Thin sections of the samples were examined to select the best minerals available for ⁴⁰Ar-³⁹Ar dating. The mineral separation was performed at the CICESE's Geochronology Laboratory. The samples were crushed, sieved and washed in distilled water. To concentrate the minerals of interest a Frantz magnetic separator was used. Finally the minerals were prepared by hand picking under the binocular microscope. Care was taken to insure that the biotite and muscovite concentrates were free of alteration. The minerals were irradiated at the Uranium enriched research reactor of McMaster University in position 5C using Cd liner. As irradiation monitor, sanidine TCR 2 of 27.87 ± 0.04 Ma was used. The argon isotopes were corrected for blank, decay of ³⁷Ar and ³⁹Ar, mass discrimination and interference reactions of Ca, K and Cl. All errors reported are 1σ. The decay constants recommended by Steiger and Jäger (1977) were used in the age calculations. Whenever it applies, the equations given by York et al. (2004) were used to calculate the best straight line. With the exception of within spectra errors, the integrated, plateau and isochron ages include the uncertainty in the irradiation monitor J.

The results obtained for aplite-granite Muscovite 2a and Muscovite 2aa are given in Fig. 3 and Table 2. Biotite 1a from the Bendenow granite-gneiss and Biotite 2a from the aplite-granite are presented in Fig. 4 and Table 3. Muscovite sample 2aa was step-heated with the argon released in 9 fractions. The first part of the age spectrum (Fig. 3a) yielded unrealistic old ages indicating the presence of excess argon. Then some argon loss is indicated by age less or equal to 153 Ma on two fractions, these represent ∼30% of the ³⁹Ar released. The last five fractions fulfill the criteria for plateau age: all but one of the ages for these fractions are in agreement within 1σ, additionally these fractions represent more than 50% of the ³⁹Ar released. The weighted mean of the individual ages of these five fractions yielded a plateau age of 179.5 ± 1.7 Ma. In the correlation diagram (Fig. 3c) the inverse of the y-intercept of the isochron calculated with plateau fractions indicates a (⁴⁰Ar/³⁶Ar)_i = 1222 confirming the presence of excess argon. Since the ⁴⁰Ar of the plateau fractions consists of almost pure radiogenic argon, the effect of excess argon in them is negligible, thus yielding an isochron age of 176.9 ± 3.9 Ma, which is statistically undistinguishable from the plateau age. Muscovite sample 2a behaved in a similar manner as Muscovite 2aa. A more detailed experiment was performed with this sample since 14 fractions were collected. Again the age spectrum (Fig. 3a) displays old ages for the first fractions followed by low ages, but in this case the shape of the age spectrum indicates a more pronounced argon loss. The plateau of Muscovite 2aa is defined by the fractions released with a laser power of greater or equal to 0.68 Watts. The ages obtained on Muscovite 2a for the argon released applying similar laser power, range between 140 and 160 Ma, displaying an up and down staircase pattern that climbs to 175 Ma on the last fraction, an age close to the plateau age defined by Muscovite 2aa. The ³⁷Ar_Ca/³⁹Ar_K diagram (see Fig. 3b) indicates that the Ca/K ratio of these muscovites is slightly different, which may explain the different behaviors between these micas with respect to their argon retentivity, muscovite 2aa displaying a less disturbed pattern than muscovite 2a on this diagram.

A very detailed step-heating experiment was performed on Biotite 1a, with a total of 24 fractions collected (Fig. 4). An extensive plateau is observed for this sample. The first part of the age spectrum (see Fig. 4a) indicates slight argon loss; however, twenty one consecutive fractions, which represent 88% of the ³⁹Ar released, yielded ages statistically undistinguishable within 1σ. The weighted mean of their individual ages yielded a plateau age of 76.8 ± 0.2 Ma. In the correlation diagram (Fig. 4c) the plateau fractions cluster around a point close to the abscissa axis. This distribution does not define an isochron line. The 76.7 ± 1.2 Ma age displayed on the correlation diagram (Fig. 4c) was calculated with the plateau fractions of Biotite 1a and a y-intercept through (⁴⁰Ar/³⁶Ar)_i = 295.5; this is equivalent to assume that the non radiogenic ⁴⁰Ar is of atmospheric composition. The best estimate for the age of Bendenow granite-gneiss is thus taken from the plateau age of 76.8 ± 0.2 Ma of Bitotite 1a. The other biotite analyzed comes from the aplite-granite, biotite 2a sample, on which a less detailed experiment was conducted; only 10 fractions were collected. The integrated age obtained on Biotite 1a is statistically undistinguishable from the integrated age of Biotite 2a with values of 72.9 ± 0.6 Ma and 72.4 ± 0.6 Ma respectively. However, Biotite 2a yielded a perturbed age spectrum. It is possible that this biotite was older and its actual age is the product of a resetting event associated to the Bendenow granite-gneiss intrusion.

