample FK-164 is an Eocene sandstone (Fig. 5a). Two grains have used for step-heating experiments. These two grains yield an overprint as young as ca. 180 Ma in the first step, and total gas ages of 272.7 ± 1.6 Ma and 406.8 ± 4.1 Ma for the main portion of the experiments. Both ages are considered to be geologically significant.

7.1. Petrographic description of samples

7.2.2. Units E of Chapedony fault From a total of 26 grains, step-heating experiments have been performed (Figs. 6, 7). Sample FK-288 is a Cretaceous sandstone taken SW Saghand (Fig. 5b). From this sample, two grains were used for step-heating experiments. Grain 2 yield a staircase pattern with an age of 114.3 ± 15.3 Ma for the first step. The total gas ages of the two grains are 176.5 ± 3.3 Ma and 176.3 ± 1.7 Ma. Only one grain of sample FK-218B, a Cretaceous (?) sandstone collected to the west of Saghand, has been used for step-heating

The petrographic composition of Jurassic to Pliocene samples varies largely through time. The petrographic data are compiled in Table 2 and representative microphotographs are shown in Fig. 5. Note that, none of the samples experienced post-sedimentary temperature higher than diagenetic respectively very-low-grade metamorphic conditions resulting in pressure solution. Therefore, no in situ metamorphic effect has to be considered in interpretation of the data.

Fig. 5. Selected photomicrographs of thin sections from sedimentary rocks chosen for dating of detrital white mica: a) Eocene arkose from west of Kalut-e-Chatak. b) Lower Cretaceous arkosic sandstone from SW Saghand. c) Cretaceous marly sandstone from west of Saghand. d) Miocene arkosic sandstone from saddle Posht-e-Badam. e) Jurassic arkosic quartz wacke from Kharanagh. f) Upper Cretaceous hybrid sandstone from the Cretaceous formation west of Chapedony. g) Pliocene lithic arenite from west of Kalut-e-Chapedony. Legend: quartz (qz), plagioclase (pl), K-feldspar (ksp), biotite (bt), calcite (cc), white mica (ms), opaque (op). h) Cretaceous(?) quartz sandstone from south of Posht-e-Badam. i) Cretaceous? carbonate wacke from Moghestan. j) Paleogene lithic hybrid arenite from south Posht-e-Badam. k) Paleogene carbonate quartz arenite from south of Posht-e-Badam. l) Paleogene lithic arenite– carbonate from west Posht-e-Badam.

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Fig. 5 (continued).

experiments (Fig. 5c). The sample yields a total gas age 826 ± 5.7 Ma, which is considered to represent a geologically significant age. From sample FK-229A, a Paleogene sandstone, four grains have been measured by the step-heating method. Only grain 3 comprises small argon loss in the first two steps with an age of 158.2 ± 9.8 Ma. The total gas ages of these four grains include 169.6 ± 1.1 Ma, 173.7 ± 1.4 Ma, 207.9 ± 1.2 and 220.3 ± 1.8 Ma. From sample FK229B collected in the same outcrop group, seven grains have been used for step-heating experiments. Five of these grains show negligible overprint, the total gas ages between 193.4 ± 1.3 and 205.2 ± 1.2 Ma Grain 4 yields a staircase pattern with a youngest age of 91.3 ± 16.4 Ma and an oldest age of 208.2 ± 2.1 Ma comprising a large portion of the experiment. Grain 3 also yields a staircase pattern with an age of 386.8 ± 9.9 Ma for the first step and a total gas age of 473.9 ± 2.5 Ma for remaining steps. From nine mica grains of sample FK-175, a Miocene red micabearing sandstone from the saddle south Posht-e-Badam (Fig. 5d), step-heating experiments were performed (Fig. 7). All nine grains gave total gas ages, which range between 171.1 ± 1.0 Ma and 174.0 ± 1.8 Ma. This proofs a uniform population from a metamorphic terrain, which was obviously not overprinted by a second tectonothermal event after 171 Ma. We suggest, therefore, that these ages are close to geologically significant ages.

7.3. Single-grain total fusion experiments Results of single-grain total fusion dating of white mica separated from samples are described from oldest to youngest stratigraphic age separated for units west and east of the Chapedony fault.

