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Mid-Cretaceous radiolarian faunas from the Ashin Ophiolite (western Central-East Iranian Microcontinent)
Abstract and Figures
The Ashin ophiolitic m elange crops out in the west of the Central-East Iranian Microcontinent (CEIM). Accurate dating the deep-water radiolarian chert deposits associated with this ophiolite provide valuable data to constrain the stratigraphic and paleotectonic evolution this Neo-Tethyan oceanic basin. Chert nodules within Upper Cretaceous limestones and chert layers overlying pillow lavas of this ophiolite were collected and examined in detail for the first time. Radiolarians indicate that the cherts accumulated in the mid-Cretaceous in a deep marine setting within the eastern branch of the Tethyan Ocean in Iran. Two distinct radiolarian faunas, one mid-Albian (~107 Ma) the other Turonian (~94 Ma) were recovered. The radiolarian microfossil ages are consistent with radiometric ages previously obtained from associated plagiogranites and quartz keratophyre. This study indicates that mid-Cretaceous radiolarian assemblages from the western CEIM are similar to other radiolarian faunas reported from Cretaceous ophiolites in Greece, Turkey, Iran (Outer Zagros Ophiolitic Belt in Iran, e.g. Khoy, Kermanshah, Neyriz, and Soulabest), and southern Tibet in China.
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... Geochemical studies on the mafic lavas of the Nain and Dehshir ophiolites (Shafaii Moghadam et al., 2008, as well as on the mantle peridotites (Mehdipour Ghazi et al., 2010;Pirnia et al., 2010) have suggested that the Nain-Baft ophiolites were originated in a backarc basin, which opened between the Sanandaj-Sirjan zone and the CEIM in the Late Cretaceous in response to the subduction of the Southern Neo-Tethys below the Sanandaj-Sirjan block (see Takin, 1972;McCall, 2002;Shahabpour, 2005;Barrier and Vrielynck, 2008). However, a recent petrological study has suggested that mantle lherzolites in the Nain ophiolites represent sub-continental mantle exhumed at an Iberia-type ocean-continent transition zone (Pirnia et al., 2018) and a recent biostratigraphic study has shown that the Ashin ophiolites are mid Cretaceous in age (Shirdashtzadeh et al., 2015). Figure 3. Albian radiolarians from sample 1010, magnification of all specimens 120Â (scale bar 100 mm). 1. Acanthocircus cf. ...
... These data will allow the type and age of the magmatic events to be constrained in detail. Several models for the geodynamic evolution of the oceanic branches of the Neo-Tethys that existed in Mesozoic times around the CEIM are available in literature (e.g., Shojaat et al., 2003;Barrier and Vrielynck, 2008;Shafaii Moghadam et al., 2009;Rossetti et al., 2010;Omrani et al., 2013;Mattei et al., 2014;Shirdashtzadeh et al., 2015). Hence, one of the main goals of this paper is to use our new data for testing and developing the extant tectonic models. ...
... Several models for the geodynamic evolution of the oceanic branches of the Neo-Tethys that were existing in Mesozoic times around the CEIM have been suggested by many authors (e.g., Shojaat et al., 2003;Barrier and Vrielynck, 2008;Shafaii Moghadam et al., 2009;Rossetti et al., 2010;Omrani et al., 2013;Mattei et al., 2014;Shirdashtzadeh et al., 2015). Rahmani et al. (2007) suggested that the Nain ophiolites were formed in an intra-oceanic island arc setting during the Late Cretaceous. ...
