1 Introduction

Tarom is a magmatic belt, with an NW–SE trend within the Alborz orogenic belt, NW Iran. This belt is characterized by intensive magmatism during the Eocene. Among the volcanic and volcaniclastic rocks, acidic and intermediate rocks are volumetrically dominant, with subordinate basic rocks. These volcano-sedimentary rocks of Eocene age are known as Karaj Formation (KF). Some intrusions have intruded into the volcanic and volcaniclastic units, mainly trending NW–SE.

The Tarom intrusions range in composition from quartz monzo-gabbro to diorite, quartz monzodiorite, monzonite, quartz monzonite, monzogranite, granodiorite, alkali granite, granite and syeno-granite (e.g., Nabatian et al. 2014, 2016; Esmaeli et al. 2015; Mehrabi et al. 2016; Kouhestani et al. 2017, 2018; Yasami et al. 2018). These I-type intrusions have high-K calc-alkaline to shoshonitic affinity and metaluminous characteristics (Irannezhadi et al. 2007; Azizi et al. 2009; Nabatian et al. 2014, 2016; Yasami et al. 2018) and show enrichment in Large Ion Lithophile Elements (LILE) and Light Rare Earth Elements (LREE) and negative anomalies in High Field Strength Elements (HFSE) and depletion in Heavy Rare Earth Elements (HREE) (Nabatian et al. 2014, 2016; Yasami et al. 2018). The Tarom plutons show subduction-related arc magmatic signatures (Nabatian et al. 2016).

The contribution of juvenile sources in the petrogenesis of the Tarom intrusions was indicated by isotope evidence. Tarom plutons parental magma derivation from the sub-continental lithospheric mantle source was suggested by geochemical and isotopic data, which were fluids and melts metasomatized from the subducted Neotethyan slab with a minor crustal contribution. Decompression melting in the final stage of Neotethyan subduction through slab rollback or shortly after the cessation of arc magmatism was created by subsequent hot asthenospheric upwelling and lithospheric extension (Nabatian et al. 2016).

Chargar area is located in the southern border of Tarom magmatic belt. There are some small intrusions in the area which are exposed in the southern and western parts of the Chargar Cu–Au deposit. In this study, two different types of intrusions were identified in the area: (1) Intrusions in the south of Chargar deposit consisting of gabbro, pyroxene quartz monzodiorite, quartz monzodiorite and quartz syenite (Type-I). (2) The intrusion in the west of the Chargar deposit with gabbro and gabbro-diorite composition (Type-II). The Type-II intrusion is mineralized and hosts disseminated chalcopyrite, pyrite, and galena, while the Type-I intrusions do not contain any mineralization.

These two types of intrusions have not previously been reported or studied. This paper presents petrological and geochemical characteristics of the barren Type-I intrusions and the first-time reported mineralized Type-II mafic intrusion in the Tarom in order to determine their petrogenesis.

2 Regional setting

Alborz Mountains are regarded as part of the Tethyan Eurasian magmatic–metallogenic belt. Eocene normal faults have been recognized in the Alborz Mountains (Guest et al. 2006), and stratigraphic evidence of Eocene subsidence in the Alborz Mountains have been explained in connection with Eocene extension (Brunet et al. 2003; Hassanzadeh et al. 2004; Vincent et al. 2005; Guest et al. 2007; Morley et al. 2009). The Eocene Karaj Formation in the Alborz Mountains comprised of volcanic, volcaniclastic, and sedimentary strata. Eocene units were intruded by plutons of mafic to acidic compositions. A 40Ar/39Ar age of 37.2 ± 0.38 Ma was yielded from biotites in intruded gabbros inside the Karaj Formation in the east–northeast Tehran (Verdel et al. 2011). These late Eocene gabbros show typical arc characteristics, especially noticeable Nb depletions (Verdel et al. 2011), perhaps the single most unique trace element, typical of arc magmatism (Gill 1981).

