The Soursat Complex in northwestern of Iran is part of the Sanandaj-Sirjan metamorphic belt. Three granitoid suites are present: Type I– syenogranite and deformed syenogranite; Type II– Turkeh Dare and Pichagchi plutons; and Type III– quartz porphyry. The granites can be assigned to a medium-K calc-alkaline to high-K calc-alkaline series.
Turkish Journal of Earth Sciences http://journals.tubitak.gov.tr/earth/ Research Article Turkish J Earth Sci (2013) 22: 1-31 © TÜBİTAK doi:10.3906/yer-1001-37 The U-Pb age, geochemistry and tectonic significance of granitoids in the Soursat Complex, Northwest Iran 1, Mahboobeh Jamshidi BADR *, Alan S COLLINS , Fariborz MASOUDI , Grant COX , Mohammad MOHAJJEL Geology Department, Payam Noor University, 19395-4697, Tehran, I.R of Iran Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, University of Adelaide, SA 5005, Australia Faculty of Earth Sciences, Shahid Beheshti University, 19839-63113, Tehran, I.R of Iran Faculty of Science, Tarbiat Modars University, 14115-175, Tehran, I.R of Iran Received: 28.01.2010 Accepted: 21.04.2011 Published Online: 04.01.2013 Printed: 25.01.2013 Abstract: The Soursat Complex in northwestern of Iran is part of the Sanandaj-Sirjan metamorphic belt Three granitoid suites are present: Type I– syenogranite and deformed syenogranite; Type II– Turkeh Dare and Pichagchi plutons; and Type III– quartz porphyry The granites can be assigned to a medium-K calc-alkaline to high-K calc-alkaline series The Type I granitoids are weakly to strongly peraluminous and belong to a S-type suite, whereas Type II plutons are metaluminous to weakly peraluminous with I-type character Type III granitoids are weakly peraluminous and can be labelled as a highly fractionated I-type suite U/Pb zircon dating of the syenogranite and deformed syenogranite from Type I and Type II granitoids by laser inductively coupled plasma mass spectrometry (LA-ICP-MS) yielded 238U/206Pb emplacement ages of ~540 Ma (543±6 Ma and 537±8 Ma) and 59.0±2.7 Ma, respectively Rare Earth Elements in the Type I granitoids are strongly fractionated with (La/Lu)N= 32 to 52 and negative Eu anomalies (Eu/Eu*= 0.09–0.72), typical for plagioclase fractionation; the primitive mantle-normalized element patterns are homogeneous with marked negative Ba, Nb, Ta, Sr and Eu anomalies Based on geochemical data, Type I granitoids are formed in a continent-continent collision, which could be related to Cadomian collision along the northern margin of Gondwana after its final amalgamation Types II and III granitoids have I-type affinities and show homogeneous and moderately fractionated REE patterns with (La/Lu)N= 17–34 Primitive mantle-normalized element patterns are homogeneous with marked negative Nb, Ta, Sm and Ti anomalies The Soursat Complex is shown to contain early- to syn-collisional granitoid magmatism of Ediacaran–Cambrian age and a subsequent group of Palaeocene I-type granitoids that are syn- to post-collisional with respect to the Arabia-Eurasia collision and are interpreted to represent roll-back during the closure of Neotethys, pre-dating the Arabia-Eurasia collision Key Words: LA-ICP-MS, zircon, NW Iran, Sanandaj-Sirjan, Soursat Complex Introduction Gondwana was assembled by the collision of about 7–8 Australia-sized Neoproterozoic continents during two main periods, first at ~650–600 Ma and secondly at ~570–520 Ma (Collins & Pisarevsky 2005) Ediacaran– Early Cambrian magmatism is suggested to have formed a widespread continental arc along the northern margin of a newly formed Gondwanan supercontinent (Ramezani & Tucker 2003; Gessner et al 2004; Hassanzadeh et al 2008) Soon after final amalgamation, Andean-type active margins formed along the Australia-Antarctica-South America sector of the Gondwana supercontinental margin (Cawood 2005; Foden et al 2006; Cawood & Buchan 2007) In the Himalayas an arc formed along the northern margin of the India sector of the Gondwana margin * Correspondence: m_ jamshidi76@yahoo.com soon after the amalgamation of Gondwana (Cawood et al 2007) This margin can be traced westwards into Iran where similar-age magmatism is documented from the basement (Ramezani & Tucker 2003; Hassanzadeh et al 2008) Neoproterozoic magmatism in Iran was followed