The Nain and Ashin ophiolites consist of Mesozoic mélange units that were emplaced in the Late Cretaceous onto the continental basement of the Central-East Iran microcontinent (CEIM). They largely consist of serpentinized peridotites slices; nonetheless, minor tectonic slices of sheeted dykes and pillow lavas - locally stratigraphically associated with radiolarian cherts - can be found in these ophiolitic mélanges. Based on their whole rock geochemistry and mineral chemistry, these rocks can be divided into two geochemical groups. The sheeted dykes and most of the pillow lavas show island arc tholeiitic (IAT) affinity, whereas a few pillow lavas from the Nain ophiolites show calc-alkaline (CA) affinity. Petrogenetic modeling based on trace elements composition indicates that both IAT and CA rocks derived from partial melting of depleted mantle sources that underwent enrichment in subduction-derived components prior to melting. Petrogenetic modeling shows that these components were represented by pure aqueous fluids, or sediment melts, or a combination of both, suggesting that the studied rocks were formed in an arc-forearc tectonic setting. Our new biostratigraphic data indicate this arc-forearc setting was active in the Early Cretaceous. Previous tectonic interpretations suggested that the Nain ophiolites formed, in a Late Cretaceous backarc basin located in the south of the CEIM (the so-called Nain-Baft basin). However, recent studies showed that the CEIM underwent a counter-clockwise rotation in the Cenozoic, which displaced the Nain and Ashin ophiolites in their present day position from an original northeastward location. This evidence combined with our new data and a comparison of the chemical features of volcanic rocks from different ophiolites around the CEIM allow us to suggest that the Nain-Ashin volcanic rocks and dykes were formed in a volcanic arc that developed on the northern margin of the CEIM during the Early Cretaceous in association with the subduction, below the CEIM, of a Neo-Tethys oceanic branch that was existing between the CEIM and the southern margin of Eurasia. As a major conclusion of this paper, a new geodynamic model for the Cretaceous evolution of the CEIM and surrounding Neo-Tethyan oceanic basins is proposed. © 2019 China University of Geosciences (Beijing) and Peking University
... All rights reserved. listwaenite, and rodingite; Shirdashtzadeh, 2014, Shirdashtzadeh, Kachovich, Aitchison, & Samadi, 2015, mixed up in a matrix of serpentinite ( Figure 1b). 40 K- 40 Ar dating of Sharkovski, Susov, & Krivyakin (1984), and geological interpretations of Shirdashtzadeh et al. (2011) and Shirdashtzadeh et al. (2014a) suggested Jurassic to early Late Cretaceous ages for the spreading magmatism in this ancient oceanic realm. ...
... 40 K- 40 Ar dating of Sharkovski, Susov, & Krivyakin (1984), and geological interpretations of Shirdashtzadeh et al. (2011) and Shirdashtzadeh et al. (2014a) suggested Jurassic to early Late Cretaceous ages for the spreading magmatism in this ancient oceanic realm. Two distinct mid-Cretaceous radiolarian faunas, one mid-Albian (~ 107 Ma) and the other Turonian (~ 94 Ma) accumulated in a deep marine setting within the eastern branch of the Tethyan Ocean in Iran (Shirdashtzadeh et al., 2015). ...
... along Nain-Baft suture zone. This south to north increasing of depth, higher partial melting degrees in the source of basic rocks from Nain than Ashin ophiolite (Shirdashtzadeh, Torabi, Samadi, 2014b), and the previous geochronological data along Nain to Baft ophiolitic zone ( Figure 1a: Nain ~ Albian, Pirnia, Saccani, Torabi, Chiari, Gorič, & Barbero, 2019; Ashin ~ mid-Albian-Turonian, Shirdashtzadeh et al., 2015) are in accordance with a south to north closure between Sanandaj-Sirjan zone and CEIM plates in the north of Nain-Baft zone. ...
