Geochemistry, zircon U-Pb age, and tectonic constraints on the Bazman granitoid complex, southeast Iran

The Bazman granitoid complex (BGC), including a large zoned pluton, intrudes into the upper Paleozoic sedimentary cover of the Lut block. It crops out on the southern slope of the Bazman volcano in Baluchestan Province of Iran. The intrusive rocks range from gabbro to various metaluminous to weakly peraluminous granites, and they are classified as I-type magmatic series.

Turkish Journal of Earth Sciences Turkish J Earth Sci (2016) 25: 311-340 © TÜBİTAK doi:10.3906/yer-1509-3 http://journals.tubitak.gov.tr/earth/ Research Article Geochemistry, zircon U-Pb age, and tectonic constraints on the Bazman granitoid complex, southeast Iran 1, Mohammad Reza GHODSI , Mohammad BOOMERI *, Sasan BAGHERI , Daizo ISHIYAMA , Fernando CORFU Department of Geology, University of Sistan and Baluchestan, Zahedan, Iran Department of Earth Science and Technology, Faculty of Engineering and Resource Science, Akita University, Akita, Japan Department of Geosciences, University of Oslo, Norway Received: 06.09.2015 Accepted/Published Online: 15.03.2016 Final Version: 09.06.2016 Abstract: The Bazman granitoid complex (BGC), including a large zoned pluton, intrudes into the upper Paleozoic sedimentary cover of the Lut block It crops out on the southern slope of the Bazman volcano in Baluchestan Province of Iran The intrusive rocks range from gabbro to various metaluminous to weakly peraluminous granites, and they are classified as I-type magmatic series They display geochemical characteristics of typical volcanic arc magmatism at continental margins Major- and trace-element variation diagrams show that fractional crystallization was the major process and crustal contamination, a subordinate process during the evolution of the BGC The decrease in CaO, MgO, Al2O3, Fe2O3, TiO2, P2O5, and Sr, as well as the increase of K2O and Rb with increasing silica, are possibly related to the fractionation of plagioclase, hornblende, apatite, and titanite, whereas the increasing K, Rb, Cs, Pb, and light rare earth elements (LREEs) can be explained by crustal contamination The BGC rocks are enriched by large ion lithophile elements (e.g., Rb, K, Cs) and the LREEs with respect to the high field strength elements (e.g., Zr, Hf, Nb, Ta, Y) and heavy rare earth elements New IDTIMS U-Pb dating performed on zircon and titanite extracted from the granitic samples indicates that the BGC was emplaced during the late Cretaceous period at 83–72 Ma by subduction of the Neo-Tethyan oceanic crust beneath the Eurasian continent Subsequently, the complex became part of the Lut block when it probably rotated counter-clockwise with respect to the Sanandaj-Sirjan zone and the Urumieh-Dokhtar volcano-plutonic belt Key words: Zircon U-Pb age, Lut block, Neo-Tethyan subduction, Bazman, Iran Introduction The role of granite and granitic magma is crucial for the understanding of the magmatic processes, continental crust evolution, and tectonic setting of many terrains (e.g., White and Chappell, 1983; Atherton, 1993; Brown, 2013) There are various complexities in the genesis of granitoid magmas, but in general they fall into mantle and crustal processes: 1) fractional crystallization of mantlederived mafic magma is a major process in producing a wide diversity of granite compositions (e.g., Bowen, 1948; Huppert and Sparks, 1988; Pitcher, 1993); 2) hightemperature metamorphism leads to partial melting of the continental crust and the formation of granites (Winkler, 1965; Chappell and White, 1974; Ashworth, 1985; Mehnert, 1987) Most granitoids originate indirectly from the mantle or consist of mixtures of continental crust and mantle components (Wyllie, 1984; Atherton, 1990; Gray and Kemp, 2009) There is also significant production of granitoid rocks in nonconvergent plate tectonic settings, * Correspondence: boomeri@hamoon.usb.ac.ir particularly some of the extensional tectonic regimes (e.g., Leake, 1990; Eby, 1992; Atherton and Petford, 1993; Vigneresse, 1995; Barbarin, 1999) Over the past two decades there has been an increasing interest in the petrogenesis and thermochronology of granite in many parts of Iran The oldest group of granitoids cropping out in central Iran is attributed to an early Cambrian magmatic belt associated with the Proto-Tethyan subduction (Ramezani and Tucker, 2003; Bagheri and Stampfli, 2008; Hassanzadeh et al., 2008) The second group of granitoids is related to the PaleoTethyan subduction in central and northern Iran (Bagheri and Stampfli, 2008; Mirnejad et al., 2013) A third group of granitoids crops out in the Sanandaj-Sirjan Zone, a Mesozoic magmatic belt (Berberian and King, 1981), that lies to the NE and parallel to the Zagros fold-thrust belt above the Neo-Tethyan subduction zone (Figure 1a) (e.g., Ahmadi Khalaji et al., 2007; Ghalamghash et al., 2009; Shahbazi et al., 2010; Tahmasbi et al., 2010; Mahmoodi et al., 2011; Esna-Ashari et al., 2012) 311 GHODSI et al / Turkish J Earth Sci 60°00´ 34°00´ 55°00´ Ur um ieh kh 28°00´ EIR Lu 11 UD B 10 Pe Sistan Suture Zone rsi an Neotethys Suture Zone Gu Reactivated Neotethys lf back-arc Suture Zone Paleotethys Suture Zone Mp Oman Sea Afghan Block b Afghanistan Ks Belt askoh R i a Chag Pakistan es ng Ra rs Pe Yz PbTb n nia Ira BGC ulf Figure 1B outline AJT Za Tf c r A ic lcan o V ran Bz Mak Jazmurian depression G ian Gb Z st Ea tar Vo Sa lca nan nodaj Plu -Si ton rja ic B nP elt lut oni cB elt SS Lut Block -Do Kd Al B Iran UD Zone Sut ure istan t he S lt of Lu of t tonic b e e lt East Plu ic B t ton Lu Plu of elt ic B on lut o-P an olc st V We Tabas Block a Caspian Sea Makran Accretionary Prisms 500Km Oman Sea 65°00´ Figure (a) Main tectonostratigraphic units of Iran, modified after Stöcklin (1977), Berberian and King (1981), Tirul et al (1983), and Bagheri and Stampfli (2008) (b) Main magmatic belts in the south and east of Iran AJT: Anarak-Jandaq terrane; Al: Alborz; BGC (Bazman granitoid complex); Bz: Bazman volcano; EIR: Eastern Iranian Ranges; Gb: Great Kavir Block; Kd: Kopeh Dagh; Ks: Kuh-e-Sultan; Lu: Lut Block; Mp: Makran accretionary prisms; Pb: Poshteh-e-Badam terrane; SSZ: Sannadaj-Sirjan Zone; Tb: Tabas Block; Tf: Taftan Volcano; UDB: Urumieh-Dokhtar volcano-plutonic belt, Yz: Yazd Block; Za: Zagros fold and thrust belt 1: Urumieh plutonic complex (Ghalghamash et al., 2009); 2: Astaneh pluton (Tahmasbi et al., 2010); 3: Alvand plutonic complex (Shahbazi et al., 2010); 4: Borojerd granitoid (Ahmadi Khalaji et al., 2007); 5: Shir-Kuh granite (Sheibi et al., 2011); 6: Sirjan granitoid; 7: Bajestan granitoid (Karimpour et al., 2011); 8: Shah Kuh granitoid (Esmaeily et al., 2005); 9: BGC; 10: Band-e-Zyarat ophiolite; 11: Dehshir-Baft ophiolite The remaining enigmatic granitoids, which occur in eastern Iran, are attributed to a variety of origins Some of those are interpreted as originated from syn-collision magmatism along the Sistan suture zone (Figure 1a) 312 (Camp and Griffis, 1982; Sadeghian et al., 2005) The granite formations exposed in the Lut block (Figure 1b), in the eastern part of central Iran, are ascribed to the subduction of the Sistan oceanic lithosphere under the GHODSI et al / Turkish J Earth Sci Lut block (Esmaeily et al., 2005; Mahmoodi et al., 2010; Arjmandzadeh et al., 2011; Zarrinkoub et al., 2012) There is also ample evidence emphasizing eastward subduction under the Afghan block (Camp and Griffis, 1982; Tirrul et al., 1983; Fotoohi Rad et al., 2005; Saccani et al., 2010; Angiboust et al., 2013) The Bazman granitoid complex (BGC) is one of the granitoid complexes that intruded the Lut block, north of the present-day Makran range (Figure 1b), in the Late Cretaceous (Berberian et al., 1982) It is composed of different types of plutonic rocks with a wide range of silica contents (Vahdati Daneshmand et al., 2004; Sahandi and Padashi, 2005) Some primary contacts among the various rock bodies are preserved; however, the metamorphosed country rocks and blocks of roof pendant are dispersed through the complex These features, along with good exposure, make the BGC suitable for studying the