Open access peer-reviewed chapter
Abstract
The Jurassic subduction of the Neo-Tethys oceanic crust under the western continental margin of the Iranian Block has led to the fragmentation of the Iranian Block in the back-arc basin, leading to the opening of three oceanic basins around it. The ophiolitic belts surrounding central Iran are the indicators of the closure of these basins. The Sabzevar-Nain Basin is one of these basins, which has been created between the micro-block of central Iran in the south and the Alborz Mountain Ranges in the north. This basin opened in the late Jurassic as a rift and then became a trough in the early Cretaceous. Finally, this basin developed into an oceanic basin in the early late Cretaceous. The sedimentation in this basin can be divided into pre-rift, syn-rift and oceanic environments. All of these sediments are strongly folded and faulted. The closure of this basin started during the Paleocene with a subduction under the southern margin of the Alborz Mountain Ranges. The collision event between the northern margin of the micro-block of central Iran and the southern margin of the Alborz Mountain Ranges occurred in the early Eocene. The result of this event was the creation of a wide collision zone, forming a volcanic arc and a back arc basin on the active of the Alborz Mountain Ranges, an ophiolitic belt, and post- collision intrusion masses that appear everywhere in the collision zone. In the point of lithology, these intrusion masses are composed of granite, diorite, and granodiorite. The magmatic activities that started in the Paleocene-early Eocene continued until early Quaternary.
Keywords
- geodynamics
- ophiolite
- collision
- volcanic arc
- syn-rift
- foreland basin
- back-arc basin
1. Introduction
It is possible to express the evolution of the Iranian crust and basins by looking at their geology and geodynamic history: the consolidation of the Iranian basement in the Gondwana mega continent; the Precambrian magmatism and metamorphism; deformation and folding in the Precambrian and early Paleozoic rocks; crustal thinning due to an extensional state during the early Paleozoic (Neo-Tethys rifting); rifting in the eastern part of Gondwana in the early Paleozoic [Silurian], (Figure 1a); the northward subduction of the paleo-Tethys oceanic crust under the southern margin of the Eurasia supercontinent in the early Paleozoic (Figure 1b); the early Cimmerian collision of the Iranian Block with the Turan Block (the southern margin of Eurasia) in the middle Paleozoic (Figure 1c); the late Paleozoic-early Jurassic crustal thinning with an instability period in the sedimentary basin after the early Cimmerian orogenic event (Figure 1d). The great event on the Iranian Block occurred after the oceanic crust subduction of the Neo- Tethys Ocean under the western margin of the Iranian Block. This subduction led to the global extension and fragmentation of the Iranian crust during the late Jurassic. After this extension and thinning, the crust of the Iranian Block broke in the back-arc basin of the Neo-Tethys Ocean in the early-middle Cretaceous. This event led to the creation of inner oceanic basins such as the Sabzevar-Nain, Nain-Baft, and Sistan-Baluch Basins (Figure 2) [1, 2, 3, 4, 5]. The ophiolitic belts surrounding central Iran are the indicators of the closure of these basins. They were over thrust on the continental margins accompanied by other materials of the accretionary prism and the continent-continent collision between the adjacent blocks.
Figure 1.
A brief history of the Iranian block evolution from the Precambrian to the late Triassic-early Jurassic; a: The rifting event in the eastern part of Gondwana; b: The subduction of the paleo-Tethys slab under the Turan block (the southern part of Eurasia); c: The early Cimmerian collision of the Iranian block with the Eurasia mega-continent and the development of the neo-Tethys Ocean; d: The crustal thinning and instability in the sedimentary basin after the early Cimmerian collision. GSC: Gondwana supercontinent; PTR: Paleo-Tethys rift; MLC: Mega Lhasa continent; IB: Iranian block; APM: Alborz passive margin; PTOC: Paleo-Tethys oceanic crust; PTT: Paleo-Tethys trench; KMA: Kopeh Dagh magmatic arc; EMC (TB): Eurasia mega-continent (the Turan block); ECCZ: Early Cimmerian collision zone; AP: Arabian plate; ZPM: Zagros passive margin; NTOC: Neo-Tethys oceanic crust; NTT: Neo-Tethys trench; PTS: Paleo-Tethys suture; ECC: Eurasia continental crust.
