Ediacaran to Jurassic geodynamic evolution of the Alborz Mountains, north Iran: geochronological data from the Gasht Metamorphic Complex

Abstract

The Alborz Mountains in north Iran underwent several tectono-metamorphic events during opening and closure of the Palaeotethys and Neotethys Oceans. These events are recorded by rare and discontinuously exposed metamorphic rocks, such as the HP-LT Asalem-Shanderman Complex and the Gasht Metamorphic Complex (GMC), that are considered to have been metamorphosed during the closure of the Palaeotethys Ocean. The GMC comprises poorly exposed metasediments and amphibolites metamorphosed under greenschist- to amphibolite-facies conditions, along with smaller volumes of granites. Different dating methods were applied to selected samples of the GMC basement to constrain the geological evolution of this part of the Alborz Mountains. A metagranite yielded two LA-ICP-MS U–Pb zircon ages of 638.4 ± 4.1 Ma and 590.3 ± 4.8 Ma that possibly date protolith crystallisation and later deformation and metamorphism, respectively, and a granite yielded a late Ediacaran 551 ± 2.5 Ma U–Pb zircon crystallisation age. A northern provenance from the basement to the South Caspian Basin can neither be established nor ruled out because no age data are available for this unit. Derivation of the GMC from Turan Block basement is unlikely, as this has a different crustal makeup and is probably composed of Paleoproterozoic and early Neoproterozoic material. The zircon ages are similar to published ages from the Arabian-Nubian Shield, indicating that this part of the Alborz basement may have belonged to the northern margin of Gondwana in the Neoproterozoic before rifting and drifting away along with other Iranian blocks (the Cimmerian terranes) during opening of the Neotethys Ocean. Chemical Th-U-total Pb ages for metamorphic monazites from two metapelite samples yielded a very large range of spot ages, of which c. 80% falls between 200 and 250 Ma, that do not allow to distinguish between Eo-Cimmerian and Main Cimmerian events in the GMC. However, they may indicate that the amphibolite-facies peak metamorphism of the GMC occurred sometime in the Triassic, in any case much later than the Carboniferous metamorphism in the neighbouring Asalem-Shanderman Metamorphic Complex to the north. Peak-metamorphic amphibole from amphibolite, retrograde white mica and foliation-defining biotite from metapelites and magmatic white mica from granite yielded much younger 175.1 ± 0.5 to 177.0 ± 0.4 Ma ⁴⁰Ar/³⁹Ar plateau ages. The Toarcian ⁴⁰Ar/³⁹Ar ages for minerals with different nominal closure temperatures reflect very rapid cooling of GMC basement below the Shemshak Group due to extension-triggered uplift. This late Toarcian to Aalenian extension event can be correlated with the regional Mid-Cimmerian unconformity of mid-Bajocian age (c. 170 Ma) that resulted from the tectonic movements causing rapid uplift and erosion. Extension probably started in the western Alborz Mountains in the Toarcian, migrated eastward, and culminated in the Aalenian in the eastern Alborz with the formation of a deep-marine basin. It was probably triggered by the onset of the subduction of Neotethys oceanic crust beneath the Central Iranian Microcontinent.

1 Introduction

The Alborz Mountain Range in north Iran is the result of several compressional and extensional tectonic events, starting with deposition of Ediacaran to lower Palaeozoic sediments at the northern margin of Gondwana, followed by opening and closure of the Palaeotethys Ocean that culminated in the mid- to late Triassic Cimmerian Orogeny (Stöcklin, 1974; Sengör, 1979; Stampfli, 2000), and late Cenozoic to ongoing compression (e.g. Rezaeian et al., 2012). The Alborz Mountains mark part of the Triassic-age Palaeotethys suture between a Cimmerian terrane to the south and either central Eurasian crust (e.g. Stöcklin, 1974) or a (in Triassic times) still isolated Turan Plate to the north (e.g. Besse et al., 1998).

In Iran, the suture zone of the Palaeotethys Ocean and the (Eo-)Cimmerian orogeny are perhaps best documented east of the Alborz Mountains, from the area around Gorgan towards the southeast into the Binalud Mountains to Torbat-e-Jam in east Iran. Here, in eastern Iran, Permian–Triassic (ultra)mafic and ophiolite complexes represent obducted and accreted remnants of the Palaeotethys Ocean marking the suture zone (Sengör, 1979; Alavi, 1991; Ruttner, 1993). K–Ar whole rock ages for low grade metasediments (Gorgan Schists), with a palynological late Ordovician protolith depositional age (Ghavidel-Syooki, 2008), cluster around c. 200 Ma and may indicate accretion and collision time (Delaloye et al., 1981). Based on detrital zircon age spectra, Chu et al. (2021b) concluded that deposition of the Mashhad Phyllite (Binalud Mountains) in a synorogenic peripheral foreland started at, or slightly before, 228 ± 3 Ma and that initial accretion of a Cimmerian terrane must have started shortly before this time.

Further to the south-east, collision was accompanied and followed by intrusion of a late Triassic to earliest Jurassic, c. 216–200 Ma (Sheikholeslami & Kouhpeyma, 2012), suite of gabbros, diorites, monzonites and granites (Karimpour et al., 2010; Mirnejad et al., 2013; Zanchetta et al., 2013; Ghavi et al., 2018; Deyhimi et al., 2020; Mahmoudi & Corfu, 2022), the youngest of which mark the end of collision and accretion. The metamorphosed volcano-sedimentary, arc-derived Darreh Anjir and Fariman complexes are considered to belong to an accretionary wedge, part of the active margin of the Palaeotethys subduction system in the Turan domain (Shafaii Moghadam et al., 2015; Topuz et al., 2018). Cimmerian compressional deformation is well recorded in the Triassic succession of the Aghdarband Basin (Zanchi et al., 2016). The 203–224 Ma Torbat-e-Jam and 200–217 Ma Mashhad granites (U–Pb zircon crystallisation ages) intruded these complexes following Cimmerian terrane accretion (Zanchetta et al., 2013; Mazaheri-Johari et al., 2022; Yang et al., 2023). Biotite and muscovite from metapelites and a pegmatite SW of Mashhad yielded 196 to 183 Ma ⁴⁰Ar/³⁹Ar step-heating ages (Sheikholeslami et al., 2019) that date post-collisional cooling and exhumation of this part of the orogen. The early Jurassic erosional products were deposited in a foreland basin or rift and also northward onto the accreted Turan Plate (e.g. Fürsich et al., 2009a; Wilmsen et al., 2009b; Poursoltani & Fürsich, 2020; Poursoltani et al., 2023).