processes involved in the evolution of granite The BGC is situated at the intersection of several volcano-plutonic belts in the southeast corner of Iran, where several tectonostratigraphic terranes (see Howell, 1989) with uncertain relationships are present (Figure 1) This location is one of the few direct sources of information that could shed light on the magmatic evolution and tectonic history of terranes, and, additionally, the recognition of terrane outlines There is little published geological information on the geology, geochemistry, and petrogenesis of the BGC The most important source of fundamental knowledge in this regard is the petrological and geochronological study of Berberian (1981), who postulated that calcalkaline magmatism was generated by the Neo-Tethyan oceanic lithosphere (Sea of Oman) that was subducted beneath the Makran Range The magmatic differentiation was considered as the principal process involved in the generation of the BGC (Berberian, 1981) More recent efforts have focused on geological mapping to separate the various granitic phases with different characteristics (Vahdati Daneshmand et al., 2004; Sahandi and Padashi, 2005) However, there are inconsistencies in the grouping of these granites between the eastern and western parts of the BGC on the published geologic maps There are two main unanswered questions related to the understanding of granite emplacement and genesis in eastern Iran First, the oldest accretionary prisms of the Makran range are composed of Eocene-Oligocene flysch (McCall, 1997, 2002), while the proposed late Cretaceous formation of the BGC would have required the initiation of subduction prior to the start of the Cretaceous period However, there is no preserved evidence, specifically in the Makran area, that proves that subduction occurred before the Cretaceous period Accordingly, considering the nature of the Makran volcanic arc, the second question is self-evident: Why are the Eocene volcanic rocks and the Oligocene-Miocene plutonic rocks, the obvious signs of the Urumieh-Dokhtar volcano-plutonic belt in Iran, almost nonexistent in the Makran volcanic arc? The study of the BGC can help us to understand several puzzling petrogenetic aspects in the region, because the key tectonic setting of the BGC is within or near several other magmatic belts in southeast Iran (Figure 1b) Finally, the data emerging from research on the BGC could be correlated with comparable plutonic rocks in adjacent tectonic units, such as the Sanandaj-Sirjan Zone in Iran (Berberian and Berberian 1981), or Lhasa and Karakorum in the Himalayas (Searle et al., 2010) Accordingly, this paper focuses on precise age determinations with ID-TIMS U-Pb dating of the most common rock types in the BGC, supported by petrography, whole-rock geochemistry, the evolution of the magmatic rocks, and its relationship to the overall tectonic setting and the changes therein The results are also critical to related discussions regarding the southern outline of the Lut block and its tectonic behavior since the late Mesozoic period Geological setting The Iranian plateau is a part of the Alpine-Himalayan orogenic system, which is one of the major structural features of the planet Earth The main tectonostratigraphic units of Iran are shown in Figure 1a All the units have been attributed to the opening and closing of the Paleo-Tethyan and Neo-Tethyan oceanic basins as a result of subduction and collision events in the northern to southern parts of Iran The BGC is located at the southeastern extremity of the Urumieh-Dokhtar volcano-plutonic belt, north of the Makran accretionary prisms, west of the Sistan suture zone (Flysch zone), and south of the Lut block where the Iranian microcontinent experienced several subduction and collision events with the Arabian plate beginning during the Late Cretaceous period (Stöcklin, 1977; Berberian and King, 1981) and continuing through to the Miocene arc stage (Shahabpour, 2005; Agard et al., 2007) to Quaternary volcanism (Farhoudi and Karig, 1977; Saadat and Stern, 2011) (Figure 1a) There are seven volcanic and/or plutonic belts intersecting each other in the southern and southeastern parts of Iran near the study area (Figure 1b) They are chronologically presented here 2.1 Sanandaj-Sirjan plutonic belt The magmatic part of the Sanandaj-Sirjan Zone comprises mainly middle-late Jurassic, and infrequently Cretaceous, plutonic rocks and other comparable extrusive rocks It is known as the Mesozoic magmatic belt of Iran and was produced by the Neo-Tethyan oceanic crust subduction (Berberian and King, 1981) 2.2 East plutonic belt of Lut A few late-Jurassic plutonic bodies, such as the Shah Kuh granite pluton (Esmaeily et al., 2005; Mahmoodi et 313 GHODSI et al / Turkish J Earth Sci al., 2010), and probable Cretaceous intrusions, such as the Bajestan granite pluton (Karimpour et al., 2011), are the main constituents of this belt (Figure 1a) There is no consensus on the origin of this plutonic belt 2.3 Chagai-Raskoh volcanic belt This is the western volcanic belt of Pakistan with an intraoceanic island-arc origin that developed between the Cretaceous period and Eocene epoch in the Neo-Tethyan Ocean and subsequently accreted to the northern active margin of Eurasia (e.g., Siddiqui et al., 1986; Nicholson et al., 2010) 2.4 Plutonic belt of the Sistan suture zone Several late Eocene-Oligocene granitoid plutons were emplaced along the north- to northwest-trending suture zone situated between the Lut and Afghan blocks (Camp and Griffis, 1982; Sadeghyan et al., 2005) Most of them are characterized by the calc-alkaline syn-collision to subduction-related magmatism that intruded during the closing of the Sistan Ocean (Tirrul et al., 1983) 2.5 Urumieh-Dokhtar volcano-plutonic belt The Urumieh-Dokhtar volcano-plutonic belt consists mainly of Eocene calc-alkaline extrusive rocks and Oligocene-Miocene granitoid intrusions (Berberian and Berberian, 1981) It extends parallel to the Zagros foldthrust belt and is the product of the subduction of the NeoTethyan oceanic lithosphere under the Iranian continental lithosphere (e.g., Berberian and King, 1981, Verdel et al., 2011) 2.6 West volcano-plutonic belt of Lut This belt includes Eocene calc-alkaline volcanic and Oligocene-Miocene plutonic rocks In spite of its similarities to the Urumieh-Dokhtar volcano-plutonic belt, the belt is oriented at a sharp angle with respect to the Neo-Tethyan suture zone, and consequently identifying its origin is problematic Several researchers have ascribed the belt to the subduction of the Sistan oceanic lithosphere underneath the Lut block (Arjmandzadeh et al., 2011; Karimpour et al., 2011) 2.7 Makran volcanic arc This arc includes several recently active strato-volcanoes, such as Bazman, Taftan, and Kuh-e-Soltan, which are situated above the Makran subduction zone, parallel to the Cenozoic Makran accretionary prisms, and north of the Jazmurian Depression in a fore arc basin geodynamic position (Farhoudi and Karig, 1977; Jacob and Quittmeyer, 1979; Saadat and Stern, 2011) Geology of the BGC The BGC consists of several plutonic bodies, including a main elliptical pluton in the western part and several small intrusions with complicated boundaries in the eastern part This complex covers an area of about 900 km2 314 (Figure 2) The BGC is strongly weathered and eroded, and it displays a topography that is lower than that of the surrounding sedimentary rocks The general geology of the study region is outlined in the 1/100,000-scale geological maps of Bazman and Maksan (Vahdati Daneshmand et al., 2004; Sahandi and Padashi, 2005), and it is presented in a more detailed map in Figure The BGC is surrounded by Paleozoic sedimentary rocks that locally underwent contact metamorphism These sedimentary rocks include the Carboniferous Sardar Formation (Cs), which consists of shale, sandstone, and limestone, and the Permian Jamal Formation (Pj) composed of siltstone, shale, sandstone, and thick bedded limestone and dolomite (Figure 3a) Blocks of crystallized carbonate and sandstone of varying sizes, originating from the Cs and Pj formations, can