Figure 2.
The breaking of the Iranian block and the creation of rifts around the micro-block of Central Iran (a), the locations of the three new basins around the micro-block of Central Iran in the back-arc of the neo-Tethys Ocean (b). AMR: Alborz Mountain ranges; AFB: Afghan block; CIB: Central Iran block; UDMA: Urumieh-Dokhtar magmatic arc; Alborz M.: Alborz Mountains; APM: Alborz passive margin; CIPM: Central Iran passive margin; NBB: Nain-Baft Basin; SNB: Sabzevar-Nain Basin; SBB: Sistan- Baluch Basin; APM: Afghan passive margin; AP: Arabian plate; NTMP: Neo-Tethys passive margin; NTMOR: Neo-Tethys mid-oceanic ridge; NTSZ: Neo-Tethys suture zone.
In this paper, the authors tried to discuss the geochronology of the opening and closing processes of the Sabzevar-Nain Basin situated between the southern margin of the Alborz Mountain Ranges and the northern margin of the micro-block of central Iran (Figure 3). The Sabzevar-Nain Basin (SNB) was formed due to an extensional system with crustal thinning accompanied by a north-south rifting which occurred in the back-arc basin of the Neo-Tethys Ocean. The large number of geological structures in this region has motivated many geologists to conduct their researches in it. Several Ph.D. dissertations [including Sadreddini [6], Alavi Tehrani [7], Dehghani [8], and Noghreyan [9]] have been conducted on the oceanic crust remnants (ophiolites) in the Sabzevar region. The first study on the Sabzevar ophiolites was conducted by Sadreddini [6]. This study depended on the petrographic characteristics of the ophiolites in the middle part of the ophiolitic range of Sabzevar. Alavi Tehrani [7] studied the geology and petrology of the ophiolitic rocks in the northwest of Sabzevar. Dehghani [8] described the gravity field and structure of the Iranian crust. Noghreyan [9] stated that the ophiolitic belt of Sabzevar was formed due to an immature arc. These studies were carried out in the frame of the Geological Survey of Iran called the ‘Geodynamic Project, [10, 11, 12]. The ophiolitic belt was distinguished by rock units including harzburgite, intrusive rocks, a sheeted dyke complex, volcano-sedimentary sequences, an ophiolitic mélange (composed of ophiolitic rocks, intrusive rocks, and oceanic sediments), and metamorphic rocks. The cover rocks were characterized by a Cenozoic sequence and Quaternary rocks. Lensch and Davoudzadeh [13] identified three types of ophiolitic rocks around the micro-block of central Iran including an ophiolitic mélange, ridge type ophiolites, and trench type ophiolites. Baghdadi [14] related the volcanism of the northern part of Sabzevar to a subduction process during Eocene. Ghassemi and Rezaei-Kahkhaei [15] stated that these rocks were the result of the partial melting of an enriched mantle by an extensional process within the arc. Khalatbari and Etessami [16] worked on the petrology and tectonomagmatic setting of the Eocene volcanic rocks in the Semnan area. They concluded that these volcanic rocks are the result of a subduction event during the Paleocene. Based on her studies on the petrology, petrography, and geochemistry of the intrusive rocks of the Sabzevar-Nain collision zone, Goharshahi [17] concluded that their exposure is due to a subduction event and its continuation after the collision (post-collision intrusion).
Figure 3.
