Foraminiferal-based paleobiogeographic reconstructions in the Carboniferous of Iran and its implications for the Neo-Tethys opening time: a synthesis

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Carboniferous deposits are sporadic and discontinuous in Iran except for Lower Carboniferous strata. The Alborz zone and central Iran have received most of the attention, being subject of stratigraphical and paleontological investigations (Bozorgnia, 1973; Vachard, 1996; Leven et al., 2006; Leven and Gorgij, 2008; Falahatgar et al., 2012, 2015; Zandkarimi et al., 2014, 2017; Vachard and Arefifard, 2015). After the description of the Early Carboniferous Mobarak and Geirud formations by Assereto (1963, 1966), foraminiferal zonations were recognized in several stratigraphic sections of central and eastern Alborz Mountains (Bozorgnia, 1973; Lys et al., 1978; Vachard, 1996; Ueno et al., 1997; Brenckle et al., 2009; Falahatgar et al., 2012, 2015; Zandkarimi et al., 2014, 2017). The Carboniferous deposits of central Iran have been studied in Tabas (Shotori, Shirgesht, and Ozbak-Kuh areas), Posht-e Badam and Yazd blocks and in the Shahreza area. Carboniferous foraminiferal associations of Central Iran have been reported from the Sardar Formation, at the North and South-East of the Tabas town, and from the Gachal Formation, Kalmard area, West of Tabas (Vachard, 1996; Leven et al., 2006; Vachard and Arefifard, 2015). There are some other records of Carboniferous foraminifera in the Yazd Block (Leven et al., 2006). In the Shahreza area only Upper Carboniferous foraminifers have been documented (Baghbani, 1993, Leven and Gorgij, 2008). In the Sabzevar zone, poorly preserved upper Pennsylvanian foraminifers have been reported (Gorgij and Leven, 2013)

The paleogeographical position of Iran during the Carboniferous has been discussed by many authors (Meissami et al., 1977; Sengör, 1990; Stampfli, 2000; Kalvoda, 2002; Torsvik and Cocks, 2004; Wendt et al., 2005; Devuyst, 2006; Brenckle et al., 2009; Muttoni et al., 2009; Zandkarimi et al., 2014, 2017; Vachard and Arefifard, 2015). In order to recognize the paleobiogeographic affinity of Iran during Carboniferous times, improved investigations of foraminiferal assemblages have been undertaken with the following results. In the Upper Paleozoic: i) foraminifers were widespread within tropical and subtropical paleolatitudinal belts (Boudagher-Fadel, 2008; Vachard et al., 2010; Davydov, 2014); ii) they were very sensitive to water temperature, and their associations display different compositions in lower latitude tropicalsubtropical belts versus intermediate and higher latitudes, i.e. temperate and boreal belts (Beavington-Penney and Racey, 2004; Murray, 2006; Davydov, 2013; Prothero, 2013); iii) foraminifers had a rapid evolution reaching a high taxonomic diversity and iv) they have adapted to abiotic factors such as the paleoenvironmental shift led by tectonics (Kalvoda, 2002). The data for this study comprise author’s own published and unpublished data gathered during years of field work in different parts of Iran and data from literature. Attention is first focused on the distribution of Carboniferous foraminifers in Iran to analyse later their affinity to either warm-water paleoequatorial belts, temperate belts, or cold-water higher latitude belts. Furthermore, the time of the opening of Neo-Tethys in Iran is discussed based on the integration of paleontological data with published data on Carboniferous volcanism and tectonics (Saccani et al., 2013; Tavakoli-Shirazi et al., 2013; Dilek et al., 2014; Moghadam et al., 2014).

Iran is located in the center of the Alpine-Himalayan orogenic belt which starts in western Europe and passes through Turkey, Iran, Afghanistan into Tibet and probably Burma and Indonesia, where it links with the circumPacific orogenic belt (Stöcklin, 1974; Moores and Twiss, 1995; Prelevi et al., 2013). Iran is divided into several tectonic units according to different stratigraphic, structural, magmatic and metamorphic features (Stöcklin, 1968; Stampfli, 1978; Berberian and King, 1981; Alavi, 1991; Golonka, 2002; Scotese, 2004; Torsvik and Cocks, 2004). For the structural divisions of Iran (Fig. 1), the tectonic maps by Alavi (1991) and Angiolini et al. (2007) are used herein.