The Tarom belt is bounded by the Roudbar and Soltanieh faults to the north and south, respectively. Eastern and northwestern parts of the Tarom run to central Alborz and Azerbaijan (Arasbaran) zones, respectively (Castro et al. 2013). Rock units in the Tarom belt mainly consist of Paleogene volcanic rocks including lava flows and pyroclastic rocks (Eocene Karaj Formation) which are intruded by late Eocene intrusive bodies (Castro et al. 2013; Nabatian et al. 2014, 2016). Intrusions are widespread throughout the Tarom subzone forming two sets of elongated NW–SE trending bodies in the north and the south.

Chargar area is located in the southern part of the Tarom magmatic–metallogenic belt within the Alborz orogenic belt (figure 1). This area is made up of volcanic and volcaniclastic rocks of Eocene age (KF) mainly with intermediate to acidic composition. Some small intrusions were noticed in the volcanic and volcaniclastic rocks. These intrusions crop out in the southern (Type-I) and western (Type-II) parts of the Chargar area (figure 2).

Figure 1
figure 1

(modified after Hirayama et al. 1965, 1966).

(a) Simplified structural map of Iran, showing the location of the Chargar area within the Cenozoic Alborz magmatic belt; (b) Generalized geological map of the Tarom magmatic belt and location of Chargar Cu–Au deposit. Based on 1:250,000 geological map of Zanjan

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Figure 2
figure 2

(modified after Hosseini et al. 2017).

Geological map of the Chargar area, revised and modified based on the new field data from 1:100,000 geological map of Abhar

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The volcanic and volcaniclastic rocks in the Chargar area consist of rhyolitic to basaltic units. Recent work has shown that they have calc-alkaline to high-K calc-alkaline and shoshonitic affinity (Mousavi Motlagh and Ghaderi 2019).

3 Chargar intrusions

The south Chargar intrusions are represented by gabbro, pyroxene quartz monzodiorite to quartz monzodiorite and quartz syenite (figure 3a). The pyroxene quartz monzodiorite intrusion is intruded by quartz syenite and cut by aplitic veins and dikes (figure 3b). The quartz syenite has a thermal effect on the pyroxene quartz monzodiorite. A common feature of the quartz syenite intrusion is its morphology (curvy outcrop), along with brecciation of adjacent host rocks and their invasion by aplitic veins and dikes (figure 3c). Interpenetrating relations include possible diapiric intrusion of the quartz syenite into the pyroxene quartz monzodiorite bodies (figure 3d). The quartz syenite forms a distinct topographically high area relative to pyroxene quartz monzodiorite (figure 3e). These field observations suggest that the quartz syenite intrusion is younger than gabbro–pyroxene quartz monzodiorite–quartz monzodiorite.

Figure 3
figure 3

Pictures of the selected studied outcrops of intrusive rocks. (a) Field outcrops of quartz syenite and pyroxene quartz monzodiorite in Chargar area (view to north). (b) Aplite vein cross-cutting the pyroxene quartz monzodiorite. (c) Aplite veins cross-cutting the quartz syenite. (d) The intrusion of the quartz syenite into the pyroxene quartz monzodiorite (view to the northeast). (e) The quartz syenite is topographically higher than pyroxene quartz monzodiorite (view to the north). (f) The Chargar west gabbro – gabbro-diorite intrusion (view to the northeast). QSy: Quartz syenite; PQMD: Pyroxene quartz monzodiorite; Gdi: Gabbro-diorite.

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The Chargar west mineralized intrusion consists of gabbro – gabbro-diorite. This intrusion intruded into the Karaj Formation and has experienced alteration (figure 3f).