The location of the oceanic crust remnants and the suture of the Sabzevar collision zone in the northern part of the micro-block of Central Iran: 1. Cenozoic volcanic rocks; 2. The intrusion of Mesozoic and Cenozoic granites and diorites; 3. The cretaceous-Paleocene ophiolites around the micro-block of Central Iran; 4. The ophiolites of the Zagros suture zone; 5. The ophiolites of the paleo-Tethys continental collision exposed in the north of Iran; 6: The Hormoz formation (pre- Cambrian) in the southeast of the Zagros Mountain ranges; 7: The trace of the paleo-Tethys suture.
During the years 1999–2002, 12 geological maps in the scale of 1:100,000 were prepared in the framework of a project in the Geological Survey of Iran. They contain important data about the petrology, stratigraphy, sedimentology, and structural geology of the geodynamic events.
2. Geological setting
The geological characteristics of the Sabzevar-Nain collision zone are described as pre- and syn-rift sediments, syn-rift magmatic activities, syn-subduction oceanic sediments, syn-subduction magmatism, syn-collision volcanism, post-collision sediments, post-collision magmatism, and post-collision volcanism. The pre-rift sediments are a thick Paleozoic and Mesozoic sequence including the sediments of continental and platform environments. The youngest pre-rift sediments of this basin are late Jurassic-early Cretaceous carbonates which have a wide distribution in the southern part of the Alborz Mountains and the northern part of central Iran (Saidi and Akbarpour [18]; Saidi and Vahdati Daneshmand [19]). Their thickness changes from 450 m in the north of Damghan to 580 m in the south of Sabzevar.
The syn-rift sequence is composed of continental and detrital facies as well as thick-bedded and massive limestone of the platform environment (Figure 4). The other syn-rift sediments are thick highly deformed flysch facies consisting of a majority of calcareous shale, some sandstone, and a few limestone lenses. These facies show the high thinning of the continental crust at the time of sedimentation in the Sabzevar-Nain trough. The basalts in the magma chamber under the rift penetrated the broken crust and flowed on the basin floor. In the convergent system between the continental crusts of the Alborz Mountain Ranges and central Iran, these basalts were folded with the deposited sediments in the basin (Salamati and Shafei [20]; Kolivand [21]; Ghaffari Nik [22]) (Figure 5).
Figure 4.
The two different facies of middle Cretaceous in the middle part of collision zone. The low lands are the syn- rift sediments derived from the continent (flysch) which are covered by upthrusting the thick bedded, massive limestone of platform environments (high lands).
Figure 5.
The physical characteristics of the syn-rift sediment during middle Cretaceous with intercalation of basalt.
The pre- and syn-collision late Cretaceous oceanic deposits consist of shale, sandstone, tuffaceous shale, limestone, radiolarian shale, pelagic limestone (Campanian-Maastrichtian in age), tuff, pillow lava, and spilitic basalt. The thickness of these sediments is estimated to be 2000 m (Figure 6) [23].
Figure 6.
The late Cretaceous oceanic deposits consist of shale, sandstone, pelagic limestone, radiolarian shale, pillow lava (Campanian- Maastrichtian).
The syn-subduction and syn-collision Paleocene-Eocene volcanic rocks of the volcanic arc in the Sabzevar region are composed of andesite, andesitic tuff, andesitic basalt, olivine alkali breccia, feeder dykes, basaltic andesite, and porphyritic andesite (Figures 7 and 8) [24].
Figure 7.
The morphology characteristics of the Paleocene-Eocene volcanic arc in the western part of Abbasabad (west of Sabzevar).
Figure 8.
The syn-subduction and syn-collision Paleoceane-Eoceane volcanic arc parallel with the ophiolitic belt, north of Sabzevar.
The post-collision foreland basin deposits consist of conglomerate, sandy limestone, gypsiferous marl, tuffaceous shale, and sandstone (Eocene and Oligo-Miocene in age). The other sediments of this basin are Miocene sandstone, conglomerate, and gypsiferous marl (Figures 9 and 10).
Figure 9.
The brown well-bedded post collision foreland deposits, north west of Sabzevar, composed of conglomerate and sandstone.
Figure 10.