be observed with pronounced resorbed margins dispersed in the main BGC granitic body Close to the contact, the sedimentary country rock associations were metamorphosed to hornfels, quartzite, and marble depending on the local lithological composition The hornfels in the aureole are intensively silicified and can be divided into biotite, hornblende, and pyroxene hornfelses These rocks are unconformably overlain by Miocene to Quaternary volcanic rocks, travertine, and recent alluvial deposits The Bazman volcanic rocks, including lava flows and pyroclastic deposits, appear with extreme thicknesses in the northern part as represented on the map (Figure 2) The BGC is a polyphase granitoid complex that can be divided into western and eastern parts The western part includes a zoned pluton with a diameter of 30 km, having a gabbro to meladiorite rim characterized by an average width of 1000 m (Figure 3b), changing inwardly to felsic rocks with a composition shifting from monzodiorite to granodiorite, to porphyritic granites in the core (Figures 3c and 3d) (Vahdati Daneshmand et al., 2004) The widespread existence of various xenoliths, remnants of roof pendant strata that reacted to the hornblende granite, can be identified in the center of the pluton In addition, the metamorphosed outcrops of the country rocks between the intrusions, especially in the eastern section, are evidence that could potentially indicate the role of a contamination process during the magmatic evolution of the BGC The gabbro occurs mainly as a narrow ribbon-shaped outcrop on the southern and western margins of the complex (Figure 3e) These plutonic phases with different lithology are cut by numerous granitic plugs and aplitic dikes (Figure 3f) The aplitic dykes apparently follow an old fracture system (N15°E) in the gabbro The enclaves have distinct boundaries with the host granite and granodiorite Enclaves are a common feature of the BGC and are mainly gabbro and diorite in composition that occur as oval bodies and irregularly shaped blobs, ranging from to 50 cm in size (Figure 3g) GHODSI et al / Turkish J Earth Sci 59˚50'0"E 60˚0'0"E 60˚10'0"E N Dare Ahuo B2 Bazman 27˚50'0"N 24 437 g7 447 11 10 Irans Ja7 Ja8 111 320 Bz-2 330 hahr 316 318 Bz-7 27˚40'0"N 160 340 50 Spost Maksan Tanak 48 Fault Road 13 Sample Location Ja13 U/Pb Dating 12 12 km 85 Bz-3 Porphyritic granite Contact Metamorphism Alluvium Basaltic lava flows Pyroxene hornfels facies Biotite, hornblende granite Andesitic lavas, dacite, tuff, and minor olivine basalt (Pliocene) Hornblende hornfels facies Biotite granite Andesitic and dacitic lavas (Miocene) Slightly metamorphosed Granodiorite Limestone of Jamal Formation (Permian) Monzodiorite to quartz monzodiorite Shale and sandstone of Sardar Formation (Carboniferous) Granite Late Cretaceous Quaternary L E G E N D Bazman Granitoid Complex Village and Town Gabbro - Diorite Figure Simplified geological map of the BGC (modified after Vahdati-Daneshman et al., 2004; Sahandi and Padashi, 2005) The eastern part of the BGC has a general NE-SW trend and consists of various types of granites, which are wrapped by each other, demonstrating a complicated pattern of emplacement No regular pattern in their distribution can be observed It seems that the main portion of eastern granites was covered by the younger products of the Bazman volcano Some parts of the eastern granitoids obviously illustrate a different mineralogical composition as they contain garnet and muscovite (Figure 3h) The BGC is cut by two relatively young fault systems (Figure 2); the first consists of en echelon, dextral strikeslip faults with a general N30°E trend distributed in the eastern part of the complex, whereas another minor system appears as sinistral strike-slip faulting with a general N45°W trend in the western part of the complex These two fault systems are similar to those observed in the Eastern Iranian Range, the so-called East Flysch Basin (Freund, 1970; Stöcklin, 1974) The Iranian-Arabian plate collision (e.g., Berberian, 1983; Mouthereau et al., 2012), was certainly the main cause of this shear deformation Overprinting of this new deformation phase onto the previous ones during the late Tertiary period crushed the BGC and resulted in the flat topography, which differs from the normal topographically irregular appearance of granites Analytical methods A total of about 300 samples from various intrusive phases, including granite, granodiorite, monzodiorite, quartz- 315 GHODSI et al / Turkish J Earth Sci Bazman Volcano Jamal Formation Diorite Sardar Formation Granodiorite Granite a Qz Hbl b Bt cm c Marble 0.8 cm Enclave d Or cm Gabbro ee Grt Aplite Granite f g Granite h Ms 0.8 cm Figure (a) Field photograph of the BGC’s rocks intruded into the sedimentary rocks, (b) diorite, (c) granodiorite, (d) porphyritic granite with pink orthoclase megacrysts, (e) contact between gabbro and marble, (f) aplitic vein/dikelet in porphyritic granite, (g) gabbroic enclave in granite, and (h) biotite-muscovite granite contains garnet Mineral abbreviations from Whitney and Evans (2010) monzodiorite, diorite, and gabbro, as well as enclaves of various compositions, were collected Thin sections of these samples were prepared and studied by optical microscopy Rock types were identified by modal analyses, based on the counting of 3000 points for each sample Twenty-one samples were analyzed for major and some minor elements by X-ray fluorescence (XRF) Trace and rare earth elements were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) The measurements using XRF and ICP-MS were carried out with a Phillips PW2404 XRF at the Faculty of Education and Human Studies and a VG Elemental PQ-3 ICP-MS at the Faculty of Engineering and Resource Sciences at Akita University, Akita, Japan Loss on ignition (LOI) was determined by heating the samples at 900 °C for h to determine relative weight loss 316 The U-Pb analyses were carried out by thermal ionization mass spectrometry isotopic dilution (IDTIMS) at the University of Oslo (Norway) The rocks were crushed and pulverized in a jaw crusher and hammer mill and the heavy minerals concentrated with a succession of Wilfley table flotation, free fall, and high gradient magnetic separation and methylene iodide density separation Further selection was carried out by hand-picking under a binocular microscope All zircon fractions were subjected to chemical abrasion, based on Mattinson (2005), but by approximately following the procedure of Schoene et al (2006) with an annealing stage of days at 900 °C, a partial dissolution step with HF (+HNO3) at ca 190 °C overnight, and a hot plate step of h in N HCl after removal of the solution and some rinsing The dissolution was carried out following Krogh (1973) as described by GHODSI et al / Turkish J Earth Sci Corfu (2004), but using a mixed 202Pb–205Pb–235U spike Data were calculated using the decay constants of Jaffey et al (1971) and corrected for initial 230Th disequilibrium (Schärer, 1984) The final ages are the weighted averages of 206 Pb/238U dates Calculations and plotting were done with the Isoplot program of Ludwig (2003) Petrography The modal compositions for the western and eastern granitoids are given in Table On the basis of the average modal percentage of quartz, orthoclase, and plagioclase, the BGC composition ranges from granite and granodiorite, through quartz monzodiorite, monzodiorite, diorite, and gabbro (Figure 4) Some of most striking petrographic characteristics of these granitoids are summarized below 5.1 The western granitoids 5.1.1 Porphyritic granite Porphyritic granites cover the central part of the western granitoids The contact of porphyritic granite with the surrounded rocks (granodiorite and diorite) is sharp They are white to pink in color and coarse-grained porphyritic in texture with very large euhedral to subhedral pink orthoclase megacrysts (Figure 3d) The orthoclase megacrysts usually contain inclusions of zircon, apatite, magnetite, biotite, and hornblende The plagioclase grains are subhedral, unzoned, and polysynthetically twinned, and weakly altered to sericite and epidote