Introduction

The Neotethys orogenic belt was developed as a result of geodynamic processes in the Mesozoic and Cenozoic eras and includes several phases of subduction, abduction, microplate accretion, continent–continent collision, and exhumation (Dercourt et al. 1986; Hafkenschied et al. 2006). The Jurassic northeastward subduction of the Neotethys occurred beneath the eastern European (i.e., Eurasian margin caused continuous active arc magmatism along the eastern Pontides, the Lesser Caucasus, and the Sanandaj–Sirjan Zone (SaSZ)) (Kazmin et al. 1986; Ustaömer and Robertson 1999; Davoudian et al. 2016). The SaSZ is a narrow belt of highly deformed and metamorphosed rocks in the Zagros orogeny with NW–SE structure trend, associated with abundant deformed and undeformed plutons, as well as widespread Mesozoic volcanics (Eftekharnejad 1981; Berberian and Berberian 1981; Mohajjel and Fergusson 2000; Babaie et al. 2001; Mohajjel et al. 2003; Azizi and Jahangiri 2008; Shahbazi et al. 2010; Azizi et al. 2011; Mahmoudi et al. 2011; Esna-Ashari et al. 2012; Azizi et al. 2014; Azizi et al. 2015a, b; Azizi et al. 2016; Davoudian et al. 2016; Hassanzadeh and Wernicke 2016; Azizi et al. 2018a, b). The volcanic rocks of the marginal subzone are interpreted to represent volcanic rocks that accumulated in a forearc basin located along the southwestern margin of the Urumieh-Dokhtar magmatic arc (Alavi 1994). These volcanic rocks are interbedded with detrital sediments such as black shale, sandstone, and sandy limestone (Zahedi et al. 1992). Kazmin et al. (1986) believed that the Mesozoic volcanic rocks of the SaSZ were formed in Jurassic. 49Ar/48Ar dating of the volcanic rocks of Shahrekord indicates 145 to 169 Ma age with calc-alkaline and toleiitic nature that was formed in island arc setting (Emami et al. 2009). Zarasvandi et al. (2015) believed that during ocean–ocean subduction in Jurassic to Cretaceous, an immature island arc developed before the closure of Neo-Tethys Ocean in SSZ, while an intercontinental rifting regime is considered to be the formation environment of the Jurassic rocks in the northern of the SaSZ (Azizi et al. 2018a, b). Also, chemical composition of Panjeh mafic and intermediate rocks, in combination with data for other gabbroic to dioritic bodies in the Ghorveh area, offers two interpretations for these (and other Jurassic igneous rocks of the SaSZ) as reflecting melts from (a) subduction-modified OIB-type source above a Neo-Tethys subduction zone or (b) plume or rift tectonics involving upwelling metasomatized mantle (Azizi et al. 2018a, b). The narrow belt of black and green–colored magmatic rocks in the Lattan Mountain extends NW–SE in the north of Chaharmahal and Bakhtiyari provinces that is parallel to the main Zagros fault and 35 km distanced from it (Fig. 1). There are no other detailed geochemical and isotopic studies on the Lattan Mountain magmatic rocks (e.g., their nature, source, and age). In this paper, petrography, whole-rock geochemistry, and Sr-Nd isotopic ratios are used to constrain Lattan Mountain magma genesis and its tectonic setting.

Fig. 1
figure 1

Geological map of the Zagros orogenic belt (modified from Alavi (2004))