This study is focused on a plagioclase-bearing spinel lherzolite from Chah Loqeh area in the Neo-Tethyan Ashin Ophiolite. It is exposed along the west of left-lateral strike-slip Dorouneh fault in the northwest of Central-East Iranian Microcontinent. Mineral chemistry (Mg#olivine < ~ 90, Cr#clinopyroxene < ~ 0.2, Cr#spinel < ~ 0.5, Al2O3Orthopyroxene > ~ 2.5 wt%, Al2O3Clinopyroxene > ~ 4.5 wt%, Al2O3Spinel > ~ 41.5 wt%, Na2Oclinopyroxene > ~ 0.11 wt%, and TiO2clinopyroxene > ~ 0.04 wt%) confirm Ashin lherzolite was originally a mid-oceanic ridge peridotite with low degrees of partial melting at spinel-peridotite facies in a lithospheric mantle level. However, some Ashin lherzolites record mantle upwelling and tectonic exhumation at plagioclase-peridotite facies during oceanic extension and diapiric motion of mantle along Nain-Baft suture zone. This mantle upwelling is evidenced by some modifications in the modal composition (i.e. subsolidus recrystallization of plagioclase and olivine between pyroxene and spinel) and mineral chemistry (e.g., increase in TiO2 and Na2O of clinopyroxene, and TiO2 and Cr# of spinel and decrease in Mg# of olivine), as a consequence of decompression during a progressive upwelling of mantle. Previous geochronological and geochemical data and increasing the depth of subsolidus plagioclase formation at plagioclase-peridotite facies from Nain ophiolite (~ 16 km) to Ashin ophiolite (~ 35 km) suggest a south to north closure for the Nain-Baft oceanic crust in the northwestern of Central-East Iranian Microcontinent.
... In the Atlantic-Mediterranean Realm, radiolarian associations are very similar. For example, associations of Albian radiolarians of Cyprus and Italy are (Erbacher, 1998); (2, 3) Atlantic, Moroccan Trough, (2) Borehole 137/138, (3) Borehole 545 (Erbacher andThurow, 1997, 1998); (4) Atlantic, Iberian Trough, Borehole 637-641 (Thurow, 1988); (5) Spain, Betic Cordillera (O'Dogherty, 1994); (6) northern Tunisia (Ben Fadhel et al., 2010); (7) Italy, Umbria-Marche Apennines (O'Dogherty, 1994); (8) western Slovenia (Gorican and Šmuc, 2004); (9) Montenegro (Gorican, 1994); (10) Poland, Western Carpathians (Bak, 1999;Bak and Bak, 2013); (11) western Turkey (Moix and Gorican, 2013); (12) Cyprus (Moni blocks) (Bragina and Bragin, 2006;Bragina and Bragin, in press); (13) northeastern Azerbaijan, northern slope of the Greater Caucasus (Aliev, 1965(Aliev, , 1968Bragina and Bragin, 2015a;Kopaevich et al., 2015); (14) central Iran (Shirdashtzadeh et al., 2015); (15) Hymalayas, Ladakh (Danelian and Robertson, 1997); (16) Malaysia, Kalimantan Jasin, 2012, 2013); (17) Pacific Ocean, East Mariana Trench, Borehole 585 (Vishnevskaya, 2001); (18, 19) Japan, Shimanto Belt (Nakaseko and Nishimura, 1981); (20) Japan, northern Hokkaido (Tumanda, 1989); (21?) eastern Kamchatka, Cape Kamchatka (Vishnevskaya, 2001;Palechek et al., 2010); (22?) Pacific Ocean, Boreholes 303-307, 310, 313 and Borehole 167 (Foreman, 1975;Moore, 1973). Localities of Boreal Superrealm: (23) Mountainous Crimea (Gorbatchik and Kazintsova, 1998); (24) northern Voronezh Anteclise (Vishnevskaya et al., 2005b); (25) Moscow Syneclise (Aliev and Smirnova, 1969;Vishnevskaya et al., 2005b); (26) western Volga-Urals Anteclise (Kazintsova and Olferiev, 1997); (27) Ulyanovsk-Saratov Depression, Mordovia (Vishnevskaya et al., 2005b); (28) Ulyanovsk-Saratov Depression, Penza Realm (Vishnevskaya, 2001); (29) Timan-Pechora Plate (Amon, 2000); (30) Transuralia (Amon, 2000); (31) West Sakhalin Mountains (Kazintsova, 1993); (32?) East Sakhalin Mountains (Vishnevskaya, 2001); (33?) southern Koryak Upland (Vishnevskaya, 2001). ...