The westward continuation of the suture zone in the Alborz Mountains is less clear. For example, Zanchi et al. (2009) interpreted the Asalem-Shanderman of blueschist- to eclogite-facies, mid-ocean ridge type metabasalts and the Gasht Metamorphic Complex (GMC) to be allochthonous remnants of a late Triassic, Eo-Cimmerian collisional orogen or south verging nappe stack that is largely composed of continental crust deformed and metamorphosed in the Carboniferous. They suggested that true ophiolites (and, hence, a suture) are not preserved here. In contrast, Omrani et al. (2013b) regarded the Shanderman Metamorphic Complex to mark part of the suture zone of a northward subducted Palaeotethys Ocean.

Regionally, the (Eo-)Cimmerian orogeny was followed by a transgression that started in the late Triassic (Norian) with deposition of the shallow marine to terrestrial, siliciclastic and carbonate-siliciclastic Shemshak Group in a subtropical climate (Fig. 1). The up to 4 km thick Shemshak Group comprises two tectono-sedimentary cycles (Nayband and Ab-e-Haji subgroups) bounded by unconformities caused by tectonic events (e.g. Fürsich et al., 2009b; Salehi et al., 2018; Seyed-Emami et al., 2021).

Fig. 1

Simplified tectono-stratigraphy of the Gasht Metamorphic Complex (GMC) and surrounding areas of the western Alborz Mountains showing the main geological events, based on Allen et al. (2003) and this study (see text for details)

Deposition of the first, late Triassic cycle—in the Alborz Mountains known as the Nayband Subgroup (Seyed-Emami et al., 2021)—followed the Eo-Cimmerian event and took place in a synorogenic peripheral foreland basin that developed on extending and subsiding crust north of the incipient subduction zone of the Neotethys Ocean (Wilmsen et al., 2009a). This cycle ended with the so-called Main-Cimmerian tectonic event at the Triassic-Jurassic boundary that comprised uplift phases resulting in erosional unconformities. The trigger was probably a combination of slab break-off of the subducted Palaeotethys Ocean and initiation of Neotethys subduction (Wilmsen et al., 2009a). The Main-Cimmerian event was followed by deposition of the largely Early Jurassic Ab-e-Haji Subgroup (Seyed-Emami et al., 2021) that constitutes the second cycle and started with post-orogenic molasses and in the Toarcian–Aalenian developed into deep marine sedimentation in rift basins that also formed in a Neotethys back-arc basin setting (Wilmsen et al., 2009a). This second cycle ended in the Middle Jurassic (mid-Bajocian) with the Mid-Cimmerian event, a tectonic phase that has been related to seafloor spreading in the South Caspian Basin (Fürsich et al., 2009b; Wilmsen et al., 2009a).

A second regional transgression led to deposition of Middle to Late Jurassic shallow marine sandstones that unconformably overlie rocks of the Shemshak Group, and that are known as the Shal Formation in the Alborz Mountains. This third, post Shemshak Group depositional cycle ended around the Jurassic-Cretaceous boundary with a not fully documented Late Cimmerian tectonic event (Fürsich et al., 2009b; Wilmsen et al., 2009a; Seyed-Emami et al., 2021).

The present average crustal thickness of the West and Central Alborz (c. 47 km) is too small to isostatically compensate the existing topography (e.g. Sodoudi et al., 2009; Rastgoo et al., 2018) which is the result of, and maintained by, intensive late Cainozoic to ongoing tectonic activity and attendant delamination of lithospheric mantle. Compressional deformation was due to the onset of collision between Arabia and Eurasia (the latter by now including the accreted Cimmerian terranes) and started in the middle Miocene. Uplift, cooling and exhumation of the West Alborz Mountains occurred variably and in several stages. It peaked in the late Miocene (e.g. Allen et al., 2003; Guest et al., 2006a; Chu et al., 2021a; Madanipour et al., 2023) and closely overlapped in time with rapid subsidence of the South Caspian Basin (Axen et al., 2001). The resulting large scale, double verging flower structure is made up of north- and south-vergent thrust-bound units (Allen et al., 2003; Guest et al., 2006a) and is bounded by deep-seated faults and thrusts to its north and south. Motaghi et al. (2018) found that the complete Talesh unit was thrust southwestwards over Neogene basins along NE-dipping faults. In addition, the Central Alborz Mountains host many ESE– to ENE–striking, right- and left-lateral strike-slip faults that accommodate most of the ongoing N-S shortening (e.g. Rashidi, 2021). During these events, parts of the metamorphic basement of the Alborz Mountains became exposed.

The GMC comprises poorly exposed metasediments and amphibolites metamorphosed under greenschist- to amphibolite-facies conditions as well as smaller volumes of granites and metagranites. The evolution of these basement rocks of the Alborz Mountains is still poorly known and the number of radiometric ages is limited. We applied different dating methods to selected samples of the GMC basement to better understand the geological evolution of this part of the Alborz Mountains.

1.1 Regional geology

The GMC is situated to the west of Rasht and east of Masuleh in Gilan/Guilan Province (Fig. 2). It extends southwestwards from the southern coast of the Caspian Sea until it abuts the NW-trending Boghrov-Dagh Fault Zone (which includes the dextral Masuleh Shear Zone; Moosavi & Rasouli-Jamadi, 2018) that separates it from Palaeocene–Eocene volcanic and plutonic rocks of the Alborz magmatic belt to the SW. The study area is situated in the (per 2018 by the UN designated) Gasht-e Rudkhan Protected Area in which the rocks are weathered and overgrown by dense forest. Hence, exposure is very poor and field relations between the main rock units are not, or barely, exposed and these were disturbed due to later, late Mesozoic tectonic events. Their relative tectonostratigraphy has to be inferred from their grade of metamorphism, degree of deformation and radiometric ages. The GMC predominantly comprises low grade metasediments (phyllites, greenschist-facies metapelites, metasandstones, subordinate calc-silicates, Clark et al., 1975) and higher grade, amphibolite-facies metasediments such as metapelitic mica schists with staurolite, kyanite, sillimanite, andalusite or garnet, quartzites and migmatitic metapelites (Moazzen et al., 2017; Razaghi et al., 2018). Thermobarometry on metapelites, gneiss and migmatite by Razaghi et al. (2018) yielded temperatures and pressures in the range of 630 °C, 8 kbar/0.8GPa to 720 °C, 6 kbar/0.6 GPa. The age of the sedimentary protolith is still uncertain. Clark et al. (1975) proposed a Neoproterozoic or early Palaeozoic age for the sedimentary rocks. However, Crawford (1977) reported Rb–Sr whole-rock isochron ages of 375 ± 12 and 382 ± 45 Ma (Devonian) of uncertain geological significance for, respectively, phyllites and porphyroblastic gneisses.

