Petrogenesis and geochemistry of the Tabas Black Land volcanic ﬁeld: implications for volcanic activity along the Nayband Fault, East Iran

The 14 Ma Tabas Black Land volcanic ﬁeld lies on the major active Nayband Fault, Western Lut Block, central-east Iran. Eroded cones and broad craters are surrounded by lavas. Eastern lavas are fault-deformed, partly covered by sediments, while western undeformed lavas stand out as inverted relief. We study the magmatic evolution of this volcanic ﬁeld and assess relationships with regional tectonics. Compositions are intraplate basaltic trachyandesite– basaltic-andesite, produced from postcollisional asthenospheric melting. Modelling indicates 5–10 wt.% partial melting of primary garnet peridotite mantle with residual garnet. Modest K 2 O values are consistent with limited source hydrous phases (e.g. phlogopite). Low P 2 O 5 /K 2 O indicates little crustal contamination. Geothermobarometry results point towards lower crust (480–760 MPa = 18–29 km) magma storage and di ﬀ erentiation, where olivine and clinopyroxene fractionated to produce di ﬀ erentiated trachybasalt to basaltic trachyandesite at magma temperatures of 1150–1200 ◦ C. Compared to other Lut Block volcanics, there is more partial melting in Tabas, with the lowest level of crustal contamination. The ongoing 14 million year period of volcanism along the Nayband Fault appears to have had the same source, migrating southwards over time from the Tabas Black Lands with decreasing partial melting. Nayaband magmatism seems to be dwindling, even if the fault itself remains active.

of the Nayband Fault. To do this, we carried out field and remote sensing mapping, and analysed field samples for whole rock chemistry, and Nd, Sr and Pb isotopes. We compare these results with data from the other volcanoes along the Nayband Fault to chart the degrees of partial melting, crustal contamination, and fractional crystallization. We compare in particular with the Gandom Beryan volcanic field, which is located in the southern part of Nayband Fault where compositions [Yousefi et al. 2017] and ages are best known [2.2-2.6 Ma; Walker et al. 2009].

Tabas Black Land description
We start with a description of the Tabas Black Land, then give the regional and local settings and finally present previous studies with Tabas placed in a syn-to post-collision geodynamic context.
The Tabas Black Land erupted very close to the Nayband Fault zone, and is part of a series of volcanic fields spread along the fault, so volcanism is probably genetically related to the fault. It is a broad lava field of nearly 300 km 2 , surrounded by several small scattered outliers ( Figure 2). The complex was erupted onto Neogene deposits in the east and folded shales and sandstones from different members of the Nayband formation to the west. Crossing the area, long trains of sand dunes climb up and down the topography and drape some western parts of the Tabas lava field (see Figure 2).
Three main eroded volcanic edifices are visible as well as some small craters in the lava fields. The western side of the field is exposed as inverted relief, with lavas raised up to 100 m above the present surface. In contrast, the eastern side consists of lavas covered by alluvial fans from the Nayband Fault hills. Smaller eastern outliers form small steep hills, where the lavas are folded and tilted by the Nayband Fault deformation ( Figure 2).
The whole assemblage of features produces a spectacular landscape going from the fault line hills, through the north east outliers, and gradually changing into a higher plateau to the south, (locally called Takhte-Nader), terminating with the southernmost outcrops jutting out above the sandy desert ( Figure 2). This landscape, which is both scientifically interesting, providing are wide range of clearly observable geological features, and extremely beautiful, is under consideration as a protected geological area, such as a geopark.
The region is also particularly susceptible to natural hazards, for example the old city of Tabas was destroyed by an earthquake in 1979 [Berberian 2014], and more scientific study, description and outreach, such as through geoheritage, could help mitigate the seismic risk, and others such as eruption and flooding.

Regional setting
This section gives the regional context for Tabas, which helps understand the broader-scale tectonic relationships. The Tabas Black Land is situated within the greater context of the Alpine-Himalayan orogenic belt [Darvishzadeh 1991]. It stands on 45 to 48 km-thick crust [Dehghani and Makris 1984] formed by folding and thrusting between continental fragments of Gondwana to the south and Laurasia to the north. The crust of the Iranian plateau is made of a mosaic of different Gondwana continental fragments which drifted hundreds to thousands of kilometres before being united during successive phases of collision [Berberian 1989].
Today, the Iranian plateau is surrounded by convergent margins, with the Arabian plate to the west and southwest, the Indian oceanic plate to the south, the Eurasian plate to the north, and the Helmand (Afghan) block to the east. The Lut Block is part of the Iranian plateau, lying between the east Iranian flysch zone (Sistan suture zone) along the Afghanistan boundary and the Tabas block to the west.
From a paleogeodynamic point of view, the Lut continental fragment was attached to the northern edge of Gondwana up to the end of the Carboniferous. After the Hercynian orogeny, during the opening of the Neotethys ocean to the south and the closure of the Paleotethys to the north of Iran, the continental fragment containing the Lut and Tabas blocks separated from Gondwana and eventually joined the southern boundary of Laurasia. At the end of the Triassic, the Lut and Tabas blocks moved separately under rotation, creating extension between them. From the upper Cretaceous, generalised compression ensued and widespread Tertiary magmatism followed, which is ongoing [Tarkian et al. 1983]. In the Lut Block, these extensive volcanics cover older formations [Nogole-Sadate 1978].
The Lut Block is bounded by the Nehbandan and Nayband Faults, two major dextral north-south strikeslip faults with a displacement of about 1.5 mm per year [Walker et al. 2009], and is responsible for significant rotation. For example, Davoudzadeh et al. [1981] and Dercourt et al. [1986] suggest an 135°anticlockwise rotation for the Lut Block after the Triassic, an idea supported by Bagheri and Stampfli [2008].