4 Materials and methods

Fresh and unaltered rocks from Chargar area were collected for petrographic studies and geochemical analysis of major and trace elements using X-Ray Fluorescence (XRF), Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES), and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) techniques. Petrographic studies on 63 microscopic thin sections and XRF analyses on 12 samples were performed at Tarbiat Modares University, Tehran, Iran. Bulk rock major elements of the Chargar samples were analyzed by a Philips PW2404 XRF. It utilizes a rhodium target X-ray tube and a 4 kW generator to provide the primary X-ray beam. Analytical uncertainties were generally better than ±5% (2r). Whole-rock major elements were also analyzed by ICP-OES on 14 samples and trace and rare earth elements by ICP-MS on 25 samples at the Zarazma Company, Iran. The samples were carefully cut into rock chips and were crushed after removal of weathering rinds. Fresh chip samples from different rock types were powdered to 200 meshes using a jaw crusher, hand-picked, and then powdered by low chrome steel disc mill. Then, they preserved for analyses after being dried in an oven at 1000°C. The major elements were analyzed using ICP-OES after fusion with lithium tetraborate and fluxmeta in a platinum crucible and dissolution by nitric acid, respectively. Analytical accuracy is generally 1 ppb for REE and trace elements. Loss On Ignition (LOI) was determined separately by routine procedures. Powdered samples were dissolved using HF, HCl, HClO4 and HNO3 mixture in Teflon beakers for 4 hr at 220°C, followed by dryness at ambient temperature. An internal standard solution was added and the spiked dissolutions were diluted with distilled water. The internal standard solution that is approved by REE standards was utilized for monitoring the signal shift during trace element measurements using an Agilent series 4500 ICP-MS. The precision of the ICP-MS measurements was generally better than 3% (2r) for most trace elements.

5 Results

5.1 Petrography

5.1.1 Type-I

The southern intrusions consist of gabbro, pyroxene quartz monzodiorite to quartz monzodiorite and quartz syenite.

Gabbro: The gabbro shows granular and sub-ophitic textures including plagioclase and clinopyroxene. Anhedral–subhedral pyroxene grew between large euhedral and subhedral plagioclase (figure 4a). Coarse-grained plagioclase crystals are replaced by calcite and sericite. Clinopyroxenes have been altered to chlorite (figure 4b). Thin chlorite veinlets cut through the gabbros.

Figure 4
figure 4

Polarized light microphotographs of gabbro, pyroxene quartz monzodiorite, and quartz monzodiorite. (a) Pyroxene grew between large plagioclase; (b) Pyroxene replaced by chlorite; (c) Pyroxene quartz monzodiorite with granular texture consists of plagioclase and clinopyroxene. Intergrowth of alkali feldspar and quartz can be seen between crystals. (d) Twinned clinopyroxene within fine-grained groundmass in pyroxene quartz monzodiorite. (e) Alteration of clinopyroxene to actinolite and opaque minerals in pyroxene quartz monzodiorite. (f) Quartz and alkali feldspar intergrowth between plagioclase crystals. (g) The granular texture of quartz monzodiorite consists of plagioclase, alkali feldspar, clinopyroxene, and hornblende. Pyroxene crystals replaced by surrounding amphibole. (h) Chlorite and opaque minerals replacing amphibole. Abbreviations from Whitney and Evans (2010). Pl: Plagioclase, Cpx: Clinopyroxene, Hbl: Hornblende, Afs: Alkali feldspar.

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Pyroxene quartz monzodiorite: Pyroxene quartz monzodiorites with granular to porphyritic textures (figure 4c–e) are characterized by significant quantities of euhedral to subhedral twined plagioclase and subhedral to anhedral clinopyroxene. Chlorite, calcite, epidote, sericite, and clay minerals are secondary and form due to alteration of original minerals. Quartz and alkali feldspar are present as small crystals between other minerals. In some places, an intergrowth of alkali feldspar and quartz forms graphic texture (figure 4c, f) which can be related to the late stage of magmatic liquid.

Quartz monzodiorite: The quartz monzodiorite has granular texture (figure 4g) and consists of euhedral to subhedral plagioclase, subhedral to anhedral hornblende, anhedral alkali feldspar, and quartz with a lesser amount of subhedral clinopyroxene and biotite. Accessory minerals are apatite, titanite, magnetite, and zircon. Secondary minerals include calcite, chlorite, sericite, and actinolite. Amphibole is replaced by chlorite and opaque minerals (figure 4h). Some clinopyroxene crystals are replaced by surrounding amphibole (figure 4g). This relationship probably resulted from late hydrous magmatic fluids reacting with pre-existing clinopyroxene to form amphibole.