... Legend: (1) land, (2) shallow seas, (3) deep basins, (4) localities of the Tethyan Superrealm, (5) localities of the Boreal Superrealm, (6) localities of the Austral Superrealm. Localities of Tethyan Superrealm: (1) California (Pessagno, 1971a(Pessagno, , 1971b(Pessagno, , 1973(Pessagno, , 1976; (2) western Atlantic, Borehole 603 (Thurow, 1988); (3) Atlantic, Moroccan Trough, Borehole 137/138 (Erbacher andThurow, 1997, 1998); (4) Atlantic, Moroccan Trough, Borehole 545 (Erbacher andThurow, 1997, 1998); (5) Atlantic, Iberian Trough, Boreholes 637-641 (Thurow, 1988); (6) eastern Equatorial Atlantic, Boreholes 959-962 (Erbacher, 1998); (7) Spain, Betic Cordillera (O'Dogherty, 1994); (8) northern Tunisia (Ben Fadhel et al., 2010); (9) Italy, Umbria-Marche Apennines (O'Dogherty, 1994); (10) Poland, Western Carpathians (Bak, 1999(Bak, , 2011Bak and Bak, 2013); (11) western Slovenia (Gorican and Šmuc, 2004); (12) Montenegro (Gorican, 1994); (13) western Turkey (Moix and Gorican, 2013); (14) Cyprus (Bragina and Bragin, 2006); (15) northeastern Azerbaijan, northern slope of the Greater Caucasus (Aliev, 1965(Aliev, , 1968Bragina and Bragin, 2015a;Kopaevich et al., 2015); (16) central Iran (Shirdashtzadeh et al., 2015); (17) Himalayas, Ladakh (Danelian and Robertson, 1997); (18) Malaysia, Kalimantan Jasin, 2012, 2013;Jasin and Tongkul, 2012); (19) Pacific Ocean, East Mariana Trench, Borehole 585 (Vishnevskaya, 2001); (20) Pacific Ocean, Hess Rise, Boreholes 463-466 (Schaaf, 1981); (21) Pacific Ocean, Boreholes 303-307, 310, 313 and Borehole 167 (Foreman, 1975;Moore, 1973); (22) Japan (Nakaseko and Nishimura, 1981); (23) Japan, northern Hokkaido (Tumanda, 1989); (24?) Cape Kamchatka (Vishnevskaya, 2001;Palechek et al., 2010). Localities of the Boreal Superrealm: (25) Voronezh Anteclise (Vishnevskaya et al., 2005b); (26) Moscow Syneclise (Aliev and Smirnova, 1969;Vishnevskaya et al., 2005b); (27) Mountainous Crimea (Gorbatchik and Kazintsova, 1998); (28) western part of the Volga-Urals Anteclise (Kazintsova and Olferiev, 1997); (29) Ulyanovsk-Saratov Depression, Mordovia (Vishnevskaya et al., 2005b); (30) Ulyanovsk-Saratov Depression, Penza oblast (Vishnevskaya, 2001); (31) Timan-Pechora Plate (Amon, 2000); (32) Transuralia (Amon, 2000); (33) West Sakhalin Mountains (Kazintsova, 1993); (34?) East Sakhalin Mountains (Vishnevskaya, 2001); (35?) southern Koryak Upland (Vishnevskaya, 2001). ...