Fig. 2

Simplified 1:1000,000 geological map of N Iran (Sahandi & Soheili, 2014) showing the location of the GMC and the locations of the published ages of crystalline rocks of the Alborz mountains

In the northern GMC, south and southwest of Gasht, small volumes of undated granites informally known as the “Gasht granites” have been subdivided into older, Precambrian, and younger granites of suspected Triassic age (e.g. Omrani et al., 2013a). In addition, many amphibolite boulders in the river bed testify of the presence of nearby metabasites which, however, have not been found in exposure. They are clearly locally derived.

Low grade metamorphic Carboniferous black shales or slates and Permian limestones and marls are found along the southwestern boundary of the GMC where they unconformably overlie the high grade GMC (Clark et al., 1975). To the southwest, these late Palaeozoic rocks are juxtaposed against magmatic (mainly volcanic) rocks of the Alborz magmatic belt that have yielded Eocene, Oligocene and (locally) late Miocene U–Pb zircon crystallisation ages (Castro et al., 2013; Chiu et al., 2013; Nabatian et al., 2014, 2016; Sepidbar et al., 2019).

The Shemshak Group overlaps the GMC with a regional angular unconformity and starts with basal conglomerates followed by Toarcian shallow-marine siliciclastic rocks with coal bearing layers, changing to carbonates in Bajocian times (Clark et al., 1975; Fürsich et al., 2009a). The sub-Shemshak Group unconformity postdates closure of the Palaeotethys Ocean and accretion of a Gondwana-derived terrane in late Carnian–early Norian times (220–210 Ma; Guest et al., 2006b; Horton et al., 2008) and the Shemshak Group is generally considered to contain the molasse of the Cimmerian orogeny (Zanchi et al., 2009; Wilmsen et al., 2009a, b).

1.2 Previously reported ages

The Ediacaran age crystalline basement saw granitoid magmatism from c. 570 Ma (perhaps as early as c. 620 Ma), followed by high grade metamorphism and deformation that had ended before intrusion of c. 550 Ma, undeformed granites. The number of published ages for crystalline rocks for the Alborz Mountains (Fig. 2) is limited. Migmatised granite SW of Rasht city yielded a 571.0 ± 4.8 Ma protolith U–Pb age for a cluster of youngest zircons, and a migmatitic granite gneiss ca. 8 km further SW yielded two concordant U–Pb zircons ages at 575.4 ± 3.6 Ma and 621.3 ± 2.7 Ma (Chu et al., 2021a). Granite from Lahijan east of Rasht has a late Ediacaran 551 ± 9 Ma U–Pb zircon age (Guest et al., 2006b).

Late Early Jurassic magmatic activity is limited to an undeformed granite intruding unknown host rocks of the GMC c. 3 km west of Gasht and that has an early Jurassic 180.5 ± 1.3 Ma U–Pb zircon crystallisation age (Chu et al., 2021a, erroneously labelled “Triassic” in their Table 2). This age is indistinguishable from the 179 ± 9.8 Ma U–Pb zircon lower intercept age of the Lisar granite c. 50 km to the north of the GMC (Mandanipour, 2023). Crawford (1977) reports 175 ± 10 Ma (probably K–Ar) ages for muscovite and a whole rock sample of tourmaline granite from the “Talesh Hills”. This age should be recalculated using the modern decay constant, but the analytical data are not available.

Younger, mid-Cretaceous magmatic activity in the GMC and in neighbouring areas was mostly basic in composition. Rezaei et al. (2023) reported 99.5 Ma U–Pb zircon ages and 96.6–97.5 Ma phlogopite ⁴⁰Ar/³⁹Ar ages of the Gasht-Masuleh cumulate and isotropic gabbros. Diorite from the western boundary of the GMC has a mid-Cretaceous 99.1 ± 0.7 Ma U–Pb zircon crystallisation age (Chu et al., 2021a) and Guest et al. (2006b) reported a 97.4 ± 1.8 Ma ²⁰⁶Pb/²³⁸U zircon crystallisation age for the diorite phase and a 96.9 ± 2.4 Ma zircon age for the (rapakivi) granite phase of the composite Nusha Pluton ca. 150 km to the ESE of the Gasht-Masuleh area. Cretaceous, 108–88 Ma U–Pb zircon ages for Javaherdasht gabbros and Talesh pillow lavas are reported by Monsef et al. (2022). Amani et al. (2024) reported a 95.6 ± 1.8 Ma U–Pb zircon age for basalt from the Talesh Mountains NW of the GMC.

Other basement units of the Alborz Mountains appear to have had a different geological evolution. Eclogite from the Asalem-Shanderman Metamorphic Complex to the north of the GMC has an early Carboniferous 353 ± 8.9 Ma U–Pb rutile and a 350.9 ± 1.2 Ma phengite ⁴⁰Ar/³⁹Ar step-heating age (Wan et al., 2021), and blueschists and mica schists from the same complex yielded ca. 345–352 Ma ⁴⁰Ar/³⁹Ar step-heating cooling ages for phengites (Rossetti et al., 2017). These ages probably date subduction of ocean crust, attendant high pressure metamorphism and subsequent cooling (Wan et al., 2021; Rossetti et al., 2017). Further to the NW, metapelites of the allochthonous Allahyarlu Complex yielded Carboniferous 333.7 ± 0.8 and 324.0 ± 0.8 Ma ⁴⁰Ar/³⁹Ar step-heating ages indicating cooling following amphibolite-facies metamorphism that was probably related to accretion of a Carboniferous active continental arc (Moazzen et al., 2020).