The quartz is anhedral, with varying sizes, and displays undulatory extinction The groundmass consists mainly of medium- grained quartz, plagioclase, and orthoclase as the main minerals, with biotite and hornblende occurring rarely as subhedral and anhedral phases Myrmekitic intergrowths are commonly seen between the quartz and plagioclase in these rocks 5.1.2 Granite Granites are coarse- to medium-grained, granular in texture, and gray in color Major minerals include K-feldspar, plagioclase, quartz, hornblende, and minor minerals such as biotite, titanite, apatite, and opaque minerals The K-feldspars are large to small subhedral to anhedral and show Carlsbad twinning The plagioclase occurs as subhedral large crystals, unzoned, polysynthetically twinned, and weakly altered to sericite The quartz is anhedral, medium-grained, and with undulatory extinction The hornblende appears as euhedral to subhedral crystals, which are the most common mafic minerals in these rocks The biotite typically occurs as subhedral to anhedral, in irregular plates, and contains inclusions of apatite and titanite 5.1.3 Granodiorite The granodioritic rocks display a variety of shapes in the western granitoids Some of them occur as a continuous thin zone around the margin of the porphyritic granites and some of them occur as a wide granodioritic zone near the southern part of the porphyritic granite (Figures and 3c) The granodiorite is mainly coarse-grained, pale gray in color, and granular in texture (Figure 5a) The plagioclase occurs as subhedral and anhedral small to large crystals, Table Modal mineralogical compositions of the BGC (Gb: gabbro, Mn: monzodiorite, Gd: granodiorite, G: granite, PG: porphyritic granite, BG: biotite granite, BHG: biotite-hornblende granite, BMG: biotite-muscovite granite)   Western granitoid Rock type Gb Sample no 12 340 Eastern granitoid Mn Gd G PG BG BHG BMG 111 13 330 Ja7 447 85 10 Plagioclase 65.7 47.1 66.6 41.5 33.9 29.8 37.3 41.4 24 Quartz 0.7 1.2 3.7 20.3 22 27.1 23.1 15.3 35 K-feldspar 4.7 17.6 26.2 34 39.6 32.2 22.7 36.7 Hornblende 31 9.8 9.2 0 10.3 Biotite 6.1 4.4 1.6 0.6 2.8 7.2 8.3 1.3 Clinopyroxene 13.5 6.6 0 0 0 Orthopyroxene 5.3 0 0 0 0 Muscovite 0 0 0 0 Garnet 0 0 0 0 Titanite 0 0.8 0.2 0.9 0.1 0.7 Opaque 1.7 5.1 1.5 1.1 0.5 0.7 0.2 1.8 Counted points 3000 3000 3000 3000 3000 3000 3000 3000 3000 317 GHODSI et al / Turkish J Earth Sci Q e iorit nod G Granite A Qz-Monzodiorite P Monzodiorite Gabbro/Diorite Figure Modal compositions of representative samples of the BGC in quartz-alkali-plagioclase (QAP) ternary diagram of Streckeisen (1976) variable in size, unzoned, polysynthetically twinned, and partially altered to sericite and epidote A few plagioclase crystals are zoned The K-feldspar is mainly anhedral in crystal shape, medium-grained, with Carlsbad twinning, and is partially altered to clay minerals The quartz is anhedral, medium to coarse-grained, and has undulatory extinction (Figure 5a) The myrmekitic intergrowths between the quartz and the plagioclase are common in the granodiorite The hornblende forms as isolated prismaticsubprismatic subhedral crystals and is the most mafic mineral in the granodiorite The hornblende is partially altered to biotite, chlorite, and titanite The primary biotite appears as long flakes and irregular plates Some mafic minerals occur as mafic clots of amphibole, titanite, and magnetite 5.1.4 Monzodiorite to quartz monzodiorite These rocks are also exposed near the margin and occur locally as small bodies They are generally coarse to medium-grained and granular in texture and consist of plagioclase, orthoclase, hornblende, and biotite as the main minerals (Figure 5b) Pyroxenes are observed in some samples Apatite, titanite, zircon, monazite, magnetite, and rutile are the main accessory minerals The secondary minerals are chlorite, sericite, epidote, and clay minerals that were formed by alteration of the main minerals The plagioclase shows large variations in size and is less altered The orthoclase occurs as subhedral to anhedral crystals and exhibits Carlsbad twinning The orthoclase contains inclusions of apatite, monazite, zircon, and acicular rutile Perthitic and myrmekitic intergrowths 318 are common in these rocks Magnetite and ilmenite are the main opaque minerals 5.1.5 Dioritic rocks The dioritic rocks occur as small bodies on the eastern margin of the western granitoids They are coarse-to medium-grained, dark gray to gray in color, and granular in texture (Figure 5c) The plagioclase grains are mainly subhedral, unzoned, and polysynthetically twinned and weakly altered to sericite A few anhedral interstitial quartz grains are present in some samples Clinopyroxene is not abundant in these rocks and seems to be replaced by amphibole Unaltered subprismatic and relicts of augite are observed in one sample (No 24, Figure 2) Hornblende crystals are the dominant mafic mineral in the dioritic rocks They form subprismatic crystals, irregular plates, or clusters The hornblendes are partially replaced by biotite, chlorite, and titanite Primary biotites are present as irregular large flakes with ragged outlines in some samples Titanite, apatite, magnetite, and ilmenite are the main accessory minerals of these rocks Orthoclase occupies the interstices between plagioclase, contains small inclusion of apatite, and often shows an anhedral shape Rounded enclaves of gabbro (1 to 20 cm in size) occur in the dioritic rocks 5.1.6 Gabbro These groups of rocks are coarse- to medium-grained and dark in color with various textures; some samples display intergranular and myrmekitic textures, whereas others have a granular texture (Figure 5d) Plagioclase crystals are the dominant felsic mineral, ranging in size from very large euhedral-subhedral laths to small anhedral crystals The anhedral interstitial crystals are pyroxene and amphibole Effects of deformation, such as strained boundaries, are present in some plagioclase laths Clinopyroxene is not abundant in these rocks and seems to be replaced by amphibole and biotite (Figure 5d) Hornblende is the dominant ferromagnesian mineral in the Bazman gabbro They form subprismatic crystals, irregular plates, or clusters A few apatite prisms and abundant large to small anhedral to subhedral grains of magnetite form the accessory minerals Clay, sericite, minor amounts of actinolite, and some biotite are the secondary minerals The gabbros are the oldest part of the BGC as they occur as enclaves within the other intrusive rocks (Figure 3g) 5.2 Eastern granitoids 5.2.1 Biotite granite Biotite granites are the most abundant granitoids in the eastern part of the BGC They are granular in texture and consist of plagioclase, K-feldspar, quartz, and biotite as the main minerals and titanite, apatite, and opaque minerals as accessory minerals (Figure 5e) Chlorite, epidote, and clay minerals are the main secondary minerals The quartz GHODSI et al / Turkish J Earth Sci a b Ttn Qz Qz Hbl Kfs Hbl Kfs Plg c d Plg Plg Plg Hbl Cpx Plg Bt Plg e f Zrn Grt Plg Ap Kfs Qz Plg Ms Figure Photomicrographs of thin sections of representative BGC rocks (cross polarized transmitted light): (a) granodiorite, (b) monzodiorite, (c) diorite (d) gabbro, (e) biotite granite, (f) biotite-muscovite granite Abbreviations: Bt = biotite; Cpx = clinopyroxene; Grt = garnet; Hbl = hornblende; Kfs = alkalifeldspar; Ms = muscovite; Plg = plagioclase; Qz = quartz; Ttn = titanite; Zrn = zircon Mineral abbreviations from Whitney and Evans (2010) is typically anhedral, coarse-grained, and has undulatory extinction Plagioclase occurs as subhedral large crystals, unzoned, and polysynthetically twinned Biotite is the only ferromagnesian phase in these rocks and occurs as medium to small anhedral crystals Some parts of this granite were intruded by porphyritic granite 5.2.2 Biotite-hornblende granite Gray, coarse-to medium-grained, granular biotitehornblende granite (granodiorite) also covers a large area in the eastern part of the BGC Major minerals include K-feldspar, plagioclase, quartz, hornblende, and biotite, and the minor minerals are titanite, apatite, and opaque minerals K-feldspars