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Zircon U-Pb Ages and Magmatic History of the Kashan Plutons in the Central Urumieh-Dokhtar Magmatic Arc, Iran: Evidence for Neotethyan Subduction during Paleogene-Neogene

  • The Kashan plutons are situated in the central part of Urumieh-Dokhtar magmatic arc recording subduction-related magmatism within the Alpine-Himalayan orogeny in Iran. These rocks consist of different calc-alkaline plutonic rocks including gabbro,gabbroic diorite,microdiorite,monzodiorite,tonalite,granodiorite,and granite. The plutons were emplaced into the Jurassic sedimentary units (Shemshak Formation) and the Eocene calc-alkaline volcanic and pyroclastic rocks. New U-Pb zircon ages show that the Kashan plutons formed during two main periods at 35.20±0.71 Ma in the Late Eocene (Priabonian) and at 18.90±0.84,19.26±0.83,19.30±1.2,and 17.3±1.8 Ma in the Early Miocene (Burdigalian). The reported events in the Kashan plutons imply the final phases of subduction-related magmatism before the collision which happened between the Arabian and Iranian plates in the Middle Miocene. The plutonic activity in the Kashan region occurred during the transition from Eocene subduction-related setting to Middle Miocene collisional setting.
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Zircon U-Pb Ages and Magmatic History of the Kashan Plutons in the Central Urumieh-Dokhtar Magmatic Arc, Iran: Evidence for Neotethyan Subduction during Paleogene-Neogene

    Corresponding author: Nematollah Rashidnejad-Omran, rashid@modares.ac.ir
  • 1. Department of Geology, Tarbiat Modares University, Tehran 14115-175, Iran
  • 2. School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
  • 3. School of Earth and Space Sciences, Peking University, Beijing 100871, China

Abstract: The Kashan plutons are situated in the central part of Urumieh-Dokhtar magmatic arc recording subduction-related magmatism within the Alpine-Himalayan orogeny in Iran. These rocks consist of different calc-alkaline plutonic rocks including gabbro,gabbroic diorite,microdiorite,monzodiorite,tonalite,granodiorite,and granite. The plutons were emplaced into the Jurassic sedimentary units (Shemshak Formation) and the Eocene calc-alkaline volcanic and pyroclastic rocks. New U-Pb zircon ages show that the Kashan plutons formed during two main periods at 35.20±0.71 Ma in the Late Eocene (Priabonian) and at 18.90±0.84,19.26±0.83,19.30±1.2,and 17.3±1.8 Ma in the Early Miocene (Burdigalian). The reported events in the Kashan plutons imply the final phases of subduction-related magmatism before the collision which happened between the Arabian and Iranian plates in the Middle Miocene. The plutonic activity in the Kashan region occurred during the transition from Eocene subduction-related setting to Middle Miocene collisional setting.

0.   INTRODUCTION
1.   GEOLOGICAL SETTING
  • The UDMA as a long volcano-plutonic belt is substantially considered as a magmatic arc extending along the SSZ and south Central Iran microcontinent (Fig. 1). The UDMA is believed to have formed above a slab of the Neotethyan oceanic lithosphere which is being subducted beneath the Iranian continental crust (with a rate of approximately 2–3 cm/year) (Alavi, 1994; Davoudzadeh and Schmidt, 1984). The UDMA comprises mafic to felsic igneous rocks (e.g., Kananian et al., 2014; Berberian and King, 1981).

    Geochemical research studies imply that the highest volume of magmatism in the UDMA is related to volcanic activities (Eocene to Oligocene; Chiu et al., 2013) involving basalt, basaltic andesite, ignimbrite and pyroclastic rocks (Alavi, 1994). Less plutonic activities including Oligo-Miocene gabbro to granite (Berberian and Berberian, 1981) also occurred throughout the UDMA (e.g., Haghipour and Aghanabati, 1985). These rocks are predominantly calc-alkaline and tholeiitic (e.g., Ahmad and Posht Kuhi, 1993; Jung et al., 1976). Granitoids in the UDMA are mostly Ⅰ-type; however, A-type granitoids have recently been reported from the same region (Dargahi et al., 2010). Calc-alkaline intrusive rocks are the oldest, while Pliocene to Quaternary pyroclastics, lavas, and sediments are the youngest rocks in the UDMA (Berberian and Berberian, 1981).

    The Kashan plutons outcropping between 33°35'N– 34°00'N and 51°10'E–51°40'E are situated in the southwest of Kashan in Central Iran in an area of about 240 km2. The Kashan plutons are mainly made of mafic to felsic plutonic rocks scattered along the UDMA. The Ghohroud granitoid (GG) is a primary exposure in the Kashan plutons (Fig. 2).

    Figure 2.  Simplified geological map of the Kashan plutons (modified after Radfar, 1993) including sample locations and U-Pb ages from current research. mdi. Microdiorite; mzdi. monzodiorite; ton. tonalite; grd. granodiorite; gr. granite; Fm.. Formation.

    According to geological studies, rock units of the study area are summarized as follows: (1) Major outcrops of Late Triassic sedimentary rocks including a sequence of argillaceous limestone; i.e., the oldest rocks in the region, (2) an Early Jurassic sequence which comprises of conglomerate, marl, and limestone (Shemshak Formation; SF), (3) an Early Cretaceous sequence of orbitolina limestone, (4) Early–Late Eocene volcanic and pyroclastic rocks composed of andesite, basalt, and rhyolite, (5) an Oligo- Miocene sequence including dark gray shale, green marl and limestone (Qom Formation; QF). This sequence is followed by Miocene andesitic to dacitic calc-alkaline volcanic rocks and pyroclastics on the top and is intruded by plutonic and subvolcanic rocks. The Kashan subvolcanic intrusions mostly crop out as small and scattered dome-shaped bodies in the south and southwest of Kashan. These rocks have intruded into some Eocene and Oligo-Miocene volcano-sedimentary and pyroclastic rocks. Most of the outcropped units in the study area are volcanic rocks including basaltic, andesitic and rhyodacitic lavas and pyroclastics with Early–Late Eocene in age (Radfar, 1993). Also, the Middle Miocene dacite and dacitic andesite adakitic rocks (Chiu et al., 2013) crop out as dome structure in the southwest of Kashan. Intrusive rocks including gabbro to granite are often associated with these volcanic rocks.