... Legend: (1) land, (2) shallow seas, (3) deep basins, (4) localities of the Tethyan Superrealm, (5) localities of the Boreal Superrealm, (6) localities of the Austral Superrealm. Localities of Tethyan Superrealm: (1) California (Pessagno, 1971a(Pessagno, , 1971b(Pessagno, , 1973(Pessagno, , 1976; (2) western Atlantic, Borehole 603 (Thurow, 1988); (3) Atlantic, Moroccan Trough, Borehole 137/138 (Erbacher andThurow, 1997, 1998); (4) Atlantic, Moroccan Trough, Borehole 545 (Erbacher andThurow, 1997, 1998); (5) Atlantic, Iberian Trough, Boreholes 637-641 (Thurow, 1988); (6) eastern Equatorial Atlantic, Boreholes 959-962 (Erbacher, 1998); (7) Spain, Betic Cordillera (O'Dogherty, 1994); (8) northern Tunisia (Ben Fadhel et al., 2010); (9) Italy, Umbria-Marche Apennines (O'Dogherty, 1994); (10) Poland, Western Carpathians (Bak, 1999(Bak, , 2011Bak and Bak, 2013); (11) western Slovenia (Gorican and Šmuc, 2004); (12) Montenegro (Gorican, 1994); (13) western Turkey (Moix and Gorican, 2013); (14) Cyprus (Bragina and Bragin, 2006); (15) northeastern Azerbaijan, northern slope of the Greater Caucasus (Aliev, 1965(Aliev, , 1968Bragina and Bragin, 2015a;Kopaevich et al., 2015); (16) central Iran (Shirdashtzadeh et al., 2015); (17) Himalayas, Ladakh (Danelian and Robertson, 1997); (18) Malaysia, Kalimantan Jasin, 2012, 2013;Jasin and Tongkul, 2012); (19) Pacific Ocean, East Mariana Trench, Borehole 585 (Vishnevskaya, 2001); (20) Pacific Ocean, Hess Rise, Boreholes 463-466 (Schaaf, 1981); (21) Pacific Ocean, Boreholes 303-307, 310, 313 and Borehole 167 (Foreman, 1975;Moore, 1973); (22) Japan (Nakaseko and Nishimura, 1981); (23) Japan, northern Hokkaido (Tumanda, 1989); (24?) Cape Kamchatka (Vishnevskaya, 2001;Palechek et al., 2010). Localities of the Boreal Superrealm: (25) Voronezh Anteclise (Vishnevskaya et al., 2005b); (26) Moscow Syneclise (Aliev and Smirnova, 1969;Vishnevskaya et al., 2005b); (27) Mountainous Crimea (Gorbatchik and Kazintsova, 1998); (28) western part of the Volga-Urals Anteclise (Kazintsova and Olferiev, 1997); (29) Ulyanovsk-Saratov Depression, Mordovia (Vishnevskaya et al., 2005b); (30) Ulyanovsk-Saratov Depression, Penza oblast (Vishnevskaya, 2001); (31) Timan-Pechora Plate (Amon, 2000); (32) Transuralia (Amon, 2000); (33) West Sakhalin Mountains (Kazintsova, 1993); (34?) East Sakhalin Mountains (Vishnevskaya, 2001); (35?) southern Koryak Upland (Vishnevskaya, 2001). ...
Radiolarian paleobiogeography for the late Albian–Santonian is proposed for the first time. The paleobiogeographic differentiation is found to be different for the Albian, Cenomanian, Turonian, and Coniacian–Santonian. The Tethyan and Boreal superrealms can be recognized for the Albian–Santonian. For the Albian–Santonian, the Tethyan Superrealm can be subdivided into realms: Atlantic-Mediterranean, Carpathian-Caucasian, and Tropical-Pacific. The boundaries of these realms changed throughout geological time. The Boreal Superrealm recognized for the Albian so far cannot be subdivided into realms, whereas in the Cenomanian it included the East European and Western Siberian realms without a clear definition of the boundaries and the Boreal-Pacific (in the North Pacific). The Boreal Superrealm is subdivided in the Turonian into two realms (European-Western Siberian and Boreal-Pacific), and in the Coniacian–Santonian, it is subdivided into three realms (European, Western Siberian, and Boreal-Pacific). The Austral Superrealm can be recognized only for the Albian and Cenomanian, and because of the lack of data, it cannot be delineated for the Turonian and Coniacian–Santonian.