Quartz syenite: This intrusion has porphyritic to porphyroidic (figure 5a) and granular textures (figure 5b) and is composed of tabular to subhedral alkali feldspar, quartz, euhedral to subhedral plagioclase and euhedral to subhedral hornblende. Plagioclase and alkali feldspar phenocrysts exist in alkali feldspar and quartz groundmass. Clinopyroxene exists as an accessory mineral. Quartz exists as an anhedral mineral in a groundmass of other minerals. The accessory minerals include zircon and apatite. Altered minerals are chlorite, clay minerals (figure 5d), and calcite. Quartz and calcite veinlets crosscut these rocks (figure 5c). Furthermore, calcite is replaced by plagioclase phenocrysts (figure 5c). Most of the alkali feldspar and plagioclase phenocrysts are altered to clay minerals.

Figure 5
figure 5

Polarized light microphotographs of quartz syenite. (a) Plagioclase phenocryst in alkali feldspar and quartz groundmass; (b) Alkali feldspar, hornblende, quartz and plagioclase with granular texture; (c) Calcite veinlets crosscutting the rocks; and (d) Argillization of feldspars and chloritization of hornblende. Abbreviations from Whitney and Evans (2010). Pl: Plagioclase, Cal: Calcite, Hbl: Hornblende, Afs: Alkali feldspar, Qz: Quartz.

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5.1.2 Type-II

Gabbro and gabbro-diorite: This intrusive rock forms small elongated bodies (figure 3f), dominantly showing granular texture and composed mainly of euhedral to subhedral plagioclase, subhedral amphibole (hornblende) and clinopyroxene (figure 6a). Secondary quartz crystals have grown between magmatic minerals. Chlorite (figure 6b), calcite and clay minerals are secondary minerals. Quartz-calcite veinlets crosscut this intrusion (figure 6c). The symplectite texture can be seen in this unit in which opaque minerals and chlorite replaced ferromagnesian minerals (figure 6d). Most remarkable is the presence of sulfide mineralization as disseminated chalcopyrite, pyrite, galena, and bornite (figure 6e, f) along with some ilmenite and titanomagnetite.

Figure 6
figure 6

(a–e) Microphotographs of gabbro – gabbro-diorite: (a) The granular texture in gabbro – gabbro-diorite. Plagioclase associated with altered hornblende, quartz and opaque mineral (XPL); (b) Hornblende in gabbro – gabbro-diorite replaced by chlorite and opaque mineral (PPL). (c) Plagioclase crystals cut by quartz-calcite veinlet (XPL); (d) Symplectite texture in which opaque minerals and chlorite replaced ferromagnesian mineral in gabbro – gabbro-diorite (PPL). (e) Sulfide mineralization (opaque minerals) in gabbro – gabbro-diorite unit. (f) Galena mineralization gabbro – gabbro-diorite unit in reflected light. Abbreviations from Whitney and Evans (2010). Pl: Plagioclase, Chl: Chlorite, Cal: Calcite, Hbl: Hornblende, Afs: Alkali feldspar, Qz: Quartz, Gn: Galena.

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5.2 Geochemistry

Major and trace element geochemistry has emerged as a reliable tool to decipher petrogenetic conditions of the rocks. The major and trace element data for the Chargar samples are given in table 1.

Table 1 (a) Major oxides in samples from the Chargar intrusions (XRF and ICP-OES). (b) Trace elements of Chargar intrusions (ICP-MS).
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In the Harker diagrams, a distinct gap exists between the two series (figure 7) which can be related to the small number of analyzed samples from the Type-II intrusion and gabbroic rocks from the Type-I intrusion. Major element variations characterize two general types of sources. Each type has a distinct chemical signature representing a separate magmatic evolution. CaO, TiO2, Fe2O3, MgO and MnO contents in Type-I decrease with increasing silica content as assumed for FC processes throughout magma evolution in the Chargar region. The K2O contents increase with increasing silica contents.

