Petrogenesis and Tectonic Significance of Miocene Volcanic Rocks in the Ahlatlı–İspir–Erzurum Region, Türkiye

Abstract

The İspir–Ahlatlı region in northeastern Türkiye, situated within the Eastern Pontides, hosts significant Miocene trachy-andesite volcanic rock exposures. This work seeks to elucidate their petrographic, geochemical, and isotopic compositions to enhance comprehension of their genesis and tectonic significance. Geochemistry reveals a transitional affinity, an enrichment in large-ion lithophile elements (LILEs), and a decrease in high-field-strength elements (HFSEs), suggesting a subduction-modified mantle source. Geochemical variations and fractional crystallisation trends indicate that the parental magma underwent significant differentiation, likely involving the fractionation of amphibole, clinopyroxene, and plagioclase. As supported by recent thermal modelling studies, the presence of intermediate volcanic rocks without associated bimodal suites in the study area may reflect elevated geothermal gradients and lithospheric delamination during post-collisional extension. The signatures indicated that the trachy-andesites originated in a post-collisional extensional environment after the closing of the Neo-Tethys Ocean and the ensuing tectonic reconfiguration of the Eastern Pontides. The reported geochemical traits correspond with post-collisional volcanic phases documented in various sectors of the Alpine–Himalayan orogenic system, such as the Eastern Pontides, the Iranian Plateau, and the Himalayan Belt, reinforcing the notion of a subduction-influenced mantle source. These findings increase the comprehension of magma formation in post-collisional settings and offer novel insights into the geodynamic context of the area. This research improves the understanding of post-collisional volcanic systems, their petrogenetic evolution, and their role in regional tectonic processes.

1. Introduction

Volcanic rocks, particularly the enigmatic trachy-andesites, are the focal point of interdisciplinary geological research. Trachy-andesites are considered enigmatic due to their complex petrographic characteristics, hybrid mantle–crust geochemical signatures, and formation within tectonically ambiguous environments. Consequently, they are important indicators for deciphering subduction-related magmatism and post-collisional tectonic evolution. Unravelling their petrogenesis not only unveils the tectonic and magmatic processes that shape the Earth’s crust but also offers profound insights for engineering and industrial applications [1,2,3]. The trachy-andesites, characterised by their hypo-crystalline and porphyritic textures, reflect dynamic igneous processes, including magma mixing, crystallisation, and differentiation. Often found in subduction-related tectonic environments, these rocks also display the interplay between crustal assimilation and mantle-derived contributions [1].

The tectonic map of Türkiye shows the major suture zones, arc systems, and continental blocks [4] and the distribution of the main Neogene volcanic fields (modified from [5]) (Figure 1). The Eastern Anatolian Volcanic Province (EAVP), considered one of the most prominent examples of post-collisional magmatism worldwide, has been tectonically and magmatically active since the early Miocene, following the collision between the Arabian and Eurasian plates [6,7,8,9,10]. This magmatic activity is characterised by a wide range of volcanic products, from mafic to felsic compositions, reflecting complex mantle–crust interactions in an extensional post-collisional regime [6]. Situated within the Alpine-Himalayan orogenic zone, this region has witnessed substantial magmatic and tectonic activity from the Late Cretaceous to the Miocene epochs. These volcanic rocks have experienced significant structural deformation, such as faulting and jointing associated with post-magmatic tectonic reactivation and alteration due to hydrothermal processes [11].

Delving into the petrogenesis of trachy-andesites in the area is not merely a scientific endeavour but a fundamental necessity. It provides crucial insights into the origin and evolution of magmatic sources and elucidates the complex relationships between the surrounding volcanic units and regional tectonic processes [12]. This comprehensive investigation is essential for advancing our understanding of the volcanic history of the Eastern Pontides and evaluating the region’s potential natural resources.

Recent research in the Carpathian-Pannonian area and Anatolia highlights the importance of integrating petrographic, geochemical, and isotopic data to understand the petrogenesis of volcanic rocks [1]. These approaches facilitate reconstructing a region’s volcanic development and explain the significance of crust–mantle interactions and tectonic settings [13]. The İspir–Ahlatlı trachy-andesites offer a valuable opportunity to investigate magmatic differentiation processes and the interaction between mantle and crustal components within a tectonically active region. Despite the considerable number of studies conducted on the Eastern Pontides, the Miocene volcanic units in the İspir area remain insufficiently characterised regarding their petrogenesis and geochemical evolution. This study addresses this gap through an integrated approach involving petrographic, geochemical, and isotopic analyses.

