Identification of Eocene-Oligocene magmatic pulses ...

Gondwana Research 57 (2018) 141–156

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Identification of Eocene-Oligocene magmatic pulses associated with flare-up in east Iran: Timing and sources Fatemeh Sepidbar a,c, Hassam Mirnejad a,b,⁎, Changqian Ma c, Hadi Shafaii Moghadam d,e a

Department of Geology, Faculty of Sciences, University of Tehran, Tehran 14155-64155, Iran Department of Geology and Environmental Earth Sciences, Miami University, OH 45056, USA State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geoscience, Wuhan 430047, China d Department of Geology, Faculty of Science, University of Mohaghegh Ardabili, Ardabil 56199-13131, Iran e School of Earth Sciences, Damghan University, Damghan 36716-41167, Iran b c

a r t i c l e

i n f o

Article history: Received 14 March 2017 Received in revised form 9 January 2018 Accepted 11 January 2018 Available online 14 February 2018 Handling Editor: I. Safonova Keywords: Zircon U–Pb ages Hf–O isotopes Eocene-Oligocene magmatism Magmatic flare–up Sangan magmatic complex

a b s t r a c t The Sangan Magmatic Complex (SMC), at the northeastern edge of the Lut block, includes a thick pile of extrusive and pyroclastic rocks, intruded by younger granitoid stocks. New zircon U–Pb ages show subaerial eruptions at ~42–44 Ma, followed by emplacement of granitoids at ~41–40 Ma. The granitoids have high K2O (~3.6–5.9 wt %), with SiO2 (~63.1 and 71.9 wt%) contents. They are metaluminous to peraluminous, calc alkaline and I-type in composition. The SMC magmatic rocks have typical high–K and shoshonitic signatures, and are characterized by enrichment in large–ion lithophile elements (LILEs) and depletion in high–field–strength elements (HFSE). Zircon εHf(t) from the SMC magmatic rocks ranges from +0.45 to +3.5 for volcanic rocks, −1.6 to +2.5 for granitoids and −4.1 to −1.4 for ignimbrites. Zircon δ18O values for the SMC are variable from +6.1 to +8.1‰, significantly higher than those of mantle–derived melts. The whole–rock εNd(t) values range between −4.5 to −3.5 for granitoids, −4.6 to −3 for volcanic rocks and −5.3 to +0.7 for ignimbrites. The whole–rock Nd and zircon Hf crustal model ages (TCDM) for the SMC magmatic rocks range between 0.8 and 1.2 Ga. All of the SMC magmatic rocks have quite similar trace element patterns, and slightly different whole–rock Nd and zircon Hf isotopic composition. High 207Pb/204Pb and 208Pb/204Pb values for the SMC rocks indicate the involvement of the thick continental crust during the formation of these rocks. Modeling of zircon O–Hf, bulk–rock trace elements, and Sr–Nd isotopes suggest the magmas were generated by interaction of mantle–derived melts with thick continental crust through AFC processes. Compiled geochronological and geochemical data from east Iranian magmatic belt and Urumieh-Dokhtar magmatic belt allow identification of Eocene-Oligocene flare-up associated with several magmatic pulses. © 2018 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction Subduction zones deliver raw materials into the Subduction Factory, where oceanic lithosphere, sediments, and seawater re–equilibrate with ambient mantle, triggering flux melting and generating magmas (Stern, 2002; Grove et al., 2006). During Phanerozoic time, continental crust grew mainly above subduction zones, both by tectonic accretion and emplacement of juvenile magmatic rocks (Condie et al., 2009). Active continental margins are sites of MASH (Melting, Assimilation, Storage, Homogenization) processes (Walker et al., 2015). There are several locales where such magmatic arcs can be studied at different erosional levels, including the Andes (upper levels) and Sierra Nevada of California (mid–crustal levels) (Pickett and Saleeby, 1993; Ruppert et al., 1998; ⁎ Corresponding author at: Department of Geology, Faculty of Sciences, University of Tehran, Tehran 14155-64155, Iran E-mail address: [email protected] (H. Mirnejad).

Ducea, 2001, 2002). The focus of this paper is on the Cenozoic magmatic arc of Iran, the eastern Iranian magmatic belt, where both upper-and lower-crustal levels are accessible. The flare-up can produce massive magmas for a duration of several millions or a few tens of millions of years (~10–20 Ma) (He et al., 2014) and is characterized by pulses of magmatism (de Silva et al., 2015). These magmatic activities are followed periodically by short– lived (~1 Ma), steady or “lull” low-volume magmatism (de Silva et al., 2015). The intense of magmatism in the flare-up stage is 3–4 times greater than that from the “lull” stage (Burns et al., 2015). Most documented magmatic pulses during a flare-up are characterized by high volumes of intermediate to felsic rocks with significant crustal inputs, including magmas with high bulk rock 87Sr/86Sr ratios and low εNd and zircon εHf values (Annen et al., 2006; Ducea, 2002; Ducea and Barton, 2007; Ducea et al., 2009; Lackey et al., 2008). The associated plutonic manifestations of magmatic pulses are large–scale batholiths, which are involved in the formation of new continental crust

https://doi.org/10.1016/j.gr.2018.01.008 1342-937X/© 2018 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

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F. Sepidbar et al. / Gondwana Research 57 (2018) 141–156

(DeCelles et al., 2009; Ducea et al., 2009; Ducea et al., 2010). These distinct pulses during one flare-up lasting several million years are sometimes associated with a peak in magmatic activity (de Silva et al., 2015). The Cenozoic magmatism of Iran is mostly concentrated in north of Zagros (Fig. 1). One of the most significant magmatic events in Iran and the surrounding regions (Turkey, Caucasus and Afghanistan) was the Eocene–Oligocene magmatic flare-up and associated pulses, resulting in widespread magmatism throughout most parts of Iran, Turkey, the Caucasus and Afghanistan (Pang et al., 2013; Verdel et al., 2011). These magmatic pulses in Iran were linked to the northeastward, oblique subduction of Neo–Tethyan oceanic lithosphere beneath Iran, followed by collapse, extension and magmatism (Moghadam et al., 2016b). Here we report an overview of the age and geodynamics of magmatism in east Iran magmatic belt (EIMB). We also evaluate the Cenozoic magmatic flare ups and pulses from east, northeast and central Iran, including the Sangan Magmatic Complex (SMC), presenting new geochronological, geochemical and isotopic data. These data allow us to better understand how Eocene-Oligocene magmatic flare-up and associated pulses dominated east Iran, ≳1000 km away from the Tethyan suture zone (Main Zagros Thrust) (Fig. 1). We also report zircon U–Pb ages and Hf–O isotopes along with mineral compositions, bulk– rock

major– and trace elements and Sr–Nd–Pb isotope data from both intrusive and extrusive rocks. These data are integrated with other data from magmatic rocks across northeast, east and central Iran to better understand the timing of Eocene-Oligocene magmatic pulses during the “flare–up” episode. The main aims of our study are to better understand the age and nature of the igneous rocks of the SMC; the exact timing and duration of the magmatic “flare–up” in this area; and the tectonic regime which controlled the ignition of magmatic pulses in east Iran during the flare-up. We also examine the age, isotope geochemistry and tectonic setting of the igneous rocks from east Iran to see if they were formed in response to northeastward subduction of Neotethyan ocean beneath the central Iran along the Zagros suture zone, or if they are related to the subduction of the Birjand–Sistan ocean beneath the Lut block. 2. Geological framework 2.1. Cenozoic magmatic belts Iran is a tectonic collage of Gondwana–derived terranes, separated by narrow belts of ophiolites (Moghadam and Stern, 2014; Moghadam