2.   FIELD RELATIONSHIPS AND PETROGRAPHY
  • Jurassic shales and sandstones (SF) are the oldest stratigraphic units of the study area. These are followed by the Eocene volcano-sedimentary and pyroclastic units overlain by the Oligo-Miocene limestones (QF). The Kashan plutons intruded these units (Fig. 2). There are Miocene volcanic and subvolcanic rocks on the top (andesitic to rhyolitic rocks and pyroclastics).

    The Kashan plutons crop out into three distinct exposures consisting of Jazeh, Jahagh, and Ghohroud (Fig. 2). Based on field observations, except for Ghohroud granitoid (GG) that is a massive intrusion, others occur as small and scattered hills and masses. The GG is composed mainly of tonalite and granodiorite with minor granite which locally led to contact metamorphism in the neighboring rocks and formation of fertile skarns and hornfels (Ahankoub, 2003). The GG has white to gray color with mainly granular and less porphyroid in texture and fine-grained in the margin. Jahagh microdiorites appear as small outcrops in the west of GG. These rocks tend to be light to dark gray and intruded into the Eocene pyroclastic rocks and Jurassic SF. The Jazeh gabbroic diorite and monzodiorite intrusions are exposed in the north of the Kashan plutons and have sharp contact with surrounding volcanic and subvolcanic rocks. The subvolcanic rocks contain dacite, microdiorite and andesite mostly as dome-shaped bodies in the south and southwest of the study area. Subvolcanic bodies have also intruded into the Ghohroud intrusion. Porphyritic trachy-andesitic and fine- grained granitic dikes are abundant in the tonalitic and granodioritic host rocks. The dikes are variable in thickness and generally display an SW-NE orientation and follow the trend of deep fault structures in the Kashan region. The Kashan plutons consist of different plutonic rocks including gabbro, gabbroic diorite, microdiorite, monzodiorite, tonalite, granodiorite, and granite. Petrographic features of these rocks are as follows.

    The gabbros are fine- to medium-grained and demonstrate an inter-granular texture. These rocks include clinopyroxene (~43 wt.%), plagioclase (~36 wt.%), biotite (~15 wt.%) and hornblende (~5 wt.%) as major minerals with minor apatite, zircon, and opaque minerals. The clinopyroxene grains anhedrally occur in the interstices among plagioclase grains and are often partly replaced by uralite. Some clinopyroxene crystals appear as relics in the core of hornblende grains, and some grains are enclosed by plagioclase as inclusion. Plagioclase is present as subhedral to euhedral tabular grains in different sizes from ~0.5 to ~2 mm. Polysynthetic twinning is common in plagioclase crystals. Some plagioclase grains are zoned. Opaque minerals (Fe-Ti oxides), zircon and apatite, occur as accessory minerals (Fig. 3a).

    Figure 3.  Microphotographs of the studied plutonic rocks (in the XPL). (a) Gabbro, (b) gabbroic diorite, (c) microdiorite, (d) tonalite, (e) granodiorite, (f) Granite, and (g) monzodiorite. Mineral abbreviations: Pl. plagioclase; Cpx. clinopyroxene; Hbl. hornblende; Bt. biotite; Kfs. K-feldspar; Qtz. quartz (Kretz, 1983).

    The gabbroic diorites consist of the same minerals as gabbros and have more hornblende than gabbros. Pyroxene and hornblende are partly to entirely uralitized in some minerals. Opaque inclusions are present with varying amounts in hornblende (Fig. 3b).

    The microdiorites, which have a microgranular texture, are fine-grained (< 0.5 mm) and contain hornblende (~43 wt.%), plagioclase (~54 wt.%) and minor clinopyroxene and biotite.

    Zircon, apatite, and magnetite occur as accessory constituents. Hornblende in the same samples is partly replaced by epidote, chlorite, and actinolite (Fig. 3c).

    The tonalites have coarse-grained size (~2.0–4.0 mm) and granular texture. The major minerals consist of plagioclase (~40 wt.%–45 wt.%), hornblende (~20 wt.%–30 wt.%), quartz (~12 wt.%–22 wt.%), biotite (~10 wt.%–15 wt.%) and alkali- feldspar (< 4 wt.%). Accessory minerals include magnetite, zircon, and apatite. Some plagioclase crystals show oscillatory zoning and hornblendes demonstrate twinning texture in some samples (Fig. 3d).

    The granodiorites, which are coarse to medium-grained and show granular texture, contain mainly feldspars (plagioclase (~30 wt.%), orthoclase (~15 wt.%)), quartz (~30 wt.%), hornblende (~15 wt.%) and biotite (~10 wt.%). Zircon, apatite, and magnetite are secondary phases. Some hornblende and biotite rims have been partly altered to chlorite and calcite. Plagioclase occasionally is replaced by sericite and epidote. Also, some plagioclase grains show oscillatory zoning and sieve texture. In some samples, large plagioclase crystals contain hornblende and/or apatite inclusions, and smaller plagioclase crystals are enclosed by alkali-feldspar as inclusion. These features imply magma mixing (Aslan, 2005; Akal and Helvaci, 1999). Zircon, apatite, and titanite occur as accessories. Calcite, chlorite, epidote, and sericite are secondary minerals in the granodiorites (Fig. 3e).

    The granites, which have a granular texture, generally crop out as leucogranitic small lenses and bodies. Quartz (~40 wt.%), alkali-feldspar (~35 wt.%), plagioclase (~15 wt.%) and biotite (~7 wt.%) are the major constituents. Quartz is locally clustered among alkali-feldspars and shows micrographic and granophyric intergrowth textures. Plagioclase and alkali-feldspar sometimes show absorbed margins and are partially altered to sericite. Biotite grains are present as brown flanks occasionally replaced by chlorite in their rims. Zircon and opaque minerals are the accessory components in these rocks (Fig. 3f).

