Calc-alkaline magmatism associated with salt diapirs in the Shurab and Garmsar back-arc areas (Central Basin, Iran): magma genesis and tectonic implications

In terms of whole-rock geochemical analyses and in agreement with the petrographic features, all representative samples of the Shurab area are classified into three groups: group 1 with mainly intergranular texture comprises basalt/trachybasalt, while groups 2 and 3 with trachytic and porphyritic textures, respectively, have basaltic trachyandesite composition.

Turkish Journal of Earth Sciences http://journals.tubitak.gov.tr/earth/ Research Article Turkish J Earth Sci (2018) 27: 294-317 © TÜBİTAK doi:10.3906/yer-1712-16 Calc-alkaline magmatism associated with salt diapirs in the Shurab and Garmsar back-arc areas (Central Basin, Iran): magma genesis and tectonic implications Somayeh FALAHATY, Mortaza SHARIFI*, Moussa NOGHREYAN, Homayon SAFAEI, Mohammad Ali MAKIZADEH Department of Geology, Faculty of Sciences, University of Isfahan, Isfahan, Iran Received: 20.12.2017 Accepted/Published Online: 29.04.2018 Final Version: 24.07.2018 Abstract: Medium- and high-K calc-alkaline magmatism of the Shurab (southeast Qom city) and Garmsar (northwest Garmsar city) areas occurred within the Lower Red Formation in the Central Basin behind the Urumieh-Dokthar Magmatic Arc In terms of whole-rock geochemical analyses and in agreement with the petrographic features, all representative samples of the Shurab area are classified into three groups: group with mainly intergranular texture comprises basalt/trachybasalt, while groups and with trachytic and porphyritic textures, respectively, have basaltic trachyandesite composition The overall major constituents are plagioclase with composition in the range from An49Ab23 to An75Ab47, clinopyroxene with composition in the range Wo43-45En39-45Fs9-15, and olivine with composition in the range from Fo58Fa31 to Fo67Fa40 Minor minerals consist of opaque minerals and K-feldspar in the range Or41-65Ab33An0.69-8 The characteristic accessories are apatite and sphene In the Garmsar area, rocks are seen as subvolcanic with mafic (basalt) and 50 intermediate (trachybasalt/basaltic trachyandesite) compositions The Garmsar area rocks represent intergranular, granular, ophitic, and subophitic textures In these rocks, major mineral phases are plagioclase and clinopyroxene The minor constituents are olivine, opaque minerals, amphibole, biotite, and quartz Apatite is the most important accessory mineral The rocks of both areas display REE patterns characterized by LREE-enriched and HREE-depleted segments typical of arc lavas Primitive mantle-normalized trace element patterns for samples of both areas exhibit high ratios of strongly incompatible elements with similar bulk partition coefficients (e.g., Th/ Ta and Th/Ce), enrichment in large-ion lithophile elements (LILEs: Cs, Ba, Rb, Th) relative to the high field-strength elements (HFSEs: Ti, Hf, Zr, and REEs), and troughs for Nb, Ta, Ti, and Zr and peaks for Cs, Th, K, and Sr, all of which are indicators for subductionrelated magmatism Subduction of the Neo-Tethys beneath the Eurasian margin led to upper mantle deformation and metasomatism Once the Arabian plate collided with the Eurasian margin, subduction ended through a slab breakoff process, and thermal flux of asthenospheric origin uprising through the slab tear induced the thermal erosion of the mantle metasomatized during the previous subduction event and triggered its partial melting Also, the late Eocene-early Oligocene collision of Eurasian with Arabian plates led to the subsidence and formation of faults and extensions in the Central Basin (i.e the Shurab and Garmsar areas) such that eruption of medium- and high-K metasomatic magmatism along these faults and extensions caused postcollision volcanism in the Central Basin Key words: Shurab and Garmsar areas, Central Basin, mafic and intermediate volcanic rocks, postcollision calc-alkaline magmatisms Introduction Back-arc basalts (BAB) occur behind the main volcanic arc, where they form a transition from arc to intraplate basalts (e.g., D’orazio et al., 2004; Kay et al., 2004; Stern, 