1.3 Sample preparation and analytical methods

The analytical methods are described in detail in Supplement S2, specifying instrumental parameters, experimental conditions, reference materials, data reduction methods and plateau criteria.

The six rock samples were cut and polished thin sections were prepared with ¼ µm fine polishing for microscopy and EPMA analytics. These samples were crushed, sieved and washed, and zircon separates were obtained by magnetic separation and heavy liquid techniques. Zircon, amphibole, biotite and white mica were purified by hand picking under a stereomicroscope at the Institute of Geosciences of the University of Potsdam.

Zircon internal textures of granite sample 16FMN09 were documented by cathode luminescence (CL) using a CL detector fitted to the electron microprobe at the University of Potsdam and zircons of metagranite sample 16FMN55B at the Institute of Geology of the Czech Academy of Science in Prague (Supplementary figure S3). Polished mounted zircons were analysed by laser ablation inductively coupled plasma mass-spectrometry at the Institute of Geology of the Czech Academy of Science in Prague. The details of the LA-ICP-MS U–Pb analyses together with isotope data and spot ages are listed in Supplement S4.

Supplements S5 and S6 present the compositional maps and chemical Th-U-total Pb dating results for monazites in two metapelite samples. Monazites compositions were determined by EPMA using a Jeol Hyperprobe at the GeoForschungsZentrum, Potsdam.

The compositions of the dated minerals (amphibole, white mica and biotite) were determined at the Institute of Geosciences of the University of Potsdam using a JEOL JXA-8200 Micro-analyser. Electron probe microanalyses are listed in Supplement S7.

Irradiation for ⁴⁰Ar/³⁹Ar dating of amphibole, biotite and white mica was done at the Oregon State TRIGA Reactor of Oregon State University and the irradiated samples were analysed by incremental step-heating analysis using a CO₂ laser and a Micromass 5400 noble gas mass spectrometer at the University of Potsdam (Supplement S8).

1.4 Samples

We sampled magmatic and high-grade metamorphic rocks of the GMC to establish crystallisation ages, the timing of amphibolite-facies metamorphism and the subsequent thermal history. The ages allow comparison of the geological history of the GMC with those of neighbouring metamorphic complexes, such as the Asalem-Shanderman Metamorphic Complex and Allahyarlu Complex.

Six rock samples were selected for different dating purposes; the sampling locations are shown in Fig. 3 and the coordinates are listed in the caption of the figure. Metagranite 16FMN55B (ca. 2 km SE of Gilvanderud) and granite 16FMN09 (ca. 1.5 km east of Abrud), suspected to be part of the Precambrian basement, were sampled for U–Pb zircon dating to establish their (protolith) crystallisation ages. ⁴⁰Ar/³⁹Ar step-heating dating of peak-metamorphic, foliation-defining biotite and retrograde white mica from the metapelites (16FMN55A and RMK5G), peak-metamorphic amphibole from amphibolite (15FMN05D), and magmatic white mica from a granite (167FMN50X4, SW of Chapul) was carried out to establish cooling and/or resetting ages (granite 16FMN09 lacks magmatic micas). Amphibolite 15FMN05D was taken from a loose block in the ENE trending valley of the Masuleh river (Rud-e-Qurubars) north of Chapul. Although amphibolite is not exposed, the block must have been derived from a local source situated somewhere upstream in the catchment area.

Fig. 3

1:250,000 scale geological map of the Gasht Metamorphic Complex (simplified after Clark et al., 1975) showing sample locations. Coordinates of the samples: 15FMN05D: 37°10′27.69ʺN–49°8′22.03ʺE, 16FMN09: 37°10′28.26ʺN–49°10′56.44ʺE, 16FMN55A and 16FMN55B: 37°8′34.05ʺN–49°4′35.45"E, 16FMN50X4: 37°6′7.19ʺN–49°8′59.55ʺE, RMK5G: 37°9′38.00ʺN–49°0′32.00ʺE

1.5 Petrography

Thin section scans of all samples are shown in supplementary figure S1.1. Granite 16FMN09 is a medium- to coarse- grained, equi- to subgranular, protocataclastic granite made up of large anhedral quartz, an- to subhedral plagioclase and an- to subhedral alkali feldspar (microcline) (Fig. 4A). Sub- to euhedral zircon and subhedral titanite, both up to 100 µm in size, occur as accessory minerals; micas are absent. Embayed quartz shows evidence for resorption and hosts zircon and titanite. Very small amounts of secondary opaque minerals form late veinlets. The large quartz and feldspar crystals partly preserve a sub- to equi-granular, coarse-grained magmatic texture (Fig. 4A). Deformation features are limited to recrystallisation of quartz, plagioclase and alkali feldspar into much smaller subgrains, kinking of polysynthetic twins and formation of deformation twins in plagioclase.

Fig. 4

Photomicrographs showing selected textures and minerals. A granite 16FMN09; microcline, plagioclase with deformation twins and recrystallised quartz (XPL). B metagranite 16FMN55B; quartz, deformed plagioclase (in centre) and recrystallised quartz (XPL). C granite 16FMN50X4; subhedral alkali feldspar, quartz and large, subhedral white mica. Note quartz with coarser grains and recrystallised grains and partly argillized feldspars (XPL). D and E metapelite 16FMN55A; nests of decussate white mica mantling staurolite poikiloblast (not shown) (D: PPL, E: XPL). F metapelite 16FMN55A; sieve-textured staurolite poikiloblast mantled by decussate white mica and minor biotite, wrapped by a composite foliation composed of finer-grained biotite—white mica foliation (PPL). G metapelite RMK5G; sieve-textured staurolite poikiloblast surrounded by white mica ± biotite, wrapped by finer-grained white mica ± biotite foliation (PPL). H metapelite RMK5G; sieve-textured staurolite poikiloblast (bottom left) and andalusite remnants (right, with high optical relief against quartz) partly replaced by decussate white mica and biotite (PPL). I amphibolite 15FMN05D; hornblende and partly argillized plagioclase (PPL, scale bar 500 µm). Mineral name abbreviations in the text, tables and figures are after Whitney & Evans (2010). PPL: plane-polarized light, XPL: cross-polarized light

Metagranite 16FMN55B shows a strongly developed planar fabric in outcrop (Supplementary figure S1.2), but in thin section lacks a penetrative fabric that is recrystallised to fine- to medium-grained, unoriented quartz and feldspar (mainly plagioclase; Fig. 4B). It lacks micas and the accessory minerals are zircon and titanite, whereas secondary quartz and calcite occur in late veinlets.