Local Geological Setting
The Lut Block extends 900 km north-south and has an average width of 150-200 km. It forms the eastern part of the central Iran microcontinent. The Nehbandan fault and East Iran flysch basin define its eastern border and the Nayband Fault forms the boundary to the west, separating it from the Tabas block. The block is limited by the Kashmar depression to the north and the Jazemurian depression to the south [Aghanabati 2001;Stöcklin and Nabavi 1973] and lies along the supposed Ural-Oman Structural lineament [Leonov 1994].  Nabavi 1976], showing major faults in purple, the study area and associated volcanic sites, and major cities.
A number of faults cut the Lut Block, and it has internal seismicity as well as Quaternary volcanic activity [Mohajer-Ashjai et al. 1975;Davoudzadeh et al. 1981;Dercourt et al. 1986]. However, despite the major boundary faults the block has remained intact, probably due to a strong metamorphic basement [Mohajer-Ashjai et al. 1975], formed in the Middle Triassic after the early Cimmerian orogenic phase [Alavi Naeini 1993]. Sedimentary strata on the Lut Block are mainly younger than Permian and consist of shallow marine carbonates, shales and sandstones [Stöcklin and Setudehnia 1971]. Continental Neogene sedimentary deposits and Quaternary sand dunes, salt flats and alluvial fans cover large areas of the block [Chance et al. 1981].
Magmatism in the Lut Block started in the Late Jurassic and continued to the Quaternary, forming a variety of volcanic, subvolcanic, and intrusive rocks [Esmaeily et al. 2005;Yousofzadeh 2009;Saadat et al. 2010;Elahpour 2011;Arjmandzadeh and Santos 2013;Su et al. 2014].

Collisional-post collisional volcanism and Lut
Block geological history The Tabas Black Land and other Lut Block volcanism provide an opportunity to study syn-and postcollisional magmatism in the context of the assemblage of Gondwanan micro-continental blocks, with ongoing, mainly strike-slip deformation [Shabanian et al. 2012]. Similar Neogene-Quaternary intraplate basaltic volcanism has been studied in different but comparable settings. Examples of well-studied recent basaltic provinces include: 1) those formed during the Carpathian collision of western and central Europe [Wilson and Downes 1991], including the Little Hungarian plain [Harangi et al. 1995], the Pannonian basin [Embey-Isztin et al. 1993;Salters et al. 1988]; 2) different sides of the Anatolian plateau in Turkey such as: [Bağci et al. 2010] for the southern side, [Alıcı et al. 2002;Aldanmaz et al. 2000] for the western side, [Kheirkhah et al. 2009;Pearce et al. 1990] for the eastern side; and 3) examples from the Arabian Harrat Ash Shaam [Weinstein et al. 2006], the Koh-e-Soltan in Pakistan [Nicholson et al. 2010], Oman [Nasir et al. 2006] and Armenia [Neill et al. 2013].
For the Iran plateau, the previous studies in eastern Lut include those along the southern and northern segments of Neh Fault [Elahpour and Heuss-Aßbichler 2017;Pang et al. 2012], western Lut, along the southern segment of the Nayband Fault, mainly focused on Gandom Beryan volcanic plain [Yousefi et al. 2017;Saadat et al. 2010;Walker et al. 2009], and western Lut  along the northern part of the Nayband Fault [Pang et al. 2012;Saadat et al. 2010;Hashemi et al. 2008]. Also within the Iran plateau there are studies on the major stratovolcanoes of Taftan [Ghazban 2004;Biabangard and Moradian 2008;Nicholson et al. 2010] and Bazman [Saadat and Stern 2011;Saadat et al. 2008;Saadat et al. 2009;Saadat et al. 2010;Saadat 2010] in the southern part of the Lut Block within the Makran arc. Existing K-Ar and 39 Ar/ 40 Ar age data indicate that eruption of alkali basalts in eastern Iran started at ∼15.5 Ma and were still happening at ∼1.6 Ma [Conrad et al. 1981;Camp and Griffis 1982;Jung et al. 1984;Walker et al. 2009]. New 39 Ar/ 40 Ar ages [Pang et al. 2012] provide additional constraints on the duration and continuity of the volcanism in eastern Iran. Their age measurement in Tabas (14.3 Ma at 32°57'09" and 57°20'27") is slightly younger than the oldest published K-Ar age (∼15 Ma) for eastern Iran Neogene-Quaternary alkali basalts and shows that the volcanism initiated at ∼14 Ma along the Nayband Fault while this onset is 3 to 4 million years later north of the Sistan-suture zone, in the eastern part of the Lut Block.