    The monzodiorites are fine- to medium-grained and demonstrate granular to intergranular textures. These rocks consist of plagioclase (~45 wt.%), hornblende (~35 wt.%), alkali- feldspar (~15 wt.%) and minor quartz. Hornblende crystals are present as anhedral to subhedral poikilitic grains enclosing fine-grained plagioclase and minor opaque. Some hornblende grains are present as subhedral to euhedral prisms that are locally altered to actinolite and chlorite. Plagioclase crystals occur as subhedral to euhedral tabular grains and demonstrate lamellar twinning. Biotite, titanite, and zircon are the accessory phases. Secondary phases include chlorite, actinolite, and calcite (Fig. 3g).

    Some characteristics of Ⅰ-type granitoids are shown by the mineralogy of the investigated rocks from the Kashan region. For example, hornblende is the common mafic mineral in all studied plutonic rocks, while no muscovite is observed. Also, the existence of apatite as inclusion is a common feature, which is typical in Ⅰ-type granitoids, whereas apatite occurs as larger crystals in S-type granitoids. Furthermore, no monazite was noticed, and titanite is the accessory component. S-type granitoids have a modal distribution similar to those of Ⅰ-type, but S-type falls in the adamellite-granite spectrum, and Ⅰ-type is categorized into the monzodiorite-tonalite-granodiorite range (Hine et al., 1978).

3.   ANALYTICAL METHODS
  • Whole-rock major element analysis was performed on 26 samples selected from the Kashan plutons by careful petrographic observation (Table 1). The samples were crushed in steel jaw crushers and then powdered in a tungsten carbide ball mill to a grain size of < 200 meshes. The whole-rock major oxides were analyzed for some samples by a Leman Prodigy Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES) at China University of Geosciences, Beijing (CUGB) and some samples using wavelength-dispersive X-ray fluorescence (XRF) spectrometry at State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. In the ICP-OES technique, analytical precisions (1ó) for most major elements on account of master standards GSR-1, GSR-3, GSR-5 (national geological standard reference materials of China) and USGS AGV-2 are better than 1% except TiO2 (~1.5 wt.%) and P2O5 (~2.0 wt.%). Loss on ignition (LOI) was gained by placing 1 g of sample powder in the furnace at 1 000 ℃ for several hours before being cooled in a desiccator and reweighed. In the XRF method, LOI was determined on ca. 0.5 g of sample powders baked at 1 000 ℃ for one hour. Analyses of international rock standards (BHVO-2, GSR-1, and GSR-3) demonstrated precision and accuracy better than 5% for major oxides.

    Sample No. G2.26 F.7 G2.55 G.4 G2.53 Gz.2 Gh.30 Jh.5 G2.23 Jv.27 Gz.24 Gz.13 G2.19 Jv.24 Gh.59 Ba.49 Gz.23 Gz.29 Jv.2 Gh.45 Gh.62 Jv.9 Gh.49 Jh.14 Gz.10 Jv.25
    Rock type gb gbd gbd gbd gbd di di mzdi di di di mzdi mzdi mzdi ton ton ton ton ton grd grd grd grd di gr gr
    SiO2 51.19 52.28 54.31 53.74 52.01 56.05 60.67 57.36 55.93 57.07 56.31 59.24 58.28 58.59 61.00 60.35 62.22 62.30 61.29 66.10 66.24 63.79 64.89 64.92 73.72 73.55
    TiO2 0.94 1.09 0.94 1.08 0.84 0.88 0.72 0.88 1.31 0.85 0.91 0.73 1.45 0.69 0.65 0.63 0.71 0.79 0.64 0.55 0.55 0.57 0.54 0.57 0.27 0.22
    Al2O3 16.58 17.99 16.76 18.27 17.75 17.46 16.23 16.32 15.57 15.64 17.26 16.69 15.44 16.45 16.27 15.80 14.96 15.15 16.37 14.58 14.64 15.51 15.05 14.73 12.94 13.60
    Fe2O3T 9.91 10.52 8.59 9.56 9.40 8.47 6.22 8.46 9.52 7.88 8.39 7.12 6.59 7.07 6.74 6.84 6.40 6.78 6.33 4.69 5.16 5.50 5.67 4.63 2.06 2.08
    MnO 0.16 0.17 0.17 0.18 0.19 0.15 0.11 0.14 0.23 0.15 0.17 0.12 0.17 0.11 0.11 0.16 0.13 0.12 0.10 0.06 0.09 0.08 0.07 0.05 0.05 0.03
    MgO 7.08 4.13 4.05 3.34 4.74 3.48 3.44 3.51 3.48 5.41 4.31 3.30 1.75 3.30 2.73 2.81 2.72 3.07 2.66 2.06 2.14 2.23 2.35 1.57 0.62 0.54
    CaO 9.72 8.97 8.96 8.66 9.11 6.86 5.35 7.32 7.04 7.26 7.34 4.92 5.72 5.87 5.29 5.60 5.57 5.73 5.54 4.10 4.05 4.83 4.55 2.99 2.12 1.45
    Na2O 3.27 3.38 3.15 3.72 3.27 3.60 4.15 3.37 3.37 3.08 3.60 4.30 3.71 3.33 3.32 3.87 3.07 3.32 3.56 3.38 3.25 3.27 3.41 3.35 3.32 3.30
    K2O 0.38 0.31 0.35 0.34 0.44 1.46 1.93 1.32 1.98 1.35 0.90 2.26 2.71 2.97 2.17 1.05 2.15 1.82 1.93 2.88 2.23 2.48 2.61 4.23 4.23 4.90
    P2O5 0.07 0.18 0.13 0.19 0.15 0.16 0.17 0.17 0.29 0.23 0.13 0.16 0.45 0.14 0.14 0.14 0.12 0.11 0.13 0.12 0.13 0.11 0.14 0.15 0.04 0.08
    LOI 0.99 0.41 0.63 0.73 1.31 0.79 0.82 0.65 1.22 1.80 0.82 1.00 2.62 0.76 0.53 2.55 1.06 0.51 0.40 0.85 0.91 1.26 0.60 2.85 0.41 0.35
    Total 100.29 99.42 98.92 99.80 99.21 99.35 99.18 99.49 99.94 100.72 100.14 99.84 98.87 98.41 99.29 99.80 99.11 99.70 98.75 99.37 99.39 99.63 99.89 100.02 99.99 99.98
    Rock types: gb. gabbro; gbd. gabbroic diorite; di. diorite; mzdi. monzodiorite; ton. tonalite; grd. granodiorite; gr. granite. Oxides are given in percentage by weight (wt.%).