White mica granite 16FMN50X4 is composed of alkali feldspar, plagioclase, quartz and white mica. Alkali feldspar occurs as large, subhedral karlsbad-twinned crystals and plagioclase as large crystals with polysynthetic twinning and that are smaller than the alkali feldspar crystals. White mica is present as large to medium-grained, subhedral grains (Fig. 4C). Quartz forms an anhedral fine-grained mass in the interstices between feldspars. The granite shows a protocataclastic texture as is clear from dynamic recrystallisation of quartz to mortar textures and subgrains, kinking of white mica cleavage planes, and deformation twins in plagioclase. Late, low temperature alteration changed feldspars to brownish submicroscopic material, probably clay minerals.

Staurolite-biotite-white mica schist 16FMN55A (Fig. 4D–F) contains a foliation defined by an- to subhedral biotite, fine-grained, an- to subhedral white mica and fine-grained quartz. Biotite is often altered to chlorite and the foliation is slightly crenulated into small open folds. Staurolite occurs as anhedral remnants of larger, sieve textured porphyroblasts (Fig. 4F), now mantled and partly replaced by intergrowths of coarse grained, an- to subhedral, decussate white mica, coarse grained quartz, and locally chlorite (Fig. 4D and E). Low temperature alteration further altered margins of staurolite remnants to submicroscopic aggregates of sericite and clay minerals. Monazite and zircon occur as accessory minerals, and feldspars appear to be absent.

Staurolite-biotite-white mica-andalusite schist RMK5G (Fig. 4G and H) is similar to the previous sample but here staurolite occurs as anhedral, sieve-textured poikiloblasts, containing inclusions of mainly ellipsoidal, subparallel oriented quartz and a few anhedral, elongated opaque minerals that are oriented parallel to the quartz inclusions. Both minerals mimic a pre- or syn-tectonic internal fabric that is at high angles to the external mica foliation (Fig. 4G). It is mantled and partly replaced by large decussate, sub- to euhedral white mica, quartz and ragged anhedral, secondary biotite that together grew in pressure shadows on staurolite (Fig. 4H). Biotite and white mica form the composite foliation in which the white micas are smaller than the late decussate white micas replacing staurolite. Accessory andalusite is present as small anhedral remnants that contain a few quartz inclusions (Fig. 4H). In addition, this sample contains layer- to spindle-shaped porphyroblasts of light brownish yellow, completely pinitised cordierite that contain randomly oriented inclusions of large opaque minerals, xeno- to hypidioblastic biotite, large hypidioblastic white mica and chlorite. However, cordierite relicts have not been preserved. Cordierite probably formed during a later stage of metamorphism under lower pressures.

Amphibolite 15FMN05D is mainly composed of amphibole and plagioclase, and contains subordinate amounts of opaque minerals that together form a granoblastic texture (Fig. 4I). Amphibole occurs as fine- to medium-grained, xenoblasts to idioblasts with a brownish green pleochroism. Some grains exhibit irregular, patchy colour zoning, and a few grains contain inclusions of small roundish quartz grains. Plagioclase is anhedral and has the same grain size as amphibole, often shows polysynthetic twinning and is locally altered to clay minerals. The opaque minerals occur as large anhedral xenoblasts, and some are mantled by strongly chloritized secondary biotite.

2 Results

2.1 LA-ICP-MS U–Pb zircon dating

The dated zircons from granite 16FMN09 are mainly euhedral long prismatic crystals of up to 110 µm long and c. 50 to 80 µm wide; some are broken due to sample processing. In the CL images, most crystals show concentric growth zoning typical for magmatic zircons and inherited zircon cores were not observed (Supplementary figure S3). U concentrations vary from 98 to 454 ppm, Th concentrations from 87 and 1700 ppm and Th/U ratios range between 0.9 and 4.7 (Supplement S4). LA-ICP-MS U–Pb dating of fourteen zircons yielded ²⁰⁶Pb/²³⁸U ages in the range 456 to 553 Ma. Two analyses are 5.6 and 3.4% discordant and have ²⁰⁶Pb/²³⁸U ages of 456 respectively 472 Ma (16fmn09_1 and _9 in Supplement S4). One 6% discordant analysis (16fmn09_3) has a 530 Ma ²⁰⁶Pb/²³⁸U age. The remaining eleven analyses are concordant and have ²⁰⁶Pb/²³⁸U ages between 548 and 553 Ma, allowing a 551 ± 2.5 Ma concordant age to be calculated with a MSWD of 0.14 (Fig. 5A).

Fig. 5

Concordia diagrams of dated zircons from A granite 16FMN09 and B and C metagranite 16FMN55B young and old cluster. Individual ages of dated spots are displayed in red, and concordia ages in blue ellipses. Data-point error ellipses are 2σ. Concordia diagrams with all analysed points are given in Supplement S3

The dated zircons from metagranite 16FMN55B are euhedral to rounded and most are long prismatic crystals up to ~ 220 µm long and 50 to 80 µm wide. Most of the zircons show concentric crystal zoning; some grains show embayment. Internal textures vary from featureless and CL-dark, to oscillatory zoning, to zircons with cores and overgrowths (Supplementary figure S3). U concentrations range from 45 to 2637 ppm, Th concentrations from 32 to 1419 ppm and Th/U ratios from 0.03 to 2.8 (average 0.6, median 0.5, mode 0.6; Supplement S4). Forty-two zircons yielded ²⁰⁶Pb/²³⁸U ages in the range 433 to 2879 Ma. The two youngest grains at 433 and 495 Ma are 4.0% and 3.2% discordant, respectively, but the next youngest grain (527 Ma) is concordant (0.4%). Four grains have 2.1 to 2.9 Ga, Paleoproterozoic to Neoarchean ages, one of which is 24% discordant. Most of the remaining ages define two clusters of which the younger one yields an age of 590.3 ± 4.8 Ma for eight spots (Fig. 5B), and the older cluster yields an age of 638.4 ± 4.1 Ma for thirteen spots that, however, show a larger scatter (Fig. 5C). Zircons of the c. 590 Ma cluster tend to be idiomorphic with well-defined oscillatory zonation, whereas those of the c. 640 Ma cluster tend to be rounded, partially resorbed grains (Supplementary figure S3).