The dark brown thick bedded post collision foreland basin deposits, over the ophiolitic rocks of Sabzevar-Nain Suture (East of Davarzan).
The syn/post-collision intrusive rocks consist of Cenozoic granite, quartz diorite, and diorite (Figures 11 and 12).
Figure 11.
The great masses of syn/post collision intrusive rocks, consist of Cenozoic granitoid, diorite and granodiorite in Kuh-e-Baharestan (south of Sheshtamad).
Figure 12.
The granite masses of syn/post collision, in age of Paleocene in the Kuh-e-Mish (south of Sabzevar).
The most recent post-collision dacite domes (Miocene and Plio-Quaternary in age) intruded into the ophiolitic belt (Figure 13), andesitic lava, tuff, and dacitic lava flow of Sabzevar (Figure 14).
Figure 13.
Youngest post collision dacitic domes penetrated into the ophiolitic rocks of Sabzevar.
Figure 14.
Young post collision dacitic domes penetrated into the foreland basin deposits, north of Mehr (North-West of Sabzevar).
3. Geodynamic setting
The architecture of the present-day Sabzevar-Nain Basin reflects the extensive tectonic regime (Figure 2a) which has occurred since the early-middle Cretaceous [2, 4]. This basin has been formed along the east-west direction in the southern part of the Alborz Mountain Ranges and the northern micro-block of central Iran. The tectonostratigraphic sequences of the sediments deposited in the basin between the late Paleozoic and Cenozoic are shown in Figure 15. These sediments are composed of shallow water deposits of early Triassic before early Cimmerian orogenic events (the continent-continent collision of the Iranian Block and Eurasia) (Figure 1c).
Figure 15.
The tectonostratigraphic sequences of the Iranian block before the middle cretaceous segmentation.
The early Triassic deposits are unconformably overlain by shale and sandstone detrital facies with an age of late Triassic-early Jurassic and from a new basin in extension after the above-mentioned orogenic events. A discontinuity can be observed in the sedimentary sequences between the late Jurassic and early-middle Cretaceous deposits (Late Cimmerian events). The first stage of rifting occurred in the early-middle Cretaceous in the basins around the central Iranian Block (Figure 2b) which were mechanically different from the Nain-Baft and Sistan-Baluch basins. The last two basins were created as pull-apart basins along two transform faults in the western and eastern parts of the micro-block of central Iran. The Sabzevar-Nain Basin was formed in the form of a classical continental rift (Figure 16a). This was the time for the micro-block of central Iran to be separated from the Alborz Mountain Ranges and to move toward the south.
Figure 16.
The geodynamic modeling of the opening (early-middle Cretaceous) and closure (middle-early late Eocene) of the Sabzevar-Nain Basin. CICC: Central Iran continental crust; S N rift: Sabzevar-Nain rift; ACC: Alborz continental crust; CIPM: Central Iran passive margin; APM: Alborz passive margin; SNMOR: Sabzevar-Nain mid oceanic ridge; SNB: Sabzevar-Nain Ocean; SNVA: Sabzevar-Nain volcanic arc; SNCZ: Sabzevar-Nain collision zone; AAM: Alborz active margin; OC: Oceanic crust; E: Eocene; K: Cretaceous.
During the middle Cretaceous, when a magma chamber was formed below the rift of Sabzevar-Nain, this basin became a large but not very wide trough (Figure 16b). After the process of intercontinental extension and the formation of the oceanic crust, the Sabzevar-Nain trough changed to an oceanic basin (Figure 16c). Simultaneously with the change of the divergent regime to the convergent regime during the Santonian-Campanian (86.3 ± 0.5–72.1 ± 0.2 Ma) [25], the oceanic crust of the Sabzevar-Nain Basin was subducted under the Alborz continental crust (Figure 16d). At this time, sedimentation reached its highest rate, whereas the intensity of deposition reached its highest rate in the Maastrichtian. Therefore, in 6.1 Ma, more than 578 m of sediments were deposited in the basin [23].