Fig. 1. Map of Iran showing the Cadomian basement rocks, Cenozoic igneous rocks and Paleozoic–Mesozoic ophiolites.


and Stern, 2015), which accreted continually to the southern margin of Eurasia (Sengor et al., 1993; Sengor et al., 1991; Sengor and Natalin, 1996). The oldest magmatic rocks in Iran are Late Neoproterozoic to Early Cambrian granites and gneissic rocks (~500–600 Ma; Cadomian), formed during Pan–African orogenic events along the northern margin of Gondwana (Hassanzadeh et al., 2008; Moghadam et al., 2015a; Rossetti et al., 2015). Cadomian exposures are documented from regions in western (Golpayegan), north–western (Khoy–Salmas, Zanjan–Takab), northeastern (Torud, Taknar), northern (Lahijan granites), and central Iran (Saghand) (Hassanzadeh et al., 2008; Moghadam et al., 2015a; Moghadam et al., 2016a) (Fig. 1). Neoproterozoic basement rocks are also pervasive in northeast–east Iran, both north and south of the SMC (Fig. 2). Most of the Late Neoproterozoic–Cambrian basement was exhumed from the middle–lower crust and was intruded by younger plutons during the Eocene–Miocene, accompanying the oblique subduction of Neo–Tethys. An excellent example of Late Neoproterozoic-Cambrian basement is found in the Saghand area of central Iran, which was exhumed during Eocene extension (Verdel et al., 2007), followed by Early Miocene erosion (Kargaranbafghi et al., 2015). Another example is provided by (Stockli et al., 2004), who reported a Late Neoproterozoic-Cambrian basement which was exhumed during Oligocene–Miocene from the Zanjan–Takab region of northwest Iran. These core complexes show an important phase of extension, which prevailed across Iran during Cenozoic time and was accompanied by a magmatic event.

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Geographically, four Cenozoic magmatic belts are recognized in Iran (Fig. 1). 1– The Alborz magmatic belt in northwest–northeast Iran, which continues into the magmatic belts of the Lesser Caucasus and Pontides (Turkey) (Castro et al., 2013; Dilek et al., 2010). The Alborz magmatic belt includes Upper Jurassic–Cretaceous flysch and platform carbonates, which grade upward into Eocene volcanic rocks, and are then intruded by shoshonitic and adakitic Oligocene–Miocene plutons (Jamali et al., 2010). A continental back–arc basin setting has been suggested for the formation of the Alborz magmatic rocks (Asiabanha and Foden, 2012). 2– The EIMB, or Gonabad–Birjand–Zahedan Magmatic Belt, to which the SMC belongs, includes a thick sequence of Late Cretaceous–Paleocene volcano–sedimentary rocks, which are overlain by Eocene–Oligocene (46–25 Ma) calc–alkaline to shoshonitic magmatic rocks (Pang et al., 2013). Westward subduction of the Birjand– Sistan oceanic lithosphere beneath the Lut block may have caused the Cretaceous magmatic activity (Tirrul et al., 1983). The Birjand– Sistan Oceanic basin closed during latest Cretaceous–early Paleocene (Zarrinkoub et al., 2012).Therefore, Paleogene magmatism post–dates the collision of the Lut–Afghan blocks and cannot be related directly to the westward subduction of Birjand–Sistan Oceanic lithosphere. Pang et al. (2013) proposed that Paleogene magmatism in EIMB occurred in a post–collisional setting and was triggered by

Fig. 2. Geological map of the Sangan Magmatic Complex. (Modified after 1:250,000 geological map of Taybad, Geological Survey of Iran.)

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convective removal of lithosphere (delamination) and resultant asthenospheric upwelling accompanying extensional collapse. Continued asthenospheric upwelling led to the Late Miocene alkali basaltic volcanism (40Ar–39Ar; 14–11 Ma) (Pang et al., 2012). 3– The Makran magmatic belt of southeast Iran is an east–west magmatic belt with three large young strato–volcanoes including Koh– e–Sultan in Pakistan, and the Taftan and Bazman volcanoes of Iran. U–Pb zircon ages of 7.5 to 0.84 Ma are reported for lavas from these volcanoes (Pang et al., 2014). The Makran magmatic belt reflects continued subduction of oceanic lithosphere beneath Makran. 4– The Urumieh–Dokhtar Magmatic Belt (UDMB) is a 50–80 km wide Andean–type arc, formed by northeastward subduction of the Neotethyan ocean beneath Iran, which began in the Late Cretaceous and continued through the Cenozoic (Berberian and King, 1981). This magmatic belt includes a thick (~4 km) pile of early calc–alkaline and late shoshonitic volcanic rocks, as well as adakitic rocks (Berberian and King, 1981). 2.2. Regional geology, sample description, and petrography The SMC at the northeast of the Lut block is the northward continuation of the EIMB. The oldest rocks in the Sangan area include a thick pile (~1500 m) of Late Neoproterozoic volcano–sedimentary rocks with limestone–marble interlayers, overlain stratigraphically by Devonian and Carboniferous dolomites, shales and calc–schists in the southern parts of the SMC (Fig. 2). These sedimentary rocks are unconformably covered by the SMC volcanic and pyroclastic rocks and/or intruded by granitoids. The SMC includes ~2000 m of volcanic, pyroclastic rocks cross-cut by plutonic rocks and can be subdivided into three major units according to the geographical position and its rock types: (1) the southern SMC unit, including volcanic and volcanoclastic rocks with minor intrusive rocks; (2) the northern SMC unit, predominantly comprising granitoid intrusions; and (3) the thermal metamorphic unit including skarns and metasomatic rocks at the contact between Eocene intrusions and Neoproterozoic-Paleozoic carbonates. Field observations indicate magmatic activity started with the explosive eruption of pyroclastic rocks, followed by a subaerial eruption of intermediate to felsic volcanic rocks. The volcanic succession in the SMC is similar to the stratovolcanoes, with alternative eruptions of pyroclastic rocks (ignimbrites) and lavas. The third magmatic activity was intrusion of granitoid melts into the middle–upper crust as well as into the thick volcanic successions and the Late Neoproterozoic–Paleozoic strata. At the contact between granitoids and Paleozoic–Late Neoproterozoic successions, there is a wide (~8 km) and thick (1000 m) thermal metamorphic aureole containing Fe skarn mineralization (Fig. 2). Below we describe the petrography and mineral assemblages of volcanic, plutonic and pyroclastic rocks in detail. 2.2.1. Pyroclastic rocks Volcanic activity in the SMC was predominantly explosive and periodic and produced a thick (2000 m) and extensive pile of pyroclastic rocks such as breccias, agglomerates, and varieties of tuffs and ignimbrites (Fig. 3A, B & C). The volcanic breccias vary from clast– to matrix–supported and are either monomictic or polymictic (Fig. 3A & B). The clasts are fine– to coarse–grained, elongated, sub–angular to subrounded and range in size from 1 to 100 mm. The clasts are trachydacitic to dacitic in composition. Crystal fragments include anhedral biotite, quartz, and feldspar. The agglomerates cover an area of ~500 to 800 m2 and mostly contain trachydacitic layers/lava sheets (Fig. 3D). Agglomerates have different types of rock fragments and contain altered alkali feldspar (30%), resorbed quartz (25%), plagioclase (10%), and biotite (3%). Tuffs and ignimbrites, covering an area of ~30 km2, consist mainly of welded shard of feldspar and quartz and are characterized by pilotaxitic texture. Tuffs mostly vary from lithic to crystal tuffs and have felsic composition (Fig. 3C). The crystal fragments in crystal tuffs include alkali