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Geological setting

The Lattan Mountain (LM) is located between the longitude 50° 42′ E to 51° 10′ E and the latitude 32° 45′ N to 32° 08′ N (Fig. 2). It lies within the highly deformed subzone of the SaSZ (Zahedi et al. 1992). The typical lithology of the study area is similar to those of the other parts of the SaSZ which are exposed around the east of Zagros Thrust Fault. In general, the stratigraphical units of the LM can be divided into two main parts that are affected by the Neotethys events: before and after the Permian (Zahedi et al. 1992). The Permian units consist of light to gray thick-bedded and extremely folded dolomite with a thickness of over several tens of centimeters and with inverse fault (Zahedi et al. 1992). Permian layers have a discontinuous erosional boundary with Triassic detrital and carbonate units, and there are some traces of Cu mineralization. The Jurassic units include volcanic rocks with dark gray limestone layers and slight sandstone, and shale that was extended from north to south of Shahrekord. This limestone is folded and displays abundant cracks and fractures, which is covered with Cretaceous limestone and sandy limestone and argillitic limestone. The alternate Miocene-Pliocene units, with various thicknesses from 1 to 2 m, include gray-green marls, gray sandstones, and light gypsum that are located with angular unconformity over older units and are below Quaternary sediments. There are ebony scotella fossils in the limestone layers (Ghasemi et al. 2005). The Quaternary sediments consist of alluvial of clay and silt. In addition, due to Cimmerian orogeny deformation, there are Mesozoic phyllites, schist, and recrystallized limestone. There are metamorphic complexes (metabasite) in the north of the LM that occurred in a-subduction zone setting during late Neoproterozoic to early Cambrian times (Malek-Mahmoudi et al. 2017), while metagranites of the north of LM were mainly produced through mixing of basaltic melts with components similar to metasedimentary source that occurred in Early Paleozoic times after the closure of the Proto-Tethys Ocean (Badr et al. 2018). In the LM, the magmatic rocks are often observed among sedimentary and metamorphic rocks (Fig. 3a). Based on the presence of fossils in the sedimentary rocks, they have been formed in a marine setting in the Jurassic times (Ghasemi et al. 2005). The igneous rocks are found as small to large singular outcrops in the Lattan Mountain area. These magmatic rocks are black to dark gray, green, and gray in color that intrude into the Jurassic to Cretaceous unites (Fig. 3b, c). The volcanic rocks are intermediate to basic terms. The subvolcanic and plutonic rocks are composed of microgabbro, microdiorite, and dolerite as sills and dikes with chilled margins.

Fig. 2
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The geological map of the LM area (Zahedi et al. 1992)

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Fig. 3
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a, b Outcrop of the magmatic rocks in Lattan Mountain. c Pyroxene in volcanic rocks

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In some parts of the LM, malachite, azurite, and hematite, and magnetite mineralization are revealed on the magmatic rocks.

Analytical methods

In this paper, whole-rock analysis for major and trace elements and 87Sr/86Sr and 143Nd/144Nd isotope ratios was performed for 9 samples. The rock samples were crushed to sizes smaller than 74 μm. Ten major and 14 trace elements were analyzed for nine samples by XRF (Shimadzu XRF-1800) at Kwansei Gakuin University. Loss of ignition (LOI) was calculated by weight difference after ignition at 950 °C. The rock powder samples and flux (Li2B4O7) were mixed in proportions of 0.7:6.0 g for major elements and 2.0:3.0 g for trace elements, and the glass beads were prepared for the XRF analysis. As for quantitative analysis of REEs and Sr-Nd isotope analysis, eight of nine samples were prepared through hydrofluoric acid treatment at Nagoya University, Japan. About 100-mg powdered sample was decomposed in HF+ HClO4 in a covered PTFE beaker. After drying, the samples were re-dissolved in 10 ml of 2–4 M HCl, and the resulting solution was split into two aliquots: one is for REE quantitative analysis and the other for the isotope analysis. The REE concentrations were measured by an Agilent 7700x ICP-MS spectrometer at Nagoya University. For Sr and Nd isotope analysis, conventional column chemistry was conducted to isolate Sr and REEs using cation-exchange resin (Bio-Rad AG50W-X8, 200–400 mesh) with an HCl eluent. Neodymium was separated from the extracted REE fraction by another cation-exchange column with α-hydroxyisobutyric acid (α-HIBA) as eluent. The isotope ratios for the eight samples were obtained using thermal ionization mass spectrometers (TIMS), VG Sector 54-30 for Sr and GVI IsoProbe-T for Nd, at Nagoya University. The mass fractionations during the Sr and Nd isotope measurements were corrected based on 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. For the samples analyzed at Nagoya University, NIST-SRM987 and JNdi-1 (Tanaka et al. 2000) were adopted as the natural Sr and Nd isotope ratio standards, respectively. The average and 2σ for isotope ratios standards are NIST-SRM987 = 0.710251 ± 0.000020 (n = 8) and JNdi-1 = 0.512114 ± 0.000002 (n = 6).The Sr and Nd isotopic ratio diagram is illustrated using the GCDkit software (Janoušek et al. 2016).