are anhedral and occupy the interstices between the other minerals Plagioclase occurs as large euhedral to subhedral crystals, unzoned, and polysynthetically twinned The quartz is anhedral, coarsegrained, and has undulatory extinction Hornblende and biotite occur as euhedral to subhedral crystals, frequently associated with each other, but with the hornblende being more abundant than the biotite Some mafic minerals occur as mafic clots of amphibole, titanite, and biotite Magnetite and ilmenite are the main opaque minerals 5.2.3 Biotite-muscovite granite These rocks are poorly exposed in the eastern part of the map area and occur as small intrusions associated with biotite granite They are white in color, coarse-grained, granular in texture, and contain garnet locally The mineral assemblages consist of quartz, orthoclase, microcline, 319 GHODSI et al / Turkish J Earth Sci plagioclase, biotite, muscovite, and garnet Garnet is the most important accessory mineral in this rock The quartz typically is anhedral, coarse-grained, and has undulatory extinction The microcline is subhedral to euhedral, coarse- to medium-grained, and contains inclusions of orthoclase, quartz, and plagioclase The plagioclase grains are mainly subhedral, unzoned, polysynthetically twinned, and occupy the interstices between other minerals The biotite and muscovite occur as subhedral crystals but the muscovite is more abundant than the biotite (Table 1) Garnet occurs as individual crystals, euhedral to subhedral, and medium-grained (Figure 5f) Geochemistry The major- and trace-element data for representative samples of the BGC’s rocks are listed in Table The Bazman intrusions vary from gabbro to granite in composition (Figure 6) 6.1 Major elements The rocks have a wide range of SiO2, from 47.15 to 81.57 wt % The more mafic samples are the gabbros, and the more silicic samples are the aplite and pegmatite dikes The abundances of Fe2O3, MgO, CaO, TiO2, MnO, and P2O5 decrease with increasing SiO2, whereas K2O and Na2O increase (Figure 7) Al2O3 has a bent trend, increasing to 60 wt % SiO2 and then decreasing from this point onward Based on the alumina saturation index (ASI = A/CNK) (molar Al2O3/(CaO+Na2O+K2O) of Shand (1947), the rocks are metaluminous to weakly peraluminous (Figure 8a) In the A/NK versus A/CNK diagram, only one sample overlaps the S-type granitoid field, and the other samples plot on the I-type field (Figure 8a) The FeOt/(FeOt+MgO) versus silica diagram (Frost et al., 2001) shows that the BGC is mainly a magnesian I-type, similar to Cordilleran batholiths (Figure 8b) The I-type geochemical character of the Bazman granitoids is supported by the presence of hornblende, magnetite, and titanite, and the absence of high-grade regional metamorphism around the BGC In the diagrams of K2O+Na2O versus SiO2 (Middlemost, 1985) and AFM, all samples of the BGC plot within the subalkaline and calc-alkaline fields (Figures and 8c) 6.2 Trace elements The abundances of large-ion lithophile elements (LILEs), such as Rb and Sr, vary systematically with increasing SiO2 (Figure 9) Rb generally increases, whereas Sr decreases with increasing SiO2 The high field-strength elements (HFSEs), such as Y, Zr, and Ti, decrease with increasing SiO2 The transition elements (Co, V, Zn) also display a negative correlation with SiO2 (Figure 9) Trace-element spider diagrams for the BGC, normalized to MORB, are presented in Figure 10 All the examined samples display considerable Ti, Hf, Y, and Yb depletion and K, Rb, Ba, and Th enrichment The trace- 320 element patterns of the granites show dissimilarities and can be divided into three patterns; one sample exhibits Sr depletion and a strong negative Ba anomaly Although most granite samples have negative Hf anomalies, there is one sample that shows a positive Hf anomaly The trace-element spider diagrams of granodiorite, quartzmonzodiorite, monzodiorite, diorite, and gabbro are quite similar All the samples show strong negative Hf and moderate negative Ti anomalies Some samples of quartzmonzodiorite, monzodiorite, and diorite show strong positive Th anomalies The rare earth element (REE) distribution patterns for the BGC are normalized to chondrite abundances (Boynton, 1984) in Figure 11 The chondrite-normalized REE patterns show that all the rock types in the BGC are enriched with light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs) These patterns also show that the granite, diorite, and gabbro have a weak negative Eu anomaly while the granodiorite and quartzmonzodiorite have a moderate negative Eu anomaly and the monzodiorite has a weak negative Eu anomaly One granite sample that contains garnet shows a strong negative Eu anomaly; the content of REEs in this sample is lower than in the other samples The REE patterns of the granites exhibit a moderate to deep negative slope from La to Ho and a moderate positive slope from Ho to Yb, and they are flat from Yb to Lu The patterns of REE for granodiorite, quartz-monzodiorite, monzodiorite, and diorite show a moderate to steep slope from La to Eu and a moderate slope from Gd to Lu The REE slope for gabbro is low to flat 6.3 U-Pb geochronology Three samples were selected for U-Pb dating (Table 3; Figures 12 and 13) Sample BZ-3 represents granodiorite, BZ-2 is monzodiorite, and BZ-7 is porphyritic granite Zircon occurs in the granodiorite (BZ-3) as partly broken prismatic to equant crystals Cores were not immediately evident, but analyses revealed the presence of somewhat older components, especially in the two fractions of residual grains that resisted the first dissolution (fractions and 5, Table 3) and were dissolved separately (Nos and 2) The CL images of the grains in BZ-3 (Figure 13) display regular and local sector zoning, but also multiple stages of intermediate resorption, which confirm the U-Pb evidence for intermediate growth stages The results suggest that the residues were enriched in an early growth component with higher Th/U They may represent antecrysts (e.g., Miller et al., 2007; Schaltegger et al., 2009) The age of emplacement of the granodiorite is best defined by the three zircon analyses with the youngest 206Pb/238U ages, which average 83.07 ± 0.30 Ma (Figure 12) Titanite occurs as brown, partly euhedral crystals rich in U (320–230 ppm) and gives a slightly younger age of 81.32 ± 0.20 Ma GHODSI et al / Turkish J Earth Sci 1.6 TiO Granite Granodiorite Quartz monzodiorite Monzodiorite Gabbro Diorite 1.2 12 0.4 0 PO MnO Na2O 0.4 0.3 MgO 0.8 0.4 Fe O 0.2 0.2 0.1 0 KO CaO 12 Al O 20 15 10 45 55 65 SiO2 75 85 45 55 65 SiO2 75 85 45 55 65 SiO 75 85 Figure Selected major oxides (wt %) vs SiO2 (w.t %) contents for the BGC rocks (Harker diagram) that was added to the base of the crust This heat caused partial melting and assimilation of the lower crustal rocks, especially evident in the eastern granitoids The crustal and mantle-derived magmas probably mixed and, as a result, dioritic magma formed This derivative dioritic magma ascended into the crust (cooling and depressurization) to produce diorite, granodiorite, and granite through assimilation and fractional crystallization (AFC) processes The strong positive Ti anomaly and high Pb content indicate that the new magma was continually altered by crustal contamination Hornblende and biotite are the most abundant ferromagnesian minerals in the western granitoid of 326 the BGC Towards the center of the western granitoid, pyroxene, hornblende, and plagioclase decrease in abundance, whereas K-feldspar and quartz gradually become the predominant minerals (Table 1) The zonation in the western granitoids of the BGC is consistent with fractional crystallization, as high temperature rocks (gabbro and diorite) were formed first on the walls and lower temperature rocks (granodiorite and granite) were crystallized later toward the center The major- and trace-element variation trends also indicate that fractional crystallization affected the evolution of the BGC The decrease in CaO, MgO, Al2O3, Fe2O3, TiO2, P2O5, and Sr, as well as the increase GHODSI et al / Turkish J Earth Sci Peralkaline 0.4 0.6 0.8 1.1 1.2 1.4 F c A-type