Volcanic studies around the Lut Block
Four volcanic areas have been studied around the Lut Block. The first one is on the north Nayband Fault [Pang et al. 2012;Saadat et al. 2010;Hashemi et al. 2008] and is our Tabas Black Land study area.
The second one, the Gandom Beryan area, is located in the southern part of the Nayband Fault [Yousefi et al. 2017;Saadat et al. 2010;Walker et al. 2009].
The third one is referred to as Sarbisheh (after the nearby city) for those from the northern part of the Neh Fault, at the opposite side of the block to our study area (Pang et al. [2012], and one sample in this study).
The fourth one is the Nehbandan volcanic area in the southern part of the Neh Fault [Pang et al. 2012]. Walker et al. [2009] concluded that the volcanic rocks were from a probable asthenospheric mantle source at depths of ∼80 km. Hashemi et al. [2008] have concluded that these intraplate basalts have a mantle source and ascended rapidly to the surface. Saadat et al. [2010] have concluded that these rocks were formed by low to moderate degrees of partial melting and suggest the involvement of EM-type mantle asthenosphere in their source. Yousefi et al. [2017] concluded that the parental magmas of Gandom Beryan volcanics might have originated in the garnet-lherzolite stability field. Pang et al. [2012] concluded that these post-collisional and intra-plate alkali basalts underwent small amounts of fractionation of olivine, clinopyroxene and Fe-Ti oxides with minor crustal contamination before eruption.

Analytical techniques
The main lithologic and structural features were mapped in the field, based on prior work using Google Earth images, which were used to select sample sites. Petrographic studies were then carried out at Payamenoor University of Birjand, and twenty nine fresh samples were chosen for geochemical analysis which was mostly carried out at Laboratoire Magmas et Volcans at Clermont Auvergne University, France.

Major element methods
The inductively coupled plasma -atomic emission spectroscopy (ICP-AES) technique was used for major elements. For this, 100 mg of sample powder was mixed with 300 mg of LiBO 2 flux in a porcelain dish, transferred to a graphite crucible machined from 25 mm diameter rods and fused for 5 minutes at about 1100 • C in an induction furnace (2 kW). The melt was poured into a disposable polystyrene beaker containing 50 mL of 1 M HNO 3 stirred by a magnetic bar. After complete dissolution of the shattered quenched melt droplets (about 15 minutes), the solution was passed through a filter paper (Whatman, # 40, 110 mm diameter) to remove graphite particles. The final volume was diluted to 200 mL with 1 M HNO 3 . Reference materials GH (for Si, Na and K) and BR (for other elements) both from CRPG, Nancy, France (Centre de Recherches Petrographiques et Geochimiques), prepared in the same way as the unknown samples, provided high points for the calibration lines, while a pure LiBO 2 solution (300 mg in 200 mL 1 M HNO 3 ) was used as the zero in every case.

Trace element methods
In addition, trace elements and REE (rare-earth element) content of 11 samples were determined by inductively coupled plasma mass spectrometry (ICP -MS) at Laboratoire Géoscience Ocean, Université de Bretagne Occidentale, France. Detailed procedure is from Barrat et al. [1996]. Basaltic samples BIR -1 and BCR -2 were selected as the calibration standards. Thulium method [Barrat et al. 1996] and BHVO -2 were used as internal and external standards to monitor accuracy during analysis respectively.

Isotope methods
Strontium-Neodymium isotope measurements on 11 samples were made by thermal ionization mass -spectrometry (TIMS, Triton, Thermo Scientific) in static mode with relay matrix rotation (virtual amplifier) on double W filaments for Nd and on a single Re filament for Sr at Laboratoire Magmas et Volcans. Chemical Sr-Nd separations were achieved using the method from Pin and Gannoun [2017]. Sr and Nd blanks for the complete procedure were <5 ng and <200 pg, respectively. Sr and Nd isotopic measurements were (1) corrected for mass-fractionation using an exponential law and 86 Sr/ 88 Sr = 0.1194 or 146 Nd/ 144 Nd = 0.7219, respectively, and (2) normalized to the value of the NIST SRM 987 standard (value: 86 Sr/ 88 Sr = 0.710245) or the value of the JNdi-1 Nd standard (value: 143 Nd/ 144 Nd = 0.512100 ± 5 (2σ ), n = 5), respectively. External reproducibility was monitored by repeated analyses of NIST SRM 987 ( 86 Sr/ 88 Sr = 0.710244 ± 10 (2σ ), n = 4) for Sr, and JNdi-1 ( 143 Nd/ 144 Nd = 0.512101 ± 10 (2σ ), n = 4) for Nd. These values were equal, within error margins, to the proposed values for each standard. We selected 6 samples for Pb isotope analysis. For this measurement, 100 mg of powder were digested in an HNO 3 -HF mixture. Pb isotopic compositions were measured by MC-ICP-MS following the procedure described by White et al. [2000] at Laboratoire Magmas et Volcans. Total procedural blanks vary below 0.240 ng of Pb, which is negligible (0.05 %) compared to the amount of Pb loaded on the columns (200 to 500 ng). We used international standards (AGV-2, BHVO-2 and BIR-1) in order to test the reproducibility of our method. Values obtained for AGV-2 are 206

Mineral Composition Methods
Mineral compositions in 6 selected samples were analysed using a CAMECA SX100 Electron Microprobe at Laboratoire Magmas et Volcans. Operating conditions were 15 kV accelerating voltage, 15 nA beam current and 10 s counting times on peak. Standardization was performed on natural and synthetic minerals and checked every 24 hours. Standards included wollastonite (Si and Ca), TiMnO 3 (Ti and Mn), Al 2 O 3 (Al), Cr 2 O 3 (Cr), Ffayalite (Fe), forsterite (Mg), albite (Na), and orthoclase (K).

