Volcanic activity in the region started in the early Miocene (21 Ma) with the emplacement of abundant Na-alkaline products and subordinate silica-oversaturated basaltic and andesitic rocks in the north sector of the Arabian foreland, within the Gaziantep Basin. In the middle-late Miocene a new pulse of magmatism occurred within this basin, while Siverek Stage Na-alkaline volcanism took place in western and southern Karacadağ, namely around the cities of Urfa, Siverek and Viransehir; as for the early Miocene lavas, some subordinate silica-oversaturated basalts are found in the Siverek Stage products22. Subsequently, at the beginning of the Pliocene through to the Pleistocene-Holocene, two volcanic phases (Karacadağ and Ovabağ Stages) characterised Karacadağ volcanic activity, with the production of abundant Na-alkaline magmas. Lastly, during the Pleistocene to recent, strike-slip related magmatism with Na-alkaline affinity developed along the northern sector of the Dead Sea Fault Zone (e.g., Karasu Valley) and a few km to the north, along the Karatas-Osmaniye strike slip fault (Osmaniye area) in the Eastern Anatolia Fault Zone. Note that even if the study samples come from different areas and cover a time span of more than 20 Ma, they have practically the same petrography and major element geochemistry.
The occurrence of large trace element and isotopic variations led us to investigate the processes responsible for these trends. In particular MgO (6.5–12.1 wt.%) and Mg# (60–74) variations may indicate that some of the studied rocks are not in equilibrium with the mantle source, and that evolutionary processes involving fractional crystallisation and/or crustal assimilation played a significant role. However, significant variations in some incompatible element ratios, as well as in radiogenic isotope compositions, cannot be ascribed to magma differentiation (Fig. 5). In particular, the range of Nb/U (17–55) and Th/Nb (0.064–0.196) spans from value typical of MORBs and OiB-HiMu lavas (Nb/U > 40 and Th/Nb < 0.1) in the most primitive samples, to typical continental crust values in the relatively more evolved samples (Fig. 5a-b). These characteristics imply digestion of small amount of crustal material during magma ascent, in some samples, especially the basaltic andesites from the older Gaziantep phase. More importantly, significant Sr-Nd isotope variations are observed in the less evolved samples (e.g., 87Sr/86Sr varying from 0.70312–0.70404 for SiO2 < 48%, Fig. 5c-d), implying the occurrence of heterogeneous sources ranging widely from depleted N-MORB type mantle to Primitive Mantle values. In addition, large variations occurring in major and trace element of most primitive samples, also suggest that melting depth and melting degree may significantly vary among and inside the various suites.
The potential mantle temperature and pressure at which magmas segregated from their peridotitic source were estimated using the major element compositions of primary melts, as described in detail in the Supplementary Materials. Results (Fig. 6) show that potential melting temperatures (T °C) vary from 1447 to 1528 °C, whereas estimated pressures (P) vary from 1.8 to 3.2 GPa, corresponding to a depth of ≈ 53–97 km (Supplementary Table 3). Thus, magma segregation occurred mostly in the garnet stability field, or in the garnet-spinel transition zone. Lower values for early Miocene Gaziantep samples may be affected by significant shift of their composition during evolutionary processes. Interestingly, potential temperatures and pressures are well correlated with both radiogenic and B isotope ratios. In particular a well-defined negative trend is observed for pressure and 87Sr/86Sr, with the younger magmas characterised by the highest pressures and the lowest Sr-isotopic compositions (Fig. 6b). These trends indicate a heterogeneous mantle source, with a less depleted domain towards the surface and a more depleted component at greater depths.
Variations in radiogenic isotopes are the most distinctive features of a mantle source shifting from Primitive to Depleted Mantle domains, and the Sr-Nd variations trends depicted by our samples are therefore quite common. However, radiogenic isotopes, B isotopes and T-P estimates together highlight two peculiarities: (i) although more depleted sources are usually shallower than those more akin to a primitive mantle, here depletion increases with depth; (ii) in theory, B isotopes should not vary significantly between Primitive and Depleted Mantle values. As B isotope fractionation at magmatic temperatures is negligible, magma fractionation responsible for depletion of the mantle can affect B contents but should not change the B isotope signature.