... This ophiolitic mélange was formed along Nain-Baft fault, between the Sanandaj-Sirjan Zone and the CEIM, some time in late Lower Cretaceous-Palaeocene (e.g., Glennie 1992;Torabi 2004;Reichert 2007;Shirdashtzadeh et al. 2010Shirdashtzadeh et al. , 2011Shirdashtzadeh et al. , 2014. The Nain ophiolitic mélange (Figure 1c) is a mixture of various oceanic sedimentary, igneous and metamorphic lithologies, obducted onto the northwestern border of Yazd Block (e.g., Shirdashtzadeh et al. 2010Shirdashtzadeh et al. , 2014Shirdashtzadeh et al. , 2015Shirdashtzadeh et al. , 2020b. Several outcrops of ophiolitic M-type granitoids, including tonalitic dykes and plagiogranites, with contrasting age and origins are exposed along this ophiolitic zone. ...
This study investigates relicts of some granitic Gondwanan basement unexpectedly outcropping in the northwest of Central-East Iranian Microcontinent (CEIM) and incorporated into an ophiolitic mélange. Based on petrographical (e.g. high modal content of muscovite (~10 vol.%), absence of hornblende, inherited zircons (>541 Ma)), geochemical (peraluminous and calc-alkaline S-type affinity, high silica, high ‘light rare earth element (LREE)/heavy rare earth element (HREE)’ ratios, negative Nb and Ti anomalies), and geochronological (magmatic zircon age ~448 Ma) results, it is an Ordovician anatectic granite formed from a sedimentary source during crustal thickening in a syn-collisional setting. It shows some signatures of metamorphic deformation (cataclastic fabric, quartz bulging recrystallization, and foliation) likely developed in the Devonian (~410 Ma). The U-Pb zircon ages from this granite are analogous to the other Ordovician collision-related magmatic events in the CEIM (Chahak to Airekan, Balvard). Our results confirm that Cadomian subduction and closure of the Proto-Tethys Ocean to the north of the Gondwana supercontinent resulted in crustal thickening during Ordovician collision-related magmatism and Devonian-Carboniferous regional metamorphism in the CEIM.
... This radiolarian fauna suggests that sampled radiolarite was deposited during the mid-Cretaceous which also determines the time of oceanic crust formation. This radiolarian fauna from the Dumak ophiolitic m elange contains a typical mid-Cretaceous radiolarian assemblage that compares well with that published in the outstanding monograph of O'Dogherty (1994), although quite similar radiolarian faunas have been also reported from Iran Pessagno et al., 2005;Babazadeh, 2007;Gharib and De Wever, 2010;Shirdashtzadeh et al., 2015;Pirnia et al., 2019), Crimea (Kopaevich, Vishnevskaya, 2016), southern Tibet (Ziabrev et al., 2003), India (Bragina and Bragin, 2013), Eastern Russia (Zyabrev, 1996(Zyabrev, , 2011Kurilov and Vishnevskaya, 2011), Malaysia (Jasin, 1992(Jasin, , 2018Asis and Jasin, 2012;Jasin and Tongkul, 2012), Indonesia (Jasin and Haile, 1996), as well as from Cuba (Vishnevskaya, 2001). ...