The process of subduction beneath the Alborz continental crust continued until the beginning of the early Eocene, while the margin of central Iran always remained a passive margin (Figure 16e). Since the early Paleocene, there have been highly intense volcanic and intrusive activities which can be observed throughout the collision zone especially in the south of Sabzevar. The oldest age determined for these volcanic and intrusive rocks is Paleocene [7, 9, 13, 14, 17, 26]. However, the age of the volcanic activity in the collision zone and the back-arc of the Sabzevar-Nain Basin varies from Paleocene to early Quaternary [20, 23, 24, 27, 28, 29, 30, 31]. The convergence and closure of the Sabzevar-Nain Basin could be due to the northward movement of the micro-block of central Iran. This event took place during the late Eocene-Oligocene (Figure 16f). The youngest deposits in the Sabzevar-Nain Basin are early Eocene flysch facies which were upthrust on both continental margins. These sediments are widely spread in the Nain region and the northwest of Bardeskan in the south of Sabzevar.
4. The structural and petrological characteristics of the Sabzevar-Nain collision zone
The best and the more complete units of the Sabzevar-Nain collision zone are well appeared around the Sabzevar. In a section from north to south, these units may be described as below:
Back arc basin in which, is deposited the detrital sediments in age of late Eocene, Oligo-Miocene and plio-Quaternary, Volcanic arc mainly composed of Eocene intermediate volcanic rocks and ophiolitic belt (Sabzevar-Nain oceanic crust remnants). Foreland basin in which the shallow water sediments of early- middle Eocene to Plio- Quaternary are accumulated., Syn-post collision intrusive masses of granite, granodiorite, diorite and granitoid, and overthrustihg of ophiolitic units (Figure 17) are also observed.
Figure 17.
The structural sections of the Sabzevar-Nain collision zone. SNBAB: Sabzevar-Nain back arc basin; SNVA: Sabzevar-Nain volcanic arc; EVSR: Eocene volcano- sedimentary rocks; SNFB: Sabzevar-Nain foreland basin; SNOCR: Sabzevar-Nain oceanic crust remnants (ophiolites); CIPM: Central Iran passive margin; AAM: Alborz active margin; JBB: Joghatay back arc basin; BFB: Bardeskan foreland basin; DF: Daruneh fault; PCDA: Post collision dacites; QVC: Quaternary volcanic Cone; EM: Eocene- Miocene deposits in foreland basin; K2f: Late Cretaceous oceanic deposits; Ev: Eocene volcanic; Q: Quaternary; M: Miocene deposits; E: Eocene foreland basin deposits; gr: Post collision granites; d: Post collision diorites.
Middle Cretaceous basalts: These rocks comprise the oldest layer of the Sabzevar-Nain oceanic crust. In fact, these basaltic flows are the highest part of the magma chamber under the Sabzevar-Nain rift. These rocks can be observed within the flysch sediments of the trough and are folded with them during continental convergence (Figure 18). These are from alkali basalt series and have a spilitic texture [20]. In the Kharturan area, the syn-rift sediments (flysch facies) and the massive carbonates of the platform environment have been exposed near each other (Figure 4).
Figure 18.
The syn-rift basaltic flows (K1v2) folded with the sediments of the Sabzevar-Nain trough. Quaternary: 1. Clay flat, 2. Sand dunes, 3. Cultivated area, 4. Mud flat, 5. Younger alluvium, 6. Older alluvium, 7. High level gravel fan, 8. Low level gravel fan, Cretaceous: 9. Crystal lithic tuff, 10. Andesitic lava (spilite), 11. Calcareous shale and limestone (turbidites), 12. Massive limestone, 13. Shaly limestone, 14. Shale, Paleocene: 15. Conglomerate and sandstone, 16. Shale.