7.3.1. Biabanak block (W of Chapedony fault) Sample FK-156 is a Jurassic molasse-type sandstone from Kharanagh, and 17 grains have been measured (Fig. 5e). The results are highly diverse and cluster around several age groups: (1) Variscan ages with 312 to 371 Ma, (2) Permian to Early Triassic ages between 242 and 289 Ma, (3) Cimmerian ages ranging from 190 to 232 Ma, and (4) two Precambrian ages (699 and 816 Ma). Sample FK-154 is a Cretaceous sandstone from the Biabanak unit exposed west of the Chapedony fault (Fig. 5f). The sample yielded highly diverse ages, which include only a few Cimmerian and Permian ages (208, 258–284 Ma) and many Variscan ages ranging between 312 and 385 Ma. A few further grains are older: 400 and 422 Ma. Sample FK-164 is an Eocene sandstone intercalated within the volcanosedimentary unit in the western hangingwall limb of the Chapedony metamorphic core complex, in the northwest of Chatak Mountains. Only a few grains have been measured: They comprise


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Fig. 6. Results of 40Ar/39Ar single-grain stepwise heating experiments of several samples from various sandstones ranging in age from Jurassic to Pliocene. Laser energy increases from left to right.

Permian, Variscan and early Paleozoic ages between 265 and 321 Ma. From sample FK-153, a Pliocene conglomerate from west Chapedony only four small mica grains were dated (Fig. 5g). They gave the ages: 256, 289, 423 and 780 Ma.

7.3.2. Units E of Chapedony fault Sample FK-176 is a Jurassic (or Cretaceous?) red molasse-type sandstone exposed to the south Posht-e-Badam, which yields the following age groups (Fig. 5h): (1) a single Variscan age of 344 Ma and (2) a

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exposed north of Saghand, too (Fig. 5i). A few grains have been measured. They include Cimmerian ages, 179, and 207 to 219 Ma, and few Variscan ages between 322 and 342 Ma. Sample FK-33, a sandstone from Kerman Conglomerate (Paleogene) (Fig. 5j), south Posh-e-Badam yields dominant group with ages of 164 to 188 Ma, and few further Cimmerian ages around 207 Ma, a single young age of 104 Ma, two Permian ages at 262 and 285 Ma, and Variscan ages ranging from ca. 324 to 379 Ma. Sample FK-34, another sandstone from Kerman Conglomerate (Paleocene–Eocene) exposed south of Posht-eBadam (Fig. 5k) yields five age groups (1) a single Early Paleozoic age at 478 Ma, (2) two Variscan ages at 323 and 335 Ma, (3) a few Permian– Early Triassic ages (Early Cimmerian) at ca. 231 and 237 Ma, (4) Late Cimmerian ages between ca. 164 and 197 Ma, and (5) few postCimmerian ages: 105, 112 and 143 Ma. Sample FK-35, collected from a sandstone layer intercalated within the Kerman Conglomerate (Paleogene) (Fig. 5l), west Posht-e-Badam yields a similar highly diverse age pattern as the other samples from the same Kerman Conglomerate. The dominant age group is between 159 and 196 Ma. Two grains are younger, at 96 and 99 Ma. In addition, Variscan and early Variscan ages ranging from 319 to 403 have been found. One grain with an age of 427 Ma is older. Sample FK-229A is a Paleogene conglomeratic sandstone that is considered to represent the Kerman Conglomerate. This sample yielded relatively diverse age populations including Cimmerian ages between 168 and 179 Ma, early Cimmerian ages between 200 and 214 Ma, a single Permian age (246 Ma), some Variscan ages (321 to 354 Ma) and a Cambrian age at 524 Ma. Sample FK-229B is a Pliocene conglomeratic cover on Paleogene marls and sandstones (sample FK-229A). This sample yielded a nearly uniform age population with ages ranging between 190 and 219 Ma. Only one grain is younger with an age of 150.2 ± 15.1 Ma. 8. Discussion Our preliminary survey on detrital white mica is the first in Central Iran. The objective of the study is to present a first order overview on age distributions and particularly of changes of ages between Jurassic, Cretaceous, Paleogene and Neogene steps of tectonic evolution. The main results include: Together from Cretaceous to Pliocene stratigraphic levels, the results of single-grain step heating experiments indicate the clear presence of Neoproterozoic (826 Ma), Early Paleozoic/ pre-Variscan (406, 473 Ma), Variscan (386 Ma), Permian (272 Ma) and two stages of Cimmerian events: 169–193 Ma and 201–220 Ma. These age groups are confirmed by plateau ages of single-grain total fusion experiments, which record largely the same populations, and in addition a Variscan age group between 308 and 385 Ma. 8.1. Distinct sources west and east of the Chapedony fault