    Table 1.  Whole-rock major element contents for the rocks from the Kashan plutons

  • Zircon separates were prepared for five samples using both magnetic and heavy liquid methods before finally hand-picking under a binocular microscope at the Geological Survey of Iran. Selected crystals were transparent, colorless, and mostly sub-angular and some grains show well-defined prisms and euhedral to subhedral habitus. The zircons were then mounted on adhesive tape, embedded in epoxy resin and polished. The U-Pb analyses were performed at the Key Laboratory of Crust-Mantle Materials and Environments at the University of Science and Technology of China (USTC), Hefei. The analytical data are presented in Table 2. U-Pb Zircon isotopic compositions were determined by a laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS; Perkin Elmer Elan DRC Ⅱ) with a microlas system (GeoLas 200M, 193 nm ArF-excimer laser). The analyses were performed with a pulse rate of 10 Hz, a beam energy of 8.5 J/cm2, and a spot diameter of 36 μm. Signal and background measuring times were 70 and 40 s, respectively. Age calculations were calibrated against zircon 91500 reference material analyzed after every five sample spots. More on the laser ablation U-Pb dating technique is provided in Liu et al. (2007). The U-Pb raw data were processed using Glitter 4.0 (Macquarie University, Australia) and the U-Pb ages were calculated by ISOPLOT 3.0 (Ludwig, 2003). In this research, seventeen spots analyzed on the zircon 91500 provide a mean 207Pb/206Pb age of 1 065±50 Ma (2SD) and a mean 238U/206Pb age of 1 062±9 Ma (2SD). The ages of the 238U/206Pb system are reported here due to its high precision for young rocks.