This research primarily aims to clarify the petrogenesis of trachy-andesites from the İspir–Ahlatlı area through an integrated approach involving petrographic studies, whole-rock geochemical characterisation, and Sr–Nd isotopic analysis. This approach aims to enhance our understanding of petrogenetic processes operating within tectonically dynamic settings. This work corresponds with previous investigations of analogous volcanic formations (e.g., Miocene volcanic units in Central, Eastern, and Western Anatolia), providing comparative insights and emphasising regional geological relationships [12,14].

This study enhances our understanding of the petrogenetic development of volcanic rocks in Eastern Anatolia. It establishes a solid foundation for future research that combines petrographic data with geochemical and isotopic analysis.

2. Geological Background

The Eastern Pontide Orogenic Belt, located along the southern margin of the Eurasian plate, represents a long-lived convergent margin shaped by the closure of branches of the Neo-Tethys Ocean. It has experienced multiple stages of magmatic activity, including Late Jurassic–Early Cretaceous arc-related volcanism, Eocene collisional magmatism, and Miocene post-collisional intrusions and volcanism. The region displays a compressional tectonic regime resulting from the convergence of the Eurasian and Arabian plates, which led to marine sedimentation, volcanic activity, and faulting [7]. Jurassic–Cretaceous sedimentary units, including sandstones, marls, and limestones, reflect a prolonged marine depositional environment. These units were subsequently intruded and overlain by Miocene trachy-andesitic volcanic rocks that played a role in developing the region’s surface geology [12].

The İspir area is within the Eastern Pontid orogenic belt in Erzurum Province, Türkiye. This area has a complex geological structure resulting from intensive tectonic and igneous activities. The İspir area, including geological records from the Mesozoic to Cenozoic eras, is particularly notable for its Miocene volcanic rocks that overlay sedimentary strata from the Jurassic–Cretaceous period. The volcanic assemblage of the region comprises trachy-andesites, reflecting notable igneous activity during the Miocene epoch. Sedimentary rocks unconformably cover the volcanic units, including limestone and sandstone, suggesting that the area had an active tectonic regime.

The İspir region lies within the Eastern Pontide Orogenic Belt and exhibits a complex geological evolution shaped by the closure of the Tethys Ocean and the subsequent post-collisional tectonic processes [15,16]. The trachy-andesites in the region are linked to heightened interactions with the continental crust and magmatic differentiation processes that transpired throughout the Late Cretaceous-Eocene [17]. The Eastern Pontides is an orogenic belt that has undergone many stages and numerous magmatic events throughout its paleotectonic history. These tectonic processes continued with the subduction of the oceanic crust during the Cretaceous, followed by continental collision in the Eocene and post-collisional extensional tectonics throughout the Oligo-Miocene [18]. Magmatism in the region is linked to arc volcanism that emerged from the northward subduction of the Neo-Tethys oceanic plate and magmatic processes intensified by crustal erosion following the collision [15].

3. Geology of the Region

The Eastern Pontid orogenic zone constitutes a notable geological corridor formed by successive marine and volcanic activities. The Jurassic–Cretaceous sedimentary formations, comprising sandstones and marls, underscore the region’s enduring maritime depositional environment. Kaygusuz [19] reported a K–Ar age of approximately 13.9 ± 0.12 Ma for post-collisional trachy-andesitic to dacitic volcanic rocks in the Ilıca region of Erzurum. In addition, Kaygusuz et al. [20] presented Sr–Nd–Pb isotopic and major–trace element geochemical data from Miocene–Pliocene volcanic successions in the Kandilli area (Erzurum). Given the close geotectonic and lithological similarities, these findings collectively support a Miocene age for the Ahlatlı–İspir volcanic rocks. During the Miocene, tectonic uplift occurred and was accompanied by the widespread emplacement of trachy-andesitic lavas, indicating significant volcanic activity (Figure 2). The Miocene volcanics crop out in the centre of the study region and consist of trachy-andesite (Figure 3). Although the Eastern Pontides have been extensively studied regarding magmatic activity and plate interactions, investigations focusing on the petrogenesis of trachy-andesites remain limited [20]. Despite significant research on the structural and geochemical characteristics of regional magmatism [21,22,23,24,25,26], the petrographic features of trachy-andesites and their relationship with tectonic processes are still poorly constrained. Addressing this gap would enhance our understanding of the temporal-spatial dynamics of magmatic events in the Eastern Pontides and their role in regional geological development.