2.2 Monazite compositions and chemical Th-U-total Pb monazite ages

The monazite composition results are described in detail in Supplement S5 and EPMA compositions are presented in Supplement S6. The monazite spot ages of both samples have very large ranges, from ca. 150 to ca. 295 Ma (Supplement S6.2), and unfortunately this large scatter does not allow a geologically significant age to be calculated. However, irrespective of the spot age calculation method [i.e. Rhede et al. (1996) vs. Suzuki and Adachi (1991) and “AgeFinder” computer program of Appel (2010)], ca. 80% of the pooled ages from both samples fall between 200 and 250 Ma. The kernel density estimate (KDE) plots calculated with IsoPlotR (Vermeesch, 2018) shows the extent of scatter but indicate mean apparent ages of ca. 230 Ma and 210 Ma (with a standard deviation of ca. 30 Ma) for samples 16FMN55A and RMK5G, respectively (Supplement S5).

2.3 Mica and amphibole compositions

The minerals dated by the ⁴⁰Ar/³⁹Ar method were analysed by electron probe microanalysis to establish their compositions and check for possible heterogeneities such as zoning and inclusions that may affect interpretation of the ⁴⁰Ar/³⁹Ar gas release spectra. The spreadsheet of Locock (2014) based on the International Mineralogical Association's amphibole nomenclature (Hawthorne et al., 2012) was used to classify amphibole compositions of amphibolite 15FMN05D (Supplement S7.1). The amphiboles are relatively homogeneous calcium amphiboles (magnesio-hornblende to pargasite; with X_Mg (= Mg/(Mg + Fe²⁺)) between 0.55 and 0.67 (average 0.60), and Si a.p.f.u. (atoms per formula unit) between 6.32 and 6.94 (average 6.55). Biotites in metapelites 15FMN05D and RMK5G are fairly homogeneous and classify as siderophyllite (Supplement S7.2). Biotite X_Mg in both samples varies from 0.42 to 0.56 a.p.f.u. (average 0.46) whereas Al_total varies from 3.51 to 3.99 a.p.f.u. (average 3.63). Most white micas have end-member compositions close to muscovite (Supplement S7.3), and those in granite 16FMN50X4 have somewhat higher Mg contents (0.11 to 0.33, average 0.24 a.p.f.u.). Elevated Fe contents of magmatic white mica in granite 16FMN50X4 cause small offsets towards the ferrimuscovite end-member. Apart from a few outliers with elevated Si contents in metapelite RMK5G, all white micas are relatively homogeneous and form clusters that fall on, or trend parallel to, the Tschermak substitution line. The micas and amphiboles have normal compositions and show no compositional zoning.

2.4 ⁴⁰ Ar/ ³⁹ Ar step-heating dating

The analytical data for the ⁴⁰Ar/³⁹Ar step-heating dating experiments are listed in Supplement S8. The ⁴⁰Ar/³⁹Ar analyses of all samples yielded plateau ages (Fig. 6) that are validated by the fact that the corresponding normal isochron correlation ages yield initial ⁴⁰Ar/³⁶Ar ratios that agree within error with the atmospheric ⁴⁰Ar/³⁶Ar ratio (298.56 ± 0.31; Lee et al., 2006). Furthermore, the normal isotope correlation ages agree with the plateau ages within 2 sigma error (Supplement S8.2).

Fig. 6

A–F ⁴⁰Ar/³⁹Ar incremental step heating age spectra, Ca/K ratios and fractions (%) of the radiogenic ⁴⁰Ar of the white mica from granite 16FMN50X4, retrograde white mica and biotite from the metapelite 16FMN55A and RMK5G and amphibole from amphibolite 15FMN05D of the GMC basement

Magmatic white mica from granite 16FMN50X4 yielded a 176.6 ± 0.4 Ma plateau age for gas fractions 1–10 (Fig. 6A). Foliation-defining biotite from staurolite-biotite-white mica schist 16FMN55A yielded a 176.1 ± 0.6 Ma plateau age for gas fractions 5–8 (Fig. 6B). Decussate white micas replacing porphyroblasts in the same sample yielded a similar 177.1 ± 0.2 Ma plateau age for gas fractions 3–12 (Fig. 6C). Biotite from St-Bt-WM schist RMK5G yielded a 175.1 ± 0.5 Ma plateau age for gas fractions 5–10 (Fig. 6D) and white mica from the same sample has a 176.1 ± 0.5 Ma plateau age for steps 2–5 (Fig. 6E). Amphibole from amphibolite 15FMN05D yielded a 176.4 ± 0.7 Ma plateau age for gas fractions 3–9 (Fig. 6F).

3 Discussion

Precambrian basement in the Alborz Mountains is very poorly exposed (e.g. Allen et al., 2003), except in the GMC and nearby areas such as Lahijan. The new ages presented in this study will contribute to understanding the history of the Ediacaran crystalline basement. We suggest that the oldest, 638.4 ± 4.1 Ma zircon cluster of metagranite 16FMN55B may date the protolith magmatic crystallisation age and that the older Neoproterozoic, Meso- and Palaeoproterozoic, and Neoarchaean zircons represent material inherited from older sources (Fig. 1). The youngest, 590.3 ± 4.8 Ma age may date the deformation and recrystallisation. Both ages are ca. 15 Ma older than those reported for a migmatised granite gneiss ca. 8 km further SW that yielded two concordant U–Pb zircons ages at 621.3 ± 2.7 Ma and 575.4 ± 3.6 Ma (Chu et al., 2021a). Migmatised granite SW of Rasht yielded an even younger 571.0 ± 4.8 Ma protolith U–Pb age for a cluster of youngest zircons (Chu et al., 2021a). The 551 ± 2.5 Ma U–Pb zircon crystallisation age for granite 16FMN09 closely overlaps that of the 551 ± 9 Ma U–Pb zircon crystallisation age of granite exposed near Lahijan east of Rasht (Lam, 2002; Guest et al., 2006b). Our and published zircon ages for this part of the Alborz Mountains indicate that the basement shows a range of ages between ca. 638 Ma (latest Cryogenian) to ca. 551 Ma (latest Ediacaran). The youngest zircon ages for these basement rocks agree with detrital zircon ages of Neoproterozoic-Cambrian clastic sediments in the Central Alborz for which the youngest age clusters (559 ± 36 Ma and 537 ± 29 Ma) indicate partial derivation from sources of very latest Neoproterozoic to Early Cambrian age (Horton et al., 2008).