The Sistan orogen (Eastern Iran) separates the Afghan and Lut continental blocks and stretches along ~700 km from north to south, at a high angle with respect to other, dominantly E-W trending Alpine-Himalayan orogens. This study reappraises the tectono-metamorphic evolution of the northern part of the orogen, as well as its significance within the Neotethyan realm. Detailed inspection of the Sistan ophiolite indicates that the Sistan Ocean was of a slow- spreading type and that, given its structural patterns, petrological characteristics and age, it opened in a transtensional setting ~125 Ma ago. Closure of the Sistan Ocean took place through a major NE-dipping subduction zone, formed no later than 90 Ma, as shown by the location and age of bimodal juvenile arc magmatism, the SW vergence of the orogen and the location and age of subducted fragments. The discovery of a metamorphic sole at the base of the ophiolite (~750°C-0.65 GPa) argues for the onset of an additional intra-oceanic thrust/subduction zone around 74-72 Ma, which resulted in the south-westward obduction and preservation of the ophiolite onto the continental Lut block. The Sistan Ocean therefore appears to have recorded two major geodynamic events that accompanied the closure of the Neotethys, i.e. the major change in kinematics at ~105 ±5 Ma and the northward migration of India from ~75-70 Ma onwards. Subsequent collision, likely started during the Paleocene and mostly completed by the Oligocene, was accompanied by a drastic change of the Eocene sedimentation yet by only moderate shortening (~30-50 km in total). Since the Late Miocene onwards, post-collisional deformation is dominated by far-field stresses related to the Zagros collision.
In Northeastern Tunisia, the Upper Albian deposits are characterized by continuous sedimentation starting with marl-limestone alternations that include a radiolarian peak with varied morphotypes and a dominance of the species Cryptamphorella conara, Dictyomitra gracilis, D. napaensis and Crucella cachensis. The objective of this work was the biostratigraphical and micropaleontological study of the Upper Albian deposits in Jabal Msella: This analysis is based on the spatio-temporal survey coupled to a quantitative analysis of the whole microfauna. Also noted, the presence of benthic foraminifera essentially the genera: Lenticulina, Dentalina, Nodosaria and Epistomina. Toward the top, the sedimentation is essentially marly and yield of planktonic foraminifera represented by the genera Rotalipora, Hedbergella and Ticinella. The presence of these markers indicates the Upper Albian. The distribution of benthic foraminifera recorded vertical variations: Thus, in the basal part of the Upper Albian of hyaline, calcareous tests (Nodosaridae) are predominant, and in the middle and upper part hyaline, calcareous tests (Lagena) and agglutinated tests (Praedothia, Ammodiscus) are present. The results can be summarized as follows:
In this study, some mantle lherzolites of Ashin ophiolite are investigated which contain evidence of a geotectonic/metamorphism during exhumation and obduction of oceanic lithosphere on the continental crust, after closure of Neo-Tethys Ocean. The petrography and geothermobarometry indicate that these 4-phase lherzolites were formed in the lithospheric mantle (at pressures ~ 21.6 to 8.6 kbar) by melt/wall rock reactions (at temperatures ~ 1012-1183 °C). Then, they were emplaced and obducted on the continental crust along the fault zone of this region, and consequently deformed. The first ductile deformation event occurred in the depth of lithosphere and resulted in high-temperature mylonitization at temperatures higher than 600 to 800 °C. Mineralogical features confirm pressure decreasing of this stage by subsolidus reaction of pyroxene and spinel and substitution of plagioclase and olivine. Therefore, petrography and thermobarometry data are indicative of the spinel to plagioclase lherzolite facies for these rocks. Finally, they partially underwent brittle and cataclastic deformation at temperatures below 600°C and lower pressures and depth during exhumation. However, most of plagioclases were replaced by with prehnite, pumpellyite, chlorite, hydrogrossular and xonotlite minerals by further alterations.