granites field 0.6 0.7 0.8 0.9 Ferroan 0.4 0.5 A/NK I-S Line FeOtotal/(FeOtotal+MgO) Peraluminous Metaluminous 1.1 b a 1.6 Magnesian 45 A/CNK 55 65 SiO2 (wt %) 75 Calc-alkaline Series 85 A M Figure (a) A/NK vs A/CNK for rocks from the BGC (Shand, 1947), (b) FeOt/(FeOt+MgO) vs SiO2 diagram with ferroan and magnesian (Frost et al., 2001) (c) AFM diagram for BGC samples (Irvine and Baragar, 1971) (see Figure for symbols) 800 V 160 Co 30 600 Zn 120 20 400 200 10 0 80 40 Y Eu 30 1.6 1.2 250 Zr 200 150 20 0.8 100 10 0.4 50 0 800 Ba 1000 Rb 600 800 Sr 800 600 400 400 200 1200 400 200 45 55 65 SiO 75 85 45 55 65 SiO 75 85 45 55 65 SiO 75 85 Figure Selected trace elements (ppm) vs SiO2 (wt %) contents for the BGC rocks 327 1000 1000 GHODSI et al / Turkish J Earth Sci 10 0.1 Sample/MORB 100 10 0.01 0.01 0.1 Sample/MORB Granodiorite 100 Granite K Rb Ba Th Ta Nb Ce Sr P Zr Hf Sm Ti Y Yb P Zr a Hf Sm Ti Y Yb Monzodiorite 0.1 0.01 0.01 0.1 1 10 10 Sample/MORB Sample/MORB 100 100 Quartz monzodiorite K Rb Ba Th Ta Nb Ce 1000 1000 Sr Sr P Zr Hf Sm Ti Y Yb P Zr Hf Sm Ti Y Yb 0.1 10 Sample/MORB 100 Gabbro 0.01 0.01 0.1 10 Sample/MORB 100 Diorite K Rb Ba Th Ta Nb Ce 1000 K Rb Ba Th Ta Nb Ce 1000 Sr Sr K Rb Ba Th Ta Nb Ce P Zr Hf Sm Ti Y Yb Sr K Rb Ba Th Ta Nb Ce P Zr Hf Sm Ti Y Yb Figure 10 MORB-normalized trace-element spider diagrams for the various phases of BGC rocks Normalization value is from Pearce (1983) of K2O and Rb with increasing silica, are related to the fractionation of plagioclase, hornblende, apatite, and titanite Plagioclase fractionation results in lower abundances of Sr and moderate negative Eu anomalies in the chondrite-normalized REE pattern of the granodiorite 328 and monzonite The fractionation of hornblende causes an increase in the LREE/HREE ratio of the residual melt, which therefore develops a concave-upward REE pattern (e.g., Romick et al., 1992) The increase in K2O and Rb with increasing silica indicates that K-feldspar was not an early GHODSI et al / Turkish J Earth Sci 1000 100 100 10 1000 Granodiorite Rock/Chondrites Granite Rock/Chondrites Rock/Chondrites 1000 10 Quartz Monzodiorite 100 10 Biotite-muscovite granite 1 1000 Diorite 100 100 10 10 1 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 1000 Rock/Chondrites Monzodiorite Rock/Chondrites Rock/Chondrites 1000 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Gabbro 100 10 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 11 Chondrite-normalized rare earth element plot of the BGC rocks Normalization value from Boynton (1984) fractionation phase The depletion of P results from the removal of apatite during fractional crystallization The negative Ti anomalies in the spider diagram are consistent with titanite fractionation The fractionation of accessory phases, such as zircon, allanite, and titanite, account for the depletion of Zr and Y Fractional crystallization, assimilation, partial melting, and associated contamination can also be distinguished by plotting a compatible versus a compatible (Co-V and Co-Sc), an incompatible versus an incompatible (Ce-La), and a compatible versus an incompatible element (Sr-Rb) (Martin, 1987; Pearce et al., 1999) The linear positive correlation in Figures 17a–17c indicates that the BGC was affected by AFC Th/Yb versus SiO2 (Figure 17d) is helpful in determining that AFC processes that occurred during formation of the BGC Geotectonic considerations Taking the geotectonic setting of the BGC into consideration, it is necessary to evaluate which one of the main tectonostratigraphic units in southeast Iran is the actual host of the BGC A thick succession of detritalcarbonate sedimentary rocks found around the BGC was metamorphosed in the various thermal metamorphic facies Until recently, only Triassic fossils had been extracted from the upper part of the sedimentary country sequence, while the older recrystallized beds, in consideration of the lithostratigraphical studies, have been attributed to the Sardar and Jamal Formations (Sahandi and Padashi, 2005) However, a few kilometers to the west of the BGC, a nearly entire sequence of Paleozoic sedimentary rocks has been recognized (Vahdati Daneshmand et al., 2004) The above described succession is similar to that of the Paleozoic/early Mesozoic platform-type deposits that can be found in most Iranian tectonic units, such as Lut, Tabas, Yazd, and Alborz (Figure 1a), belonging to a larger terrane named the Cimmerian block (Şengör, 1984) The continental platform deposits surrounding the BGC are obviously different from the deposits identified in the East Iranian Ranges and the Makran accretionary prisms These terrains are characterized by Tertiary deep-water siliciclastics and turbidites Therefore, we assert that a continental terrane such as the Lut block, the nearest terrane to the BGC, is the real host of this complex Considering the BGC’s origin, on the basis of the geochemistry represented above, the presence of a subduction zone under the Lut block during the Late Cretaceous epoch would have been necessary Consequently, the magmatic belts of the west volcano-plutonic belt of Lut and the east plutonic belt of Lut are the most significant candidates for the main magmatic arc of this subduction zone Based on previously published Rb-Sr ages of 74–64 Ma, the Bazman batholith was considered to be the result of the subduction of the Oman oceanic crust beneath central Iran during the Late Cretaceous/Early Paleocene (Berberian et al., 1982) Our new geochronological finding shows that one granodiorite sample derived from the mesocratic outer part of the complex yields an age of about 83 Ma, while one porphyritic granite sample from 329 330 BZ-3 Z tips ca4 [3] Z fr ca5 [6] Z sp [1] Z tips [10] Z fr [9] Z fr [1] Z fr [1] T [10] T [12] BZ-2 Z fr ca14 [5] Z tip [1] Z lp-tip [1] Z lp[8] Z fr [2] Z eq [5] T [7] T [2] BZ-7 Z tips [5] Z sp [6] Z sp [1] Z tips [2] Z sp [5] Z tip [1] T [6] T [2] 16 10 28 21 193 125 140 45 14 149 242 118 63 60 15 32 38 1 188 203 (b) 746 930 906 454 815 786 98 76 556 311 185 232 874 192 456 492 43 257 51 163 147 413 1189 231 320 (b) [ppm] [µg] (a) U Weight Properties 0.37 0.49 0.66 0.44 0.36 0.36 2.94 4.01 0.66 0.74 0.94 0.65 0.42 0.57 0.55 0.72 0.70 1.01 0.61 0.67 0.58 0.69 0.54 3.55 2.86 (c) Th/U Concentrations     Pbc   1.04 1.00 1.42 1.37 0.98 0.98 (d) 1.8 2.0 0.7 103.2 2.5 1.3 198.7 124.5 31.5 7.9 1.8 3.5 5.5 3.7 89.7 83.1 2.0 4.6 4.9 1.3 1.8 1.7 8.6 183.5 197.7 (d) [ppm] [pg] Pbi U 4671 3421 894 106 4865 2558 86 73 2012 1443 1160 7973 12682 4912 272 300 105 109 147 3283 2556 214 131 207 280 (e) 0.07530 0.07538 0.07474 0.07483 0.07416 0.07407 0.07459 0.07440 0.08474 0.08437 0.08387 0.08363 0.08366 0.08372 0.08276 0.08388 0.08528 0.08769 0.08841 0.08572 0.08547 0.08508 0.08387 0.08233 0.08391 (f) 235 Pb 204 207 Pb/ 206 Pb/ Atomic ratios 0.00023 0.00026 0.00075 0.00607 0.00022 0.00031 0.00384 0.00465 0.00027 0.00044 0.00061 0.00021 0.00023 0.00022 0.00119 0.00127 0.00844 0.00371 0.00259 0.00038 0.00034 0.00327 0.00266 0.00178 0.00118 (f) [abs] ±2 σ 0.011466 0.011448 0.011449 0.011343 0.011313 0.011300 0.011362 0.011300 0.012893 0.012853 0.012789 0.012731 0.012727 0.012724 0.012639 0.012650 0.013184 0.013117 0.013097 0.012998 0.012974 0.012927 0.012923 0.012681 0.012701 (f, g) U 238 Pb/ 206 0.000024 0.000023 0.000038 0.000053 0.000025 0.000024 0.000069 0.000082 0.000031 0.000044 0.000026 0.000027 0.000031 0.000027 0.000033 0.000034 0.000083 0.000053 0.000043 0.000036 0.000026 0.000053 0.000061 0.000058 0.000038 (f) [abs] ±2 σ 0.75 0.68 0.50 0.77 0.83 0.61 0.09 0.06 0.81 0.76 0.45 0.93 0.95 0.89 0.22 0.30 0.75 0.47 0.44 0.76 0.61 0.52 0.30 0.27 0.25 (f) rho 0.04763 0.04776 0.04735 0.04785 0.04754 0.04754 0.04761 0.04775 0.04767 0.04761 0.04756 0.04764 0.04768 0.04772 0.04749 0.04809 0.04692 0.04849 0.04896 0.04783 0.04778 0.04773 0.04707 0.04709 0.04791 (f, g) Pb 206 Pb/ 207 0.00010 0.00012 0.00042 0.00372 0.00008 0.00016 0.00244 0.00299 0.00009 0.00016 0.00031 0.00005 0.00004 0.00006 0.00067 0.00070 0.00443 0.00197 0.00137 0.00014 0.00015 0.00174 0.00144 0.00098 0.00065 (f) [abs] ±2 σ U 73.50 73.38 73.38 72.71 72.52 72.44 72.83 72.44 82.58 