by deposition of several kilometres of Palaeozoic and Mesozoic sedimentary and volcanic strata It is, therefore, worthwhile to consider the mechanisms and timing of exhumation of the exposed Neoproterozoic–Early Cambrian granites within the Zagros collision zone The basement is entirely concealed beneath 8–12 km of passivemargin sedimentary rocks of the Arabian platform, with the exception of small basement fragments brought up in the Hormuz salt diapirs (Stöcklin & Setudehnia 1970) BADR et al / Turkish J Earth Sci No significant crystalline basement has been exhumed within the external or internal portions of the Zagros fold and thrust belt, although basement involvement has been postulated (e.g., McQuarrie 2004) However, exposures of granitic and metamorphic basement rocks are scattered throughout the remainder of central and northern Iran The Soursat metamorphic complex in north-west Iran (Figure 1a–c) is one of the structurally, exhumed domal basement complexes containing Neoproterozoic– Early Cambrian granites and mylonites The complex is characterized by low-angle faults, juxtaposing crystalline basement against younger, low-grade metamorphic or non-metamorphic rocks The presence of clear deformation phases in the Soursat complex makes it possible to construct the relative sequence of magmatic events in the Gondwana margin, which was followed by magmatism of the ArabiaEurasia collision during the closure of Tethys However, the complex is largely unknown in terms of the age and tectonic significance of its basement and the history of its exhumation In this research we used U-Pb Laser Inductively Coupled Mass Spectrometry (LA-ICP-MS) age data and focused on dating granitoids in the Soursat complex in order to understand the absolute age of the magmatic events in the complex We also investigated the evolution of the magmatism in the north Gondwana margin within the Zagros collision zone Geological setting Most palaeogeographic and plate tectonic reconstructions for the margin of Gondwana facing the Palaeo- and Neotethys oceans propose that crustal blocks making up the collage of Iranian microplates rifted away from Gondwana during the Palaeozoic (e.g., Şengör & Natal’in 1996; Stampfli 2000; Stampfli et al 2001; Stampfli & Borel 2002) In these reconstructions, a series of crustal fragments separated from the northern margin of Gondwana during the opening of Palaeotethys, forming what is referred to as the Cimmerian superterrane (Şengör & Natal’in 1996) Subsequently, the individual fragments of this Cimmerian superterrane, such as the Sanandaj-Sirjan, Alborz, central Iranian, and Lut blocks were accreted to Laurasia during the Cimmerian orogeny as the result of northward subduction and closure of Palaeotethys and the synchronous opening of the Neotethys in the Late Triassic (Şengör 1987; Stampfli 2000) The crustal Iranian blocks were assigned a Gondwana affinity based on flora and excellent correlation of Palaeozoic facies with basins in Arabia or India (e.g., Ghavidel Syooki 1995; Stampfli 2000) The Sanandaj-Sirjan Zone is a complex NW–SEtrending metamorphic zone that extends over 1500 km in length and 150–200 km in width It separates the stable Central Iran block from the Arabian microcontinent The Sanandaj-Sirjan Zone was deformed and exhumed during the Cretaceous–Palaeogene continental collision between the Afro-Arabian continent and Central Iran (e.g., Şengör & Natal’in 1996; Mohajjel & Fergusson 2000; Mohajjel et al 2003), but preserves rocks within it that have experienced long histories The Sanandaj-Sirjan Zone preserves evidence of subduction of Tethyan sea floor under Central Iran prior to the Middle Triassic (e.g., Berberian & King 1981) Late Triassic extension-related successions are found throughout the zone (Mohajjel et al 2003) However, the structure of the Sanandaj-Sirjan Zone was imposed during the collision of Arabia and Eurasia and the subsequent southward propagation of the fold-thrust belt (Alavi 1994), with the Sanandaj-Sirjan Zone acting as a deformable backstop (Macquarie 2004) The Sanandaj-Sirjan Zone mainly consists of metamorphic complexes and granitic intrusions The metamorphic complexes have been considered to be Precambrian (Zahedi et al 1992), Palaeozoic (Sabzehei 1996, for lower parts of the Sanandaj-Sirjan Zone) and even Mesozoic in age (Mohajjel & Fergusson 2000) The grade of metamorphism is reported to be greenschist and lower to upper amphibolite facies (e.g., Berberian & King 1981; Sabzehei 1996; Mohajjel & Fergusson 2000) Highpressure metamorphism is also reported (Davoudian et al 2008) The Soursat Complex is one of these inliers and is located north of the Takab-Shahin Dezh road (Figure 2) where it is in tectonic contact with Precambrian and Palaeozoic rocks The Soursat Complex mainly consists of metamorphic rocks and different granitic intrusions Two main geological units have been defined in the metamorphic rocks The protoliths of these units are Precambrian to Palaeozoic sedimentary rocks (Kholghi Khasraghi 1994): (1) the upper Precambrian Kahar formation, which consists of slate, sandstone and some acidic volcanic rocks, locally metamorphosed to greenschist-amphibolite facies; (2) Precambrian–Cambrian and Ordovician dolomite (Bayandor and Soltaniyeh formations), sandstone, shale and dolomitic limestone (Barut, Lalun and Mila formations) Structurally, three major ductile deformation phases, Dn, Dn+1 and Dn+2, have been recorded within the complex, each of which coincides with igneous activity (Jamshidi Badr 2001; Ghasemi 2001) These ductile deformation phases are followed by sinistral or dextral shearing and late tectonic faulting along the Pichagchi fault, the central shear zone, and the southern shear zone (Jamshidi Badr 2001; Ghasemi 2001; Ghasemi & Poor Kermani 2009) The emplacement of the Pichagchi granitoid was controlled by BADR et al / Turkish J Earth Sci a b Pacific Ocean StudyArea c study area Sanadaj Sirjan zone marginal subzone ophiolite subzone completely deformed bisotun subzone subzone radiolarite subzone upper Mesozoic rocks (mostly under Neogene -Quaternary cover) Hamadan phyllite upper Palaeozoiclower Mesozoic rocks Palaeozoic-lower Mesozoic rocks upper precambrianlower Palaeozoic rocks Urumieh-Dokhtar magmatic arc ophiollite and ophiolitic melange trust fault strike-slip fault fault contact Figure (a) Palaeogeographical reconstruction at ~545 Ma after the amalgamation of eastern and western Gondwana (modified after Dalziel 1997) In this time period, deformation within the northern portion of the East African Orogen (hatched pattern) postdates the main collisional tectonism and is dominated by post-orogenic dextral transcurrent deformation and extension (b) Geological map of Iran and surrounding regions showing mountain belts, Zagros fold-thrust belt (after Guest et al 2006) White circle indicates study area (c) Location of the Sanandaj-Sirjan belt in Iran (after Mohajjel & Fergusson 2003) 36°38ʹ30ʺ BADR et al / Turkish J Earth Sci Figure Geological map of the Soursat region, with position of granitoids suites in schist and sedimentary country rocks a N40°E strike-slip fault The main schistosity (Sn+1) and foliation is parallel to the strike of the Pichagchi fault and is broadly parallel to the foliation strike in the surrounding southern granitoids (Turkeh Dare granitic gneiss) (Figure 2) Based on their lithological features and different compositions, three suites of plutonic rocks can be distinguished • Type I: the syenogranite intrusions, which crop out in the northeast of the Soursat Complex (Figure 2) These massive and homogeneous rocks are leucocratic and have concordant contacts with the surrounding schist Elongated domains and lamellae of the deformed syenogranite with mylonitic foliation alternating with stripes of undeformed syenogranite are also present Structural transitions between both rock types were BADR et al / Turkish J Earth Sci observed Deformed syenogranites are foliated (Dn), sheared and mylonitized and have ribbons of mica around K-feldspar minerals with augen-type structure These are termed augen gneiss in Figures 5a, b & 6a, b • Type II: a set of granitic rocks crop out to the south and north of the Soursat Complex (Figure 2) These are the Turkeh Dare and Pichagchi plutons, respectively The Turkeh Dare pluton has locally preserved well-defined magmatic layering (Dn) and preserves a solid-state foliation (Dn+1) The magmatic layers are made of interbanded felsic (quartz, plagioclase and K-feldspar) and mafic (biotite and amphibole) bands The Pichagchi pluton is exposed as a massive and almost homogeneous intrusion with only limited magmatic layering (Dn) preserved in the NE part of the studied region where it is parallel to the schistosity (Sn) in the surrounding metasedimentary schists The age of the Pichagchi pluton was first reported as Late Cretaceous–Palaeocene on the Shahin Dezh geological map (scale 1:100000) (Kholghi Khasraghi 1994) based on stratigraphy and field observation More recently, a K-Ar whole rock age was reported that yielded an age of ~75 Ma that suggests emplacement was Upper Cretaceous (Kholghi Khasraghi 2004) • Type III: a quartz porphyry intrusive phase, which is represented by porphyritic granite, leucogranite and forms an outcrop in the central part of the Soursat Complex A thermal aureole around the intrusions affected the previously regionally metamorphosed schists The heat of the intrusion was a foundation for the development of andalusite and cordierite in the host schists This confirms the younger age for the intrusive body compared to the age of regional metamorphism (M1) Analytical methods 3.1 Laser ablation ICP-MS dating U-Pb analysis of zircons from samples Sh-119 and Sh-31, a syenogranite and an augen gneiss respectively (Type I), and three samples Sh-109, Sh-110 and Sh-111 from the Turkeh Dare pluton (Type II) were conducted using the laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) at the University of Adelaide (Australia) Zircons were separated using conventional methods that include crushing, sieving, magnetic separation and floatation More than fifty zircon grains were handpicked under a binocular microscope The zircons were then set in synthetic resin mounts, polished and cleaned in a warm HNO3 ultrasonic bath Cathodolumuniscence (CL) and back-scattered electron (BSE) imaging were carried out to help characterize any compositional variation within individual zircons Equipment and operating conditions for zircon analysis were identical to those reported by Payne et al (2006) A spot size of 30 μm and repetition rate of Hz was used for U-Pb data acquisition, producing a laser power density of ~8 J/cm2 Zircon ages were calculated using the GEMOC GJ-1 zircon standard to correct for U-Pb fractionation (TIMS normalization data 207Pb/206Pb= 608.3 Ma, 206Pb/238U= 600.7 Ma and 207Pb/235U= 602.2 Ma; Jackson et al 2004), and the GLITTER software for data reduction (Van Achterbergh et al 2001) Over the duration of this study the reported average normalized ages for GJ-1 were 609±10, 600.2±2.7 and 601.9±2.4 Ma for the 207 Pb/206Pb, 206Pb/238U and 207Pb/235U ratios, respectively (n= 24) 3.2 Whole-rock major and trace element analyses A total of fifteen samples were selected for whole-rock major element and trace-element analysis The samples were analyzed by ICP-MS at ACME Laboratories in Canada, using the method package group 4B and group 1T-MS The results are presented in Table Representative samples from the Soursat complex were analyzed by X-Ray Fluorescence at Tarbiat Moallem University (Iran) for major (det Lim 0.01 %) and some trace elements (det Lim 1ppm) Thirteen and eight samples were from the Turkeh Dare and Pichagchi plutons respectively; four were from Quartz Porphyry, two from syenogranite and one was from associated augen gneiss The results are shown in Tables & 3.3 Microprobe Analyses Mineral analyses were collected using a Cameca SX100 electron microprobe at the Iran Mineral Processing Research Center The quantitative analyses of selected minerals were performed with a 15keV accelerating voltage, a 10nA beam current and a 2–5 µm beam size The counting time at each peak was 20–30 s Most analyses represent averages of three or more individual spot analyses from feldspar, amphibole and biotite crystals Representative analytical data are listed in Tables 5, & Results and discussion 4.1 Petrography and mineral chemistry Type I (Syenogranite) – Medium- to coarse-grained syenogranite is whitish to light grey in colour The main minerals in Type I granitoids are K-feldspar (60–45 vol %), quartz (20–28 vol %), plagioclase (20–25 vol %), and biotite (5–20 vol %) Common accessory minerals are apatite, magnetite