Late Cretaceous deep-sea sediments: These rocks are widely exposed in the middle part of the collision zone especially in the north and southwest of Kuh-e-Baharestan (Baharestan Mountain) in the south of Sabzevar. These sediments are composed of siliceous sandstone, shale and tuffaceous sandstone with some pillow lava, spilitic vesicular lava, green tuff, radiolarian shale, and pelagic limestone. These deep sediments are strongly folded and faulted. They are a combination of thrusting slices and their boundaries with other rock units are thrust faults.
Oceanic crust remnants: The ophiolitic rocks of the Sabzevar-Nain oceanic crust appear in three areas of the collision zone. The southern exposure is near the northern margin of central Iran among the Eocene flysch and volcanic rocks (Figure 19). Here, they are composed of ultramafic rocks, diabase, plagiogranite, and gabbro. The second one is exposed with a fault contact in the middle part of Kuh-e Mish just in the southern flank of the great post-collision exposures of diorite. At their southern limit, these ophiolitic rocks are upthrust on Miocene marls. Lithologically, they are composed of harzburgite, serpentinite, dunite, diabase, and gabbro (Figure 20) [7, 13, 23, 32]. The main remnants of the oceanic crust in the suture zone (Figure 17) in the north of Sabzevar are exposed as a mountain range. The length of this mountain range is more than 480 km from east to west and its width is about 20 km. In its southern limits, this ophiolitic range is thrust over the detrital sediments of the foreland basin (Figure 21). However, in its northern part, it is limited to the volcanic arc (Figure 17). From a lithological point of view, it is composed of harzburgite, lherzolite, dunite, rodingite, serpentinized harzburgite, gabbro, diabase, a complex of sheeted dykes, submarine andesitic basalt, pillow lava, amphibolite, amphibolite schists, serpentinized peridotite, glaucophane schists, hornblende schist, garnet-muscovite schists, epidote-muscovite-chlorite schist, epidote-tremolite-actinolite schists, marble, pegmatite gabbro, monzodiorite, diorite, quartz diorite, and granodiorite [7, 9, 13, 24, 26, 27, 28, 33] (Figure 22). Obviously, the last intrusions are related to post-collision intrusion events (Figure 23) [17].
Figure 19.
Southern exposure of ophiolitic rocks near the northern margin of Central Iran composed of ultramafic rocks, diabase, plagiogranite and gabbro.
Figure 20.
The second ophiolitic rocks are exposed by thrust faults in the middle part of Kuh- e- Mish. In the southern limit, they are upthrust on the Miocene marls.
Figure 21.
The main remnant of oceanic crust in the suture zone in the north of Sabzevar. These ophiolitic rocks are thrusted over the foreland basin deposits.
Figure 22.
The ophiolitic rocks, situated in the northeast of Sabzevar, composed of harzburgite, lherzolite, dunite, rodengite, serpentinite, gabbro, diabase and pillow basaltic lava.
Figure 23.
The last intrusion related to syn- post collision activities (Kuh- e- Mish) (south of Sabzevar).
Volcanic arc: The Sabzevar-Nain volcanic arc is exposed on the Alborz continental margin very close to the suture zone and parallel to the ophiolitic range. This can be due to the 30–35-degree angle of the oceanic crust subducted under the Alborz continental crust. Its length is about 660 km from the west of Torbat-e-Jam in the east of Iran to the north of Semnan in the central southern part of the Alborz Mountain Ranges. Its width in the west of Abbasabad near Miamey is a little more than 10 km. The lithological composition of this arc changes from east to west. In the Sabzevar region, it is reported as red dacite, pyroclastic rocks, massive micrite, and trachyandesite (Oligocene in age). The Eocene volcanic rocks are siliceous tuff interbedded with andesite, porphyritic andesite, basalt, agglomerate andesite basalt to trachyandesite basalt, volcanic breccia, andesite lava, and lithic crystal tuff (Figure 24) [23, 27, 28, 30]. In the western part of the arc in the north of Semnan, the volcanic rocks from bottom to top include intermediate-basic lava with a composition of andesite basalt to phitic andesite. In some parts, this sequence is crosscut by andesitic dykes. The eruptions of andesitic-basaltic lava sometimes enter the shallow water environment and produce brecciated hyaloclastites with sandstone, shale, and limestone. In the higher parts of the sequence, there is some intermediate lava with a phyric andesite composition which is crosscut by quartz feldspathic dykes. Briefly, the Eocene volcanic rocks in the western part of the volcanic arc consist of basalt, basaltic andesite, andesite, dacite, riodacite, riolite, and tuff. Based on their geological, petrographic, and geochemical study (2018), Khalatbari and Etessami concluded that these volcanic rocks are the result of a subduction event during the Paleocene and early-middle Eocene (Figure 25).