The combined evidence (Azizi & Whattam, 2022; Moazzen et al., 2023) indicates that, as with other Iranian terranes, part of the Alborz Mountains basement belonged to the northern margin of Gondwana in the latest Ediacaran to early Cambrian. Azizi and Whattam (2022) also pointed out that 570–530 Ma old granitoids in Iran formed in an extensional tectonic regime. The older, c. 638–590 Ma zircon ages reported from the GMC (Chu et al., 2021a; this study) may reflect subduction related arc magmatism (i.e. a true Cadomian event) and ages in this range are known from the Arabian-Nubian Shield (e.g. Johnson et al., 2011; Cox et al., 2019; Eyal et al., 2019; Dessouky et al., 2021). A northern provenance from the basement to the South Caspian Basin can neither be established nor ruled out because no age data are available for its basement rocks. In contrast, derivation of the GMC from Turan Block basement is unlikely, as the Turan Block has a different crustal makeup and is probably composed of Paleoproterozoic (2.0–1.8 Ga) and early Neoproterozoic material (Chu et al., 2021b).

Much later, fragments of the Gondwana margin rifted and drifted away from Gondwana due to the opening of the Neotethys, probably in the Permian (Muttoni et al., 2009; Berra & Angiolini, 2014), or even earlier in the Early Carboniferous (Jamei et al., 2021) along with other Iranian blocks, forming the so-called Cimmerian terranes.

Despite the very large monazite spot age range (c. 150 to c. 295 Ma, Supplement S6) and irrespective of the age calculation method (i.e. Rhede et al. (1996) vs. Suzuki & Adachi (1991)), ca. 80% of the pooled ages from both samples fall between 200 and 250 Ma, which probably indicates a Triassic age of amphibolite-facies metamorphism of the GMC metapelites. However, they do not allow to distinguish between Eo-Cimmerian and a Main Cimmerian events in the GMC. The mean apparent ages of ca. 230 Ma and 210 Ma (± 30 Ma standard deviation) for samples 16FMN55A and RMK5G, respectively (Supplement S5), are also too imprecise to distinguish between the two Triassic events. In any case, the GMC monazite ages indicate that this event was in all likelihood unrelated to both the Carboniferous high pressure metamorphism in the Asalem-Shanderman Metamorphic Complex and the amphibolite-facies metamorphism in the Allahyarlu Complex to the NW of the GMC. Wan et al. (2021) suggested that the ages of the Asalem-Shanderman Metamorphic Complex document subduction of Devonian-age Palaeotethys ocean crust in the early Carboniferous at the southern margin of Eurasia. Zanchi et al. (2009) postulated that the western part of the Alborz Mountains/Talesh Mountains belongs to southern Transcaucasia, a block or terrane west of the Alborz Mountains that had accreted to the southern margin of Eurasia during the Late Palaeozoic after full closure of the Palaeotethys Ocean. In contrast, Eo-Cimmerian accretion and collision events in central and, especially, east Iran were delayed until the late Triassic or even early Jurassic due to the wide character of the convergence zone in these parts (Fig. 1).

The Gasht-Masuleh and Asalem-Shanderman metamorphic complexes are situated quite close to each other (ca. 30 km); each must have belonged to geological units with different histories as is clear from their different metamorphic grades, time of peak metamorphism and cooling ages. The Asalem-Shanderman Metamorphic Complex accreted in the Carboniferous, which was followed by accretion of the GMC in the Triassic. It is possible that the low temperature heating steps of the ⁴⁰Ar/³⁹Ar age spectra of phengites from the Asalem-Shanderman Metamorphic Complex that yield 240–260 Ma apparent ages (Rossetti et al., 2017) may be due to partial loss of argon triggered by a thermal overprint during Triassic accretion of the GMC or similar units to the southeast. Later, the GMC and Asalem-Shanderman metamorphic complexes along with Mesozoic units were displaced left laterally along the Lahijan Fault Zone to the east of GMC (e.g. Allen et al., 2003; Safari et al., 2013) and it is possible that the initial position of the GMC was more to the south of the Asalem-Shanderman complex.

The ca. 175–177 Ma (Toarcian, late Lower Jurassic) ⁴⁰Ar/³⁹Ar plateau ages for magmatic, peak-metamorphic and retrograde metamorphic minerals from granite, metapelites and amphibolite closely overlap within analytical uncertainty. They are unexpectedly young and therefore unlikely to be post-peak metamorphic (i.e. post collisional) cooling ages. When not rapidly uplifted to much higher crustal levels shortly after metamorphism, regionally metamorphosed rocks can be expected to cool slowly and pass sequentially through the nominal closure temperatures of, respectively, hornblende (580°–480 °C, average c. 550 °C; Harrison, 1982), white mica (400–425 °C; Harrison et al., 2009) and biotite (340°–280 °C; McDougall & Harrison, 1999). This clearly did not occur in the GMC. The time span between the amphibolite facies metamorphism sometime between 200 and 250 Ma (monazite ages; closure temperature c, 500–800 °C, Smith & Giletti, 1997) and the 175–177 Ma ⁴⁰Ar/³⁹Ar ages appears too large to reflect slow cooling at depth from peak metamorphic temperatures (c. 630–720 °C, Razaghi et al., 2018) to closure temperatures of the dated metamorphic minerals, especially biotite. Instead, the fact that minerals with different closure temperatures and irrespective of being magmatic or peak- or retrograde metamorphic all have ⁴⁰Ar/³⁹Ar ages in the narrow interval 175–177 Ma, indicates very fast cooling through the temperature range ca. 550–300 °C in early/mid-Jurassic times. We repeat, that petrographically late minerals, such as andalusite, are retrogressed to decussate white micas that yield a ⁴⁰Ar/³⁹Ar age that is indistinguishable from the ages of the other minerals.

The most likely interpretation is that after accretion and amphibolite-facies metamorphism in the (mid?) Triassic, the rocks of the GMC remained at depth and above the closure temperature of at least hornblende (c. 550°C). They were uplifted and (perhaps even exhumed) in an early / mid-Jurassic tectonic event during which all minerals passed through their respective closure temperatures within a very short time (i.e. very rapid cooling).