Petrography
The in situ appearance of Tabas lava is black and blocky, generally massive, but sometimes vesicular, giving rise to the local name "Martian land". Near the surface of flows, we observe vesicles up to 3 cm in diameter, frequently filled by white zeolites, carbonates and silicate minerals. The most common textures of these rocks are aphyric, microgranular to slightly porphyritic with microlitic and microlitic-glassy mesostasis. Plagioclase microlites form fluidal to subfluidal textures, sometimes organized around rounded mafic phenocrysts. Opaque minerals (Ti-magnetite and Ilmenite) are abundant. Photomicrographs of two of these rocks (sample 14, a typical basaltic trachyandesite: Figure 4A, B; and sample 25, one of the more differentiated samples (trachyandesite): Figure 4C, D) show that the phenocryst content is generally low, mostly about 5 vol.%, with a few samples having up to 20 vol.% (Figure 4A). The phenocrysts mainly consist of olivine (0.5 -1.5 mm in diameter), clinopyroxene (0.8-1 mm in diameter) and plagioclase (1-1.5 mm in diameter).
Coarse euhedral and subhedral olivine phenocrysts are generally iddingsitized along internal cracks and margins, and fresh olivine crystals are rarely seen (Figure 4B). Clinopyroxene phenocrysts are scattered in the mesostasis, which also contains plenty of smaller clinopyroxene (cpx) crystals. Plagioclase phenocrysts are occasionally present in some of the more differentiated basaltic-andesite samples ( Figure 4C, D). In these samples, mafic phenocrysts are uncommon and the lack of peripheral reactive edges on the olivine crystals is an important indication of an alkaline nature. In sample 25 two distinct compositional zones were distinguished, with one apparently being an injection of a more evolved magma containing coarse plagioclase phenocrysts ( Figure 4C, D).

Mineral Chemistry, thermometry and barometry
Olivine: The composition of olivine phenocrysts from Tabas ranges from Fo 49 to Fo 82 . Ol ivine phenocrysts are normally zoned, and compositions are similar between samples, with only a few samples presenting high forsteritic values (mineral data given in Supplementary Material). Clinopyroxene: Clinopyroxene phenocrysts are zoned and predominantly augitic (Wo 41 , En 48 , Fs 11 to Wo 36 , En 44 , Fs 20 ) with little difference between samples ( Figure 5).
Plagioclase and alkali feldspar: The compositional range of plagioclase from Tabas is An 63 -Ab 37 to An 50 -Ab 50 (labradorite) and there is no or only very weak zoning. Rare K-feldspar occurs in the groundmass of some samples.

Thermo-barometry
In the paragenesis observed in the basalts, clinopyroxene is the only mineral that can be used for barometry, based on pressure-dependant jadeite substitution [e.g. Putirka 2008]. It is also the best thermometer, because its structure allows for more possible substitutions than olivine or plagioclase.
Two clinopyroxene phenocrysts in sample 23  Putirka [2008]). Based on the clinopyroxene/melt equilibrium, these phenocrysts crystallized at a pressure of 620 ± 140 MPa [Neave and Putirka 2017] and a temperature of 1169± 45 • C (Eq. 33 of Putirka [2008]). Uncertainties reported here are standard errors of estimates for the barometer and thermometer calibrations, as they are significantly larger (about 5 times) than analytical uncertainties and data dispersion. The temperature is consistent with a pyroxenesaturation temperature of 1174 • C (Eq. 34 of Putirka et al. [2009]) for sample 23. Since samples are almost aphanitic, and phenocryst cores are in equilibrium with the bulk rock composition, we can assume that their bulk composition is close to a liquid composition. The saturation temperature is thus the temperature at which the magma started crystallizing clinopyroxene. Pyroxene-saturation temperatures for all the bulk rocks vary from 1146 • C to 1178 • C with an average of 1159 ± 49 • C. Thermometry and barometry based only on the composition of clinopyroxene is usually less accurate, but calculations based on clinopyroxene phenocrysts from all six samples gives an average pressure of 410 ± 310 MPa (Eq. 32a of Putirka [2008]) and an average temperature of 1152 ± 58 • C (Eq. 32d of Putirka [2008], recalibrated after Nimis and Taylor [2000]), consistent with the values given by the more accurate clinopyroxene/melt equations.
The early olivine phenocrysts can also be used as a thermometer. Samples are sparsely porphyritic and show no evidence of xenocrysts. Olivine cores in sample 23 are in equilibrium with the liquid composition (Fe/Mg Kd of 0.32), strongly suggesting that the bulk rock is a true melt composition from which the olivine (and clinopyroxene) crystallized. Olivine saturation temperatures for this sample vary between 1177 • C [Sugawara 2000] and 1220 • C [Ford et al. 1983]. Assuming the other samples were also olivine-saturated melts, they would have crystallized olivine at average temperatures of 1148 • C [Sugawara 2000] and 1191 • C [Ford et al. 1983].