Figure 7
figure 7

Harker diagrams of CaO, Fe2O3, TiO2, MgO, MnO, Al2O3, P2O5, Na2O and K2O vs. SiO2 for the two rock types from Chargar. Blue filled square: southern intrusions (Type-I), red filled circle: western gabbro – gabbro-diorite intrusion (Type-II).

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The Chargar intrusive rocks display wide compositional variations. Two groups of rocks were divided based on the existent mineralization, their locations, major and trace elements: Type-I and Type-II. Based on major element geochemistry, the Chargar Type-I and Type-II intrusions were classified applying the classification diagram of Middlemost (1985) (figure 8a). This diagram indicates that the southern plutonic rocks (Type-I) plot in the acid to intermediate fields (monzodiorite, monzonite, quartz monzonite, granodiorite, and diorite), while the western plutonic rocks (Type-II) plot in the basic field (gabbroic diorite and monzodiorite) (figure 8a). Intrusive rocks of Chargar area mainly have quartz monzonite composition (figure 8a). The Chargar intrusive rocks are plotted in the sub-alkaline field. The sub-alkaline series can be separated using AFM triangle diagram into calc-alkaline and tholeiite series (Irvine and Baragar 1971). All samples from the Chargar area demonstrate calc-alkaline nature (figure 8b).

Figure 8
figure 8

(a) Rock classification diagram (Middlemost 1985) for Chargar intrusions. Samples from Chargar plot into the monzodiorite, monzonite, quartz monzonite, granodiorite and diorite (Type-I) and gabbroic diorite and monzodiorite (Type-II) fields. (b) AFM diagram (Irvine and Baragar 1971) for samples from Chargar intrusions. Blue filled square: southern intrusions (Type-I), red filled circle: western gabbro – gabbro-diorite intrusion (Type-II).

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The Chargar intrusive rocks have high K2O contents so that most of the samples plot in the high-K calc-alkaline field in a K2O vs. SiO2 classification diagram (Peccerillo and Taylor 1976; figure 9a). Based on Th vs. Co diagram (Hastie et al. 2007), all samples plot in the calc-alkaline field (figure 9b). The Chargar intrusive bodies are metaluminous, with the alumina saturation index values (A/CNK) of <1 (figure 10). Usually, the rocks display I-type characteristics involving a positive correlation between Rb and Th (White and Chappell 1977; Chappell 1999), which is also confirmed by petrographic studies where hornblende is present as a major mafic phase, that is mostly absent in S-type granitoids.

Figure 9
figure 9

(a) Plot of the Chargar samples on the K2O vs. SiO2 diagram (Peccerillo and Taylor 1976). Most of the samples are plotted in the high-K calc-alkaline series field; (b) Plot of Chargar samples on the Th vs. Co classification diagram (Hastie et al. 2007). Blue filled square: southern intrusions (Type-I), red filled circle: western gabbro – gabbro-diorite intrusion (Type-II).

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Figure 10
figure 10

Plot of the Chargar samples on the A/NK vs. A/CNK diagram (Shand 1943) [A/NK=molar Al2O3/(Na2O+K2O); A/CNK=molar Al2O3/(CaO+Na2O+K2O)]. Blue filled square: southern intrusions (Type-I), red filled circle: western gabbro – gabbro-diorite intrusion (Type-II).

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In the Zr vs. SiO2 diagram (Collins et al. 1982) and Na2O vs. K2O diagram (Chappell and White 2001), the Chargar intrusive samples plot in the I-type granite field (figure 11). The hornblende, biotite, and apatite presence; high CaO and Na2O ratios (White and Chappell 1983; Chappell and Stephens 1988; Chappell and White 2001); LILE and LREE enrichment, and HFSE depletion affirm their I-type nature (e.g., Dall’Agnol et al. 2012). Three main mechanisms are proposed for the formation of I-type magmas in subduction zones (e.g., Altherr et al. 2000): (1) partial melting of subducted oceanic crust, (2) metasomatized mantle wedge melting above subducted slab, and (3) partial melting of lower crustal rocks because of the injection of hot mantle magmas.