feldspar (~32%), quartz (20%) and plagioclase (b2%). Coarse–grained quartz crystals show resorbed texture. 2.2.2. Volcanic rocks Intermediate to felsic lavas, covering an area of 10 km2 in the southern part of the SMC, are petrographically subdivided into dacites, trachydacites and trachytes. Trachydacites occur both as thick (~10 m) lava sheets between the pyroclastic rocks and as volcanic domes (Fig. 3D & E). The trachydacites have a weak trachytic texture, characterized by oriented fine–grained feldspars in the groundmass. The trachydacite sheets contain abundant plagioclase (12–15%) and minor biotite phenocrysts (2–4%), which float in a light brown fine–grained groundmass of K–feldspar, quartz, and magnetite. The rocks in lava domes have porphyritic textures and contain sanidine (4–5%) and biotite (5%) phenocrysts in a light grayish matrix. Quartz phenocrysts are rare. Zircon, apatite, allanite and magnetite are accessory mineral phases in trachydacites while chlorite is a secondary component. Dacites are less abundant than trachydacites and occur as lava flows. The dacites have porphyritic textures and contain phenocrysts of plagioclase (15–20%), biotite (5–7%), sanidine (~5%), and quartz (1–2%) set in a matrix composed mainly of glass, quartz, and sanidine microlites. Plagioclase is present as randomly oriented, tabular crystals and shows alteration to kaolinite and sericite. Coarse–grained plagioclase in some dacitic lavas is characterized by disequilibrium dusty and/or sieve textures, which are an indicator of rapid decompression during the eruption of magmas and/or signify magma mixing (Nelson and Montana, 1992). Trachytes occur both as lava flows and as volcanic domes (Fig. 3F). The main rock–forming minerals in trachytes are sanidine (10–15%), plagioclase (4–5%) and biotite (4–5%) phenocrysts, in a fine–grained, microcrystalline groundmass with trachytic texture. 2.2.3. Intrusive rocks Granitoids are exposed as stocks in the northern part of the SMC (Fig. 2). Field observations show that the Eocene granitoids crosscut both Late Neoproterozoic–Paleozoic sedimentary rocks and Cenozoic volcanic and pyroclastic rocks (Fig. 3G & H). Granitoids can be subdivided into granites and syenogranites. Syenogranites are extensive in the northern part of the SMC and exhibit a porphyritic texture with a medium–grained matrix. They have abundant euhedral to subhedral phenocrysts of K–feldspar (40–42%), quartz (25–30%), plagioclase (22– 24%), amphibole and/or biotite (4–5%). Subhedral to anhedral K–feldspar (0.5–2 mm) is usually perthitic. Quartz occurs in two generations; both as euhedral, early–formed crystals and anhedral late–stage grains. Magnetite, ilmenite, zircon, apatite ± titanite are the principal accessory minerals. Granites display medium to coarse–grained granular and porphyritic textures. They contain quartz (30–35%), plagioclase (32–35%), K–feldspar (28–30%), and biotite (5%). Magnetite, zircon, apatite, and titanite are accessory minerals. There are two types of plagioclase in the granitic rocks, including medium– (0.5 mm) and large–sized phenocrysts (~2 mm). Some of the K–feldspars are weakly altered, particularly to clay minerals. Quartz is medium–grained and shows undulose extinction. Biotite is occasionally replaced by chlorite and calcite. The mineralogies of the syenogranite and granite are quite similar, although the former has less quartz and plagioclase than the granites. 3. Results Details on the analytical procedures are given in Supplementary analytical methods. 3.1. Major– and trace–element geochemistry Major– and trace–element compositions of the SMC magmatic rocks are given in Supplementary Table 1. In the total alkalis vs silica diagram of (Lebas et al., 1986) (normalized to 100% on an anhydrous basis), the


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Fig. 3. Photographs of SMC igneous rock outcrops. A and B – outcrops of the SMC volcanic breccias. C – Close–up view of the tuff. D – Trachydacite lava sheets intercalated with pyroclastic rocks. E and F – Trachydacite and trachyte volcanic domes respectively. G and H - volcanic rocks are crosscut by granitoids.

Sangan granitoids plot in the trachydacite and dacite fields which are extrusive equivalent of syenogranite and granite (Fig. 4A). In Quartz–Alkali Feldspar–Plagioclase (QAP) diagram (after Streckeisen, 1979) (Fig. 4B), the Sangan granitoids plot predominantly in the fields of syenogranites and monzogranites, whereas the volcanic rocks plot in the fields of trachydacite, trachyte and dacite (Fig. 4A). Pyroclastic rocks including tuffs and ignimbrites have trachydacitic to rhyolitic compositions. Syenogranites and granites are characterized by high SiO2 (64.4–71.9 wt%), Al2O3 (13.3–16.9 wt%) and K2O (3.6–5.1 wt%), and K2O/Na2O ratios (0.8–1.3). Similar to granitoids, the volcanic rocks of the SMC which is characterized by high contents of SiO2 (63.1– 68.4 wt%), Al2O3 (15.2–17.7 wt%), K2O (4–5.9 wt%), has high K2O/ Na2O ratios (1.3–2.1). The SMC magmatic rocks have higher K2O contents than those from low–K calc–alkaline and calc–alkaline rocks and are similar to high–K calc–alkaline and shoshonitic rocks (Fig. 4C). The