    Spot/tracks U Pb U/Th 207Pb/206Pb 1s 207Pb/235U 1s 206Pb/238U 1s 208Pb/232Th 1s
    Jh.5 (microdiorite)
    Jh.5-2 1 311 11.2 0.82 0.064 1 0.005 8 0.056 099 0.003 86 0.005 556 0.000 163 0.001 796 0.000 069
    Jh.5-4 845 6.6 1.57 0.078 6 0.007 8 0.068 961 0.005 17 0.005 557 0.000 178 0.002 254 0.000 16
    Jh.5-5 592 6.8 1.18 0.177 3 0.018 7 0.175 201 0.018 056 0.006 199 0.000 241 0.003 94 0.000 327
    Jh.5-7 1 368 11.6 0.9 0.052 1 0.004 8 0.045 352 0.002 914 0.005 687 0.000 157 0.001 864 0.000 074
    Jh.5-8 1 877 15.9 0.74 0.046 0.003 6 0.036 315 0.002 682 0.005 286 0.000 13 0.001 778 0.000 07
    Jh.5-9 1 012 7.8 1.1 0.051 0.005 4 0.047 559 0.003 205 0.005 524 0.000 154 0.001 958 0.000 172
    Jh.5-10 755 5.8 1.07 0.054 6 0.006 8 0.058 331 0.004 232 0.005 343 0.000 176 0.001 826 0.000 092
    Jh.5-11 858 6.7 1.07 0.060 1 0.007 8 0.057 287 0.003 629 0.005 395 0.000 16 0.001 838 0.000 086
    Jh.5-13 674 5.8 1.46 0.129 8 0.013 1 0.091 238 0.006 949 0.005 537 0.000 195 0.003 163 0.000 203
    Jh.5-14 2 296 19.7 0.71 0.056 4 0.003 4 0.043 476 0.002 516 0.005 536 0.000 109 0.001 773 0.000 005
    Jh.5-15 2 362 19.7 1.86 0.108 6 0.007 9 0.086 139 0.007 023 0.005 45 0.000 131 0.004 497 0.000 468
    Jh.5-16 1 368 11.6 0.97 0.065 3 0.005 4 0.056 88 0.003 348 0.005 865 0.000 144 0.001 996 0.000 078
    Jh.5-18 1 457 11.5 0.99 0.041 7 0.004 3 0.037 357 0.002 961 0.005 633 0.000 159 0.001 691 0.000 079
    Jh.5-19 7 438 69.5 1.29 0.123 9 0.006 1 0.096 598 0.004 697 0.005 688 0.000 116 0.003 186 0.000 115
    Jh.5-21 2 879 22 2.63 0.162 5 0.010 2 0.128 429 0.008 515 0.005 694 0. 000164 0.004 69 0.000 354
    Jh.5-22 1 186 12.7 1.077 0.115 3 0.017 1 0.952 89 0.013 047 0.005 896 0.000 136 0.002 621 0.000 111
    Jh.5-24 1 498 11.5 1.043 0.832 4 0.007 6 0.059 44 0.004 637 0.005 332 0.000 205 0.002 008 0.000 12
    Jv.2 (tonalite)
    Jv2-1 685 3.9 1.5 0.238 5 0.066 3 0.091 403 0.009 485 0.003 202 0.000 187 0.002 388 0.000 183
    Jv2-2 1 008 5.1 1.15 0.083 3 0.011 9 0.049 647 0.003 406 0.003 161 0.000 13 0.001 427 0.000 181
    Jv2-3 865 3.9 1.4 0.087 9 0.012 1 0.048 042 0.003 756 0.002 989 0.000 142 0.001 397 0.000 114
    Jv2-4 813 3.7 1.37 0.081 5 0.013 6 0.050 372 0.003 951 0.003 178 0.000 174 0.001 266 0.000 123
    Jv2-7 1 043 4.8 1.17 0.092 0.010 3 0.044 107 0.002 941 0.003 066 0.000 102 0.001 128 0.000 069
    Jv2-10 620 2.5 2 0.108 4 0.017 7 0.007 135 0.006 528 0.000 311 0.000 126 0.001 778 0.000 158
    Jv2-18 1 154 5.4 1.06 0.069 8 0.011 2 0.046 93 0.004 418 0.003 184 0.000 135 0.001 109 0.000 083
    Jv2-19 494 2.2 1.71 0.067 9 0.012 0.059 171 0.003 935 0.003 334 0.000 156 0.001 803 0.000 131
    Jv2-20 984 4.8 1.28 0.095 7 0.013 2 0.053 379 0.004 516 0.003 322 0.000 123 0.001 504 0.000 113
    Jv2-21 517 2.4 1.56 0.132 2 0.026 0.072 016 0.006 075 0.003 072 0.000 17 0.001 9 0.000 144
    Jv2-22 804 3.5 1.38 0.067 4 0.011 3 0.041 217 0.003 53 0.003 007 0.000 149 0.001 33 0.000 228
    Jv.2 (tonalite)
    Jv2-23 1 062 4.9 1.24 0.120 4 0.022 5 0.059 461 0.007 432 0.003 113 0.000 14 0.001 295 0.000 086
    Jv2-25 3 110 10.2 14.8 0.057 0.004 2 0.023 255 0.001 659 0.002 846 0.000 071 0.001 929 0.000 157
    Jv2-26 669 2.8 1.28 0.065 3 0.010 8 0.052 25 0.003 568 0.003 06 0.000 129 0.001 046 0.000 092
    Jv2-29 532 2.2 1.65 0.079 9 0.010 9 0.067 991 0.004 534 0.003 031 0.000 127 0.001 48 0.000 098
    Jv2-30 949 3.8 2.1 0.085 4 0.009 4 0.044 698 0.003 222 0.002 958 0.000 109 0.001 562 0.000 112
    Jv2-31 664 2.8 1.94 0.076 3 0.013 2 0.059 196 0.003 687 0.003 265 0.000 206 0.001 391 0.000 113
    Jv2-32 785 3.2 2.07 0.066 4 0.012 3 0.048 036 0.003 583 0.003 15 0.000 176 0.001 399 0.000 117
    Jv2-35 818 3.7 1.29 861 0.022 9 0.040 823 0.002 158 0.003 243 0.000 118 0.001 325 0.000 084
    Jv2-40 734 3.5 1.96 0.193 5 0.029 6 0.078 141 0.004 39 0.003 152 0.000 125 0.002 066 0.000 11
    G2.19 (monzodiorite)
    G2-19-1 620 3.8 1.33 0.128 4 0.029 2 0.697 08 0.006 537 0.003 933 0.000 19 0.001 73 0.000 352
    G2-19-2 677 3.4 1.25 0.07 0.009 9 0.047 913 0.002 826 0.003 561 0.000 135 0.000 977 0.000 068
    G2-19-3 894 3.3 2.08 0.041 3 0.005 4 0.026 103 0.001 716 0.002 959 0.000 099 0.000 731 0.000 054
    G2-19-4 971 3.8 1.46 0.058 5 0.007 3 0.029 467 0.002 218 0.002 819 0.000 119 0.000 566 0.000 051
    G2-19-5 995 4.7 1.68 0.065 8 0.008 8 0.034 465 0.003 044 0.003 296 0.000 154 0.001 38 0.000 145
    Gh.49 (granodiorite)
    Gh.49-1 666 3.1 1.8 0.169 53 0.033 3 0.089 571 0.006 919 0.003 113 0.000 156 0.001 958 0.000 171
    Gh.49-2 1 366 5.9 1.15 0.097 6 0.016 5 0.042 653 0.004 21 0.002 941 0.000 135 0.001 148 0.000 08
    Gh.49-3 1 153 4.5 1.71 0.075 5 0.014 3 0.046 371 0.007 825 0.003 028 0.000 091 0.001 045 0.000 066
    Gh.49-4 607 2.9 2.17 0.204 5 0.030 3 0.088 541 0.007 435 0.003 28 0.000 193 0.002 32 0.000 243
    Gh.49-5 996 4.2 1.43 0.076 1 0.011 5 0.047 577 0.007 741 0.003 025 0.000 092 0.012 46 0.000 068
    Gh.49-6 1 010 4.2 1.53 0.063 4 0.009 0.040 247 0.002 747 0.003 279 0.000 148 0.000 952 0.000 073
    Gh.49-7 819 3.3 1.58 0.067 7 0.011 3 0.045 467 0.003 648 0.003 0.000 126 0.001 196 0.000 071
    Gh.49-8 767 3.1 1.63 0.070 1 0.012 4 0.046 009 0.004 047 0.003 112 0.000 157 0.001 273 0.000 106
    Gh.49-9 1 205 5.7 1.53 0.098 6 0.010 6 0.053 255 0.003 393 0.003 211 0.000 115 0.001 594 0.000 103
    Gh.49-10 827 2.9 2.26 0.051 4 0.009 5 0.044 245 0.002 624 0.002 941 0.000 106 0.001 434 0.000 113
    Gh.49-11 457 1.9 2.11 0.097 1 0.022 5 0.079 278 0.005 635 0.003 171 0.000 194 0.001 859 0.000 146
    Gh.49 (granodiorite)
    Gh.49-12 375 1.8 2.17 0.144 6 0.017 6 0.099 975 0.006 042 0.003 233 0.000 159 0.003 027 0.000 208
    Gh.49-13 531 2.2 1.38 0.075 0.015 5 0.060 271 0.003 516 0.003 166 0.000 129 0.001 269 0.000 077
    Gh.49-14 699 3.2 1.82 0.109 9 0.011 6 0.064 098 0.003 811 0.003 19 0.000 121 0.001 79 0.000 111
    Gh.49-15 627 5.5 1.47 0.167 0.016 6 0.087 978 0.006 022 0.003 399 0.000 127 0.002 144 0.000 123
    Gh.49-16 377 1.7 2 0.221 2 0.103 7 0.144 313 0.028 698 0.003 225 0.000 197 0.002 323 0.000 172
    Gh.49-17 489 2.5 1.8 0.273 9 0.053 0.109 819 0.008 683 0.003 285 0.000 157 0.002 298 0.000 161
    Gh.49-18 536 2 1.83 0.096 9 0.021 1 0.058 706 0.005 598 0.002 992 0.000 218 0.001 44 0.000 149
    Gh.49 (granodiorite)
    Gh.49-19 711 3.1 1.9 0.058 0.007 1 0.047 75 0.002 762 0.003 24 0.000 111 0.001 765 0.000 113
    Gh.49-20 602 2.5 1.59 0.074 7 0.011 3 0.048 855 0.002 829 0.002 991 0.000 147 0.001 383 0.000 091
    Jv.25 (granite)
    Jv.25-2 920 5.3 1.56 0.131 56 0.024 57 0.069 669 0.008 165 0.003 863 0.000 219 0.001 846 0.000 194
    Jv.25-10 1 507 6.1 1.83 0.064 28 0.013 375 0.043 428 0.008 029 0.003 124 0.000 096 0.001 188 0.000 073
    Jv.25-12 729 3.5 2.29 0.065 6 0.013 39 0.064 637 0.005 126 0.003 578 0.000 145 0.001 993 0.000 154
    Jv.25-13 688 2.7 1.89 0.092 22 0.016 722 0.058 473 0.004 661 0.003 075 0.000 195 0.001 334 0.000 138
    Jv.25-14 649 2.5 2.12 0.061 34 0.010 89 0.048 205 0.002 86 0.003 279 0.000 124 0.001 125 0.000 116
    Jv.25-15 667 3.2 2.29 0.087 87 0.011 02 0.052 358 0.002 794 0.003 542 0.000 148 0.001 912 0.000 146
    Jv.25-18 1 023 4.3 1.66 0.081 77 0.017 76 0.052 087 0.009 298 0.003 301 0.000 126 0.001 217 0.000 078
    Jv.25-19 1 286 5.6 3.32 0.140 23 0.032 14 0.066 835 0.015 307 0.003 44 0.000 207 0.002 072 0.000 252
    Jv.25-21 1 091 4.7 1.44 0.062 98 0.011 45 0.040 089 0.003 233 0.003 277 0.000 116 0.001 243 0.000 087
    Jv.25-22 658 2.7 1.39 0.072 74 0.008 78 0.049 975 0.002 735 0.003 199 0.000 135 0.001 226 0.000 083
    Jv.25-23 535 2.2 1.82 0.069 22 0.012 36 0.056 499 0.002 884 0.003 195 0.000 134 0.001 58 0.000 096
    Jv.25-24 617 7.8 2.35 0.392 65 0.093 28 0.232 313 0.056 511 0.004 732 0.000 067 0.013 732 0.005 997
    Jv.25-26 1 396 5.7 1.55 0.043 08 0.004 35 0.030 37 0.001 683 0.003 118 0.000 085 0.001 072 0.000 063
    Jv.25-28 499 2.4 2.26 0.078 54 0.013 72 0.063 404 0.003 844 0.003 751 0.000 242 0.002 487 0.000 207