The B isotope composition of the pristine mantle is a conundrum: because the pristine mantle is bereft of B, mantle peridotites should have a very low B content, especially because the mobility of B in fluids causes a significant shift of B isotopic values in mantle rocks with either a minimal metasomatic imprint or even a very low degree of alteration. The best estimate for the B isotope composition of the pristine mantle comes from MORBs or OIB sourced in the uncontaminated mantle8. The δ11B values of MORBs were recently measured accurately in both fresh unaltered MORBs sourced from Depleted Mantle31 and some OIBs sourced from uncontaminated mantle akin to Primitive Mantle33. Fresh MORBs vary in a narrow interval, δ11B= -7 ± 1‰, and most OIBs fall in a similar range, δ11B= -7 ± 3‰. This indicates that the mantle is relatively homogeneous in terms of the B stable isotope composition, unless it is modified by recycling of subducted material. Slightly heavier values are available for Hawaiian rocks (-3 to -5‰)33. In contrast, OIB-type intraplate Na-alkali basalts from Western Anatolia, sourced in the mantle under the continental lithosphere, retain higher values (-1 to -3‰)9. The South-East Anatolian basalts, even excluding the two most evolved samples affected by AFC processes (i.e., the two early Miocene samples from Gaziantep, Fig. 4), display a continuous trend of variation from typical MORB-OIB values to values like those of Western Anatolian basalts (-8 to -2‰). In addition, although oceanic basalts show no correlation between B isotopes and radiogenic isotopes (Fig. 4b), B isotope variations in SE Anatolia primitive basalts are fairly well correlated with Sr isotopes, as well as temperature and pressure estimates (Fig. 6). We can conclude that the subcontinental asthenospheric mantle, at least the one present under the Anatolia and Arabia regions, is heterogeneous even when considering B stable isotopes, and that there is a somewhat continuous shift between a Réunion-like more enriched source and a West Anatolian-like more depleted source (Fig. 4b). To what extent this mantle can be considered pristine and to what degree these variations are due to recycling of old (or very old) crustal and lithospheric material is still a matter of debate. However, recycling of shallow material in the deep mantle should produce an increase in B concentrations, whereas SE Anatolian basalts display very low B contents (as monitored by the B/Nb ratio, Fig. 4a), especially in samples retaining a higher B isotopic signature. As shown in Fig. 4a, the B isotope composition of some of these samples is similar to that of Armenian volcanism, sourced from a metasomatized mantle, but the latter have significantly higher B/Nb ratios due to the occurrence of significant amount of amphibole in the mantle source34.
Previous studies on volcanism in the North Arabian Plate hypothesized the presence of hydrous phases in the mantle source, such as amphibole and/or phlogopite-rich veins, or a combination of the two, as also supported by the occurrence of metasomatised lithospheric mantle xenoliths and amphibole-rich cumulate crustal xenoliths27,35. Studied samples are characterised by a pronounced negative Pb anomaly in the Primitive Mantle-normalised trace elements (Fig. 2), which may indicate the presence of residual amphibole and/or phlogopite in the source, given that both amphibole or phlogopite are considered to represent a main repository for Pb in the mantle36. Thus, hydrous phases may occur in the mantle source of the studied rocks; it follows that this mantle was, at some time, probably enriched by recycling of crustal and lithospheric material. The occurrence of amphibole in the mantle source paragenesis of the studied rocks should also significantly affect their B isotope composition, given that amphibole is a main B repository in the mantle and can retain B for a very long time34 .