SMC magmatic rocks are characterized by higher Sr (96–424 ppm), but variable Y (13–86 ppm) and lower Yb (1.9–3.4 ppm) contents than those from adakites in the Sabzevar. The Sr/Y (2.6–19.9) ratios of the SMC magmatic rocks are also different from those of adakites from Sabzevar (Moghadam et al., 2014a) and Birjand (Pang et al., 2013), but are similar to normal arc rocks through central Iran (Verdel et al., 2011) (Fig. 4D). Chondrite–normalized rare earth element (REE) patterns for the SMC granitoids show enrichment in light REEs (LREEs) compared to heavy REEs (HREEs) ((La/Yb)n = 8.81–33.11) and minor negative Eu anomalies (Eu/Eu⁎ = 0.45–0.80) (Fig. 5A). In a NMORB–normalized (Sun and McDonough, 1989) multi–element diagram, enrichment in LILE + Th and U, and negative anomalies in HFSE such as Nb, Ti and P, are conspicuous (Fig. 5B). Volcanic rocks show LREE–fractionated patterns with (La/Yb)n = 10.4–21.3 and negative Eu anomalies (Eu/Eu⁎

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Fig. 4. (A) Total alkalis vs SiO2 (Lebas et al., 1986), (B) Quartz–Alkali feldspar–Plagioclase (QAP) normative classification diagram for Sangan granitoids. (C) SiO2 vs K2O diagram (Peccerillo and Taylor, 1975), (D) Sr/Y vs Y (Defant and Drummond, 1990) for the SMC igneous rocks. Data for Birjand igneous rocks are from Pang et al. (2013), Sabzevar and Kashmar rocks from Moghadam et al. (2014a) and Moghadam et al. (2015b), central Iran from Verdel et al. (2011) and EIMB from Mohammadi et al. (2016) and this study.

= 0.67–0.73) in the chondrite–normalized REE diagram (Fig. 5C). These rocks are enriched in LILE + Th, U and depleted in HFSE in the NMORB– normalized multi–element diagram (Fig. 5D). Pyroclastic rocks including tuffs and ignimbrites are also enriched in LREEs and LILEs relative to HREEs a and HFSEs ((La/Yb)n = 8.33–17.2) with a more a pronounced negative Eu anomaly, compared to granitoids and volcanic rocks (Fig. 5E & F).

3.2. Whole rock Sr–Nd–Pb isotopes The Sr–Nd–Pb isotopic compositions of the SMC magmatic rocks are given in Supplementary Table 2 and plotted in Fig. 6. The initial 87Sr/86Sr (t = 42 Ma) ratios of granitoids vary between 0.70667 and 0.70795 and their εNd(t) values range between −3.5 and −4.5. The (87Sr/86Sr)t ratios of trachydacites (0.70606–0.70789), trachytes (0.70827) and dacites (0.70679) are similar to those of granitoids. The εNd(t)values of volcanic rocks vary from −3 to −4, displaying a similar narrow range with granitoid rocks. The initial 87Sr/86Sr and εNd(t)of the pyroclastic rocks vary from 0.71158 to 0.70339 and +0.7 to −5.3,respectively. Nd model ages (TDM; Depaolo et al., 1982) for the SMC magmatic rocks vary from 0.6 to ~1 Ga (Supplementary Table 2). All the SMC high–K magmatic rocks plot within the enriched quadrant of the conventional Nd–Sr isotope diagram, except for a single sample of tuff SG6–7 (Fig. 6A). The SMC igneous rocks show affinity to the shoshonitic rocks of Saray, northwest Iran and to the high–K calc–alkaline rocks of central Iran (Moghadam et al., 2014b; Sarjoughian et al., 2012) (Fig. 6B).

The 206Pb/204Pb and 208Pb/204Pb values of granitoids and volcanic rocks vary from 17.91 to 18.55 and 38.92 to 39.48, respectively (Supplementary Table 2). The samples are characterized by highly radiogenic 207 Pb/204Pb ratios, ranging between 15.63 and 15.67. The rocks plot above the Northern Hemisphere Reference Line (NHRL; Hart (1984)), in terms of both 207Pb/204Pb and 208Pb/204Pb (Fig. 6C & D). The highest 207 Pb/204Pb and 208Pb/204Pb values overlap those of the Global Subducting Sediments (GLOSS; Plank and Langmuir, 1998) and Enriched Mantle ‘2’ (EM II; Jackson and Dasgupta, 2008) reservoirs. The EM II mantle signature is commonly regarded as representing inheritance from recycling slab sediments (Worner et al., 1986), and/or involvement of the metasomatized sub–continental lithospheric mantle, SCLM. 3.3. U–Pb zircon geochronology Zircons from trachyte (1 sample), trachydacite (2 samples), dacite (1 sample), syenogranite (1 sample) and ignimbrite (1 sample) have been dated. 3.3.1. Trachyte (sample SG3–7) Zircon grains from the sample range in length from 100 to 200 μm, with length to width ratios between 1:1 and 2:1. Most zircons are euhedral and exhibit oscillatory zoning (Fig. 7A). These zircons have low to relatively high U (241.6–1802 ppm) and Th (292–2040 ppm) contents and Th/U ratios of 0.9 to 1.5. They display depletion in LREE with La(n)/Yb(n) = 0.00002–0.07, slight negative Eu anomalies with Eu/Eu⁎ = 0.07–0.14 and positive Ce anomalies with Ce/Ce⁎ = 8–1100


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Fig. 5. Chondrite–normalized rare earth element (left) and N–MORB–normalized trace element patterns (right) for the SMC magmatic rocks. Chondrite and N–MORB normalized values are taken from Sun and McDonough (1989).

(Fig. 8A), typical of magmatically crystallized zircons (Hoskin, 1998, 2005; Hoskin and Ireland, 2000). Eighteen analyzed grains form a cluster with a weighted mean 206Pb/238U age of 42.4 ± 0.6 Ma (MSWD = 0.8) (Fig. 9A). Two zircon grains show older 206Pb/238U ages of ~72 and 97 Ma, and are interpreted as xenocrystic zircons.

weighted mean 206Pb/238U ages of 43.5 ± 0.6 Ma (MSWD = 0.5) and 43.6 ± 0.9 Ma (MSWD = 0.4) (Fig. 9C & D).

Pb/238U age of

3.3.4. Syenogranite (sample GRA–1) Zircon separates from syenogranite have broken euhedral to short prismatic shapes and are mostly medium–grained (200 μm long). In CL images, most grains are weakly to moderately zoned (Fig. 7E). Eleven spots have been analyzed for this sample (Supplementary Table 3). U, Th contents and Th/U ratio range from 273 to 1043 ppm and 388 to 1106 ppm, respectively, and Th/U ratio changes from 0.5 to 2.4 Zircons display LREE depletion with La(n)/Yb(n) = 0.00006–0.17, slight negative Eu anomalies (Eu/Eu⁎ = 0.06–0.15) and positive Ce anomalies (Ce/Ce⁎ = 4–959) (Fig. 8E) (Supplementary Table 4). The weighted mean 206 Pb/238U age is 41.5 ± 0.6 Ma (MSWD = 1.1) (Fig. 9E), which is 1 to 2 Myr younger than zircon ages from the volcanic rocks.