and zircon K-feldspar occurs in subhedral megacrysts (Or87–99; Ab0.8–12), but scarce interstitial crystals are also found, perthite exsolutions are common and myrmekites are frequently observed at the rims adjacent to plagioclase Inequigranular quartz forms elongated crystals with undulose extinction and in some cases chessboard pattern subgrains Lath-shaped plagioclase is mainly oligoclase with subordinate albite (An21–28; Ab70–77; Or90% concordant and range in 206 Pb/238U age from 627±8 Ma to 529±7 Ma A weighted mean of all data younger than 600 Ma yielded a 206Pb/238U age of 543±6 Ma (95% confidence, MSWD= 1.6), which is interpreted as the emplacement age of the syenogranite 19 BADR et al / Turkish J Earth Sci b a Qtz Bt Kfs Qtz Kfs Bt cm cm c light band dark band Figure (a) Syenogranite including K-feldspar, quartz, plagioclase and biotite from Type I granitoids; (b) augen gneiss including a K-feldspar porphyroclast with undulose extinction; a mantle of recrystallised feldspar with isolated polycrystalline quartz ribbons surrounds the porphyroclast; (c) magmatic banding from Type II granitoids: a dark band quartz, plagioclase, K-feldspar and a light band also contains hornblende and biotite Sample Sh-31, an augen gneiss, was collected from about km north of the village of Khan Qoli (N36°38′11″, E46°52′00″) CL images of the analysed zircons show euhedral, long prismatic crystals with oscillatory zoning They show no metamorphic overgrowth, and are of magmatic origin (Figure 3d) Analyses yielded 206Pb/238U ages within the range from ~800 Ma to 528±7 Ma A weighted mean of the 206Pb/238U ages from the youngest six >90% concordant analyses produced an age of 537±8 Ma (95% confidence, MSWD= 1.3) Based on CL, images show that all the zircons have distinct oscillatory zoning, suggesting that their age should be interpreted as that of the emplacement of the protolith of the gneiss (Figure 3c, d) The older ages are interpreted to be inherited from zircons within the original granite protolith The age of type (II granitoids) – Three Samples from the Turkeh Dare pluton were analyzed in this study Sample Sh-109 (N36°30′17″, E46°46′33″) from the light 20 band, Sample Sh-111 (N36°30′32″, E46°51′15″) from the dark band, and Sample Sh-110 (N36°29′39″, E46°43′13″) from a homogeneous part of the pluton were analyzed Zircons from these samples are pink, prismatic, up to 300 μm long and perfectly euhedral Most crystals have broad CL bands and show oscillatory zoning (Figure 4d–f) Eleven analyses of zircons from Sh-109 (the leucocratic granite layer) yielded a 206Pb/238U age of 54.6±1.6 Ma (95% confidence, MSWD= 4.7) (Figure 4a) Largely discordant data from Sh-110 (homogenous part of the pluton) yielded a 206Pb/238U age of 56.2±2.1 Ma (95% confidence, MSWD= 8.4) The large uncertainty expressed by the MSWD indicates a combination of lead loss and common Pb within the analyses that cannot be resolved with the LAICP-MS technique (Figure 4b) Sh-111 (mesocratic layer) yielded the best data, with a weighted mean 206Pb/238U age of 59.0±2.7 Ma (95% confidence, MSWD= 1.5) (Figure 4c) These three ages from the Turkeh Dare pluton, 54.6±1.6 Ma, 56.2±2.1 Ma and 59.0±2.7 Ma are interpreted BADR et al / Turkish J Earth Sci a b Pl Qtz Bt Kfs Kfs 1mm 0.5mm c d Bt Qtz Am Pl 1mm 0.5mm f e Qtz Pl Bt Qtz 1mm Pl 0.5mm Figure (a) Syenogranite (sample Sh-119) from Type I granitoids including quartz, K-feldspar, plagioclase and biotite; (b) augen gneiss (Sh-31) with similar minerals to Sh-119, but with a different texture K-feldspar crystals changed to porphyroclasts K-feldspar and quartz is recrystallised as ribbons surrounding the porphyroclast; (c) microphotograph of dark band (sample Sh-111) from Type II granitoids The rock contains quartz, plagioclase, K-feldspar, hornblende and biotite Note that the hornblende and biotite are oriented; (d) sample Sh-110 from a homogenous area of Type II granitoids Felsic minerals are more abundant than in Sh-111; (e) Type III granitoid including mainly quartz, plagioclase, biotite, and K-feldspar; (f) myrmekitic texture seen in centre top of sample Sh-150 from type III granitoids 21 BADR et al / Turkish J Earth Sci Annite Siderophyllite b Peraluminous Calcic group, (Na+K)A>0.5; Ti