Magmatism and Geodynamics
Volcanism in the study area is quite peculiar because it results from a number of distinct magmatic pulses showing similar geochemical characters, emplaced in a relatively limited area, over a very long period of more than 20 Ma, and in varied tectonic settings, e.g., in close proximity to continental transform fault systems, close to the Arabia-Africa-Anatolia triple junction, or in the foreland of the Arabia-Eurasia convergence. These lavas, despite their wide age span (> 20 Ma) and their different tectonic location, share similar petrographic and geochemical characteristics and limited and continuous variations in their mantle source. This rather unusual, peculiar feature of this magmatism is also found in other older alkali basalts clustering close to the western margin of the Arabian Plate, from southeastern Turkey to Yemen and several authors noted that all these magmas are characterised by similar geochemical and isotopic compositions resembling the typical Afar fingerprint, and suggested that volcanism has with time migrated northward from the Afar Plume to the Arabian Plate. For some authors37, this migration is linked to the flattening of the Afar plume head and its northward channelling below Arabia. In contrast, Shaw et al.38 stated that the magmas emplaced in the Harrat Ash Shaam (Jordan) were geochemically and isotopically akin to the late Cenozoic volcanism throughout the Arabian Plate, but quite different from magmas emplaced in the south-western portion of the Arabian Plate (Yemen), and that only the latter were eventually affected by the Afar plume. Pliocene to Pleistocene magmatism of the Harrat Ash Shaam has been tentatively ascribed to heating of the base of the lithosphere by an anomalously hot sublithospheric mantle39, whereas Sobolev et al.40 proposed a direct relationship between the Dead Sea Fault Zone-Red Sea Rift system and the Afar plume to explain igneous activity along the fault zone. These authors explained the asymmetry and the difference in topography between the western and eastern sides of the Red Sea and Dead Sea Fault with the presence of a hot mantle adjacent to the eastern shoulder of the rift and a cold lithosphere beneath the western side, with the boundary of the thermal asthenosphere below the African Plate deeper than the one beneath the Arabian Plate. Doglioni et al.41,42 noticed that, in general, the eastern side of continental rift zones is usually more elevated than the western one. Since the mantle becomes depleted in Fe after melting, it moves “eastward” relative to the lithosphere: the higher topography of the eastern side of the rift is therefore interpreted in terms of isostatic adjustment (lower thermal subsidence). Due to the net rotation of the lithosphere, the depleted and lighter sub-ridge mantle may eventually transit43 beneath continental rifts, uplifting its eastern shoulder. In this view, no heat anomalies are required to explain the high topography of the eastern margins of both the Red Sea and Dead Sea faults.
A very long-lasting (20 Ma) homogeneous magmatism produced a large amount of alkali basalts on the western portion of the Arabian Plate, close to the Africa-Arabia margin, from the Oligocene to the Pleistocene and from the southwest (i.e., Yemen) to the northwest (south-eastern Turkey). The occurrence and persistence of an asthenospheric mantle source under the study area is interpreted in the framework of the dynamics between the Arabian and African plates, which led to the formation of the Dead Sea Fault Zone to the north, and the Red Sea Rift to the south. The divergence between these two plates is still in an embryonic stage, and close to the study area the margin is transform, with a minimal extensional component. Beneath the Red Sea Rift (and in its northern termination represented by the DSFZ), a hot, buoyant asthenospheric mantle reaches the rift zone and partially melts (Fig. 7). Since the Red Sea Rift was and still is in an immature stage, the upwelling asthenosphere does not produce large amounts of basaltic melts in the axial zone. This effect is even stronger under the DSFZ, which is a transcurrent rather than a passive margin. While some magmas come to the surface at the plate margin, portions of still hot, fertile asthenosphere continue their eastward migration and are then stored beneath the western portion of Arabian lithosphere, which is much thinner (100 km)44 than the African one (up to 175 km)45. Given the large difference in thickness between the two plates, the eastward migration of the asthenosphere implies its upwelling, creating favourable conditions for further partial melting induced by decompression. As highlighted by analogue modelling, magmas are frequently stored not only in the axial zone of rifts, but tend to migrate towards the rift shoulders46. Neither rift processes nor extensional tectonics have developed in the study area. However, local tensional and transtensional activity has provided some pathways for the ascent and decompression melting of this fertile asthenosphere, generating scattered magmas in the Gaziantep Basin and a wide, persistent (> 11 Ma) magma chamber under the Karacadağ shield volcano. More recently, the development of transform fault zones at the north-eastern margin of the Arabian Plate, here represented by the northern termination of the DSFZ, favoured upwelling of the asthenospheric mantle and the formation of Na-alkaline magmas in the Osmaniye and Karasu volcanic fields during the Pleistocene. In agreement with this model, primary magmas in the study area have a clear asthenospheric geochemical imprint and are thought to have formed at an estimated depth of 100–140 km. During its north-eastward migration, the upper asthenosphere, i.e. the low-velocity layer beneath the Arabian Plate, may have retained larger amounts of partial melt, which remained stagnant at the slab flexure, where the north-eastern Arabian Plate subducts beneath Eurasia.