3.3.3. Trachydacites (samples SG8–1 & SG8–5) Zircon grains in trachydacites are similar to zircons of dacites and trachytes. They are euhedral to subhedral and exhibit oscillatory zoning (Fig. 7C & D). They have low to high U (200–2025 ppm) and Th (370– 2900 ppm) contents and their Th/U ratios vary from 0.3 to 2.1 (mean 1.3). These zircons also show LREE depletion with La(n)/Yb(n) = 0.00002–0.017, slight negative Eu anomalies (Eu/Eu⁎ = 0.02–0.1) and positive Ce anomalies (Ce/Ce⁎ = 7.9–1370) (Fig. 8C & D). Most of the zircons have high contents of common lead. Two trachydacites yield

3.3.5. Ignimbrite (sample SG–12) Zircons from fine–grained agglomerates are euhedral to subhedral, 100 to 300 μm in length, and have length to width ratios between 1:1 and 3:1. CL images reveal that most zircons have well–developed oscillatory zoning (Fig. 7F). All analyzed grains have low to relatively high U (928–2651 ppm) and Th (396–800 ppm) contents and Th/U ratio (0.2– 0.4) which fall within the range representative of magmatic zircon. Zircons display slight LREE depletion with La(n)/Yb(n) = 0.0001–0.042, negative Eu anomalies (Eu/Eu⁎ = 0.0001–0.2) and positive Ce anomalies (Ce/Ce⁎ = 15–170) (Fig. 8F). The weighted mean 206Pb/238U age is

3.3.2. Dacite (sample SG5–7–1) Zircon grains from the dacite sample range in length from 100 to 200 μm, with length to width ratios between 1:1 and 2:1. Zircons are mostly euhedral and exhibit oscillatory zoning (Fig. 7B). Nineteen analyzed zircons show moderate to relatively high U (635–1590 ppm), and Th (614–3100 ppm) contents and Th/U ratios of 0.9 to 1.9. They are depleted in LREE with La(n)/Yb(n) = 0.00001–0.006, slight negative Eu anomalies (Eu/Eu⁎ = 0.02–0.09) and positive Ce anomalies (Ce/Ce⁎ = 27–850) (Fig. 8B). They yield a weighted mean 42.7 ± 0.5 Ma (MSWD = 0.6) (Fig. 9B).

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Fig. 6. εNd vs 87Sr/86Sr (A), 143Nd/144Nd vs 206Pb/204Pb (B), 208Pb/204Pb vs 206Pb/204Pb (C) and 207Pb/204Pb vs 206Pb/204Pb (D) diagrams for the SMC magmatic rocks. Mantle components including HIMU, EM I, EM II, DMM are from Zindler and Hart (1986); subduction–related, juvenile magmas and thick continental crust–derived rocks are from Defant et al. (1992) and Zhao and Zheng (2009) respectively. Data for Lut–Sistan magmatic rocks are from Pang et al. (2013), data for Alborz from Asiabanha and Foden (2012), central Iran from Verdel et al. (2011). Kashmar granites, Saray shoshonites and Sabzevar adakites & ophiolite data are from Moghadam et al. (2014a), Moghadam et al. (2014b), Moghadam et al. (2015b), and Moghadam et al. (2016b).

Fig. 7. CL images of zircon grains from the SMC igneous rocks. Red and yellow circles show analytical spots for U–Pb and Hf isotopes, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


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Fig. 8. Zircon trace elements plots from the SMC igneous rocks (chondrite normalized values are taken from Sun and McDonough (1989)).

42.6 ± 0.5 Ma (MSWD = 1.1) (Fig. 9F), similar to the ages obtained from other volcanic rocks.

involved during generation and evolution of the SMC magmatic rocks; 3–the implications these have on the crustal evolution of northeast Iran, EIMB and UDMB.

3.4. Zircon Hf–O isotopes 4.1. Pulses of arc magmatism in Iran The measured 176Lu/177Hf and 176Hf/177Hf ratios and δ18O values of zircons in SMC zircons are summarized in Supplementary Table 5. Zircon εHf vs U–Pb ages are shown in Fig. 10A. The εHf(t) values for zircons from volcanic rocks are relatively uniform and show positive values, varying between +0.45 and +3.5 (Fig. 10A). On the other hand, ignimbrite zircons have negative εHf (t) values; −4.1 to −1.4. Zircons from syenogranites have variable εHf(t) values, between −1.6 to +2.5. TCDM values of the SMC zircons vary between ~0.8 to 1.2 Ga (using 176 Lu/177Hf = 0.015; Griffin et al., 2004). The δ18O value of zircons from the volcanic rocks varies between 6.1 and 8.1‰, whereas syenogranites show δ18O values of 6.1 to 7.4‰ (Fig. 10B). These values are significantly higher than the δ18O typical of zircons from mantle–derived melts (~5.3‰; Eiler et al., 2000). 4. Discussion Our results provide new insights into three aspects of magmatism in northeast–east Iran 1–magmatic pulses and flare up during arc magmatism; 2–the magma sources and petrogenetic processes

The Eocene–Oligocene magmatic flare-up is one of the most significant magmatic events through Iran which lasted from 55 Ma to 25 Ma (Figs. 11 & 12) (Berberian and King, 1981; Camp and Griffis, 1982; Verdel et al., 2011). Several distinct magmatic pulses can be recognized during this flare-up. The new zircon U–Pb data obtained in this study, along with the age data reported in the literatures, reveal the geochronological framework of the Eocene-Oligocene magmatic pulses during this flare-up, in northeast Iran, UDMB and EIMB, as summarized below: In northeast Iran, the plutonic and volcanic rocks having adakitic and calc-alkaline characteristics (ca. 45–30 Ma; Moghadam et al., unpublished data) are common in the Arghash-Chah Salar regions. The older plutonic and volcanic rocks (ca. 55.4 Ma; Alaminia et al., 2013) with alkali-calcic, magnesian and metaluminous characteristics are also common. The new zircon U–Pb age data from igneous rocks indicates that the Eocene-Oligocene magmatic flare-up in northeast Iran includes one pulse which lasted between 55 and 30 Ma (Alaminia et al., 2013; Moghadam et al., 2015b), with a peak at 41–40 Ma (Moghadam et al., 2015b) (Fig. 11A).

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Fig. 9. U–Pb zircon concordia and weighted mean age diagrams for the SMC magmatic rocks.