The paleogeographic reconstruction of Iran for the Late Paleozoic has always been controversial. The absence of Variscan orogeny in Iran and the resemblance of most Paleozoic deposits to their coeval strata in the Arabian Plate led to the idea that Iran had a Gondwanan affinity during the Late Paleozoic. The Late Paleozoic was a period of major tectonic events. By the Early Permian, the supercontinent Pangea had coalesced, the Paleo-Tethys ocean was closed, and Cimmerian terranes including present-day Turkey, Iran, Afghanistan, South Pamir, Karakoram, Tibet, Qiantang, Burma, Thailand and Malaysia broke-off from the northern margin of Gondwana and drifted northward across the Paleo-Tethys as a result of the Neo-Tethys opening to collide finally with the Eurasian margin in the Late Triassic, giving rise to the Eo- Cimmerian Orogen (Sengör, 1984; Scotese and Langford, 1995; Besse et al., 1998; Metcalfe, 1999; Angiolini et al., 2003; Kent and Muttoni, 2003; Ruban et al., 2007; Muttoni et al., 2009; Berra and Angiolini, 2014).

The taxonomic composition of Carboniferous foraminifers in Iran has been studied since the 1960s by Iranian and European workers. But, there is no consensus on their paleobiogeography. The results of previous studies suggest that these foraminifera were related to either the southern margin of the Paleo-Tethys (Meissami et al., 1977; Vachard, 1980, 1996; Stampfli, 2000; Torsvik and Cocks, 2004; Brenckle et al., 2009; Zandkarimi et al., 2014; Qiao et al., 2016) or the northern margin (Kalvoda, 2002; Devuyst, 2006). According to the paleomagnetic data from the Alborz zone, Iran shows an Apparent Polar Wandering (APW), well-matched with that of West Gondwana (Besse et al., 1998; Muttoni et al., 2009). These data suggest a paleolatitude for Alborz of about 40-45º South in the Early Carboniferous. Considering these data, North Iran was positioned in the northern margin of Gondwana during this period (Angiolini et al., 2007; Brenckle et al., 2009; Bahrammanesh et al., 2011; Falahatgar et al., 2015; Qiao et al., 2016).

Paleontological evidence stands in opposition to the presumed 40-45º South location of North Iran in the Early Carboniferous. The smaller foraminifers and brachiopods reported from the Lower Carboniferous of Alborz are indicative of warm water (Brenckle et al., 2009; Bahrammanesh et al., 2011). Warm water faunas from Iran may be explained by a counterclockwise oceanic current that conducted equatorial warm Paleo-Tethyan waters to the peri-Gondwanan blocks, including Iran at the southern margin of the Paleo-Tethys (Kiessling et al., 1999; Brenckle et al., 2009; Bahrammanesh et al., 2011). Additionally, during the Early Carboniferous there were no glacial events to hamper this warm surface current gyres and the dispersion of taxa from the tropics towards intermediate latitudes. Although the climate began to cool in the late Visean because of the onset of the late Visean glacial event (Melvin et al., 2010), warm water and cosmopolitan faunas of this age in Alborz (Bozorgnia, 1973; Zandkarimi et al., 2014) confirm the continuation of the warm surface-current gyres in the southern border of the Paleo-Tethys (Brenckle et al., 2009).

Zandkarimi et al. (2014) and Falahatgar et al. (2015) speculated that Alborz was located in the southern margin of the Paleo-Tethys up to the early Visean, based on the presence of foraminifers with typical elements of peri- Gondwanan affinity. They hypothesized that since the Paleo-Tethys was narrow in the early Visean, the peri- Gondwanan terranes in the southern border of the Paleo- Tethys were easily connected with those of the northern margin. Moreover, these authors noted that the mid Visean foraminiferal assemblages in Alborz were similar to those of the northern margin of Paleo-Tethys and concluded that Alborz rifted from the northern margin of Gondwana and migrated to the North in the early mid Visean. They considered this as the first stage of the Cimmerian terranes assembly.

The upper Tournaisian-lower Visean foraminifers of Posht-e Badam, central Iran, are similar to those of Alborz (Vachard and Arefifard, 2015). It has also been mentioned that the foraminiferal species identified in the Lower Carboniferous of the Posht-e Badam Block have common features with both the northern and southern margins of the Paleo-Tethys. Vachard and Arefifard (2015) suggested that since Alborz has been attributed to the peri-Gondwanan margin during the late Tournaisian-early Visean by many authors, the Posht-e Badam Block was probably located in the southern margin of the Paleo-Tethys as well.