82.33 81.92 81.55 81.53 81.51 80.97 81.04 84.43 84.01 83.88 83.25 83.10 82.80 82.77 81.23 81.36 (f, g) [Ma] 238 Pb/ 206 Ages ±2 σ 0.15 0.15 0.24 0.34 0.16 0.15 0.44 0.52 0.20 0.28 0.17 0.17 0.19 0.17 0.21 0.21 0.53 0.34 0.27 0.23 0.16 0.33 0.39 0.37 0.24 (f) [abs] 73.72 73.79 73.18 73.27 72.64 72.55 73.04 72.87 82.59 82.24 81.77 81.55 81.58 81.63 80.73 81.79 83.10 85.35 86.02 83.51 83.27 82.91 81.78 80.33 81.81 (f) [Ma] U Pb/ 235 207 ±2 σ 0.22 0.25 0.70 5.72 0.21 0.29 3.63 4.39 0.26 0.41 0.57 0.20 0.21 0.21 1.11 1.19 7.87 3.46 2.41 0.35 0.32 3.06 2.49 1.66 1.11 (f) [abs] a) Z = zircon (all zircon grains treated with chemical abrasion (Mattinson, 2005)); T = titanite (not abraded; fr = fragment; sp = short-prismatic; lp = long-prismatic; ca4, ca5, and ca14 = leftovers after dissolution of Nr 4, 5, and 14, respectively (see text) b) Weight and concentrations are known to better than 10%, except for those near the ca µg limit of resolution of the balance c) Th/U model ratio inferred from 208/206 ratio and age of sample d) Pbi = initial common Pb; Pbc = total common Pb in sample (initial + blank) e) Raw data, corrected for fractionation and spike f) Corrected for fractionation, spike, blank (206/204 = 18.3; 207/204 = 15.555), and initial common Pb (based on Stacey and Kramers, 1975); error calculated by propagating the main sources of uncertainty The U-Pb ratio of the spike used for this work is adapted to 206Pb/238U for the ET100 solution as obtained with the ET2535 spike at NIGL g) Corrected for 230Th disequilibrium according to Schärer (1984) and assuming Th/U magma = 18 19 20 21 22 23 24 25 10 11 12 13 14 15 16 17   Nr Table U-Pb data, Bazman granitoid complex GHODSI et al / Turkish J Earth Sci GHODSI et al / Turkish J Earth Sci BZ-3 BZ-2 0.0131 zircon - youngest analyses 84 84 81.53 ± 0.10 Ma MSWD = 0.054 83 83 0.0129 82 82 zircon - youngest analyses 83.07 ± 0.30 Ma MSWD = 2.5 0.0127 81 81 titanite - analyses 81.32 ± 0.20 Ma titanite - analyses MSWD = 0.35 81.00 ± 0.15 Ma MSWD = 0.22 0.0125 0.082 0.086 0.082 0.086 73.8 0.090 BZ-7 z i r c o n - youngest analyses analyses 73.4 206Pb/238U 0.01145 72.50 ± 0.10 Ma MSWD = 1.12 73.0 0.01135 72.6 207Pb/235U 72.2 0.01125 ti ta n i te- analyses analyses 72.67 ± 0.34 Ma 71.8 0.01115 0.070 MSWD = 1.3 0.074 0.078 0.082 Figure 12 Concordia diagrams displaying U-Pb data from the Bazman granitoid complex Ellipses represent 2σ errors; full lines indicate zircon and dashed lines titanite The calculated ages are weighted averages of 206Pb/238U ages The oldest analysis of sample BZ-3 (No in Table 3) is not shown as it plots outside the chosen diagram the inner felsic area shows an age of 72 Ma Accordingly, one could expect that the gabbro-diorite samples from the marginal part of the main zoned pluton would have probable ages older than 83 Ma The existence of frequent mafic xenoliths in the granites confirms this inference As a consequence, the BGC must have started to crystallize during its emplacement sometime before the Santonian age Therefore, a subduction zone must have been established under the Lut block several million years before the Late Cretaceous epoch 331 GHODSI et al / Turkish J Earth Sci BZ- BZ- BZ- Figure 13 CL-images of zircon The figure displays typical internal textures of zircon in the four samples from the BGC They generally show well-developed euhedral growth zoning, and locally sector zoning, but in part also with good evidence of magmatic resorption followed by new crystallization periods, consistent with the prolonged crystallization history shown by the U-Pb data The crystals in all pictures range in size from 300 to 100 μm in length Late Mesozoic subduction-related granitoids similar to those in the BGC had not been found in Iran previously, except for the Bajestan granitoid (Karimpour et al., 2011), with an age of about 77 Ma, emplaced to the north of Lut (Figure 1a) The Shah Kuh granitoid, dated at about 162 332 Ma (Esmaeily et al., 2005), also crops out in the central part of the Lut block (Figure 1a) Conversely, most of the Cretaceous magmatism observed in submarine deposits is associated with the ophiolitic assemblage emplaced in the accretionary prisms and tectonic mélanges occurring 1000 Rb WPG 10 VAG 10 100 Y+Nb 1000 VAG ORG ORG 10 Rb syn-COLG 100 WPG syn-COLG 100 1000 GHODSI et al / Turkish J Earth Sci Ta+Yb 10 100 Figure 14 (a, b) Rb vs (Y+Nb) and Rb vs (Ta+Yb) diagram of Pearce et al (1984); ORG, ocean-ridge granites; syn-COLG, syncollisional granites (S-type); VAG, volcanic arc granites (I-type); WPG, within-plate granites (A-type) to the east and south of the Lut block (McCall, 1985) Accordingly, if we accept that the BGC is situated in line with the above corresponding granites in the Lut block, then our complex will not belong to any of the plutonic belts of the Sistan suture zone and the Chagei-Raskoh volcanic belt that is located east of the Lut block The origin of the oceanic lithosphere under the Lut is still a matter of debate The Birjand ophiolite, recently dated at 113–107 Ma as the remnants of the Sistan Ocean (a minor Neo-Tethyan seaway) (Zarrinkoub et al., 2012), is one of the candidate oceanic lithospheres that could have subducted under of the Lut block (Arjmandzadeh et al., 2011) Surprisingly, however, the age of the termination of this subduction was reported as 86 Ma (Zarrinkoub et al., 2012) This inference cannot explain the widespread existence of the thick pile of Eocene volcanic and OligoMiocene plutonic rocks of the Lut In spite of this reality, if we accept such interpretations regarding the age of the Sistan suture zone formation, we must search for another subduction zone as the source of the BGC 1000 Continental Margin Arc La/Yb 100 10 Island Arc Primitive Island Arc 0.1 10 100 1000 Th/Yb Figure 15 Th/Yb vs La/Yb diagram after Condie (1989) for the various phases of BGC rocks (see Figure for symbols) The Urumieh-Dokhtar volcano-plutonic belt, the most important magmatic arc in Iran, is characterized by voluminous Eocene calc-alkaline volcano-plutonic products (e.g., Berberian and Berberian, 1981) However, it surprisingly lacks Eocene volcanic rocks at its southeastern extremity in the south of the Lut block (Aghanabati, 1993) Hence, how can we possibly extend this belt to the Bazman area? Our doubts regarding this issue are also intensified by other observations Most significantly, at the southeastern end of the Zagros Mountains, where the promontory edge of the Arabian plate indented the Sanandaj-Sirjan Zone during the Neogene period, the Dehshir-Baft ophiolitic belt has been dextrally displaced tens of kilometers through the Band-e-Zyarat ophiolite (Hassanipak et al., 1996) into the inner Makran ophiolitic zone (McCall, 1997) (see Figure 1a) It is, however, not possible to see such a displacement in the Urumieh-Dokhtar volcano-plutonic belt, which extends parallel to the Zagros/Makran belt Consequently, in our opinion, the proposed continuation of the UrumiehDokhtar volcano-plutonic belt to the Bazman area, in spite of its continuity, is not correct In view of the fact that late Cenozoic volcanic rocks are the most widespread products of the subduction of the Semail oceanic plate under the Lut block, the volcanic rocks of Bazman, previously attributed to the Urumieh-Dokhtar volcano-plutonic belt, must instead be assigned to the Makran volcanic arc On the other hand, according to the fact that the Eocene and older accretionary prisms in the Makran ranges are not extensive to the south of the inner Makran ophiolitic belt in respect to the Sistan suture zone, it is difficult to imagine a subduction zone under Markran before the Eocene-Oligocene time To create Cretaceous suprasubduction zone magmatism, it would have been necessary to establish a subduction zone at least a few million years before the Late Cretaceous period (Figure 18a) However, it might be conceivable that the conditions 333 a 0.8 Metagreywackes 0.6 Experimental melts of amphibolites Metabasaltic to metatonalites 0.4 Felsic pelites 15 10 Experimental melts of Felsic pelites Metagreywackes Amphibolites 10 15 20 25 30 35 Al2O3+FeO+MgO+TiO2 0.4 0.6 0.8 1.2 10 