Major and trace elements
Tables 1 and 2 present the major and trace element compositions of 29 Tabas samples and one sample from Sarbisheh.
Tabas: They are close in major element composition, with about 51-55 wt.% SiO 2 , Na 2 O+K 2 O between ∼5 and 6, MgO ranging from ∼4 to 6.5 wt.% and Mg# from ∼46 to 57 (Table 1) [C]-[D] Two distinct compositional parts in one of the more differentiated samples, sample 25, whose middle part along a yellow line seems to be a new injection of a more evolved magma with larger plagioclase phenocrysts, XPL and PPL respectively. canic rocks is mainly basaltic trachyandesite, but a few samples are basaltic andesite. The Tabas volcanic rocks are weakly alkaline, and straddle the divide between the alkaline and subalkaline series. Strong correlations between major elements (in particular the negative correlation between SiO 2 and MgO) con firm that they are cogenetic. The samples are sodic according to the K 2 O wt.% versus Na 2 O wt.% diagram (Middlemost [1985], Na 2 O/K 2 O = 2.34-4.94 wt.%) and generally fall into the alkaline category, while some samples have a sub-alkaline nature ( Figure 6). Samples have moderate TiO 2 (below 2.25 wt.%) and Al 2 O 3 content (14.7-15.51 wt.%).
Compared with published data from the northern Nayband Fault [Hashemi et al. 2008;Saadat et al. 2010;Pang et al. 2012], our samples have higher MgO and lower CaO at a given SiO 2 (Figure 7). Other major element concentrations are similar (Figure 7). However, Tabas major element compositions are clearly distinct when compared with the Gandom Beryan volcanics on the southern part of the Nayband Fault [Yousefi et al. 2017;Saadat et al. 2010;Walker et al. 2009]. The Tabas samples have lower Na 2 O+K 2 O (5.13-6.40), higher SiO 2 (50.85-55.24) and lower MgO (4.05-6.57). Sarbisheh: Our single sample collected from the Sarbisheh area plots in the trachybasalt field ( Figure 6). The sample has a similar major element composition to literature data [Pang et al. 2012]. Compared to lavas emitted in the southern part of the Nehbandan fault, Sarbisheh samples have lower SiO 2 , Na 2 O+K 2 O, Al 2 O 3 and higher MgO (wt.%). In contrast to the Na 2 O+K 2 O vs. SiO 2 negative correlation observed along the Nayband Fault, the positive correlation observed for samples from Sarbisheh and Nehbandan may reflect a genetic link between these samples.
Trace and rare element concentrations of Tabas volcanics are presented in Table 2  Nickel is a sensitive indicator of olivine fractionation/accumulation from basaltic magmas because of its high partition coefficient (mineral/melt concentration) and these data suggest that decreasing MgO and Ni is mostly due to olivine fractionation. The Sc, Eu, Ba and Nb concentrations are fairly constant at about 14, 1.6, 265 and 28 (ppm) with SiO 2 (Figure 7). Altogether, these trace element variations seem to indicate that this melt is not primitive and has been affected at least by olivine crystallization. The Hf/3-Th-Nb/16 triangular diagram [Wood 1980, Figure 8] and Zr/Y vs. Zr [Pearce and Norry 1979, not shown] indicates a within-plate alkali basalt situation. The primitive mantle normalized trace element patterns (Figure 9) show enrichment in large ion lithophile elements (LILE) with Sr concentrations ranging from 400 to 578 ppm, Ba from 196 to 328 ppm and Rb from 18 to 30 ppm. We observe a relative depletion in U and Th concentrations, with concentrations ranging from 0.40 to 0.57 and from 1.34 to 2.01, respectively. These relative depletions suggest either the presence of a residual phase with Th and U partition coefficients (D Th = Th mineral /Th melt ; D U = U mineral /U melt ) >1 in the source or early crystallization of a mineral phase with great affinity for U and Th. All Tabas samples show LREE/HREE (light and heavy rare-earth elements, respectively) enrichment with a La/Yb ratio of 9.19-16.11, and depletion in HREE, with Dy/Yb between 2.8 and 3.1, classically interpreted as reflecting residual garnet in the source. These rocks also have high field strength element enrichment (HFSE) and enrichment in Nb relative to LILE (Ba and Sr). b) Samples from Nehbandan are richer in HFSEs and LREEs than those from Tabas and they show positive anomalies for both Pb and Sr, which are signs of crustal contamination [Weaver and Tarney 1984]. c) Samples from both the northern and southern parts of the Nayband Fault (Tabas and Gandom Beryan) show a positive Nb anomaly, confirming their similar mantle source, but Gandom Beryan samples also have positive anomalies for Th which indicates either a change in the Th behaviour during partial melting and/or crystallization or crustal contamination. d) Samples from Sarbisheh show negative and positive anomalies for Nb and Pb respectively. This shows that these mantle derived melts were affected by assimilation during ascent to the surface. All these compositions were corrected for radiogenic decay assuming a common age of 14 Ma. Age corrected ratios are also given in Table 3 and are used in the diagrams (Figures 10 and 11). The effect of this correction is limited (see Table 3). Hereafter, we present and dis-cuss only the time corrected ratio. The results show that Tabas volcanic rocks have a horizontal trend on the Sr-Nd isotope plot (Figure 10). Tabas samples plot just above the mantle array (higher 143 Nd/ 144 Nd at a given 87 Sr/ 86 Sr). Such variations in Sr-isotopes might reflect crustal assimilation (discussed later). Sr-Nd isotopic ratios are clearly distinct from MORB reference values and have an Ocean Island Basalt (OIB)-like signature, as also observed at Gandom Beryan [Yousefi et al. 2017].
To better illustrate the time adjusted ( 143 Nd/ 144 Nd) t vs. ( 87 Sr/ 86 Sr) t array, we also plotted Nd and Sr isotopic ratios of Dehsalm granitoids (33 ± 1 Ma; Arjmandzadeh and Santos [2013]) and Paleogene volcanic rocks [Pang et al. 2013], both from the central domain of the Lut Block. The results show that all the magmatic rocks of the Lut Block have similar Sr-Nd isotope compositions suggesting a similar source ( Figure 10A). For comparison, Bianchini et al. [2008] have suggested  mann 1997;Rollinson 1993;Zindler and Hart 1986;Saadat et al. 2010] shows an EM-1 like affinity above the Northern Hemisphere reference line (Enriched Mantle type -1 component, see Figure 10). It is difficult to compare our data in detail with those from the literature such as Saadat et al. [2010] and Yousefi et al. [2017], because their exact analytical procedures and normalization are not available.