2004; Ramos and Kay, 2006) Different types of magma are recognized in back-arc basins (Tarney et al., 1981; Thompson et al., 1984) Both midocean ridge basalt (MORB) and ocean-island basalt magmas characterize oceanic back-arc basins, whereas shoshonites associated with high-K calc-alkaline and ultrapotassic magmatisms characterize continental back-arc environments The basalts from oceanic back-arc settings have transitional geochemical characteristics between MORB and islandarc tholeiites, while the basalts from continental back-arc * Correspondence: sharifi_mortaza@yahoo.com 294 settings exhibit transitional geochemical characteristics between arc and intraplate basalts (Tarney et al., 1981; Saunders and Tarney, 1984; D’orazio et al., 2004; Kay et al 2004; Stern, 2004; Ramos and Kay, 2006) Over the last three decades, many studies have focused on one of the most important occurrences of mafic magmatism in continental back-arc settings Magmas that erupted in continental back-arc regions are particularly interesting as they potentially sample very different physical reservoirs, such as the asthenospheric wedge (supraslab mantle) close and far from the arc, the subslab asthenospheric mantle, the continental crust (both lower and upper), and the continental lithospheric mantle FALAHATY et al / Turkish J Earth Sci Back-arc basalts from Patagonia in Argentina from Paleocene to Holocene in age (i.e southern extra-Andean Patagonia) behind the Southern Volcanic Zone arc constitute one of the largest Cenozoic continental basaltic provinces on earth (Kay et al., 2004) The origin of these basalts has been related to mechanical perturbations of the subcontinental mantle as a consequence of subduction of the oceanic lithosphere below the South American continental plate (Skewes and Stern, 1979) In Iran, continental back-arc magmatism occurred behind the Urumieh-Dokthar Magmatic Arc (UDMA) from the Eocene to recent age, such as magmatism of the Shurab (southeast Qom city) and Garmsar (northwest Garmsar city) areas The present study investigates the petrogenesis of back-arc magmatism in the Shurab and Garmsar areas, which are spatially associated with salt diapirs Our aim is to use the magma geochemistry of the Shurab and Garmsar areas to evaluate the composition of the unmodified mantle source and to assess and identify the responsible agents for the source mantle metasomatism Geological setting The Iranian Plateau, located in the collision zone between the Arabian and Eurasian plates, is one of the world’s best examples of an early stage of continent–continent collision The basement of the plateau comprises a mosaic of tectonic blocks, i.e the Sanandaj-Sirjan Zone (SSZ), the Central Iranian Microplate/Microcontinent (also CentralEast-Iran Microplate (CEIM)), a basement block known as the NW-Iran Block (Allen et al., 2011), and the Great Kavir Block (Figure 1) (Morley et al., 2009; Allen et al., 2011) The Central Basin, located between the fold-andthrust belt of the Alborz Mountains and the CEIM, is characterized by a flat-lying topography with occasional low hills It developed on a basement composed of the NW-Iran Block, the Eocene UDMA (northern part of the SSZ), the northwestern part of the CEIM (Morley et al., 2009; Allen et al., 2011), and the southern part of the Alborz fold-and-thrust belt (Figure 1) The Central Basin, interpreted as a back-arc basin by Hassanzadeh et al (2002), has remained a poorly documented part of the Iranian Plateau Figure Simplified tectonic map of Iran with main fault systems (Morley et al., 2009 after Allen et al., 2011) Positions of the study areas are marked by the numbers (Shurab area) and (Garmsar area) 295 FALAHATY et al / Turkish J Earth Sci The Central Basin comprises two subbasins: a NW-SE trending arm including the Saveh-Qom area and a NE-SW trending arm east of Qom, near Semnan (Morley et al., 2009) The Shurab area (southeast Qom city, characterized by number 1) lies within the NW-SE trending arm and the Garmsar area (west Garmsar city characterized by number 2) lies within the NE-SW trending arm (Figure 2) (Bouzari et al., 2013) Geology of the Central Basin Three main stratigraphic