Fig. 10. Zircon εHf vs 206Pb/238U ages (A) and δ18O (B) diagrams for the SMC magmatic rocks. Data for Sabzevar ophiolites are from Moghadam et al. (2014a), Cadomian rocks from Moghadam et al. (2015a) and Kashmar granites from Moghadam et al. (2015b). Hfpm/Hfc = ratio of Hf concentration in the parental magma (pm) to Hf concentration in crustal (c) rocks.


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Fig. 11. Histograms of age results from (A) the northeast Iran, (B) the EIMB and (C) UDMB along with (D) the spatial and temporal distributions of the magmatic rocks are clearly shown from the data collected and tested in this study and literature (E) histograms of TDMC ages of the Cenozoic magmatic zircons in Iran. (The data from UDMB are from Chiu et al., 2013; Chiu et al., 2017. The data from EIMB are from Pang et al., 2013; Mohammadi et al., 2016; Hosseinkhani et al., 2017.)

The Eocene-Oligocene magmatic flare-up and associated pulses were also identified by the zircon U–Pb dates on igneous rocks from the central (57–25 Ma) and southeast of UDMB (50–24 Ma) (Verdel et al., 2011) (Fig. 11B). These ages suggest that two magmatic pulses

occurred in UDMB, one from 55 to 42 Ma with a peak in Middle Eocene (~44 Ma) and the other from 40 to 22 Ma with two peaks in Late Eocene (~38–36 Ma) and Early Oligocene (32–30 Ma) (Fig. 11B). Eocene-Oligocene magmatic flare-up and associated pulses are also dominant within EIMB, with zircon U–Pb and 40Ar-39Ar ages varying from Eocene to Late Oligocene (46–25 Ma) (Pang et al., 2013) (Fig. 11B). The new U–Pb ages acquired during this study for the SMC magmatic rocks show that the granitoids and volcanic rocks were crystallized between 44 and 41 Ma, close to or slightly older than the Eocene igneous rocks from western parts of the Birjand–Zahedan suture zone (40 Ma) (Hosseinkhani et al., 2017). This indicates that the 4 Myr magmatic activity of SMC is part of Eocene-Oligocene magmatic flare-up in the EIMB (Fig. 11C and D), with no significant differences in TDMC ages for all zircons from these areas (Fig. 11E). 4.2. Magma source and petrogenesis

Fig. 12. Simplified chart showing the key magmatic events vs age in UDMB, EIMB, Sabzevar suture zone and Lut block. See text for references to each geologic event.

The SMC rocks consist mainly of pyroclastic rocks intercalated with lava (~42–44 Ma) and intruded by younger granitoid stocks (~41 Ma). Intermediate and silicic lavas and tuffs comprise N90 vol% of the SMC magmatic rocks. Basalts and mafic intrusive rocks are absent. All SMC intermediate to felsic rocks have high–K calc–alkaline to shoshonitic signatures and show geochemical characteristics of I–type granitic rocks. Geochemical signatures of the SMC igneous rocks indicate arc magmatism, including depletion in Nb and enrichment in LILEs (e.g.,

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Cs, Rb, Th, U, Sr, K), LREEs (e.g., La, Ce) (Pearce and Peate, 1995; Thirlwall et al., 1994) and high Ba/La (8.0–27.8) and low Nb/La (0.25–0.71) ratios compared to N–MORB (Sun and McDonough, 1989). Overall, the intermediate to felsic rocks of the SMC rocks have high concentrations of Zr, Hf and Y, typical of continental–arc magmas. Although these geochemical characteristics imply a crustal contribution, the relative depletion of Nb and other HFSEs relative to the LREEs, and enrichment of LILEs may be related to downgoing slabs or inherited from anatexis of an existing arc in a subduction regime. Due to lack of S-type granite, it is inferred that crustal anatexis of upper crust cannot be ruled in the petrogenesis. However, the role of the lower existing mafic continental crust, because it is isotopically and geochemically very difficult to distinguish between mantle melt and melt evolving from the partial melting of the juvenile lower crust, difficult to evaluate, but cannot be completely ruled out. Isotopically, the SMC intrusive and extrusive rocks are cogenetic. Except variation in Sr isotope composition of the pyroclastic rocks, variations in whole rock Nd and zircon Hf can be ascribed to the different degrees of the juvenile melt interaction with continental crust. Zircon trace–element data also indicate that these rocks did not equilibrate with residual garnet, and that plagioclase was not a major fractionating phase during the magmatic evolution (excluding the ignimbrite with conspicuous negative Eu anomalies). It is generally believed that high–K I–type felsic rocks may be derived from melting of hydrous intermediate to mafic high–K meta–igneous rocks (e.g., Roberts and Clemens, 1993; Sisson et al., 2005). Alternatively, high-K, I-type magmas may form by mixing of mantle– derived magmas with crustal melts (e.g., Hildreth et al., 1991; Huang et al., 2013). Moreover, fractional crystallization (FC) of mantle–derived magmas is an alternative mechanism (Grove et al., 2005). With the exception of a single ignimbrite, the SMC rocks show high SiO2 contents (62–74 wt%), low MgO and radiogenic Sr–Nd–Pb isotopes, suggesting they did not equilibrate with the upper mantle. This criterion, and the lack of the mafic counterparts in the study area, argues against derivation of the SMC magmas from mantle melts via FC and/or mixing of mantle–derived magmas with crustal–derived materials. The Nd isotopic compositions of the SMC magmatic rocks are quite similar (except sample SG6–7) and suggest cogenetic relationships. Some pyroclastic rocks have unusual lower (~0.703) or higher (87Sr/86Sr)t content (N0.710) (Supplementary Table 2), that could be related to hydrothermal alteration and the variable whole–rock Rb/Sr ratios of these rocks (87Rb/87Sr has been calculated using whole–rock Rb and Sr data obtained from ICP–MS). Based on the Sr–Nd–Pb isotope characteristics and the enriched LILE and K signature of the SMC rocks, the source is thought to be an enriched SCLM that resembles EM II. However, the lack of basaltic shoshonitic rocks, the melting products of the SCLM, cannot be easily addressed with this hypothesis. Therefore, the Sr–Nd–Pb isotopic signatures and trace element characteristics of the SMC indicate interaction of the mantle–derived magmas with thick continental crust during ascent, storage and evolution of the SMC magmas. The wide range of Hf isotopic composition of the SMC zircon (εHf = −4.1 to +3.5) precludes a simple evolution of the SMC magmas by fractional crystallization, but other mechanisms such as wall–rock assimilation can potentially explain the observed Hf isotope variation. Therefore, the heterogeneous distribution of the zircon Hf–isotope data, as well the isotopically heavy O isotopes of zircon (Fig. 10A) and the whole–rock Nd isotopic compositions of the SMC rocks indicate that mantle–derived magma and pre–existing crustal material were involved in their genesis. Isotopically, the mantle melt would be similar to the melts of the Sabzevar ophiolite, and indicates their derivation from a source with time–integrated depletion (high εHf). Whole–rock Nd and zircon Hf–isotope model ages suggest a 0.8–1.2 Ga old continental crust to have interacted with juvenile magmas, but as the SMC magmatic rocks are mixing products of this interaction, the continental crust involved might be older than 1.2 Ga. Crustal outcrops with these ages (~1.2 Ga) are absent in northeast–east Iran, but as the Late Neoproterozoic (Cadomian)