    Table 2.  U-Pb dating data for Kashan plutonic rocks

4.   RESULTS
  • Major element contents of the representative samples from the Kashan plutons are presented in Table 1. In the R1-R2 diagram revised by De la Roche et al. (1980), the plutonic rocks plot in the gabbro, gabbroic diorite, diorite, monzodiorite, tonalite, granodiorite, and granite fields (Fig. 4a). Modal classification of these rocks shows gabbro to granite compositions are compatible with the R1-R2 diagram (Fig. 4b). These rocks show a sub-alkaline affinity and plot in the calc-alkaline field (Fig. 4c). On the K2O vs. SiO2 diagram, the felsic and intermediate samples plot in the medium to high-K calc-alkaline fields whereas the mafic samples belong to the tholeiitic series (Fig. 4d). According to the ASI [molar Al2O3/(CaO+K2O+Na2O)], varying from 0.68 to 1.02, on the A/CNK diagram, these rocks plot as metaluminous and indicate Ⅰ-type nature (Figs. 4e4f).

    Figure 4.  (a) Chemical classification R1 versus R2 diagram (De la Roche et al., 1980) and (b) modal classification QAPF diagram (Streckeisen, 1978) distinguish mafic to felsic compositions for the Kashan plutonic rocks; (c) AFM diagram (Irvine and Baragar, 1971) for identification of tholeiitic-calc-alkaline affinity of the plutonic rocks from the studied area; (d) K2O versus SiO2 diagram (Peccerillo and Taylor, 1976); (e) A/NK versus A/CNK diagram (A/NK=molar Al2O3/(Na2O+K2O) and A/CNK=molar Al2O3/(CaO+Na2O+K2O)) (Shand, 1943); (f) Na2O versus K2O diagram with the separation I-S type granites (Chappell and White, 1974).

  • U-Pb zircon method provides ages for five samples from the Kashan plutons. They include microdiorite (Jh.5), tonalite (Jv.2), granodiorite (Gh.49), monzodiorite (G2.19) and granite (Jv.25). Locations of the analyzed samples are shown in Fig. 2.

    The zircon prismatic crystals have variable lengths from ~100 to ~300 μm. Most grains exhibit weak to moderate magmatic oscillatory zoning in CL images (Fig. 5). The results of LA-ICP-MS analyses are summarized in Table 2 and are presented in Fig. 6. The errors on individual analyses are cited as 1σ. About 30 zircon grains were mounted from the microdiorite sample (Jh.5), and 17 points were ablated for dating. These data points yield a lower intercept at 35.20±0.71 Ma (Priabonian), (decay constant error included: MSWD=1.4, probability of fit=0.21) (Fig. 6a). The Jv.2 was taken from a tonalite sample, and 20 zircon grain analyses yield a lower intercept at 18.90±0.84 Ma (Burdigalian); MSWD=1.01, probability of fit=0.45 (Fig. 6b). Twenty-four zircon grains were selected from a granodiorite sample (Gh.49) and provided 20 data points that define a lower intercept age of 19.26±0.83 Ma (Burdigalian); MSWD=0.74, probability of fit=0.78 (Fig. 6c). Sample Jv.25 was collected from a granite. Fourteen data points were provided from 67 separated zircon grains and yield a lower intercept at 19.30±1.2 Ma (Burdigalian); MSWD=1.13, the probability of fit=0.33 (Fig. 6e). Finally, five zircons were selected from a monzodiorite sample (G2.19) that yields a lower intercept age of 17.3±1.8 Ma (Burdigalian), (decay-constant error included: MSWD=1.18, the probability of fit=0.33) (Fig. 6f). Most of the data points are not concordant, probably because the zircons are young and have very low 207Pb contents. It is difficult to measure the 207Pb/235U ratios by LA-ICP-MS technique and the 207Pb/235U ages obtained are not reliable, making the data points away from the concordia curve. Therefore, in this study, we used the 206Pb/238U ages for the young zircons (usually younger than 1 000 Ma).