Bashkirian-Moscovian foraminiferal assemblages have been examined both in North and central Iran, but beyond these regions there are no reports in Iran, possibly due to the lack of deposition or erosion. In the Tang-e Darchaleh section, near the town of Shahreza, Moscovian foraminifers were reported and showed biogeographic affinity with the eastern European platform (Baghbani, 1993). The Bashkirian-Moscovian foraminiferal associations in North and central Iran reveal great similarities. They also show strong relationship with coeval foraminifers from Turkey, central Asia, Donets, Russian Platform and the Urals located in the northern margin of the Paleo-Tethys during this period (Kalvoda, 2002; Leven et al., 2006; Dzhenchuravea and Okuyucu, 2007).

Vachard (1980, 1996) compared the Carboniferous deposits in the Tabas region, Central Iran, with those of central Afghanistan, and the ones in Alborz strata with the Band-e Bayan sequences in northern Afghanistan. He concluded that there were no similarities between the Carboniferous basins in North and central Iran. Leven et al. (2006) objected to Vachard’s correlations and suggested that there is no faunal evidence to support these comparisons. They argued that Bashkirian-Asselian foraminifers in Iran are not endemic and both central and North Iran belonged to the same basin in the Bashkirian- Moscovian time.

There is no evidence of Kasimovian foraminifers in Iran, may be as a result of a regional regression and subsequent subaerial erosion. The uppermost Gzhelian foraminiferal faunas in Alborz, central Iran and the Sabzevar zone contain typical elements of the northern margin of the Paleo-Tethys and have resemblance to those of the Carnic Alps, Donets Basin, Eastern European Platform, Darvaz and Fergana (Leven and Gorgij, 2006a,b; Gaetani et al., 2009; Leven and Gorgij, 2011a,b,c; Gorgij and Leven, 2013).

Based on the available data on Lower Carboniferous foraminifera, it is plausible that Iran was located in the southern margin of the Paleo-Tethys and displayed a peri- Gondwanan faunal biogeographic affinity in the Early Carboniferous (Fig. 3). The presence of faunal elements of both northern and southern Paleo-Tethys supports this idea (Bagheri and Stampfli, 2008; Brenckle et al., 2009; Bahrammanesh et al., 2011; Berra and Angiolini, 2014; Vachard and Arefifard, 2015). Furthermore, the concurrent occurrences of both faunas may be explained, either by the southerly circulation of the warm-water Paleo- Tethyan current along the northern margin of Gondwana (Brenckle et al., 2009; Bahrammanesh et al., 2011) or by the narrowness of the Paleo-Tethys ocean allowing the connection of the peri-Gondwanan faunas with those of the southern margin of Laurassia (Zandkarimi et al., 2014; Vachard and Arefifard, 2015; Falahatgar et al., 2015). The paleogeographic position of Iran as part of Gondwana in the Early Carboniferous is also supported by the strong accordance of paleomagnetic poles of Iran and West Gondwana, during the Ordovician-Early Carboniferous (Muttoni et al., 2009). In fact, based on paleomagnetic data, Iran has a similar drifting history as the Arabian Plate from Late Ordovician to Early Carboniferous (Muttoni et al., 2009). The Ordovician-Carboniferous Gondwana apparent polar wander path shows that the Arabian Plate, as part of Africa, sustained an important plate motion, drifting from a high southern latitude in the Late Ordovician to a subequatorial latitude in the Early Devonian and then moved to intermediate southern latitudes in Late Devonian- Early Carboniferous times (McElhinny et al., 2003).

The faunal assemblages of the Late Carboniferous in Iran are clearly different from those of the peri-Gondwanan realm. They differ both in taxonomic composition as in climatic affinity. The Bashkirian-Moscovian and upper Gzhelian fusulinids in Alborz, central Iran and the Sabzevar zone indicate warm water conditions and are typical of the northern margin of the Paleo-Tethys. This may be linked to the rifting of Iran from the northern margin of Gondwana in early Late Carboniferous and its migration northwards. The movement of Iran toward the North, to a low latitude position with a warm climate is suggested by the occurrence of the Upper Carboniferous coal-bearing sandstones of the Sardar Formation in central Iran (Stöcklin et al., 1965). Coal is one of the most important paleoclimatic indicators of humid climate conditions. These sandstones are rich in quartz and poor in Al.O. indicating humid to semi-humid climate. Moreover, the geochemical analyses of the Sardar Formation sandstones suggested a high degree of chemical weathering, indicative of warm and humid conditions during the deposition of these sandstones (Khanehbad et al., 2012). Besides, the paleogeographic reconstruction of Iran based on paleomagnetic data suggests a 30ºS position for Iran (Muttoni et al., 2009). The whole dataset suggests a warm and humid climate during the Late Carboniferous in central Iran.