metamorphosed volcano–sedimentary rocks host the SMC rocks, we assume the Cadomian (500–600 Ma) lower crust as the continental assimilant (Fig. 10A). The lack of inherited cores in the zircons used for U–Pb dating makes it difficult to be more precise about the continental crust involved. The existence of xenocrystic zircons depends on factors such as zircon size, magma temperature and magma chemistry (Watson and Cherniak, 1997; Watson et al., 2006), and it could be argued that the SMC magmas have dissolved any xenocrystic zircons. The high temperature and high water content of the subduction–related high–K magmas favor the absence of inherited zircons (Watson, 1996). To further evaluate the possibility of source mixing and/or assimilation– fractional crystallization (AFC) processes during generation and evolution of the SMC igneous rocks, modeling of trace elements (La, Th, Nb) and isotopic data was done using of mantle–derived juvenile melts (Sabzevar ophiolite lava) and Cadomian–like lower crust as end–members. In the modeling the fractionation phases were assumed to be typical of POAM (plagioclase, olivine + orthopyroxene, amphibole (+Cpx) and magnetite) fractionation, which is typical in arc environments for the generation of felsic melts from a mafic parent (Gill, 1984). The starting melt (Sabzevar lava) is isotopically identical to the basaltic–andesitic lavas from EIMB, with 87Sr/86Sr = 0.7044 and 143Nd/144Nd = 0.5128 (Pang et al., 2013). The isotopic modeling (Fig. 13) shows that the SMC igneous rocks follow the AFC trend in the La, Th and Nb vs 143 Nd/144Nd plots, with the exception of sample SG6–7 which is a tuff with slightly more radiogenic Nd. In La, Th, 143Nd/144Nd vs 87Sr/86Sr plots, all samples, except pyroclastic rocks with unusually low or high 87 Sr/86Sr values, follow the AFC trend, although some samples have higher 87Sr/86Sr, which might show interaction of parent magmas with an assimilant with higher 87Sr/86Sr than the local Cadomian crust. Therefore, both whole–rock trace–element/isotope and zircon Hf–O isotopic modeling are consistent with a AFC process involving interaction between a thick continental crust and a mantle–like melt. The Lut block, which hosts the SMC magmatic rocks, contains abundant outcrops of Cadomian basement. Geophysical data show that this block is thick enough in its NE parts (Moho depth at ~40 km, EntezarSaadat et al., 2017) to sustain AFC processes. The Kashmar granites from NE Iran (Moghadam et al., 2015b), show lower degrees of assimilation of Cadomian crust than the SMC rocks (Fig. 13). The basaltic–andesitic lavas from the EIMB with juvenile isotopic signatures (Pang et al., 2013), may indicate ponding of the mafic magmatic end–member in the deep crust. However, the higher content of HFSEs and REEs in the SMC rocks may show the enrichment of these elements in their mafic parent and hence suggest inheritance of their signature from an asthenospheric component. 4.3. Crustal architecture in the Iranian arcs The Hf “crustal” model ages, or TDMC, of zircons from Eocene-Oligocene magmatic rocks in the northeast Iran, EIMB and UDMB are used to estimate timing since their host magmas were derived from the presumed depleted-mantle source (cf. Griffin et al., 2002). The studied Eocene-Oligocene magmatic rocks from these regions have various origins from different magmatic domains. Therefore, comparison of Hf isotopic results for these magmatic zircons of UDMB and EIMB as well as northeast Iran is used to evaluate their sources (Fig. 11E). The results show that there are no significant differences in the origins of various rocks from these magmatic zircons. As a result, all measured Hf isotopic compositions of magmatic zircons in this study, Zahedan and Shah Kuh granites from EIMB are compiled together with Eocene-Oligocene magmatic zircon from the UDMB and northeast Iran (Fig. 11D). For the following discussion, the calculated TDMC ages of all zircons from different domains of Iran are shown as histograms (Fig. 11E). The combined TDMC age histogram of magmatic zircons from UDMB mainly ranges from 1.2 to 0.2 Ga with a major peak at ∼0.5 Ga, whereas those from EIMB and northeast Iran dominantly ranges from 1.2 to 0.8 with a major peak at ∼0.8 Ga.


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Fig. 13. La, Th, and Nb vs 143Nd/144Nd and La, Th, 143Nd/144Nd vs 87Sr/86Sr plots with modeled FC, AFC and mixing (r = 0.4) curves to demonstrate the assimilation–fractional crystallization pathways between a depleted mantle–derived starting melt (Sabzevar ophiolite lava with La = 8.9, Th = 2.1, Nb = 2.1, 143Nd/144Nd = 0.5128 and 87Sr/86Sr = 0.7043) and a Cadomian granitic gneiss from central Iran (La = 21.9, Th = 11.7, Nb = 10, 143Nd/144Nd = 0.5118 and 87Sr/86Sr = 0.7139). An average POAM fractionating composition of feldspar (20%) + olivine (15%), Opx (25%), Cpx (20%) + amphibole (10%) + magnetite & apatite (10%) according to Gill (1984) is used in these models. The composition of average Cadomian lower and upper crust is according to Moghadam et al. (2015a, 2015b). Sabzevar lava composition is from Moghadam et al. (2014a, 2014b). The Kashmar granitoid and east Iran magmatic rocks, Cretaceous–Eocene are shown for comparison.

The Eocene magmatic rocks (55–33 Ma) are extensively exposed in the UDMB, EIMB and northeast Iran (Fig. 12). In the UDMB, the Eocene rocks crop out as parts of the extensive Neotethyan subduction-related magmatic activity (Agard et al., 2011; Verdel et al., 2011). They are predominantly granitic to gabbroic and are characterized by mainly positive zircon εHf(t) of −2 to +15 (Fig. 10A). However, in the northeast Iran (Moghadam et al., 2015a, 2015b) and EIMB (Mohammadi et al., 2016), the same Eocene magmatic rocks are dominated by negative zircon εHf(t) (−7 to +4) (Fig. 10A), and are significantly less than those from UDMB. In addition, both the northeast Iran and EIMB are part of Eocene-Oligocene magmatic flare-up at ~42 Ma (Fig. 11A and C) whereas those from UDMB lasted from ~55 to 25 Ma with increased contribution from mantle-derived melts from northeast Iran and EIMB to UDMB. Such events for UDMB were interpreted to have been a result of the slab breakoff of the Neo-Tethys Ocean lithosphere beneath the central Iran (Verdel et al., 2011), while those from EIMB were resulted from post-collisional collapse and associated extension in the Lut Block (Pang et al., 2013). 4.4. Geodynamic implications The Eocene–Oligocene magmatic flare-up in Iran represents magmatic pulses (Verdel et al., 2011; Pang et al., 2013), during a ~20– 25 m.yr. time period (Chiu et al., 2013; Pang et al., 2013) (Fig. 11),