Fig. 7. Results of 40Ar/39Ar single-grain stepwise heating experiments of sample FK-175, a Miocene sandstone. Laser energy increases from left to right.

dominant group with ages ranging from 159 to 198 Ma. Sample FK-288, a Lower Cretaceous sandstone taken SW of Saghand yields a relatively uniform groups with ages 171 to 203 Ma. Sample FK-175, a Miocene red mica-bearing sandstone from the saddle S Posht-e-Badam, yields a uniform age group between 166 and 182 Ma. Sample FK-218B, a Cretaceous sandstone exposed north of Saghand, also yields a large uniform age group with ages ranging between 159 and 197 Ma. Only a few further grains gave deviating ages including 243, 272, 403 and 423 Ma. Sample FK-182 is a Cretaceous (?) sandstone

Jurassic and Cretaceous sandstones from W and E of the Chapedony fault yield different age spectra: Jurassic and Eocene formations of the Biabanak unit exposed to the west of the Chapedony fault yield dominant Variscan ages (ca. 308–385 Ma), whereas Cretaceous–Eocene formations in the east are characterized by dominant Cimmerian/ Indosinian ages (220–169 Ma). Consequently, the Chapedony fault plays an important regional role and is, therefore, a sort of terrane boundary. The Cretaceous to Eocene cover of the Biabanak unit most likely belongs of a Variscan basement unit. The source of the Variscan micas are probably derived from the Variscan accretionary complex exposed in the Anarak–Jandaq areas further NW (Fig. 2). 8.2. Neoproterozoic paleogeographic relationships of Central Iran The oldest but scarce detrital white mica grains have ages ranging from 524 to 826 Ma heralding a Late Precambrian and Cambrian crystalline basement or cannibalism from older clastic successions. These old


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Fig. 8. Results of 40Ar/39Ar single-grain total fusion experiments. The samples are shown in the approach of Sircombe (2004).

ages can be assigned to Neoproterozoic tectonic events, and represent a key for large-scale correlation. No real Precambrian metamorphic basement older than ca. 600 Ma has been identified in western Central Iran, and numerous reworked orthogneisses found in the working region (Ramezani and Tucker, 2003) originate from calc-alkaline granodiorite and diorite free of white mica. Therefore, we think that these ages rather record cannibalism from older, Paleozoic sedimentary rocks, which locally occur in the wider region. This interpretation needs proof by dating of older, Silurian–Devonian sedimentary rocks in the future. The very small proportion of detrital mica with Precambrian ages in the hinterland suggests a nearly full Phanerozoic nature of metamorphic crust exposed in Central Iran. 8.3. Prototethyan relationships The presence of Middle Ordovician to Silurian ages in a major number of samples is most interesting. There is no such tectonothermal event recorded in the wider hinterland. This event needs further confirmation by dating of detrital white mica from Silurian–Devonian sedimentary rocks. It remains unclear whether the age group results from a plutonometamorphic basement, which is not yet identified in Central Iran or from cannibalism from Triassic to Carboniferous sedimentary rocks. We suggest that a Prototethyan orogen must be the source of Mid Paleozoic micas. 8.4. Variscan events Variscan ages are widespread, and support the recent findings of the Variscan accretionary unit in the Anarak–Jandaq further northwest