Neotethyan rifting started in Carboniferous times along the eastern Gondwana margin from India (Garzanti and Sciunnach, 1997) to Oman (Al-Belushi et al., 1996). The continental breakup and the onset of the oceanic lithosphere formation followed from the Himalayas towards northern Oman in mid-Early Permian time (Garzanti, 1999; Angiolini et al., 2003).

Indications of the early phases of the Neo-Tethys opening during the Carboniferous and rifting of the Cimmerian terranes from northern Gondwana are recorded in the Misho magmatic complex and Ghushchi complex in northwestern Iran (Saccani et al., 2013; Dilek et al., 2014; Moghadam et al., 2014). Using SIMS U-Pb zircon ages, the Misho gabbros are dated at 356.7±3.4Ma and the gabbronorites and granites of the Ghushchi complex yield an age of ~320Ma. These igneous rocks may provide good evidence of the timing of the Neo-Tethys opening in Iran and cast doubts about the previous ideas of the rifting of Neo-Tethys in Iran in the Early Permian (Angiolini et al., 2007; Gaetani et al., 2009; Muttoni et al., 2009).

FIGURE 3
Early Carboniferous paleogeographic map simplified and modified from Domeier and Torsvik (2014) showing the location of Alborz and central Iran in the southern margin of the Paleo-Tethys in the Early Carboniferous. A: Annamia, K: Kazakhstania, NC: North China, SC: South China, SS: Sanandaj-Sirjan.

Proofs about Carboniferous magmatic activities and extensional deformation, related to the Variscan orogeny are increasing in Iran. For instance, an angular unconformity below the Upper Carboniferous(?)- Lower Permian Faraghan Formation in the High Zagros Belt suggests a connection with the Variscan orogeny (Tavakoli-Shirazi et al., 2013). However, the North to North-East trending faults and tilted blocks related to this Variscan unconformity show no signal of compressional deformation. The Ghushchi complex and Khalifan A-type granites in northwestern Iran are thought to be in conjunction with crustal movements in an extensional tectonic setting (Moghadam et al., 2014). Zanchi et al. (2015), according to the radiometric ages of the trondhjemite dikes and stocks that intruded the accretionary wedge of the Anarak metamorphic complex in central Iran, it was proposed that the subduction of the Paleo- Tethys happened during the Late Carboniferous. These authors suggested that the Anarak metamorphic complex developed along the southern Eurasian margin and was part of an allochthonous crustal fragment belonging to the Variscan belt before the Cimmerian collision of Iran.

Therefore, the Neo-Tethys opening and rifting of Iran from the northern margin of Gondwana has probably occurred sometime between the latest Visean to early Late Carboniferous (Bashkirian) (Fig. 4). Evidence for this conclusion are: i) the northern Paleo-Tethyan affinity of the foraminiferal associations in the late Visean-late Sepukhovian in Alborz, ii) the unique similarity between the Bashkirian-Kasimovian and late Gzhelian fusulinid assemblages of Iran to the northern margin of the Paleo- Tethys and iii) the Carboniferous magmatic activities in northwestern Iran and normal faulting and tilted blocks in the High Zagros Belt which, taken together, support extensional deformation related to the Variscan orogeny.

FIGURE 4
Late Carboniferous paleogeographic map simplified and modified from Domeier and Torsvik (2014) showing the rifting of Alborz and central Iran from the northern margin of the Gondwana in the Late Carboniferous. KK: Karakoram, NR: Neo-Tethys Ridge. For the other abbreviations see caption of Figure 3

• In the Early Carboniferous, foraminiferal associations in North and central Iran had a cosmopolitan character with elements from both the southern and northern margins of Paleo-Tethys. These faunal assemblages confirm the paleoposition of Iran in intermediate southern latitudes (ca. 30ºS) and still forming part of Gondwana. This foraminiferal-based paleogeographic reconstruction of Iran is consistent with the results of paleomagnetic data by Muttoni et al. (2009).

• In the Late Carboniferous, data from both smaller foraminifer and fusulinid assemblages in North and central Iran reveal the similarity of these faunas with their northern Paleo-Tethys counterparts. This can be inferred as a sign of the commencement of the separation of Iran from the northern margin of Gondwana and its movement towards the North.

• The movement of Iran towards the North and its low latitude position in the Gzhelian also is inferred from the occurrences of the coal-bearing sandstones of the Upper Carboniferous Sardar Formation.

• The Neo-Tethys opening in Iran occurred during the Late Carboniferous, based on the foraminiferal associations and magmatic activities in North-West Iran. This conclusion contrast with previous ideas on the onset of Neo-Tethys opening between northern Gondwana and the Cimmerian terranes during the Permian/ and or Triassic.