units are present in the Central Basin: the Lower Red Formation (LRF; Oligocene), the Qom Formation (late Oligocene-early Miocene), and the Upper Red Formation (early Miocene-early Pliocene?) (Furrer and Sonder, 1955; Gansser, 1955; Abaie et al., 1964) The Central Basin deposits overlie ~3 km of thickness of Eocene arc volcanics and volcaniclastics with subordinate marine carbonates and evaporites (Berberian and King, 1981; Bina et al., 1986) The Eocene sequence unconformably overlies Cretaceous and Jurassic sedimentary and metasedimentary rocks The Eocene succession commences with a basal conglomerate and coarse clastics, followed by a predominantly calc-alkaline volcanic series that dominates the Eocene stratigraphy (Stocklin, 1968) Interbedded with the volcanics and volcaniclastics are limestones (some nummulitic) and evaporites, indicating that the volcanism occurred close to sea level The Eocene section was deformed, uplifted, and eroded prior to deposition of the Oligocene-Miocene sedimentary rocks of the Central Basin (Huber, 1952; Gansser, 1955) Deposition of LRF lithologies in the Central Basin during the Oligocene was accompanied by episodic magmatism (Jahangiri, 2007), which is the topic of this study During the late Miocene-Pliocene, the salt at the base of the LRF, a halite-dominated evaporite sequence of commonly several hundred meters thick, became unstable and began to move, resulting in the transporting of salt and the igneous rocks associated with salt to the surface (Figures 3a and 3b) (Morley et al., 2009) Simplified geological maps of the studied Shurab and Garmsar areas in the Central Basin are shown in Figures and Figure Satellite image of the Central Basin Positions of the study areas are marked by the numbers (Shurab area) and (Garmsar area) 296 FALAHATY et al / Turkish J Earth Sci Figure Igneous rocks associated with salt diapir in (a) the Shurab and (b) the Garmsar areas Figure Simplified geological map of the Shurab area in the NW-SE trending arm of the Central Basin, part of the geology map of Aran, scale 1:100,000 (Amini and Emami, 1996) The study area is marked by a rectangle Analytical methods 4.1 Whole-rock major and trace element analyses For bulk rock analyses, secondary veins and alteration rims were carefully removed by sawing Bulk rock major elements of the Shurab samples were analyzed by X-ray fluorescence (XRF) in fused beads at the Andalusian Institute of Earth Sciences (IACT, Granada, Spain) using a BRUKER D8 Advance XRF instrument equipped with six analyzers (LiF200, LiF220, Ge, PE, PX1, PX2) Within-run precision (% RSD), measured by repeated analyses of USGS reference materials as external standards, is better than 0.5% for all elements except Na, for which it is 1.5% 297 Figure Simplified geological map of the Garmsar area in the NE-SW trending arm of the Central Basin based on the Garmsar 1:100,000 geological map (Amini and Rashid, 2004) with some changes (Sarizan et al., 2014) FALAHATY et al / Turkish J Earth Sci 298 FALAHATY et al / Turkish J Earth Sci Whole-rock trace elements (Rb, Sr, Y, Zr, Nb, Cs, Ba, REE, Hf, Ta, Pb, Th, and U) of the Shurab samples were analyzed with a Triple Quadruple Agilent 8800 ICP-MS at the IACT Sample digestion was performed following the HF-HClO4 digestion procedure described by Garrido et al (2000) Element concentrations were determined by external calibration, except for Hf, which was calculated using Zr measured by XRF and the chondritic Zr/Hf ratio The compositions of the granite reference sample GS-N, analyzed as unknown during the analytical runs, show good agreement with the working values of this international standard (GeoReM database: http://georem mpch-mainz.gwdg.de/) Major and trace element analyses of samples of the Garmsar area were made by inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS) at Lab West Laboratory, Australia 4.2 Mineralogical analyses Mineralogical analyses of the Shurab samples were conducted with a wavelength-dispersive electron probe microanalyzer (JEOL JXA-8800R) in the Cooperative Centre of Kanazawa University, Japan The analyses were performed under an accelerating voltage of 15 kV