with an Andean–type belt of intrusive and extrusive rocks in the UDMB and EIMB. Break–off of the subducted continent–ocean transitional lithosphere beneath the Zagros Mountains (e.g., Molinaro et al., 2005) and/or lithospheric thickening with partial delamination to the northeast of the Zagros (e.g., Hatzfeld and Molnar, 2010) are suggested to have caused these magmatic pulses throughout Iran. During Late Cretaceous–Early Oligocene time, continuous convergence between Arabia and Iran led to the closure of the southern Neotethyan basin, emplacing the Late Cretaceous Zagros Iranian ophiolites during the transition from a compressional to an extensional convergent plate margin (Agard et al., 2011; Rossetti et al., 2014). In Iran, this extension, including the closure of the southern Neotethyan basin, was followed by orogenic collapse during middle Eocene–Early Oligocene. Middle Eocene–Early Oligocene extension and lithospheric thinning might have been accompanied by decompression melting of upwelling hydrous asthenosphere (Verdel et al., 2011; Verdel et al., 2007). The extension may also have been accompanied by lithospheric delamination, further stimulating extension and rapid exhumation of the central Iranian core complexes. Magmatism within the SMC along with the EIMB was synchronous with magmatism within the UDMB. However, the Eocene–Miocene magmatic rocks in the eastern side of the Lut block and within the Birjand–Zahedan suture zone (the EIMB), including the SMC, are linked to the subduction of the Birjand–Zahedan oceanic basin beneath the Lut block (Pang et al., 2013) and are different from the UDMB magmatic

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rocks which are related to the subduction of the Neo-Tethys oceanic basin beneath the central Iranian micro continent. The occurrence of the SMC magmatic rocks in a subduction–related regime or through a post–collisional setting is a matter of debate. Zircon from the Birjand– Zahedan suprasubduction zone–type gabbros have yielded ages of 113–107 Ma (Zarrinkoub et al., 2012), showing that subduction was active during early–late Cretaceous time. High–P rocks give ages of ~89– 86 Ma (Bröcker et al., 2013), denoting the exhumation of the metamorphosed oceanic slab in the Coniacian (Late Cretaceous). The collision between the Lut and Afghan block is suggested to have occurred during latest Cretaceous time (ca. 60 Ma) and therefore, the Paleogene magmatism in the EIMB arose after closure of the Birjand–Zahedan oceanic basin, in a post–collisional setting (Angiboust et al., 2013; Zarrinkoub et al., 2012) (Fig. 12). This magmatism could be triggered by the convective removal of lithosphere (delamination) and resultant asthenospheric upwelling accompanying extensional collapse (Pang et al., 2013). Generally, such magmatic pulses are common in extensional settings above subduction zones and/or in post–collisional settings. High–K calc–alkaline felsic rocks are rare in anorogenic settings but common in convergent margin environments, particularly in post–collisional settings after basin closure and continental collision (e.g., Roberts and Clemens, 1993; Barbarin, 1999; Kemp et al., 2009). The most favorable tectonic setting for the formation of these rocks is the post–orogenic collapse following crustal thickening by continental collision and intraplate extension (e.g., Barbarin, 1999; Roberts and Clemens, 1993; Jiang and Li, 2014; Li et al., 2007). The new isotopic data from this study emphasize that the SMC igneous rocks are connected to the pooling of mafic magmas in the continental crust of the Lut block, producing thermal anomalies and reworking of the Lut crust (Fig. 14).

Available data demonstrate both similarities and differences between the UDMB and EIMB, including: (1) Most of the magmatic pulses in the UDMB were related to the subduction of the Neotethys beneath central Iran before collision between Arabia and Iran (Chiu et al., 2013), whereas those from EIMB occurred in a post–collisional setting, after collision of the Lut block with Afghan block. (2) The Eocene-Oligocene magmatic rocks, both in EIMB and UDAB, are petrologically and isotopically heterogeneous. They vary from calc–alkaline to shoshonitic and even adakitic rocks and have isotopic signatures showing interaction of mantle–derived juvenile melts with continental crust (Moghadam et al., 2015b). (3) Although mantle–derived juvenile rocks are present in both the UDMB and the EIMB, the juvenile signature (high zircon εHf and whole rock εNd values) seems to increase towards the Miocene in the UDMB relative to EIMB. 5. Conclusions 1– The Eocene SMC magmatic rocks are part of a voluminous (~N400 km) magmatic belt in east Iran, the Eastern Iranian Magmatic Belt. This belt is recognized as an Eocene–Oligocene flareup and associated pulses dominated by calc–alkaline and shoshonitic rocks. 2– The SMC magmatic rocks are characterized by high K2O and enrichment in LREEs and LILEs, along with depletion in HFSEs. These rocks are geochemically high–K to shoshonitic rocks and resemble those erupting along active continental margins. 3– The SMC intrusive, extrusive and pyroclastic rocks have similar zircon U–Pb ages and Sr–Nd–Pb isotope compositions, suggesting they are cogenetic and erupted during a ~4 Myr interval. 4– The whole–rock Sr–Nd–Pb and zircon Hf–O isotope data show that the SMC magmatic rocks probably resulted from significant interaction of juvenile melts with old (~0.8–1.2 Ga) continental crust through AFC process. 5– The magmatic flare-up in EIMB including the SMC was triggered by convective removal of lithosphere and resultant asthenospheric upwelling, accompanying extensional collapse. Supplementary data to this article can be found online at https://doi. org/10.1016/j.gr.2018.01.008. Acknowledgments This paper is part of PhD dissertation of the first author, supported financially by Faculty of Earth Sciences, China University of Geosciences, Wuhan, (2012CB416802) China. We gratefully acknowledge the financial support from the National Science Foundation of China (grant number: 41272079). University of Tehran and Iranian Mines and Mining Industries Development and Renovation Organization (IMIDRO) are thanked for support during the course of this study. We would like to thank Dr. Golmohammadi and Sangan Iron Ore Complex for the generous support and the access to the mine. W.L. Griffin is thanked for editing of the text. We would like to thank Prof. Inna Safonova for editorial handling of the paper, as well as Dr. Hassanzadeh, Dr. Milidragovic and two anonymous reviewers for their constructive comments. References

Fig. 14. Schematic diagram for magmatic events and AFC processes along the EIMB including the Sangan Magmatic Complex during Eocene–Oligocene time. The magmatic events was triggered by post–collisional collapse in combination with asthenosphere upwelling and thermal pulses due to the underplating of mafic magmas beneath the thickened crust of the Lut block, resulting in AFC processes and reworking of ~1.2 Ga crust. (Modified after de Silva et al. (2015).)

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