Evolution of the Central Iranian basement

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    Shurow and Tashk formations. The prevailing metamorphism, from low grade to high grade, is pre-Pan African Orogeny (i.e. from 2400 to

    faults, which appear to be inherited from earlier geological

    scarcely exposed, form the basement of the region (Stocklin,1968, 1974; Nabavi, 1976; Berberian, 1976) and this orogenic

    Since the different orogenic phases recognized in the

    represent the crust of a Precambrian calc-alkaline island arc(Stocklin, 1972; Haghipour, 1974, 1977; Berberian and King,1981). If this interpretation is correct the nearly northsoutharcuate mountain belts in east Central Iran could represent the

    (20phase, considered by many authors to be an episode of platecollision, terminated between 600 and 550 Ma in Arabiatimes. Each fragment differs in its sedimentary sequence,nature, and age of magmatism and metamorphism, and instructural character and intensity of deformation. This paperdescribes the evolution and effects of Precambrian orogenicphases in Central Iran.

    The continental crust of Iran was metamorphosed, grani-tized, folded and faulted during the Late Precambrian by thePan African Orogeny. Metamorphosed rocks, which are

    crystalline shield of Arabia (Greenwood et al., 1976) are notrecognized in Iran, no detailed correlation can be made betweenthe basements of Iran and Arabia. Hence they may havedifferent pedigrees and the consolidation of Iranian basement isnot well understood. The Precambrian Chapedony and PoshteBadam formations of east Central Iran, which consist of meta-greywacke, meta-diorite, meta-andesite, amphibolite, pyroxe-nites, serpentinite, and calc-alkaline intrusive rocks, mayregime. InfracambrianPhanerozoic sedimentary rocks were then deposited on the basement complexes. The Iranian basement complexes areextensively overprinted by Pan African Orogeny and younger igneous events. 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

    Keywords: Central Iran; Precambrian basement; Cratonization; Paleo-suture; Pan African Orogeny

    1. Introduction

    The Iranian plateau extends over a number of continentalfragments welded together along suture zones of oceaniccharacter. The fragments are delineated by major boundary

    (Brown and Coleman, 1972; Greenwood et al., 1975; Frischand Al-Shanti, 1977). Following these movements the UpperPrecambrianCambrian Hormoz salt was deposited in basins,parts of which now lie along the north and eastern side of theArabian Peninsula.Iranian coherent platform at the end of Pan African Orogeny, Central Ira570 Ma). The presence of greenstone belts, a paleo-suture zone and ophiolitic rocks (i.e. from 2100 to 1500 Ma) around the high-grademetamorphic rocks of the Chapedony Formation, provide evidence that cratonization forming the Iranian basement occurred during the Paleo- andMesoproterozoic. After formation of the basement complexes, metamorphism of the Precambrian formations and establishment of the Arabo

    n broke up during a NeoproterozoicEarly Cambrian extensional tectonicEvolution of the Cen

    Alireza

    Geology Department, Payame Noor University o

    Received 21 October 200Available online

    Abstract

    Most of the InfracambrianLower Paleozoic sedimentary rocks occuregion along the Chapedony and Poshte Badam faults, the basemedepositional, erosional, metamorphic and magmatic events. In this regio

    Gondwana Research 12 Corresponding author. Fax: +98 311 738 1002.E-mail address: anadimi@armanisp.net.

    1342-937X/$ - see front matter 2006 International Association for Gondwana Rdoi:10.1016/j.gr.2006.10.012al Iranian basement

    adimi

    ahan, Kohandej, PoB. 81465-617, Isfahan, Iran

    ccepted 18 October 2006ecember 2006

    g in Iran cover a crystalline basement. In Central Iran, and the Saghandcomplexes show structural vestiges of Precambrian deformational,he basement complexes contain the Chapedony, Poshte Badam, Boneh

    07) 324333www.elsevier.com/locate/groriginal pattern of the Precambrian arcs.After the metamorphism of the Precambrian formations and

    the establishment of the AraboIranian platform at the end of

    esearch. Published by Elsevier B.V. All rights reserved.

  • esePan African Orogeny, compressional tectonic activity endedwith intrusion of granites and alkaline volcanism. The UpperPrecambrian alkali-enriched Doran granites of Iran seem to beequivalents of the 600 Ma younger granites of Arabia(Stocklin et al., 1964; Schmidt et al., 1973, 1978; Sillitoe,1979). The Doran granite cuts the Upper Precambrian low-grade metamorphic rocks of the Kahar Formation and iscovered by Lower Cambrian sediments (Stocklin et al., 1964).

    The Late Precambrian volcanics also include andesite, basaltand tuff. These widespread post-orogenic volcanic rocks,which overlie the Precambrian metamorphic rocks and areoverlain by NeoproterozoicCambrian sediments, may be thesignature of rifting of the AraboIranian continental crust duringa phase of extensional tectonism associated with formation ofthe epicontinental platform from Arabia to Alborz prior todeposition of the Upper PrecambrianCambrian succession.Following Late Precambrian (Pan African) orogeny and con-solidation of the basement, the Precambrian craton of Iran,Pakistan, central Afghanistan, southeastern Turkey and Arabiabecame a relatively stable continental platform with epiconti-nental shelf deposits (mainly clastics) and exhibited a lack ofmajor magmatism or folding. This regime presumably lasteduntil Late Paleozoic times, although some epirogenic move-ments occurred in Late SilurianEarly Devonian times (Berber-ian, 1983).