Petrography

Intermediate to basic volcanic, subvolcanic, and plutonic rocks found in Lattan Mountain include andesite, andesitic-basalt, basalt, dolerite, microgabbro, and microdiorite. The volcanic rocks are composed of plagioclase, pyroxene, amphibole, and olivine as major minerals and biotite, apatite, and opaque as accessory ones. The main texture of the volcanic rocks is porphyritic, hypocrystalline porphyritic, hyalo-porphyritic, microgranular, and hyalo-microlithic porphyritic. The microgabbro and microdiorite have small outcrops as sills and dikes with porphyritic to microgranular texture. Their mineralogical constituents of the rocks consist of plagioclase, clinopyroxene, amphibole, and olivine as the major minerals and biotite, apatite, and opaque as accessory minerals. Olivine is mostly observed as anhedral and/or as corroded crystals. The clinopyroxene is mostly observed as microphenocryst and phenocryst. Plagioclase and clinopyroxene minerals are almost fresh, while orthopyroxene minerals are replaced by smectite-chlorite ones (Fig. 4a). The plagioclases are mostly subhedral. The megacryst and phenocryst of the plagioclase exhibit polysynthetic twinning (up to 60%). The basalts consist of plagioclase laths, olivine, and pyroxene glomeroporphyritic aggregations embedded in a glass-poor groundmass that contains small rounded vesicles filled with smectite + epidote + chalcedony. The size of the plagioclase laths ranges from 0.1 to 1 mm (Fig. 4b). The andesites have a number of plagioclase grains with “fritted” rims with a various widths from millimeters to centimeters. Fritted or sieve textures in plagioclase formed in oxidation condition of the subduction setting (Dwijesh et al. 2011). The plagioclase is the main mineral as phenocryst which is mostly replaced by epidote, and clay minerals in the matrix (Fig. 4d–f). Amphibole is common (modal abundances up to 6%). The dolerites, with ophitic to intersertal textures, are composed of plagioclase laths enclosed in anhedral to subhedral clinopyroxene. The clinopyroxene and Fe–Ti oxide occur as large crystals up to 1 and 0.5 cm, respectively (Fig. 4f). The amphibole crystal contains opaque rims. The amphibole is completely pseudomorphed by finely crystalline opaque minerals that indicate oxidation condition and high oxygen fugacity (Popp et al. 2006). There is evidence of the low-grade zeolite and prehnite–pumpellyite facies metamorphism of the igneous rocks while their texture is preserved.

Fig. 4
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Photographs of the LM. a Pyroxene phenocryst in microgabbro (XPL). b Coarse-grained olivine in basalt (XPL). c Phenocryst plagioclase and epidote in andesitic basalt (XPL). d Secondary epidote in andesite. e Plagioclase in andesite (XPL). f Cpx in andesite (XPL). Mineral abbreviations are from Whitney and Evans (2010)

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Results

Whole-rock geochemistry

Whole-rock geochemical compositions for the 9 samples of the LM magmatic rocks are presented in Table 1. Most of analyzed rocks show SiO2 in a range of 46–55 wt%; however, minor dacitic rocks are observed (i.e., sample La5, SiO2 66.9 wt%). Based on the total of alkalis vs. silica classification diagram of TAS diagram (after Le Bas 2000), the LM rocks plot dominantly in the fields of basalt, basalt andesite, and basaltic trackyandesite and dacite (Fig. 5a). In the AFM diagram (Irvine and Baragar 1971), the Lattan Mountain samples plot in the calc-alkaline domain (Fig. 5b). The Harker diagrams (Harker 1909) show almost negative correlations between SiO2 and Al2O3, Fe2O3, TiO2, CaO, MnO, MgO, Na2O, and K2O (Fig. 6). Also, the Lattan Mountain igneous rocks show markedly decreasing values of La, Sr, Eu, Rb, Ba, Zr, Y, and Yb with increasing SiO2 content (Fig. 7). The chondrite-normalized (Boynton 1984) REE patterns of the samples show LREE enrichment (Fig. 8a). The samples show weak negative Eu anomalies (Eu/Eu* = 0.27–0.33). In the primitive mantle-normalized spider diagram (McDonough and Sun 1995), all of the samples display clear enrichment in Rb, Pb, Sr, and Y and variable depletion in Ba, Th, Zr, and Ti (Fig. 8b). In the diagrams of Th vs. Yb (Barrett and Maclean 1999) and La vs. Yb (Ross and Bédard 2009), the samples were plotted in calc-alkaline to transitional fields (Fig. 9a, b). In Zr/Al2O3 vs. TiO2/Al2O3 and La/Yb vs. Th/Yb diagrams (Condie 1989), most of the Lattan Mountain magmatic rocks distribute in the field of arc-related setting (Fig. 10a).