Material from the partially molten layer may ascend as diapirs if the balance between vertical flow (buoyancy) and lateral flow (thinning/extrusion) allows a diapir to rise to shallow levels without significant cooling (Teyssier and Whitney, 2002). The buoyancy forces arising from thickened crust within the transpression have caused the emplacement of the Bendenow granite-gneiss (Sarkarinejad and Alizadeh, 2009). As shown by Fletcher (1995) and Ramsay (1967), gneiss domes without spatial association of synkinematic plutons could develop by formation of doubly plunging antiforms. The Bendenow granite-gneiss in the core of the Tutak gneiss dome is a synkinematic batholith that has been emplaced by forming of doubly plunging anticline (Sarkarinejad and Alizadeh, 2009).

Dips are steeper in the north-central part of the range in part because the foliation is locally steeper near the intrusive contact of the pluton (Alizadeh, 2008). The sub-vertical orientation of the foliation in the contact rocks between aplite sub-dome and schists in the Tutak complex is parallel (mimicking the dome shape), to illustrate the injection of aplite-granite pluton into schist facies in the core.

The prograde and retrograde metamorphism in the Tutak gneiss dome has occurred at two stages of conjugate deformations, which are related to the different proportions of pure shear and simple shear components, respectively. During the prolonged stress field of deformation and high temperatures in the feldspar grain of the aplite-granite, plagioclase twining produced lattice misorientation and line dislocations and lattice glide. The orogenic process was reactivated by intrusion of the new compressional granitic magma inside the Tutak complex and metamorphosed the first stage granite (aplite-granite) and new sediments (Alizadeh, 2008).

Thrust up within the sediments, are pieces of the basement. Amphibolite-facies in the Tutak complex originally derived from the basic and ultrabasic rocks of down going metamorphosed oceanic slab so, exhibit a retrograde metamorphism (Nuri, 2005). Later deformation under greenschist facies conditions resulted in retrogression, greenschist ultramylonite, and the formation of extensive chloritic breccia localized at the top of the shear zone. Multiple microfractures in the feldspar porphyroblasts also document the retrograde metamorphism in the Tutak complex.

The aplite is the oldest granite, which belongs to the old continental basement of the Iranian shield (Alizadeh, 2008). It existed long before the Neo-Tethyan Ocean opened and the other granite had been emplaced at the contact between the African-Arabian continent and Iranian microcontinent, within the transpressional tectonic regime (Sarkarinejad et al., 2008; Sarkarinejad and Alizadeh, 2009). Fig. 5 represents the subduction and closing of the Neo-Tethyan Ocean and structural evolution of the Tutak gneiss dome.

According to the proposed model, the sequence of the Tutak complex has deposited on the continental slope that lays to the west of it, before the generation of Neo-Tethys. At the Late Carboniferous, the Paleo-Tethys started subduction (Golonka, 2004) then, by forming of the Neo-Tethyan Ocean, the studied area was upraised and the Surian complex, at the Permian, laid on the Tutak complex unconformably. After final closure of the Paleo-Tethys, the Neo-Tethyan started to subduct underneath Eurasia (Sepehr and Cosgrove, 2004). As the Neo-Tethyan Ocean started to be subducted beneath central Iran at 179.5 ± 1.7 Ma, fore-arc basin sediments, consisting first of sandstones and shales, and then of carbonate rocks that were deposited on top of the older sediments, get metamorphosed. After the subduction and Arabia-central Iran contact at 76.8 ± 0.2 Ma, the new stage of granite invaded the pre-existing facies by emplacement in bivergent wedges (Sarkarinejad and Alizadeh, 2009). By continuous deformation, enhanced heat flow along the Dehbid shear fault system (Surian and Mazayjan) in wrench corridor enabled orogen-parallel extension at NW–SE trend and formation of the syn-compressional plutonism.

The ⁴⁰Ar-³⁹Ar plateau ages of early generations of biotite from the lower metamorphic melange at Neyriz area (Fig. 1), are 119.95 ± 0.88 Ma and 112.58 ± 0.66 Ma. This Late Aptian age is related to early thrusting and formation of HP-LT metamorphic rocks. The dating of two amphibole samples from the amphibolite yields a weighted mean age of 91.23 ± 0.89 Ma. This Turonian–Cenomanian age suggests a later metamorphic event associated with subduction and obduction of the Neyriz ophiolite and later lateral extrusion of HP–LT metamorphic rocks along the inclined Zagros accretionary prism (Sarkarinejad et al., 2009).

There are two ophiolite belts; the Khoy-Neyriz and Naien-Baft surrounding the studied area (Ghasemi and Talbot, 2006). The first ophiolite belt has been obducted over the synthetic thrust faults aligned in the Zagros Thrust System and the other has been thrusted by forming the back-thrust. The presence of compositionally diverse extrusive rocks (e.g. basaltic andesite and andesite) intercalated with a variety of the Upper Cretaceous sedimentary rocks indicates that the Naien ophiolite has both island arc and intra-oceanic components (Hassanipak and Ghazi, 2000).

The closing of the Neo-Tethyan Ocean develops at various rates in the east and west of Iran (Talebian and Jackson, 2004); so convergence of the Arabian platform and central Iranian microcontinent represents an oblique convergence (without interruption by now). Opening of Red Sea and Gulf of Aden resulted in the rotation of Arabia with respect to Africa (Nubia and Somalia) since 30 Ma (Bonatti, 1987; Guiraud and Bosworth, 1999; Hempton, 1987).