Figure 24.
The Sabzevar- Nain volcanic arc exposed on the Alborz continental margin close to the suture zone, mostly composed of intermediate volcanic rocks.
Figure 25.
Location of the studied samples on the diagrams determination of the tectonomagmatic environment a) Th-Hf/3-Nb/16 diagram [
In their geological, petrographic, and geochemical studies, Baghdadi [14] and Shahosseini and Ghassemi [39] reached the same conclusion regarding the north and west of Sabzevar, respectively. In their petrochemical and tectonic setting study of the Davarzan-Abassabad volcanic (DAEV) rocks, Ghassemi and Rezaei-Kahkhaei [15] stated that these volcanic rocks are the product of the partial melting of an enriched mantle by an extensional event within the arc (Figures 26–29).
Figure 26.
a) Classification diagrams of volcanic rocks (after [
Figure 27.
Chondrite (a) and primitive mantle (b) normalized spider diagrams of DAEV rocks (normalization values are from [
Figure 28.
Tectonic discriminant diagrams for the DAEV rocks. a) Ta/Yb vs. Th/Yb (after [
Figure 29.
a) Sm/Yb vs. Ce/Sm plot used to identify the mantel source for DAEV rocks (after [
Post-collision intrusive masses: The post-collision intrusive rocks are cropped out in the form of small and large masses everywhere in the collision zone. The intrusive masses within the ophiolitic belt in the north of Sabzevar are usually small scale. The greatest masses appear in the middle parts of the collision zone in the south and southeast of Sabzevar in the mountains called Borj-Kuh, Kuh-e-Mish, and Kuh-e-Baharestan. The petrological studies on the small intrusive bodies within the ophiolites have shown that they are composed of granite, granophyre, granitoid [29], granite, quartz diorite, diorite, microdiorite [31], micromonzonite, diorite, granodiorite, granite [28], granite, micromonzonite [27], granite intruded by monzodiorite, quartz diorite, monzodiorite, granite, granodiorite [24], quartz monzonite, and quartz monzodiorite [20]. The age determined for these rocks varies from the Paleocene to early Eocene.
Large intrusive masses have the following characteristics: Granite (post-Paleocene), microgranite to granite (late Cretaceous), diorite, monzodiorite (middle-late Cretaceous) in Kuh-e-Baharestan and Kuh-e-Mish (Figure 30) [23] which intrude into the late Cretaceous oceanic sediments (Figure 31) as well as granite, granodiorite, diorite, and monzonite which are crosscut by microdioritic and diorite-monzonite dykes [53]. Based on lithological, petrological, and geochemical studies on the intrusive rocks of Kuh-e-Mish, it has been demonstrated that they are a mass of granitoid of which a major part is granodiorite and a minor part is tonalite in terms of composition. This granitoid mass is mostly calc-alkaline. It is meta-alumina to slightly per-alumina in terms of the amount of aluminum and it is a HSS (a true hybrid) in terms of origin. The mentioned magma probably originated from the upper mantle and the base of the crust [17]. Goharshahi also believed that the depth of magma production was about 73 km or a little more and its replacement depth was less than 4 km. This intrusive mass was of the granitoid type and was associated both with the volcanic arc granitoid (VAG) of the continental margin and subduction and its continuation after collision (post-collision events). The estimated time of collision was probably Eocene and the time of granitoid production was about 37 Ma or more (Figure 32) [17].