Table 1 Whole-rock composition of LM samples
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Fig. 5
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Chemical classification diagrams for the LM samples: a Le Bas (2000). b AFM diagram (Irvine and Baragar 1971). Black circle, mafic and intermediate samples; red circle, dacite sample

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Fig. 6
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Harker (1909) diagrams of LM samples. Black circle, mafic and intermediate samples; red circle, dacite sample

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Fig. 7
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Harker diagrams of the LM samples (Harker 1909). Black circle, Mafic and intermediate samples; red circle, dacite sample

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Fig. 8
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a Chondrite-normalized REE patterns (Boynton 1984). b Primitive mantle-normalized extended trace element spider patterns of the LM samples (McDonough and Sun 1995). Black circle, mafic and intermediate samples; red circle, dacite sample

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Fig. 9
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For the LM samples: a Th vs. Yb diagram (Barrett and MacLean 1999); b La vs. Yb diagram (Ross and Bédard 2009). Black circle, mafic and intermediate samples; red circle, dacite sample

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Fig. 10
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Tectonic setting discrimination diagrams showing arc-related tectonic setting for these rocks. a Zr/Al2O3 vs. TiO2/Al2O3. b La/Yb vs. Th/Yb (Condie 1989)

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LOI loss on ignition; Eu/Eu* = (Eu)N/((Sm)N × (Gd)N)1/2 (after McLennan 1989)

Sr-Nd isotope geochemistry

The whole-rock Rb-Sr isochron diagram for the LM rocks is shown in Fig. 11a and b. All of the plots showed a clear isochron with an age of 152 ± 37 Ma and an initial ratio of 0.7048 (MSDW = 0.19). Isochron samples without dacite showed an age of 152 ± 14 Ma and an initial ratio of 0.7048 (MSDW = 0.19). Isotopic data of the igneous rocks in the LM area at 152 Ma displayed homogeneous values for the initial isotope ratios: εNdt values were from − 0.08 to + 2.19 and Sr isotope ratios (87Sr/86Sr)i were between 0.7047 and 0.7051 (Table 2). In the 143Nd/144Nd–87Sr/86Sr diagram, all of the samples were plotted in the mantle array field extending to the BSE nature (Fig. 12).

Fig. 11
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Whole-rock Rb-Sr isochron diagram for the LM rocks. a 8 samples without dacite. b All of samples. Black circle, mafic and intermediate samples; red circle, dacite sample

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Table 2 Sr and Nd isotopic compositions of the LM samples
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Fig. 12
figure 12

εNdi vs. 87Sr/86Sr diagram for the Lattan Mountain samples (DePaolo and Wasserburg 1976). DM, depleted mantle; PREMA, primary mantle; EMI, enriched mantle (type I); EMII, enriched mantle (type II); HIMU, anomaly high 238U/204Pb mantle; BSE, bulk silicate earth; PREMA, prevalent mantle; black circle, mafic and intermediate samples; red circle, dacite sample

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The Nd and Sr natural isotope ratios were normalized based on the 146Nd/144Nd and 86Sr/88Sr = 0.1194. The average and 2σ for isotope ratios standards are NIST-SRM987 = 0.710251 ± 0.000020 (n = 8) and JNdi-1 = 0.512114 ± 0.000002 (n = 6). The CHUR (chondritic uniform reservoir) values, 143Nd/144Nd = 0.51247, were used to calculate the εNd. εNd(t) (DePaolo and Wasserburg 1976) were calculated based on the following: (143Nd/144Nd)TCHUR = 0.512638–0.1967(eλT − 1)