d 0.2 Felsic pelites CaO+MgO+FeO+TiO2 c 25 20 Metagreywackes 15 Amphibolites 10 Metagrewackes (Na2O+K2O)/ (FeO+MgO+TiO2) 0.2 Peraluminous Leucogranites Al2O3/(FeO+MgO+TiO2) Felsic pelites Molar Al2O3/(MgO+FeOtot) CaO/(FeO+MgO+TiO)2 b GHODSI et al / Turkish J Earth Sci 10 12 14 16 18 Na2O+K2O+FeO+MgO+TiO2 Figure 16 Samples plotted on the (a) CaO/(FeO + MgO + TiO2) vs CaO + FeO + MgO + TiO2, (b) Al2O3/(MgO + FeOtot) vs CaO/ (MgO + FeOtot), (c) Al2O3/(FeO + MgO + TiO2) vs Al2O3 + FeO + MgO + TiO2, and (d) (Na2O + K2O)/(FeO + MgO + TiO2) vs Na2O + K2O + FeO + MgO + TiO2 (Patino Douce, 1999) See Figure for sample symbols of subduction, paleo-stress field, and/or position of the main involved plates have been changed since the Eocene epoch By way of comparison, there are undeniable similarities between the large Mesozoic plutons and associated metamorphic rocks of the Sanandaj-Sirjan zone and the comparable plutons from the East plutonic belt of Lut On the other hand, taking this analogy further to the UrumiehDokhtar volcano-plutonic belt and the west volcanic belt of Lut, it directs us to find a significant correspondence between the two magmatic belts In this connection, other researchers have already discussed the comparable stratigraphy and tectonic history of the Sanandaj-Sirjan Zone and the Lut block (Davoudzadeh and Schmidt, 1983; Davoudzadeh and Weber-Diefenbach, 1987) Consequently, it is difficult to resist the conclusion that the east plutonic belt of Lut itself could be an interrupted part 334 of the Sanandaj-Sirjan Zone, which has rotated counterclockwise more than 90° The evidence, such as a lack of Eocene volcanic rocks at the southeastern extremity of the Urumieh-Dokhtar volcano-plutonic belt, the absence of accretionary prisms older than Eocene-Oligocene in the south of the Lut block, and a wide interruption between the Sanandaj-Sirjan Zone and the Lut block, documents the anticlockwise rotation of the Lut block around a vertical axis, rotation that was confirmed by the old paleomagnetic studies (Conrad et al., 1981; Davoudzadeh et al., 1981) This rotation may have been caused by westward extrusion of the Afghan block (Tapponier et al., 1981), when the Indian plate indented Eurasia during the Eocene epoch (Bagheri, 2008) (Figure 18b) This rotation was probably accompanied by southward displacement of the Lut block, the outward slab rollback of the Semail oceanic GHODSI et al / Turkish J Earth Sci a 1000 100 b FC AFC 100 Co Ce 10 10 1 La 10 100 10 1000 100 V c 100 20 d Co Th/Yb 15 10 10 1 10 100 Sc 45 55 65 75 85 SiO2 Figure 17 (a) Logarithmic concentration for an incompatible element vs incompatible (in ppm), (b) and (c) compatible elements vs compatible (in ppm), (d) Th/Yb vs SiO2 (wt %) diagrams illustrating that AFC processes have an important role during evolution of the BGC See Figure for sample symbols plate under the Makran ranges (Stampfli and Borel, 2002), collapse of the arc-trench basin, and temporary cessation of Eocene volcanism It is also worth mentioning that Eocene volcanism continued along both the Urumieh-Dokhtar volcano-plutonic belt and the west volcano-plutonic belt of Lut The continuation of convergence and establishment of a new stage of subduction since the Neogene brought a new period of magmatism through the Makran volcanic arc, when the rotation process was completed Perhaps, according to the age, geochemistry, and tectonic setting of the Gangdese Batholith (e.g., Schärer et al., 1984; Searle et al., 2010), a pluton that was emplaced in the Lhasa terrane to the south of Tibet and north of the Indus-Xangbo suture zone of the Himalayas, it is the best choice for comparison with the BGC This resemblance may be explained by the fact that the Neo-Tethyan subduction zone has been developed all along southern Eurasia from western Iran to eastern Tibet Conclusions The BGC has a broad compositional range from gabbro to granite The rocks are metaluminous to slightly peraluminous, calc-alkaline series, I-type granite, and display geochemical characteristics typical of volcanic arc granites related to a continental margin setting The BGC rocks are enriched in LILEs (Rb, K, Cs) and LREEs, with respect to HFSEs (Zr, Hf, Nb, Ta, Y) and HREEs Majorand trace-element variation trends provide evidence that fractional crystallization and assimilation occurred during the evolution of the BGC The new ID-TIMS U-Pb dating indicates that the granite, granodiorite, and monzodiorite were formed during the late Cretaceous period, the dated 335 GHODSI et al / Turkish J Earth Sci a 45° 50° Gc East Black Sea 60° 65° 70° South Caspian Basin Tc Po Al Ta Ss 25° Paleo-Tethys suture zone Kd Gk Yz Tb BGC Bb Fr Lu Hm Spon Tang Semail Ocean Kh Gondwanian domain and associated island-arc Ophiolite obduction 50° Gc South Caspian Basin Tc Al Ta Ss 25° Db Aj Pb Sa Tb Lu Yz Mk ? 45° 50° θ=30 To Bj Ssz ° 55° 0° 25° Bb Fr Hm Ka Kh 20° ψ= 1° K Fa arako ult ru m θ= latform fghan P North A Fa ult s gro Ch am an Za Arabia Tagh Altyanult F 75° 30° Sz eh F Dorun CEIM at 7° Her ψ = Fault 15° Kd Gk Internal shear zone 20° External shear zone ? 70° 70° an EBS 30° 55° 60° 65° Scythian-Turanian domain 50° an Oc e 45° 65° 60° 55° Fault Magmatic activity Strike-slip shear zone Sis t b 15° Indian Plate Subduction zone Arabian Plate 45° 25° Pa Ka Eurasian domain Kr Hk 20° Thinned continental and/or oceanic crust Intra-arc basin 20° Ky in as -Tajik B n Turkesta Sz Aj Cimmerian Block Neo-Tethys 15° 75°30° Chaman 30° 55° Scythian-Turanian domain 15° India Indian plate 65° 60° 70° Figure 18 Schematic geodynamic reconstruction at (a) Late Cretaceous-Early Paleocene and (b) Eocene (Bagheri, 2007) Small pattern at bottom: plane-strain slip-line fields for a wedge-shape indenter and indented rigid plastic media (modified after Tapponier and Molnar, 1976) Deformational fields are indicative of the geometry of strike-slip faulting induced by collision in an idealized and oversimplified condition Aj: Anarak-Jandaq terrane, Al: Alborz, Bb: Band-e-Bayan, BGC: the Bazman Granitoid Complex, Bj: Birjand, CEIM: the Central-East Iranian Microcontinent, Db: Dehshir-Baft, EBS: East Black Sea, Fr: Farah Rud, Gc: Great Caucasus, Gk: Great Kavir Block, Hm: Helmand, Hk: Hendu Kush, Ka: Kandahar, Kd: Kopeh Dagh, Kh: Kohistan, Kr: Kermanshah, Ky: Khoy, Lu: Lut, Mk: Makran, Pa: Pamir, Pb: Posht-e-Badam, Po: Pontide, Sa: Saghand, Ss: the Sanandaj-Sirjan Zone, Sz: Sabzevar, Ssz: Sistan suture zone, Ta: Taurus, Tb: Tabas, Tc: Transcaucasus, To: Torbat, Yz: Yazd 336 GHODSI et al / Turkish J Earth Sci phases spanning a period between 83 and 72 Ma Therefore, the diorite and gabbro, which are the oldest phases of the BGC, have an age greater than 83 Ma A combination of geological, geochemical, and geochronological data for the BGC, situated in the southern part of the Lut block, suggests that this complex probably belongs to the East plutonic belt of Lut (the Sanandaj-Sirjan Zone) and is related to the subduction of the Neo-Tethyan oceanic crust beneath the Lut block during the late Cretaceous period Acknowledgments The first author thanks the members of the Department of Earth Science and Technology, Faculty of Engineering and Resource Science for their hospitality and support during his 5-month stay at Akita University Kazuo Nakashima at Yamagata University is thanked The manuscript was considerably improved by David R Lentz Financial support from the Ministry of Sciences, Research, and Technology of Iran is acknowledged We acknowledge the editor and reviewers for their constructive criticism and comments References Agard P, Jolivet L, Vrielynck B, Burov E, Monie P (2007) Plate acceleration: the obduction trigger? 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Mesozoic plutons and associated metamorphic rocks of the Sanandaj-Sirjan zone and the comparable plutons from the East plutonic belt of Lut On the other hand, taking this analogy further to the UrumiehDokhtar... location is one of the few direct sources of information that could shed light on the magmatic evolution and tectonic history of terranes, and, additionally, the recognition of terrane outlines There