The Middle Jurassic event in northern Iran (Alborz) is known as the Mid-Cimmerian event and is well recognized, particularly in the northeastern part of the Alborz Mountains (Fürsich et al., 2005). Shemshak Basin shallowing due to uplift and erosion was followed by a second pulse of extension in the Middle Toarcian to early Late Aalenian that caused basin subsidence and deposition of black shales (Fig. 7). This extensional stage has generally been associated with the opening of the South Caspian Basin (e.g. Fürsich et al., 2009a). The Toarcian, 175–177 Ma ⁴⁰Ar/³⁹Ar mineral ages of the GMC must reflect uplift-related rapid cooling during a Middle Jurassic phase of extension that in the Alborz Mountains proceeded from west (e.g. GMC, this study) to east (e.g. upper Shemshak Group in the southern Alborz, Wilmsen et al., 2009a), and is known as the Alborz rift basin stage (Fürsich et al., 2009b). Wilmsen et al. (2009a) attributed this event to continuing subduction of the Neotethys Ocean to the south and the opening of large back-arc-rift basins along the Eurasian margin, such as the South Caspian Basin (see Fig. 3 of Wilmsen et al. (2009a) and references therein).

Fig. 7

Simplified model for Toarcian-Aalenian extension, GMC uplift (as rift shoulder?), cooling, possible exhumation and concomitant basin formation in the western Alborz Mountains (after Fig. 3 of Wilmsen et al., 2009a)

An alternative explanation, namely a mid-Jurassic high temperature overprint resetting hornblende, white mica and biotite having post-collisional, presumably mid- to late-Triassic cooling ages still cannot explain their overlapping ages, which in this case too, can only have been caused by very rapid cooling. Furthermore, possible heat sources are not obvious in the GMC. Gabbros that intruded in mid-Cretaceous times are small and too young (ca. 99 Ma, Rezaei et al., 2023). The presence of a 180 ± 1.3 Ma undeformed granite that intrudes Precambrian granitoid gneisses of the southeastern part of the GMC (Chu et al., 2021a) indicates some Middle Jurassic magmatic activity, but this intrusion is too small even to be shown on the 1:250,000 scale Bandar-e-Anzali (formerly: Bandar-e-Pahlavi) geological map (Clark et al., 1977). Its formation and emplacement may be related to the mid-Jurassic extension that also uplifted the GMC. A different granite type in the northern part of the GMC (known as the Gasht granites) comprises mainly S-type granites containing muscovite and tourmaline, has a peraluminous composition and originated by low degrees of partial melting of muscovite-rich metapelites at mid-crustal levels under low water activities, probably during the collapse of previously thickened crust (Omrani et al., 2013a). However, no crystallisation ages are known for these granites that nevertheless are labelled Triassic on the geological map (Clark et al., 1977). Finally, fluids could also have reset the K–Ar systems, but apart from local fluid-induced retrogression, there is no evidence for large scale fluid introduction in the GMC.

The GMC saw compression and amphibolite-facies metamorphism sometime in the Triassic (Fig. 8A), which in the Gasht area was followed in the Lower Jurassic by deposition of Shemshak Group fine-grained carbonate sediments. Hence, no orogenic event can have been responsible for the rapid uplift and cooling of the GMC basement in the mid-Jurassic. Similarly, Zanchi et al. (2009) noted that the Asalem-Shanderman Metamorphic Complex to the northwest of the GMC was exposed sometime in the Early Jurassic as Shemshak Group sediments here contain clasts of serpentinite, eclogite and other metamorphic rocks.

Fig. 8

A and B Palaeotectonic reconstruction showing the approximate position of the GMC (star symbol) in the Late Norian and Toarcian (modified from Barrier et al., 2018). SCB: South Caspian Basin

Mid-Jurassic extension (Fig. 8B) was not limited to the Iranian Alborz Mountains, but is less evident in the western Alborz Mountains, in the Caucasus region and Western Anatolia (Rolland, 2017). Following Permian–Triassic arc magmatism, late Triassic collision of the Iranian Cimmerian block with Eurasia led to extension within the overriding plate, culminating in the opening of the continental back-arc Caucasus Basin. Trench retreat was followed by arc magmatism in the Early-Middle Jurassic (ca. 140–180 Ma) and accompanied by back-arc spreading (c. 160–180 Ma, ⁴⁰Ar/³⁹Ar and U–Pb zircon ages, Vasey et al., 2021) as well as by shallow marine sedimentation (Adamia, 2011).

4 Conclusions

The c. 638 to c. 551 Ma zircons ages reported from this part of the Alborz Mountains (this study; Chu et al., 2021a) indicate that the GMC basement once belonged to the northern margin of Gondwana. Moreover, the late Ediacaran 551 ± 2.5 U–Pb zircon crystallisation age of GMC granite is very similar to published ages from other Iranian Cimmerian terranes and such granitoids were probably generated during a stage of crustal extension at the Gondwana margin (e.g. Azizi & Whattam, 2022), part of which rifted away due to the opening of the Neotethys Ocean, thus constituting a Cimmerian microcontinent (or part thereof) that drifted towards the southern margin of Eurasia. Accretion and collision associated with peak metamorphism in Triassic times. Despite their large range, monazite ages also show that the GMC record a geological evolution that is completely different from that of the Asalem-Shanderman Complex to its northwest. The Asalem-Shanderman Complex probably records the latest Devonian to early Carboniferous subduction event, and the GMC the Triassic accretion-collision event. Much younger and overlapping 175–177 Ma, Toarcian ⁴⁰Ar/³⁹Ar ages for both pro- and retrograde minerals (amphibole, white mica and biotite) with different closure temperatures, reflect the thermal response of the GMC basement below the Shemshak Group to mid-Jurassic extension-triggered uplift. This event is associated with a mid-Jurassic extensional tectonic phase, known as the Alborz rift basin stage. Extension probably started in the western Alborz Mountains in the Toarcian, migrated eastwards, and culminated in the Aalenian in the eastern Alborz with the formation of a deep-marine basin. It may have been triggered by the onset of subduction of Neotethys oceanic crust beneath the Central Iranian Microcontinent to the south, or opening of the South Caspian Basin to the north (e.g. Wilmsen et al., 2009b).