Figure 11
figure 11

(a) Plot of the Chargar samples on the Zr vs. SiO2 diagram (Collins et al. 1982); (b) Plot of the Chargar samples on the Na2O vs. K2O diagram (Chappell and White 2001). Blue filled square: southern intrusions (Type-I), red filled circle: western gabbro – gabbro-diorite intrusion (Type-II).

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In the primitive mantle-normalized spider diagrams, all of the Chargar Type-I and Type-II intrusive rocks display LILE (e.g., Cs, Ba, K, and Pb) enrichment and negative anomalies of HFSE (e.g., Nb, Zr, and Ti). Depletion of HFSEs implies the formation of this suite in a subduction zone setting; location of the HFSEs retain by the subducting slab (Saunders et al. 1980; McDonough 1991). A pronounced negative Zr anomaly for the Chargar samples is pointing out of a subduction setting (Wang et al. 2013). Characteristics of some gabbroic samples which show weak positive Eu anomaly in the chondrite-normalized REE pattern and the weak positive Ti anomaly in the primitive mantle-normalized spider diagram (figure 12a) reflect the abundance of cumulative plagioclase (Bau 1991) and presence of ilmenite or titanomagnetite (Desta et al. 2015).

Figure 12
figure 12

(a) Primitive mantle-normalized (McDonough and Sun 1995) multi-element spider diagram for the Chargar intrusions. (b) Chondrite-normalized (Nakamura 1974) REE patterns for the Chargar intrusions. Blue filled square: southern intrusions (Type-I), red filled circle: western gabbro – gabbro-diorite intrusion (Type-II).

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According to Zheng (2012), positive Pb anomalies on a primitive mantle-normalized spider diagram (Sun and McDonough 1989) are ascribed to mantle wedge metasomatism in subduction zones by continental crust sediments and also attributed to the continental contamination (Kamber et al. 2002) (figure 12a). Two models for metasomatism are proposed in these zones: a model described by slab-derived fluids and another by hydrous melt. The first has the ability to carry water-soluble elements, not the water-insoluble elements, but hydrous melts are able to transport both water-soluble and insoluble elements. Rb/Y, Nb/Y, Nb/Zr, and Th/Zr ratios are used to determine the type of metasomatized media (Kepezhinskas et al. 1997). The samples from the Chargar area indicate high values of Rb/Y and Th/Zr, and lower values of Nb/Y and Nb/Zr, indicating hydrous melt superiority in the parental magma.

Chondrite-normalized REE patterns of the Chargar intrusive rocks show LREE enrichment, flat HREE pattern and relatively high LREE/HREE ratios (figure 12b). Chondrite-normalized REE patterns show a fractionated trend, variable negative Eu anomalies and flat HREEs which are typical of calc-alkaline magmas. The flat HREE pattern indicates that magma formed at a depth where amphibole was as a stable phase (Rapp and Watson 1995; Barth et al. 2002). Negative Eu anomalies in most of the samples are attributed to plagioclase fractionation, while two gabbroic samples indicate weak positive Eu anomalies which can be related to plagioclase concentration.

5.3 Tectonic setting

All of the Chargar intrusions show high LILE enrichment and negative anomalies of Nb and Ti, mostly along with positive Pb anomaly which are characteristics commonly found in rocks typical of subduction zone settings (e.g., Hawkesworth et al. 1993; Pearce and Peate 1995). A continental arc is the suggested setting for these rocks based on the primitive mantle-normalized spider diagram where trace element signatures of these rocks show similarity with continental arc samples.