    Figure 5.  Zircon crystals in microdiorite (a), tonalite (b), granodiorite (c), granite (d), and monzodiorite (e) dated by U-Pb. The white bars are 200 μm long.

    Figure 6.  Concordia diagrams for zircons from the Kashan plutons. See text for discussion on the diagrams.

5.   DISCUSSION
  • According to previous dating investigations on the plutonic bodies in the central part of UDMA (Table 3 and yellow box in Fig. 1), Tertiary calc-alkaline plutonism happened along the Neotethyan subduction at ~53–17 Ma. The oldest Tertiary arc magmatism in the central part of UDMA is related to the Tafresh region at 54.7±3.1 Ma (Verdel et al., 2011), the Ardestan at 53.9±0.5 Ma (Sarjoughian and Kananian, 2017) and the northwest of Kashan (Mozvash) at 50.1±0.45 (Honarmand et al., 2013). Chiu et al. (2013) reported that most of calc-alkaline plutonism happened during the Miocene in the central and southern parts of UDMA and suggested that the geochemical composition of magmatism started changing from calc-alkaline to adakitic affinity with the collision between Arabia and Eurasia in southeastern part at ~10 Ma. In the northern UDMA, the end of subduction-related calc-alkaline plutonism is reported for Turkey (such as Pontides) in the Early Paleocene (Sahin et al., 2004) and for Arasbaran (NW Iran) in the Oligocene (Aghazadeh et al., 2011). In contrast, in the southern UDMA, the end of magmatism is reported for the Anar-Sirjan region (Chiu et al., 2013) and the Kerman region (Shafiei et al., 2009) at ~6–10 and ~8 Ma, respectively. Also, Chiu et al. (2013) considered the Middle Miocene adakitic magmatism (~16 Ma; in Kashan) as the final phase of arc magmatism in the central UDMA which attributed to the collision-related crustal thickening. There are various viewpoints on the timing for collision of Arabian and Central Iranian continental crusts, for instance Late Cretaceous (Mohajjel and Fergusson, 2000; Alavi, 1994), Eocene (Hempton, 1987), Late Eocene (Dargahi et al., 2010), Eocene–Oligocene (Horton et al., 2008; Hooper et al., 1994), Oligocene (Yilmaz, 1993), Oligocene–Miocene (Agard et al., 2005; Berberian et al., 1982), Middle Miocene (Chiu et al., 2013; Azizi and Moinvaziri, 2009; Sen et al., 2004; McQuarrie et al., 2003) and Pliocene (Stöcklin, 1968). Chiu et al. (2013) and Simmonds et al. (2017) mentioned that the end of arc magmatism in the UDMA occurred continually from the NW to the SE and suggested that this southeastward end of the magmatism was related to the oblique and therefore diachronous collision of Arabia and Eurasia along the Zagros suture zone. The results of these studies show that the age of calc-alkaline plutonism and the timing of collision stepped down to the southeastward of the UDMA.

    Sample No. Locality Rock type Age (Ma) References
    10-IR-ZS-106 Qom Granodiorite 17.4 Chiu et al. (2013)
    08-IR-SZ-43 Kashan Diorite 16.9
    08-IR-SZ-42 Microdiorite 33.4
    HN5-5 Niyasar Microdiorite 50.1 Honarmand et al. (2013)
    HN2-66 Quartzdiorite 18.2
    HN5-15 Tonalite 18.2
    HN5-45 Tonalite 17.9
    08-IR-SZ-32 Natanz Gabbro 21.2 Chiu et al. (2013)
    08-IR-SZ-36 Granite 19.6
    08-IR-SZ-31 Granodiorite 20.2
    C-15-3 Nodoushan Diorite porphyry 24.1 Shahsavari Alavijeh et al. (2017)
    C-15-5 Granite 30
    C-15-6 Granite 40.4
    C-15-2 Diorite 30.5
    C-15-4 Granodiorite 24.4
    DK Ardestan Diorite 53.9 Sarjoughian and Kananian (2017)
    GK Granodiorite 51.1
    MH Granodiorite 36.8
    ZF Granodiorite 24.6
    FS Granodiorite 20.5
    MA11 Granodiorite 24.6 Babazadeh et al. (2017)
    HZ4a Tonalite 20.5
    FK1a Tonalite 22.3
    a Rb-Sr age for biotite and feldspar from the Ardestan tonalitic rocks.

    Table 3.  Summary of zircon U-Pb ages for plutonic rocks from the central part of UDMA

    The volcanic rocks in the UDMA, with large and widespread outcrops, mostly formed during the Eocene–Oligocene (Chiu et al., 2013; Omrani et al., 2008; Shahabpour, 2007; Emami, 2000; Berberian and King, 1981; Farhoudi, 1978). In the Kashan region, these rocks are related to Middle–Late Eocene calc-alkaline andesites (~30 Ma) and Middle Miocene adakitic andesites (~16 Ma) (Chiu et al., 2013). Most of the plutonic activities were related to Early Miocene in the UDMA, specifically in its central and southern parts (Chiu et al., 2013; Berberian and Berberian, 1981; and the present study). Although the peak of the magmatism (dominantly volcanism) was related to the Eocene–Oligocene time, the plutonism in the Kashan region appeared to be more intense in the Early Miocene than the Eocene–Oligocene. There is a magmatic gap in the Oligocene time in the Kashan region. Omrani et al. (2008) suggested a magmatic inactivity in the UDMA from the Oligocene to Early Miocene. They concluded that the collision between Arabia and Eurasia started at that time and suggested that the younger Neogene magmatism was related to slab break-off and thermal re-equilibration in a post-collisional setting. Agard et al. (2011) proposed the Oligocene time as an inactive stage of strongly reduced or no magmatism and suggested that the Late Paleocene–Eocene experienced a magmatic shift from the SSZ to UDMA and an increase in magmatic activity, probably as a consequence of restart of oceanic slab subduction below the Iranian Plate. Ghorbani et al. (2014) concluded that the Oligocene–Miocene basic magmatism in the central UDMA was related to the asthenospheric mantle upwelling caused or followed by a slab rollback in a subduction-related mantle wedge and considered the concomitant adakitic magma of Miocene age as further evidence for the slab rollback model. Therefore, based on the mentioned geological studies and zircon U-Pb ages introduced in this study (~17–35 Ma; i.e., Late Eocene–Early Miocene), it seems that the Oligocene magmatic gap has probably been related to slab rollback in the Kashan region. The subduction-related calc-alkaline magmatism in this region has continued with the resumption of igneous activities in the Early Miocene and adakitic magmas formed as final products of collisional magmatism in the Middle Miocene as a result of slab break-off. The crustal thickening model for the genesis of adakitic rocks, immediately after the Early Miocene magmatic activity seems to be unreasonable. Therefore, we suggest a slab break-off model for adakites in the Kashan region which happened in the Middle Miocene due to the collision of the Arabian and Central Iranian continental crusts.