    2. Basement complexes in Central Iran

    The term basement complex is used here for the set of rocksunderlying the Pan African unconformity and comprisingmostly metamorphic or igneous rocks (with the age of thebottom of the related rock cover being variable, ranging in mostcases from 570 to 550 Ma). These basement complexes showstructural vestiges of old Precambrian deformational, deposi-tional, erosional, metamorphic andmagmatic events. The relatedold features are more or less modified by younger deformationaland thermal overprints.

    Precambrian terranes are exposed in many places of Iran(Fig. 1). In the Saghand Region, Central Iran, basement com-plexes occur as the oldest lithostratigraphic element (Nadimi,2005).

    2.1. Central Iran

    The Central Iranian Terrane is surrounded by fold-and-thrustbelts, within the AlpineHimalayan orogenic system of westernAsia. It is located between the Turkish syntax to the west and theAlborz and KopehDagh Ranges to the north, the Zagros andMakran Ranges to the west and south and the East Iran Ranges tothe east. Being situated to the northeast of the ZagrosMakranNeo-Tethyan suture and its sub-parallel Cenozoic magmatic arc(UrumiehDokhtarMagmatic Arc), the Central Iranian Terrane isan area of continuous continental deformation in response to theongoing convergence between the Arabian (Gondwana) and

    A. Nadimi / Gondwana REurasian (Turan) plates (Figs. 1 and 2).The Central Iranian Terrane consists, from east to west, of

    three major crustal domains: the Lut Block, Tabas Block andYazd Block (Alavi, 1991) (Fig. 2) separated by a series ofintersecting regional-scale faults. The Lut Block is locatedbetween the Nehbandan Fault to the east and Nayband Fault tothe west. The Tabas Block is surrounded by the Nayband Faultto the east and the Kalmard and Kuhbanan Faults to the west.The Yazd Block is located between the Kuhbanan Fault to theeast, the Biabanak Fault to the north and the Rafsanjan andShahre Babak Faults to the south and west.

    Although the stratified cover rocks can be correlated betweenthe different blocks, locally significant facies and/or thicknessvariations occur across the domain boundaries. The easternregion of the Yazd Block, between the Yazd and Tabas blocks,provides remarkable exposures of the deeper sections of theCentral Iranian platform strata, among which Late Neoproter-ozoic and Lower Paleozoic rocks are abundant. Interestingly,the type localities of all of the Precambrian to Carboniferousformations in greater Central Iran (Stocklin and Setudehnia,1977) have been selected from within this relatively narrowbelt. The crystalline rocks that underlie the above formationshave been uplifted and widely exposed in the vicinity of thetown of Saghand.

    2.2. The Saghand Region

    The Saghand Region closely follows the trends of thepredominant fault structures of the Central Iranian Terrane,many of which are seismically active or were active in the nearpast. Three first-order fault systems are identified within theCentral Iranian Terrane: a north-trending system (e.g. Naybandand Nehbandan faults), a northeast system (e.g. Poshte Badamand Kalmard faults) and a northwest system (e.g. Kuhbanan andRafsanjan faults). A combination of strikeslip (right-lateral)and reverse (thrust) movements associated with these faults hasgenerated a complex pattern of regional deformation involvingcrustal shortening, horizontal block rotation and localized uplift(Jackson and McKenzie, 1984). The Saghand region, inparticular, is situated at the intersection of northeast and north-west trending fault systems where the most extensive docu-mented outcrops of basement rocks in Central Iran are exposed(Ramezani and Tucker, 2003).

    In the Saghand Region, there are three roughly NNE-trending lithotectonic domains, bordered from east to west bythe Poshte Badam, Chatak and Chapedony faults, respectively(Fig. 3a). Rocks of the Eastern Lithotectonic Domain rangefrom lower-amphibolite-facies gneisses (Boneh Shurow For-mation) to virtually unmetamorphosed sedimentary rocks,whereas, the Western Lithotectonic Domain is dominated byhigh-grade gray gneisses and migmatites (Chapedony Forma-tion). Rocks of the Central Lithotectonic Domain include avariety of metamorphic rocks (Poshte Badam Formation) andigneous plutons by and large different from those in the othertwo domains. Each lithotectonic domain contains a suite ofplutonic rocks of distinct age distribution ranging in lithologyfrom granite to quartzdiorite and, locally, diorite. In this re-

    325arch 12 (2007) 324333gion, basement complexes are about 10 km thick; they underliethe Infracambrian unconformity and contain the Chapedony,Poshte Badam, Boneh Shurow, and Tashk formations (Fig. 4).