Trace elements are used to separate different tectonic settings of granitoids (e.g., Harris et al. 1986; Pearce 1996). Based on the Yb/Ta vs. Rb diagram (Pearce 1996), all of Chargar granitoid samples fall into the volcanic arc granitoid (VAG) field (figure 13a). On the Ta vs. Th and Th/Yb vs. Ta/Yb tectonic setting discrimination diagrams (Gorton and Schandl 2000), most Chargar granitoids plot in the field of active continental margin (figure 13b, c). Based on Zr vs. Y diagram (Muller and Groves 1997), Chargar granitoids plot in the field of arc or subduction-related suites (figure 13d). Furthermore, on the Zr*3–Nb*50–Ce/P2O5 triangular diagram (Muller and Groves 1997), most Chargar samples plot in the continental arc setting and a few samples plot on post-collisional arc setting (figure 13e). In general, it can be concluded that all samples are related to subduction calc-alkaline magmatism.

Figure 13
figure 13

Tectonic setting discrimination diagrams of Chargar intrusive rocks. (a) Rb vs. Y+Nb tectonic setting discrimination diagram (Pearce 1996) with fields of volcanic arc (VAG), syn-collisional (syn-COLG), within-plate (WPG), and oceanic ridge (ORG) granites. Samples from the Chargar granitoids plot in the VAG field; (b) Th/Yb vs. Ta/Yb diagram (Gorton and Schandl 2000). OA = Oceanic Arcs, ACM= Active Continental Margins, WPVZ= Within-Plate Volcanic Zones, WPB= Within-Plate Basalts, MORB= Mid-Ocean Ridge Basalts; (c) Th vs. Ta diagram (Gorton and Schandl 2000); (d) Y vs. Zr plot (Muller and Groves 1997); (e) Zr×3 – Nb×50 – Ce/P2O5 triangular diagram (Muller and Groves 1997). Blue filled square: southern intrusions (Type-I), red filled circle: western gabbro – gabbro-diorite intrusion (Type-II).

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6 Discussion

Geochemical variations in magmatic rocks can determine magmatic processes such as fractional crystallization of the parental magma. In the Ba–Sr diagram (Arslan and Aslan 2006), the Chargar intrusions show non-linear trends which imply modifications in the fractionating assemblage of more complex processes. However, the general trend can indicate plagioclase fractionation (figure 14).

Figure 14
figure 14

Ba vs. Sr diagram (Arslan and Aslan 2006) and plots for the Chargar intrusions. The vectors indicate the influence of fractionation of plagioclase (Plg), potassium feldspar (K-fel), biotite (Bi) and hornblende (Hbl) on the composition of the residual liquids. Blue filled square: southern intrusions (Type-I), red filled circle: western gabbro – gabbro-diorite intrusion (Type-II).

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The geochemical characteristics of the Chargar intrusions suggest that these are derived by partial melting from a metasomatized sub-continental lithospheric mantle source in a subduction zone. The geochemical features of some samples imply their shoshonitic affinity and emphasize a post-collisional zone (e.g., Liégeois et al. 1998). They also can be produced from an active subduction zone (Blatter et al. 2001; Bonev and Beccaletto 2007), metasomatized sub-continental lithospheric mantle (Aldanmaz et al. 2000; Seghedi et al. 2004), or assimilation of crustal rocks (Meen 1987; Feeley and Cosca 2003).

Ba/La vs. Th/Yb diagram (Hou et al. 2012) for the Chargar intrusions indicates the addition of the fluids from the subducted slab into their source region and slab-derived enrichment (figure 15). In this diagram, the Chargar Type-I samples reveal elevated Th/Yb with relatively narrow Ba/La variation. Usually, magmatic rocks derived from a metasomatized mantle source by a fluid component are reasonable to have high Ba and Pb concentrations and high Ba/La and Ba/Th ratios, but magmas with a strong imprint of sediment-derived melts in their source region should have high Th contents and high Th/Yb and Nb/Y ratios (Hawkesworth et al. 1997; Woodhead et al. 2001; Kelemen et al. 2003; Zhao et al. 2009; Guo et al. 2013, 2015; Liu et al. 2014). Thus, the high Th/Yb ratios for the samples indicate the metasomatized mantle source for the Chargar intrusions by sediment-derived melts (figure 15). In contrast, the high Ba and Pb contents, and high Ba/La ratios are related to an enriched mantle source by fluids.