Crustal contamination
Taking the geological setting into consideration, we might expect the Tabas samples to be contaminated by the underlying continental crust [Baker et al. 1996;Jung et al. 2011]. In order to test the potential effects of crustal contamination, we look at potentially sensitive indicators amongst the major elements, trace elements and Sr isotopes.
First, in the same manner as Miller et al. [2000], we use the average value of P 2 O 5 /K 2 O for Tabas, which is 0.31, overlapping the range given by Hashemi et al. [2008], indicating very low contamination by crustal material. In addition, on the TAS diagram (see Figure 6), the few Tabas volcanic rocks plot as sub-alkaline basaltic-andesite due to the decrease in total alkalis (K 2 O+Na 2 O) wt.% with increasing SiO 2 wt.%. We think that this anomaly might also be due to a low level of crustal contamination, which appeared in only a few of the more differentiated samples. The values are well below the value of P 2 O 5 /K 2 O = 0.4 that is considered to separate contaminated samples from uncontaminated ones by Miller et al. [2000]. The average value of P 2 O 5 /K 2 O for Gandom Beryan is 0.35 [Yousefi et al. 2017] and indicates slightly more crustal contamination for this area.
Second, La/Nb ratios in mantle-derived rocks are sensitive to crustal contamination given the high LREE/HFSE characteristics of typical continental crust [Taylor and McLennan 1985]. Tabas samples have La/Nb ratios (average 0.58) clearly distinct from uncontaminated upper mantle derived melts represented by MORB reference values (1.2). We observe a horizontal trend between La/Nb and SiO 2 wt.% (Figure 11). This trend is interpreted as reflecting a predominant role of fractional crystallization at Tabas compared to crustal contamination, while the latter is more obvious in the Nehbandan lavas [Pang et al. 2012] (Figure 11).
Third, the lack of clear correlation between ( 87 Sr/ 86 Sr) t and SiO 2 suggests again that crustal Figure 7: Variation diagram of MgO, Al 2 O 3 , CaO, K 2 O and Na 2 O major oxides in wt.% versus SiO 2 wt.% and Ni, Sc, Eu, Ba and Nb in ppm vs. SiO 2 wt.% for all Neogene-Quarternary volcanic rocks studied from the Lut Block borders. Legend is the same as in Figure 6. contamination is limited. We did not model an AFC (assimilation and fractional crystallization) process [DePaolo 1981] in this study because we do not have strong constraints on the compositions of primitive magmas, nor on those of potential crustal assimilants. Overall, we consider that all evidence points to restricted crustal contamination at Tabas, suggesting either that melt rose up to the surface very quickly or remained isolated from the continental crust.

Fractional Crystallization
In order to investigate genetic relationships, we classified and plotted all available data from eastern Iran Neogene-Quaternary volcanic rocks in Figures 7  and 11. The results show a common source, and also that fractionation cannot be the main factor in the evolution of volcanic rocks erupted along the Nayband Fault, including Gandom Beryan and Tabas. This is also illustrated by the negative slope of the Gandom Beryan and Tabas field in the TAS diagram ( Figure 6).
The effects of limited crystallization are observed at Tabas. Relatively low concentrations of Ni and Cr (<100 and <200 ppm, respectively) associated with a negative correlation of Ni with SiO 2 (Figure 7) are interpreted as resulting from olivine fractionation. The lack of negative Eu anomaly in normalized REE patterns (Figure 9) indicates that plagioclase crystallization is limited. The petrographic and microprobe analyses also  In summary, the Tabas parental melt is not primitive and has undergone some fractionation with a very small degree of crustal contamination. Samples do not form a strong fractional crystallisation trend, and likely fractionated individually from a series of magmas linked by variable degrees of melting (see below).

Source composition
The strong fractionation between middle and heavy REE of the alkali basalts points to residual garnet in the source, because it is almost the only phase in mantle mineralogy capable of fractionating these two sets of elements [Rollinson 1993;Hauri et al. 1994]. For Tabas samples, the La/Yb vs. Dy/Yb diagram shows an enriched and garnet bearing source for Tabas [Lucassen et al. 2007]. Also, Kheirkhah et al. [2015] point to low Y (<25 ppm) and Yb (< 2 ppm) concentrations being an indicator of garnet as a residual phase. The average amounts of these are 18.88 and 1.29 respectively in Tabas. Furthermore, a low Lu/Hf ratio range (0.037-0.051) also shows that the magma is associated with residual garnet [Piccirillo and Melford 1988].
The high La/Yb values (between 9 to 16) and low Lu/Hf (0.03-0.05), together suggest that the primary basaltic magma could have come from melting in the presence of residual garnet and therefore at least some of the melting must have occurred at depth below the spinel to garnet transition in mantle peridotite [Beard and Johnson 1997;Thirlwall et al. 1994;Frey et al. 1978]. Therefore all aspects suggest a garnet peridotite as mantle source for the melting beneath the Lut Block. This suggests an asthenospheric root to the Nayband Fault where deep mantle flow along the lithospheric boundary could have channelled melts.
Finally, the modest K 2 O values (<1.5 wt.% and 1.25 wt.% as average for Tabas) indicate the absence of significant amounts of hydrous phases (e.g. phlogopite) in the mantle source.