  • ese326 A. Nadimi / Gondwana RGravimetric studies in Central Iran show that the continentalcrust thickness is 40 to 48 km (Dehghani and Markis, 1983).

    2.2.1. The Chapedony FormationThis formation, located mainly between the Chapedony

    Fault to the west and the Chatak Fault to the east, is the oldestbasement complex in Iran. The Chapedony Formation is com-posed predominantly of a variety of high-grade gneisses andassociated migmatites (Hushmandzadeh, 1969; Haghipour andPelissier, 1977), including rocks of granulite facies, and isbordered by rocks of greenschist facies or lower grade at theeastern side. Both types are intruded by younger granitic

    Fig. 1. Geodynamic map of Central Iran and adjacent regions. Solid rectangle indChapedony; 2 Poshte Badam; 3 Bafq; 4 Mute; 5 Golpayegan; 6 Aligodaarch 12 (2007) 324333plutons. Reported 207Pb/206Pb zircon ages of the ChapedonyFormation range from 2140.9 Ma to 48.2 Ma (Ramezani andTucker, 2003) and Rb/Sr whole rock ages are around 2382 Ma(Haghipour, 1974).

    The Chapedony Formation occurs in three main outcrops:the Kalute Chatak, Kalute Chapedony and Neybaz Mountain(Fig. 3a). The formation is approximately 4000 m thick andcontains schist, amphibolite, marble, quartzite, gneiss, ribbongneiss, migmatite, anatectic granite and scapolitic rocks allshowing migmatitic textures. Gneiss and granite formed at thelower levels of the Chapedony sequence probably from sedi-mentary protoliths called the Old Series (which is not seen on

    icates the Saghand region. Outcrops of Precambrian rocks are numbered: 1 rz; 7 Zanjan; 8 Karaj Valley; 9 Gorgan.

  • eseA. Nadimi / Gondwana Rthe surface) during the Chapedonian high-grade metamorphism(Haghipour, 1974).

    2.2.2. The Poshte Badam FormationThe diverse assemblage of mainly dark-colored, medium-

    grade, metamorphic rocks exposed to the west of the PoshteBadam Fault is known as the Poshte Badam Formation(Haghipour and Pelissier, 1977). The main body of the formation

    Fig. 2. Simplified structural map of Central Iran and constituent crustal blocks (com2003). Rectangle delineates the study area of Fig. 3.327arch 12 (2007) 324333in the northern part of the Central Domain overlies high-graderocks of the Chapedony Formation along the Chatak Fault(Fig. 3a and b). A variable association of greenstones, schists,meta-greywacke, marble, gneisses, amphibolites, pyroxenites,serpentinite, meta-basalt and conglomerate comprises the bulk ofthe Poshte Badam Formation and it is severely disrupted as theresult of thrusting and by intrusion of granitoid plutons; hence thedispersed outcrops of the formation generally lack continuity.

    piled from Jackson and McKenzie, 1984; Alavi, 1991; Ramezani and Tucker,

  • Fig. 3. (a) Geological map of the Saghand Region (modified after Haghipour, 1977). (b) and (c) cross sections from the basement complexes.

    328 A. Nadimi / Gondwana Research 12 (2007) 324333

  • Taurus Block in southern Turkey, thus presents a good matchfor the Boneh Shurow granitic gneiss in the Saghand region(Fig. 5).

    2.2.4. The Tashk FormationThe well-stratified sequence of weakly metamorphosed,

    sedimentary and volcanic/ volcaniclastic rocks that occur to theeast of, and sub-parallel to, the BonehShurow Formation isknown as the Tashk Formation (Haghipour and Pelissier, 1977).The exposed section of the formation in its type locality atTashk Mountain is estimated to reach about 2000 meters. Theformation has four main outcrops: the Poshte Sorkh, Tashk,Zamanabad and Polo mountains (Fig. 3).

    The bulk of the Tashk Formation consists of dark greenish-gray greywackes, schist, quartzitic schist, quartzite, marble,amphibolite, gneiss, slaty-shale, sandstone locally interbeddedwith arkosic arenites, argillites, tuffaceous deposits and newly

    329esearch 12 (2007) 324333In addition to the intercalated dolomitic marble layers withinthe Poshte Badam Formation, there are massive crystallinemarble sheets that form many crests and hilltops in the northernpart of the Central Domain. Close field examination indicatesthat the latter are almost invariably in thrust contact with rocks(for example, greenstones) of the Poshte Badam Formation andtherefore should be considered as a separate lithologic unit(Ramezani and Tucker, 2003). The presence of meta-mafic andmeta-ultramafic rocks induced some geologists in the past toconsider the Poshte Badam Formation as an oceanic crustmantle transition (Haghipour, 1974).

    2.2.3. The Boneh Shurow FormationThe Boneh Shurow Formation is the most widely exposed

    metamorphic unit of the Eastern Domain with ridge-formingoutcrops directly adjacent to, and to the east of, the Poshte

    Fig. 4. Generalised stratigraphic section of the basement complexes in theSaghand region.

    A. Nadimi / Gondwana RBa...

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