(Bagheri and Stampfli, 2008; Zanchi et al., 2015). Jurassic and Cretaceous sandstones from the west and east of the Chapedony fault yield different age spectra, with a dominance of Variscan ages (ca. 308– 385 Ma) compared to coeval sandstones from the units east of the Chapedony fault (eastern Yazd block), where Variscan ages are subordinate and Cimmerian ages predominate. This result underlines a major block boundary identified as the Chapedony fault, which is in extension of a fault previously proposed by Bagheri and Stampfli (2008). On a larger scale, dating detrital zircons of Permian sandstones with the U–Pb method, Horton et al. (2008) reported two age Variscan groups of 370 and 325 Ma. This implies likely the presence of some Variscan magmatic rocks in the western parts of Iran. Zanchetta et al. (2009) found Variscan eclogites in NW Iran. Wendt et al. (2005) report some deformation of sedimentary successions in Late Carboniferous time. Thus most authors believe that accretionary processes occurred in this segment of the Paleotethys. 8.5. Two stages of Cimmerian orogeny Unexpected is also the strong evidence for two stages of Cimmerian events, which are visible in our data sets from Cretaceous and Paleogene sandstones, a cluster around 170 Ma and another at ca. 205 Ma. The two age clusters suggest a two-stage Cimmerian evolution of the Posht-eBadam and Boneh Shurow complexes. New 40Ar/39Ar white mica ages (Kargaranbafghi et al., 2012b; Masoodi et al., 2013) from the Posht-eBadam and Boneh Shurow complexes virtually argue in a similar direction. On a larger scale, these two steps of Cimmerian are well known particularly in the Anarak region and northern parts of Iran. The Upper Triassic to Middle Jurassic Shemshak Fm. separates these two

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steps Cimmerian orogeny or is, respectively, synchronous with the earlier stage of Cimmerian metamorphism (Rahmati-Ilkhchi et al., 2010; Rahmati-Ilkhchi et al., 2011; Fürsich et al., 2009), whereas the older Cimmerian orogenic event is about contemporaneous with the Upper Anisian synorogenic flysch in the Anarak region (Balini et al., 2009). The Paleotethys was closed during these events (e.g., Muttoni et al., 2009). The youngest micas in Paleogene conglomerates that have an age of ca. 100 Ma are most likely derived from the base of the Posht-e-Badam complex as similar ages have been found there (Kargaranbafghi et al., 2012b). The Jurassic molasse-type sandstone only shows the presence of the older Cimmerian age group and the record of the younger stage is missing. Consequently, the level or area of the younger age group was not exposed in the source area zone of the molasse deposition. This suggests a lag time exceeding more than 20–30 Ma (the error is due to uncertainty of the exact stratigraphic age). Interestingly, the younger age group has been found in the Jurassic (or Cretaceous?) sample FK-176, which argues for a separate source area exposing the younger Late Cimmerian age group (at ca. 170 Ma). 8.6. Why the Chapedony metamorphic core complex is not recorded? No record of the uplifted Eocene Chapedony metamorphic core complex has been found in Eocene and Pliocene clastic rocks. This finding is most likely due to the scarcity of white mica in that complex as finegrained white mica only occurs in the Neybaz–Chatak shear zone along the hangingwall boundary of that complex. 9. Conclusions A first order survey of 40Ar/39Ar dating of detrital white mica from Jurassic to Pliocene sandstones has been carried out in order to reveal the tectonic evolution of blocks in Central Iran. The principal results include: (1) Scattered evidence for Neoproterozoic and Panafrican tectonothermal events in the hinterland. (2) Jurassic and Cretaceous sandstones from west and east of the Chapedony fault yield different age spectra, with a dominance of Variscan ages (ca. 308–385 Ma) compared to coeval sandstones from the block east of the Chapedony fault, where Variscan ages are subordinate and Cimmerian ages predominate. (3) Two stages of Cimmerian events are visible in our data set from Cretaceous and Paleogene sandstones, a cluster around 170 Ma and at ca. 205 Ma. These clusters suggest a two-stage Cimmerian evolution of the Posht-e-Badam and Boneh Shurow complexes. (4) The youngest micas in Paleogene conglomerates have an age of ca. 100 Ma and are most likely derived from the base of the Posht-e-Badam complex. (5) No record of the uplifted Eocene Chapedony metamorphic core complex has been found in Eocene and Pliocene clastic rocks, most likely due to the low amount of white mica. (6) Although the nature of sedimentary basins is changing from Jurassic to Pliocene times, not much shift is visible in the composition of age spectra of detrital mica. This indicates the early stages of slow exhumation of the source rocks and pervasive nature of the Cimmerian tectonic events in Central Iran.

Acknowledgements The authors are grateful to the anonymous reviewers for their constructive comments that significantly improved this manuscript as well as the journal editor for providing useful comments. Appreciation is expressed to Dr. Abdolrahim Houshmandzadeh for the fieldwork, support

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