Geochemical and tectonic context
The Tabas source can be further constrained by combining trace elements and Sr-Nd-Pb isotopes. Positive (εNd) t coupled with light REE enrichment of the rocks points to a mantle source with long-term light REE depletion, which has only recently become enriched and subsequently melted to form the magmas.
This melting context may be associated with localized stretching on the strike-slip faults that bottom into zones of ductile shear in the mantle. Such melt migration and concentration has been observed in exposed mantle and in xenoliths [e.g. O'Driscoll et al. 2015;Herwegh et al. 2016;Trestrail et al. 2017]. The small scale of the east Iranian alkaline basaltic eruptions, their relatively wide area of coverage and the long time-span (15 million years) across the whole area are consistent with mechanisms involving adiabatic upwelling and melting along the faults [Pang et al. 2012]. Taking into account the homogeneity of the Sr-Nd-Pb isotopes throughout the Lut Block, we consider that the Tabas source is the same as described by Yousefi et al. [2017] and Saadat et al. [2010] for the southern Nayband Fault and Pang et al. [2012], Saadat et al. [2010] andHashemi et al. [2008] for the northern Nayband Fault samples.
We note that the Nd isotopic compositions show very little variation across all the western Lut volcanics, while the Sr isotopic contents, at a given Nd isotopic value, increase from south to north along the Nayband Fault. Based on observations for the Sr-Nd-Pb isotope data, and after eliminating the role of extensive crustal contamination, we suggest that all volcanism in the Lut Block has the same source composition consisting of a mantle enriched by a EM-1 component ( Figure 10).

Partial melting
Several features argue for low degrees of partial melting for Tabas. First, Rollinson [1993] used Sc abundance as an important indicator of low degrees of melt-Petrogenesis and geochemistry of the Tabas Black Land volcanic field Elahpour et al, 2019  Sun and McDonough [1989]).
ing in alkali basalts. Most east Iranian alkali basalts have Sc contents ranging from 10 to 25 ppm, which compares to a range of 35-40 ppm for primitive MORB [Pearce et al. 1990]. The Sc content varies from 12 to 15 ppm in Tabas.
Second, by comparing Tabas and Gandom Beryan REE patterns, the lower LREE and higher Ba concentrations of Tabas suggest that the degree of partial melting is higher at Tabas than at Gandom Beryan. The La/Yb ratios of Tabas volcanic rocks increase with La concentrations. The average value of La/Yb ratios for the younger Gandom Beryan lavas is 32.35, which is much higher than for the older Tabas lavas (9-15 with the average of 12.62). This difference in the La/Yb ratios could be the result of different degrees of partial melting in the mantle source.
Third, according to Fitton and Dunlop [1985] the trace element compositions of sodic alkali basalts typically show evidence that their parental magma were produced by small degrees of partial melting (<5 wt.%) of either active (plume) or passively upwelling asthenospheric mantle. Using similar parameters previously used by Pang et al. [2012] to model partial melting processes, we show that the Tabas lavas formed from 5 to 10 wt.% partial melting of a garnet peridotite mantle source [ Figure 12 after Walter 1998;McKenzie and O'Nions 1991;Sun and McDonough 1989;Shaw 1970].
As described above, the Lut Block is surrounded by active fault systems, which have been involved in up to 400 million years of mobile belt evolution. There are a number of theories about the convergence history around its margins. Recently Elahpour and Heuss-Aßbichler [2017] dated a dacitic complex from the northern part of the Lut Block (33°, 19' N and 59°, 42', 05' E') using 39 Ar/ 40 Ar. The age of this volcanism is 20.7 Ma, Late Oligocene to Early Miocene, synchronous with Savian orogenic movements [Fodor et al. 1999]. Considering Batchelor and Bowden [1985], this dated dacitic sample falls in the pre-collision orogenic situation, and they believed that the final collision event between the continental fragments occurred in middle Miocene, and after that, the continuing convergence resulted in crustal shortening. Thus the Tabas field was erupted during collision and subsequent shortening along the Lut boundary. This deformation may be ongoing, since in addition to the deformation of the eastern part of the field and the seismic activity, there are hydrothermal springs along the Nayband Fault such as Dig-e-Rostam, 275 km southwest of Birjand and Abe-Garm-va-Sard, 10 km southeast of Tabas Black Land, which are signs of Nayband Fault activity during the Quaternary [Elahpour 2015;Kluver et al. 1983].
We have shown that the Tabas samples have relative enrichment in high field strength elements and do not display any depletion in Nb in comparison with LILE (Sr and Ba; Figure 9). This and the HFSE/LREE ratio of more than 1 suggests an asthenospheric origin [Smith et al. 1999].
The (εNd) t values are positive with an average close  Hart et al. [1992]. Field of depleted MORB mantle (DMM) is after Rollinson [1993]. Additional data sources: Dehsalm granitoids [Arjmandzadeh and Santos 2013], Paleogene Lut Block volcanism [Pang et al. 2013]. Other symbols are the same as in Figure 6. [B] ( 208 Pb/ 204 Pb) t versus ( 206 Pb/ 204 Pb) t with the same data (except Dehsalm granitoids). For comparison, we have plotted the Northern Hemisphere Reference Line (NHRL) and a field for EM-2 drawn with data from Society Island and a field for EM-1 drawn with data from Kerguelen Island (GeoROC database). See Hofmann [1997] for definition of mantle components (EM-1; EM-2).
to 1 (0.94), and, as Pang et al. [2012] argued, the positive (εNd) t values and LREE enrichment of the Late Cenozoic alkali basalts in eastern Iran (Lut-Sistan region) represent a LREE-depleted mantle source. This source was enriched in LREE and then melted to form the parental melts of Lut-Sistan alkali basalts. Such enrichment in eastern Iran could be related to an observed seismic low velocity region in the western Lut, where melts or volatile-rich fluids may have been released [Niu 2008;Humphreys and Niu 2009] or from the subsolidus peridotite just before asthenospheric melting [Zou and Zindler 1996].
We suggest that the degree of partial melting may have decreased with time from north to south along the Nayband Fault. Further to the south, a very young complex of highly alkaline mafic lamproites (<1 Ma; Galeh Hasanali craters, 140 km southeast of Kerman) possibly results from even lower degrees of mantle melting, and occurs where the Nayband Fault intersects the Zagros Mountains [Saadat et al. 2009]. The positive correlation in our data between (Th/Yb) and (Nb/Yb) ratios [Pearce and Peate 1995] confirms partial melting as the genetic evolution of all these volcanics from Tabas to Galeh Hasanali.  Kheirkhah et al. 2015] shows the effects of fractional crystallization and crustal contamination on volcanic eruptions along the Nayband and Neh faults (both sides of the Lut Block). Legend is the same as in Figure 6.