and a beam current of 15 nA Natural and synthetic minerals of known compositions were used as standards Petrography and mineral chemistry For this study we selected about 180 rock samples from several localities close to each other in the Shurab and Garmsar areas (Figures and 5) and only the least altered specimens were chosen for whole-rock analysis 5.1 Volcanic rocks of the Shurab area Rocks of the Shurab area are seen as volcanic with mafic composition In hand specimen, rocks of the Shurab area are green and gray Thin-section studies showed three rock groups: group with mainly intergranular texture (Figure 6a), group with trachytic texture (Figure 6b), and group with porphyritic texture (Figure 6c) Major mineral phases in the rock samples of groups 1, 2, and include, in decreasing order of abundance, 40%– 50% plagioclase, 30%–40% clinopyroxene, and 10%–15% olivine as phenocrysts and microphenocrysts of variable sizes Minor minerals consist of opaque minerals and K-feldspar Accessory minerals are apatite (as inclusions in plagioclase and clinopyroxene) and sphene (in the groundmass) Melt inclusions are seen frequently in plagioclase, clinopyroxene, and olivine A small number of samples are vesicular and in such a case that vesicles are filled by carbonates, quartz, prehnite, and zeolites, amygdaloidal textures are also found in the rock samples of groups 1, 2, and Plagioclase crystals are euhedral, twinned according to albite/albite-Carlsbad, and converted to prehnite in the majority of the cases Rare plagioclase crystals are characterized by oscillatory zoning The often centimeter-size clinopyroxene phenocrysts are euhedral-subhedral, fresh, larger than olivine, and brown to weakly purplish These minerals generally occur in glomeroporphyritic aggregates (Figure 6d) and those in the groundmass are swallow-tailed Clinopyroxene glomeroporphyritic aggregates are formed in the magma chamber and on the intratelluric stage They contain olivine and opaque minerals in some cases Olivine phenocrysts are mainly subhedral and embayed in some cases The majority of the olivines, which include the opaque minerals, are partially replaced by lowtemperature iddingsite (±bowlingite) K-feldspar minerals occur as subhedral to anhedral grains They often show dusty surfaces due to alteration to clay materials Opaque minerals are polygonal (equidimensional and prismatic), shown by jagged edges in most cases Apatite minerals appear as very long tiny needles and are enclosed by plagioclase and clinopyroxene minerals The acicular apatite indicates a rapid growth within a quench environment (Zorpi et al., 1989; Didier, 1991; Best, 2003) 5.2 Subvolcanic rocks of the Garmsar area Petrographic features of the Garmsar area rocks were studied by Sarizan (2014) In the Garmsar area, rocks are seen as subvolcanic with mafic and intermediate compositions Intermediate rocks are less frequent than mafic rocks in this area Under the microscope, subvolcanic rocks with mafic and intermediate compositions represent ophitic (Figure 6e), subophitic (Figure 6f), intergranular (Figure 6g), and granular (Figure 6h) textures Major mineral phases include 40–60 vol.% plagioclase and 20–30 vol.% clinopyroxene (smaller amounts in intermediate rocks) as phenocrysts and microphenocrysts of variable sizes Minor minerals consist of olivine and opaque and brownish glass often in the groundmass In addition to olivine and opaque and brownish glass, amphibole, biotite, and quartz are also considered as minor minerals in intermediate rocks Apatite is the most important accessory mineral in the Garmsar area rocks Plagioclases are the most abundant minerals in the rocks of the Garmsar area These minerals are euhedral to subhedral; mainly show polysynthetic twinning, zoning, and a sieve texture; are embayed in some cases; and are often converted to chlorite, epidote, calcite, and mainly prehnite minerals By calculating the average percentage of normative anorthite, the combination of labradorite and andesine-labradorite was obtained for plagioclase in mafic and intermediate rocks of the Garmsar area, respectively 299 FALAHATY et al / Turkish J Earth Sci Figure