Discussion

The LM igneous rocks are basic to intermediate, with a feature of calc-alkaline series. The major element trends for Al2O3, Fe2O3, TiO2, CaO, and MgO in the Harker diagrams had a negative correlation, which indicate to chemical evolution of magma by crustal contamination. Also, Sr, Rb, Ba, Y, Zr, Lu, and La behavior ratio to SiO2 indicates alteration and an incompatibility of the element as the magma differentiation increases (Barclay and Carmichael 2004; Moore and Carmichael 1998; Sisson and Grove 1993). The Sr, Rb, Pb enrichment indicates presence a subducted oceanic plate for the occurrence of the magmatic rocks (Wang et al. 2016). In Zr/Al2O3 vs. TiO2/Al2O3 and La/Yb vs. Th/Yb diagrams (Condie 1989), most of the Lattan Mountain magmatic rocks distribute in the field of arc-related setting (Fig. 10a). There are three potential sources for magma production in the arc magmatism increasing LREE relative to HREE, including (1) low partial melting of the mantle wedge source (Almeida et al. 2007), (2) crustal contamination of the magma (Almeida et al. 2007), and (3) released fluid or melt of slab (Winter 2001). The released melt and aqueous fluids from subducted slabs can enrich the mantle wedge by metasomatism (Kepezhinskas et al. 1995; McInnes et al. 2001; Pearce and Peate 1995; Rapp et al. 1999). The Th/Yb vs. Th and La/Yb vs. La diagrams show that partial melting is a dominant process in magma generation. It is confirmed by depletion of Ti with respect to other HFS elements that can be explained by derivation from a source with small-degree partial melts. The behavior Th in the diagram (Fig. 7a) shows an increase of the Th with the increase of the sample silica that due to sedimentary of the subducted slab (Gorton and Schandl 2000). In addition, Th behavior is due to high separation coefficient values (0.15) by amphibole in the andesitic melt (Rollinson 1993). Low Sr/Y ratio (8.81–68.54 ppm) indicated to mantel wedge as the major factor in the magmatic source (Munker et al. 2004) and low Zr/Y < 3 indicated to arc island as geology setting (Pearce and Norry 1979). Also, the Nd/Pb (< 10) and Ce/Pb (< 10) indicate presence of the slab-derived fluids (Bonev and Stampfli 2008). The La/Yb > 6 shows calc-alkaline to transitional nature of the LM magmatic rocks (Barrett and MacLean 1999). According to the Sr-Nd data, these rocks are formed by a magma source similar to BSE. All of the data show that LM igneous rocks are formed by subduction zone in 152 Ma years ago. This age is correlated with the magmatism of Jurassic–Cretaceous SaSZ. Arc magmatism is the most distinctive component in the SaSZ and includes voluminous calc-alkaline plutons and volcanic rocks, mainly at Jurassic age around 170 Ma (Hassanzadeh and Wernicke 2016). Azizi and Asahara (2013) attributed the temporal cessation and spatial shift in magmatism to a Jurassic arc-continent collision. These authors suggested that the intra-oceanic forearc is no longer present because it was removed by subsequent tectonic erosion during Cenozoic subduction, continental collision, and strike-slip faulting (Mohajjel and Fergusson 2000). Yajam et al. (2015) suggest that the calc-alkaline I-type to alkaline A-type transition in the SSZ was the result of a change from compressional subduction and arc collision to extensional rifting. As mentioned above, Azizi and Asahara (2013) suggested that an island arc collided with the SaSZ in the Late Jurassic. As a result of any of the suggested processes, the Late Jurassic calc-alkaline reflects a perturbation of the northeastward subduction of Neotethys beneath the Iranian sector of Eurasia, Laurasia.

Conclusion

Volcanic (basalt, andesite, basaltic andesite, and dacite), subvolcanic (dolerite), and plutonic (microdiorite and microgabbro) rocks in LM show calc-alkaline to transitional affinity. Isotopic composition of samples is similar to mantel array magmas with the affinity to BSE. LM magmatic rocks were formed by subduction process and the closure of the Neotethys ocean plate at around 152 Ma similar to other parts of the SaSZ such as the north of SaSZ in the arc setting. The dominant process for occurrence is partial melting of mantle wedge by released fluid or melt from sedimentary rocks.