Conclusions
Based on this study, the Tabas Black Land monogenetic volcanic field has a sodic alkaline affinity. These mafic rocks are an example of within-plate magmatism within an active continental collision zone without involvement of a mantle plume or subduction components in their petrogenesis. This post-collisional magmatism involves asthenospheric magma generation and is probably genetically related to the Nayband fault, the deep lithospheric structure forming the western margin of the Lut Block. The Nayband Fault became a right lateral strike-slip fault during the Miocene, and the deep ductile shear at the lithospheric root of the fault may have contributed to the melting conditions, the shear zones also allowing the Tabas Black Land magmas to be tapped.
Tabas formed as the result of 5-10 wt.% partial melting of an EM-type garnet peridotite mantle source. High La/Yb values (between 9 to 16) and low Lu/Hf (0.03-0.05), confirm residual garnet. Therefore, at least some portions of the melts must have originated at depths below the spinel to garnet transition in mantle peridotite [Beard and Johnson 1997;Thirlwall et al. Figure 12: Modelling of the degree of partial melting for late Cenozoic alkali basalts, eastern Iran by covariation of Sm/Yb and La/Sm. Melting trajectories are calculated using the non-modal batch melting equations of Shaw [1970]. The model assumes melting of garnet peridotite with mode and melt mode of ol 0.6 + opx 0.2 + cpx 0.1 + gt 0.1 and ol 0.03 + opx 0.16 + cpx 0.88 + gr 0.09 , respectively [after Walter 1998]. Depleted mantle MORB (DMM) and primitive mantle (PM) are after McKenzie and O'Nions [1991] and Sun and Mc-Donough [1989] respectively. Iranian enriched mantle (IEM) is a hypothetical enriched mantle reservoir extrapolated linearly from DMM and PM [after Pang et al. 2012]. Blue and red curves are partial melting trajectories of PM and IEM, respectively; numbers beside the curves denote degrees of melting in percent. Mineral-matrix partition coefficients are from McKenzie and O'Nions [1991]. Legend is the same as in Figure 6. 1994;Frey et al. 1978].
The moderate K 2 O contents (1.25 wt.% as average) indicate the limited presence of hydrous phases (e.g. phlogopite) in the mantle source.
Geothermobarometry results point towards magma storage and differentiation occuring in the lower crust (480-760 MPa = 18-29 km), where olivine and clinopyroxene fractionated from primary mantle melts to produce the trachybasalt to basaltic trachyandesite magmas at initial temperatures of 1150-1200 • C. This magma then ascended directly to the surface with little further crystallization.
Young lamproitic eruptions are present further to the south along the Nayband Fault (Galeh Hasanali volcanic field-see location on Figure 1). These rocks were formed by lower degrees of melting than the Tabas rocks, and have a deep metasomatized source. The degree of partial melting thus decreases from north to south as the volcanics become younger. In addition, the depth of melting increases from north to south, and also the pathway for magma ascent changes from some storage in the lower crust in the north to mantle depths in the south.
This study shows that Nayband volcanism initially started at Tabas with higher degrees of partial melting, and that the fault-related tapping of asthenospheric magma appears to have dwindled and progressed southwards with time. Thus, volcanic hazards are now restricted to the south, while seismic hazards are still present along the length of the fault. from Birjand cultural heritage management for her assistance. Support for the project came from a Payamenoor University Sabbatical Grant. We thank Chantal Bosq, Delphine Auclair for their analytical support concerning Sr-Nd isotopes and Claire Fonquernie for her help in the sample preparation. This work has become an initial component of the UNESCO Geosciences IGCP project 'Geoheritage for Geohazard Resilience' (No. 692), for a Tabas Black Lands geoheritage project.

Author contributions
Esmail Elahpour undertook fieldwork, sampling and sample preparation, and cooperated with mineral and chemical analyses. François Nauret took charge of trace elements and isotopic work. Etienne Médard took charge of the electron micro-probe analyses, petrology and temperature pressure estimates. Mhammmed Benbakkar took charge of the major elements. Gabrielle Quiénnec did the remote sensing mapping and assisted in all phases of sample prep and petrological analysis. Finally, Benjamin van Wyk de Vries was in charge of overall coordination of the project, remote sensing, and geodynamics. All authors contributed to writing and discussing the results.

Data availability
Data are available on the GeoROC database and Fileshare, as well as directly from the authors on request. Mineral data given in Supplementary Material alongside the online version of this article.

Copyright notice
© The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License, which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.