    The results of this study show a range of age for the plutonism from Early Eocene to Early Miocene similar to earlier studies on similar rocks in the central part of UDMA. These results outline more accurately the Neotethyan subduction- related magmatic evolution from the Eocene to Miocene times. The geochronological studies in the Kashan region represent two distinct plutonic episodes associated with the development of Tertiary magmatism in the Kashan plutons during subduction of Neotethys oceanic lithosphere beneath the Iranian continental crust. The first plutonic event is related to Late Eocene magmatism responsible for emplacement of intermediate rocks (microdiorite) at ~35 Ma, simultaneously with the peak of UDMA magmatism. The second one is more widespread and voluminous, and isassociated with Early Miocene magmatism and related to emplacement of felsic rocks such as tonalite, granodiorite, granite (GG) and also monzodiorite at ~17–19 Ma.

  • The UDMA demonstrates geochemical features comparable to those of Andean or Cordilleran type active continental margins such as the Eastern Pontides in Turkey or the Sierra Nevada plutons in the United States (Dewey et al., 1973). Based on the data for plutonic rocks in the west of Kashan (NW of our study area), Honarmand et al. (2014) suggested that the plutonic rocks formed from a melt with a combination of ~60%–70% of a lower crust-derived melt and ~30%–40% of sub-continental lithospheric mantle melt. Using geochemical evidence for the GG, Ghasemi and Tabatabaei Manesh (2015) showed that the initial magma which formed these rocks was the result of partial melting of a lower crust that mixed with mantle-derived melt and produced a hybrid magma. Based on bulk rock Sr-Nd isotopes, Babazadeh et al. (2017) proposed that the plutonic rocks of South Ardestan resulted from a mixture of ~94% mantle-derived melt and ~6% crustal rocks. Kananian et al. (2014) suggested the formation of Kuh-e-Dom granitoid (NE Ardestan) as interaction processes of lower crust- derived and mantle-derived melts in the Nain region. Haschke et al. (2010) demonstrated similar isotopic signature for Early Miocene granitoid in Natanz and suggested an interaction of mantle-dominant melts with less contribution from the Cadomian crust. Verdel et al.(2011, 2007) introduced a magmatic "flare up" for the Eocene–Oligocene magmatic peak which is common in extensional and/or post-collisional settings.

    The wide range of U-Pb ages in the Kashan plutonic rocks shows the lack of genetic relationship between the older microdiorite and younger felsic rocks. However, our data for the central UDMA including the Kashan plutons demonstrate that there are no substantial differences among Eocene–Miocene igneous rocks in Iran. The collected data confirm contributions from continental crust and mantle melts to produce UDMA melts. The comparison with other calk-alkaline plutons suggests that the events related to the evolution of the UDMA during subduction of the Neotethyan might have commenced from the Early Eocene until Early Miocene time. Also, the activity reported from the Kashan plutons shows the final phases of subduction-related magmatism before the final collision of the Arabia and Eurasia. Moreover, our U-Pb zircon ages of ~17–35 Ma for the Kashan pre-collisional Ⅰ-type granitoids imply that the Arabian-Eurasian continental collision related to the Neotethys subduction likely occurred later than the Early Miocene.

6.   CONCLUSIONS
  • The results of major element analyses and U-Pb dating of plutonic rocks from the Kashan region lead to the following conclusions.

    Petrographic and whole-rock geochemical data of the Kashan plutonic rocks have an extended compositional and lithological range from gabbro to granite that shows metaluminous, Ⅰ-type and medium- to high-K calc-alkaline affinity.

    Zircon U-Pb dating reveals two main episodes of plutonism in the Kashan region at the Late Eocene (~35 Ma) (Priabonian) and the Early Miocene (~17–19 Ma) (Burdigalian). The intermediate rocks emplaced in the Late Eocene, matching with the highest subduction-related igneous activity in Iran. In the Early Miocene, the felsic rocks emplaced as the final products of subduction-related calc-alkaline magmatism.

    The results of this study show a more precise view on the evolution of subduction-related calc-alkaline plutonism in the central part of UDMA which must have initiated since the Early Eocene until at least Early Miocene times.

    The Early Miocene plutonism happened during the change from subduction-related to a collisional setting in the UDMA and before the occurrence of adakitic magmatism in the Middle Miocene.

    According to zircon U-Pb ages of ~17–35 Ma, calc- alkaline affinity and pre-collisional signature, the subduction- related magmatism reached its peak in Eocene to Miocene much longer than perceived before just occurring in Eocene. So, a most likely Middle Miocene time is proposed for the start of the continental collision between Arabia and Eurasia.

ACKNOWLEDGMENTS
  • This manuscript comes from the first author's Ph.D. thesis at Tarbiat Modares University (TMU), Tehran, Iran. The TMU Research Grant Council funded field studies. This study is partly supported by grants from the University of Science and Technology of China, Hefei, and Peking University, Beijing. Badieh Shahsavari Alavijeh is thanked for her help in the manuscript preparation. We would like to acknowledge two anonymous reviewers for their constructive reviews on the manuscript. Ge Yao is appreciated for careful editorial handling of the manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1258-1.

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