Photomicrographs of the Shurab and Garmsar areas rocks: (a), (b), (c) Intergranular (XPL), trachytic (PPL) and porphyritic (XPL) textures in rocks of groups 1, 2, and of the Shurab area, respectively (d) Clinopyroxene and olivine glomeroporphyritic aggregates in the Shurab area rocks (XPL) (e), (f) Ophitic (XPL) and subophitic (XPL) textures in mafic rocks of the Garmsar area, respectively (g), (h) Intergranular (XPL) and granular (XPL) textures in intermediate rocks of the Garmsar area, respectively (Cpx: clinopyroxene, Ol: olivine, Pl: plagioclase, Opaq: opaque, and Chl: chlorite (Kretz, 1983)) 300 FALAHATY et al / Turkish J Earth Sci Table Microprobe analyses of plagioclase minerals in volcanic rocks of the Shurab area (Central Basin) Sample Analysis Plagioclase Plagioclase Plagioclase SiO2 49.57 52.53 55.91 TiO2 0.05 0.08 0.05 Al2O3 31.64 29.25 27.47 FeO* 0.78 0.86 0.66 CaO 15.29 12.63 9.94 Na2O 2.70 4.23 5.33 K2O 0.17 0.30 0.52 Total 100.20 99.87 99.88 Oxygens 32 32 32 Si 9.069 9.582 10.099 Ti 0.006 0.011 0.007 Al 6.823 6.289 5.848 Fe(II) 0.119 0.13 0.1 Ca 2.997 2.469 1.923 Na 0.956 1.495 1.868 K 0.04 0.068 0.119 Cations 20.011 20.044 19.964 An 75.05 61.23 49.19 Ab 23.95 37.08 47.77 Or 1.01 1.69 3.03 An: Anorthite, Ab: albite, Or: orthoclase Clinopyroxene is the most abundant mineral in the Garmsar area rocks after plagioclase Clinopyroxene phenocrysts are euhedral to subhedral, sometimes reveal zonation, and are rarely converted to actinolitic hornblende Clinopyroxenes formed in glomeroporphyritic aggregates are embayed in some cases and rarely exhibit hourglass twinning The crystals of olivine minerals are largely anhedral and are seen to be embayed Olivine phenocrysts are mainly replaced by iddingsite and show a skeletal texture in some cases Primary opaque minerals are seen to be euhedral to subhedral in groundmass Secondary opaque minerals are produced by clinopyroxene, olivine, and biotite hydrothermal alteration Amphiboles are anhedral to subhedral In most cases, these minerals are completely altered to chlorite and are indistinguishable Biotite minerals are coarse to fine in size, are often replaced by chlorite, and are embayed in some cases Quartz minerals are seen in both primary and secondary forms Most quartz minerals in the intermediate rocks of the Garmsar area are secondary and together with calcite have filled the cavities Apatite minerals with needle shapes and prismatic forms are the most important accessory minerals in the Garmsar rocks and are seen as inclusions in clinopyroxene, plagioclase, and biotite minerals These minerals are also found between minerals derived from alteration, such as chlorite, epidote, and calcite Microprobe analyses of minerals (Tables 1–4) showed the presence of plagioclase with composition in the range from An49Ab23 to An75Ab47 (labradorite to bytownite; Figure 7a), clinopyroxene with composition in the range Wo43En39-45Fs9-15 (diopside; Figure 7b), olivine classified as 45 hyalosiderite in the range from Fo58Fa31 to Fo67Fa40 (Figure 7c), and K-feldspar in the range Or41-65Ab33-50An0.69-8 (with mainly sanidine) in rock samples of the Shurab area Whole-rock geochemistry 6.1 Major elements Whole-rock major geochemical analyses of all representative samples of the Shurab and Garmsar areas (Tables and 6) show high and variable Al2O3 concentrations (from 16.51 to 17.6 wt.% in group samples of the Shurab area, from 18.14 to 18.57 wt.% in group samples of the Shurab area, from 17.07 to 18 wt.% in group samples of the Shurab area, and from 15.9 to 19.92 wt.% in the Garmsar mafic samples), high amounts of CaO (from 6.98 to 10.39 wt.% in group samples of the Shurab area, from 4.22 to 9.17 wt.% in group samples of the Shurab area, from 3.83 to 8.42 wt.% in group samples of the Shurab area, and from 6.56 to 11.47 wt.% in the Garmsar mafic samples), and low TiO2 contents (from to 1.19 wt.% in group samples of Shurab, from 0.98 to wt.% in group samples of Shurab, from 0.95 to 1.08 wt.% in group samples of Shurab, and in the Garmsar mafic samples from 0.9 to 1.2 wt.%), supporting a calc-alkaline composition Low TiO2 contents in rocks of both Shurab and Garmsar areas (e.g.,