Figure 15
figure 15

Ba/La vs. Th/Yb diagram (modified after Hou et al. 2012) for the Chargar intrusions. Blue filled square: southern intrusions (Type-I), red filled circle: western gabbro – gabbro-diorite intrusion (Type-II).

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The high (La/Sm)N (~>3.5) and low (Tb/Yb)N values for Chargar intrusions suggest their possible generation by partial melting of a mantle source with amphibole-bearing spinel peridotite composition (figure 16). Incompatible element ratios like La/Sm and Sm/Yb are influenced by partial melting and fractional crystallization processes. When partial melting is lower, La/Sm would be high. In general, high La/Yb ratios indicate smaller melt fractions and/or garnet as the main residual phase and low La/Yb ratios show a dominated melting regime by larger melt fraction and/or spinel as the predominant residual phase (Yang et al. 2007).

Figure 16
figure 16

(La/Sm)N vs. (Tb/Yb)N diagram (Wang et al. 2002) for the Chargar intrusions. The horizontal dashed line separates the fields of garnet-bearing peridotite and spinel-bearing peridotite; Blue filled square: southern intrusions (Type-I), red filled circle: western gabbro – gabbro-diorite intrusion (Type-II).

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The relationship between the Chargar intrusions and their mineralogy and geochemistry suggest that they have differentiated from a common parent magma. It seems that the parent magma of the Chargar intrusions was basic in composition, and the gabbros represent an accumulation of early-formed crystals from this magma. This accumulation of early-formed crystals was separated from the remaining liquid by pressure due to crustal movements, and this liquid was injected into higher levels in the crust and formed the gabbro, pyroxene quartz monzodiorite, quartz monzodiorite, and the quartz syenite, respectively. The concept of the origin for the quartz syenite as the product of crystallization of a magmatic liquid from which the products of early crystallization had been removed is supported by certain criteria developed by Bowen (1937).

Positive Eu anomaly corresponds to plagioclase-rich gabbroic rocks. It seems that the gabbroic intrusions could be the cumulative parts of the magma chamber for the Chargar intrusions. Such cumulative assemblages imply post-collisional transtensional movements (Desta et al. 2015). Furthermore, the positive Eu anomaly is attributed to the mixing of a mantle component with crustal melts (Mazzucchelli et al. 1992). It is suggested that the primary magma for the Chargar gabbro – gabbro-diorite (Type-II) should be derived from the lithospheric mantle intensively modified by continental crust.

7 Conclusions


Two different groups of rocks were separated in the Chargar area of the Tarom belt in Alborz, based on mineralization occurrence as well as major, trace and REE geochemistry.


The Type-I rocks are composed of gabbro–monzodiorite–quartz syenite compositions. They are calc-alkaline and metaluminous and in the diagram of the major elements a negative correlation of SiO2 relative to Al2O3, Fe2O3, MgO, CaO and TiO2 and a positive correlation with K2O. Type-I rocks are enriched in LREE against HREE. In the primitive mantle-normalized spider diagram, negative Ti, Nb, and Zr anomalies and a mostly positive anomaly of Pb can be seen. The LIL elements Cs and Ba are enriched relative to HFSE. The crustal assimilation/contamination together with fractional crystallization was an important process. Geochemical signatures have a similar appearance to continental arc setting.

The Type-II rocks are mafic metaluminous. Type-II rocks were possibly generated at the beginning of the development of the arc setting or it could have been related to small extensions in the continental arc environment. The sample with the lowest SiO2 has moderate MgO content (5.13%). These attributes suggest their derivation, not from a primary magma, but experiencing fractional crystallization probably associated with crustal assimilation. The crustal assimilation as an important process changed the original magma composition. Affected rocks by intense partial melting will show special evidence for this melt extraction. Strong evidence for the melt extraction and fractionation of basaltic magmas in the lower crust would be a positive Eu anomaly (Rudnick 1992). Therefore, intrusive bodies in the Chargar area were caused by the subduction of the Arabian plate beneath the central Iran plate and their subsequent collision.