Figure 30.
Low elevation of granite masses (post-Paleocene) and microgranite to granite (late Cretaceous) in vicinity of Estaj village (south of Kouh-e-Baharestan).
Figure 31.
Large intrusive masses of diorite, monzodiorite (late Cretaceous to middle Paleocene) which intruded into the late Cretaceous oceanic sediments north of Anjoman Village (south of Sabzevar).
Figure 32.
Geochemistry, chemical classification and location of the post collision granitoids (after [
The youngest post-collision volcanic activities: The age of the youngest volcanic activities in the collision zone varies from Miocene to early Quaternary. They are mainly composed of dacite and most of their outcrops are observed in the ophiolites of northern Sabzevar and also behind the volcanic arc in the Joghatay back-arc basin. Dacites are subvolcanic rocks which mostly appear at the intersections of faults. Young dacites crosscut all the older rocks and sediments in the three forms of dome, plug, and dike. From the petrological point of view, they can be classified into the three groups of biotite bearing dacite, amphibole-bearing dacite, and dacite with a small amount of dark minerals [24]. In the western parts of Sabzevar ophiolites, dacitic domes are more abundant both in number and size (Figure 33) [24].
Figure 33.
The youngest post- collision volcanic activities in the collision zone (Miocene to early quaternary).
In the northernmost part of the collision zone, dacite rocks show an andesitic-dacite composition and appear in the form of domes and lava flows. These lavas cover a large area of the northern margin of the Joghatay back-arc basin. The youngest volcanic cones (Plio-Quaternary in age) can be found in this area ([31]; Radfar [57]) (Figure 34 and 35).
Figure 34.
The youngest volcanic cones (Plio-quaternary in age) and the distribution of lava flows in the northernmost part of the Sabzevar-Nain collision zone (after [
Figure 35.
A Plio- quaternary volcanic cone in the northern most part of the collision zone.
5. Conclusion
Various processes and tectonic events have been influential on the evolution of the Iranian crust. The major events in this regard can be divided into two stages. One of the events in the second stage is the thinning of the crust during the late Jurassic, leading to a period of instability. After this, the Iranian crust broke and then created interior oceanic basins such as Sabzevar-Nain, Nain-Baft, and Sistan-Baluch Basins [2, 4, 5]. The processes of the opening and closure of the Sabzevar-Nain Basin (SNB) began with crustal thinning in the late Jurassic with an east–west trend between the micro-block of central Iran in the south and the Alborz Mountain Ranges in the north. As a deep trough in the middle Cretaceous, the Sabzavar-Nain rift became an oceanic basin at the beginning of the late Cretaceous. At the end of the late Cretaceous, the Sabzevar-Nain Basin reached its final expansion. With the subduction of the oceanic crust under the continental margin of Alborz, the closure of the Sabzevar-Nain Basin began in the early Paleocene. After the collision of the continental margins of central Iran and the Alborz Mountain Ranges, the basin was closed in the middle Eocene. Based on the geometry of the oceanic crust (the direction of subduction is perpendicular to the trench direction), we propose that this continent-continent collision can be classified as a classical collision. Our reasons based on the structural elements of the collision zone are ophiolitic rocks exposed in the suture, remnants of deep oceanic sediments upthrust on both margins, a volcanic arc parallel to the trend of the suture, a back-arc basin, and finally post-collision intrusive masses.
Acknowledgments
We thank Mohammad Reza Mirzaie and Mostafa Khoshduni Farahani for accompanying us in the first field trip. We warmly thank Dr. Fodazi for his petrological advice. We appreciate Mr. Jafarian for providing us with data about the Sheshtamad area and Kuh-e-Bahrestan. Our thanks are also extended to the kind people of the region between Nain and Sabzevar for their hospitality, aids, and information. Finally, we thank Miss Shahosseini for drawing the figures.
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