Some dark green fragments of amphibolites are found within skarns at the south of Nain Ophiolite at the northeast of Nain city. They are similar to ortho-amphibolites (metamorphosed basic rocks) of this ophiolite in hand specimen, but mineralogically they are composed of amphibole (magnesio-hornblende, Mg#>0.95), clinopyroxene (diopside, Mg#~0.61), garnet (grossular – andradite, with Grs~63-87 And~12-35), quartz, and minor amount of calcite and wollastonite. Accessory minerals are including chlorite and prehnite, mostly filling the fractures. Field studies, petrography and mineral chemistry indicate that amphibolitic fragments mineralogically differ from the skarns and ortho-amphibolites of this ophiolite; so they can be considered as olistoliths with sedimentary origin (calcic marls of sea floor), turned into amphibolitic rocks (para-amphibolite) during the regional metamorphism at amphibolite - granulite facies and low oxygen fugacity.
The Neyriz ophiolite complex in south central Iran is an assemblage of Upper Cretaceous crustal and mantle ophiolite sequences and related pelite, thinbedded limestone, chert, and mélange with Permian-Cretaceous mega-clasts. The complex is thrust on shallow marine limestone of the Sarvak Formation to the southwest, and it is overthrust by arc-related volcanic and volcaniclastic rocks and forearc basin sediments from the northeast. The tectonic history of the ophiolite complex involved (i) rifting of the Afro-Arabian plate due to a late Carboniferous-early Permian uplift, which led to block-faulting along Precambrian basement faults and opening of the narrow Neo-Tethyan oceanic basin, (ii) sea-floor spreading, starting probably from Triassic, that produced an oceanic crust, and (iii) SW-NE directed contraction and arc-related volcanism, in the Late Cretaceous, through which slivers of the Neo-Tethyan oceanic lithosphere and overlying sediments were accreted into an accretionary prism, and then thrust onto the passive, block-faulted northeastern margin of the Afro-Arabian plate, along a northeast-dipping subduction zone. The close correspondence between the cooling ages of the ophiolitic gabbro and plagiogranite, deposition of thin limestone interbedded with radiolarian chert, and metamorphism of mylonitic amphibolite along the sole detachment of the ophiolitic complex, suggests that the emplacement of the complex occurred in a relatively short period of time during which a segment of young rocks of a subducting mid-ocean ridge was thrust under accreted contemporaneous and older sedimentary cover. Continued contraction led to offscraping and/or underplating of pelagic, abyssal, and slope facies, deposited along the leading, passive margin of Afro-Arabian plate, under the crustal and mantle ophiolite sequences, forming an accretionary prism. Tholeiitic, arc-related basalt and volcaniclastic rocks covered the prism in the forearc region. The prism grew by incorporating slivers of Permian-Upper Cretaceous limestone, which capped normal- faulted blocks of the passive margin, and radiolarian chert and pelite which were deposited between the blocks, forming the Hajiabad mélange. The final collision of the Eurasian and Arabian plates during the Miocene, which started when the Arabian plate separated from Africa, led to internal thrusting of the complex and the development of the cataclastic Zagros Crush zone immediately to the northeast of the ophiolite belt.
Radiolarian faunas from Leg 89 are scarce and poorly preserved. Upper Cretaceous assemblages are reworked. The oldest and lowest assemblage recognized was of the Archaeospongoprunum cortinaensis Zone in Hole 585. The biochronology of two levels in Hole 462A are revised as follows: the lowest assemblage of species useful for biochronology is of the Cecrops septemporatus Zone (early Hauterivian); the age of a higher sample is late Tithonian-early Berriasian, indicating rweorking of older sediment into the Nauru Basin.-Author
The Birjand ophiolite range belongs to the inner subbelt where is found along the east border of the Lut block in Sahlabad province (Sistan ocean). Two distinct radiolarites (Eshkaftu and Fanud radiolarites) were examined in this range. The timing of ophiolite range remained questionable until now. The Albian-Cenomanian radiolarian assemblages have been distinguished for the first time from red argillaceous cherts in this region. It is concluded that during the Early-Late Cretaceous, the Sistan ocean was connected to the other Tethyan oceanic branches. The radiolarian rocks of Sahlabad province can be partly time-equivalent to the Soulabest radiolarite (Gazik province).