4. Analytical Methods

Petrographic analyses were carried out using a Leica DM 2500 P polarising microscope, and thin section photomicrographs were captured using a Leica Camera C3 These analyses were carried out at the Thin Section Laboratory of the Department of Geological Engineering, Fırat University (Elazığ, Türkiye).

Whole-rock analyses were conducted on powdered ground using an agate mortar milling device. The oxides of major elements were determined by a Bruker S8 Tiger X-ray fluorescence (XRF) spectrometer with wavelength ranges from 0.01 to 12 nm; the analytical uncertainty is usually 5%. The Elan DRC-e (PerkinElmer, Waltham, MA, USA) Inductively Coupled Plasma–Mass Spectrometry (ICP-MS) was used for trace element analyses. International USGS rock reference standards BCR-2 (basalt) and RGM-2 (rhyolite) were used for calibration and quality control. The detection limits for major oxides measured by XRF are typically between 0.01 and 0.05 wt.%, and for trace elements measured by ICP-MS, they range from 0.01 to 0.1 ppm. A two-step digestion process used approximately 50 mg of powdered sample: (1) 6 mL of 37% HCl, 2 mL of 65% HNO₃, and 1 mL of 38%–40% HF acid mixer placed in a pressure- and temperature-controlled Teflon beaker in a Bergh off Microwave at 135 °C; (2) 6 mL of 5% boric acid solution was added to the step one solution for ICP-MS analyses. Loss on ignition (LOI) values were determined by heating the powdered samples in a muffle furnace at 1050 °C for two hours. These analyses were carried out at ITU/JAL, İstanbul Technical University (İstanbul, Türkiye).

Whole-rock Sr–Nd isotopes were performed using an MC-ICP-MS analytical instrument (Neptune Plus, Thermo Scientific, Waltham, MA, USA) in the Wuhan Sample Solution Analytical Technology Ltd. laboratory in Wuhan, China. The Neptune Plus, a double-focusing MC-ICP-MS, was equipped with seven fixed electron multiplier ICs and nine Faraday cups fitted with 1011 Ω resistors. The USGS standards BCR-2 (basalt) and RGM-2 (rhyolite) were analysed with our samples. The measured isotopic ratios are ⁸⁷Sr/⁸⁶Sr = 0.705034 ± 0.000014 (2σ, n = 4) for BCR-2 and ⁸⁷Sr/⁸⁶Sr = 0.704192 ± 0.000010 (2σ, n = 4) for RGM-2, and ¹⁴³Nd/¹⁴⁴Nd = 0.512644 ± 0.000015 (2σ, n = 6) for BCR-2 and ¹⁴³Nd/¹⁴⁴Nd = 0.512810 ± 0.000015 (2σ, n = 4) for RGM-2, which are consistent with the recommended values [27].

5. Results

5.1. Petrography

The polarised microscope image illustrates the textural and mineralogical features of the trachy-andesite, highlighting the distribution of its main rock-forming minerals—including plagioclase, biotite, clinopyroxene, amphibole, and opaque phases—as well as accessory phases such as zircon, apatite, and sphene. Trachy-andesites mostly show intersertal, intergranular, glomeroporphyritic, and subophitic textures. The trachy-andesite displays a porphyritic texture, where plagioclase, biotite, clinopyroxene, and amphibole occur predominantly as euhedral to subhedral phenocrysts, often exhibiting zoning and twinning. The groundmass consists of microlitic plagioclase and dispersed opaque minerals within an intersertal to intergranular matrix. No antecrystic or xenocrystic phases were identified. Biotite crystals can be readily identified by their brown colour, strong pleochroism, and perfect basal cleavage, whereas clinopyroxene and opaque minerals display a heterogeneous distribution within the matrix. The proportion and spatial configuration of these minerals within the matrix provide valuable insights into the rock’s cooling history, crystallisation sequence, and associated mineralisation processes.

Plagioclase (28–30 vol.%) is the most abundant mineral in trachy-andesites, typically occurring in the form of labradorite to andesine (intermediate plagioclase). It constitutes a significant percentage of the mineral content of the rock. Plagioclase occurs predominantly as phenocrysts, as crystals significantly larger than the surrounding groundmass. These euhedral to subhedral phenocrysts range in size from approximately 0.5 mm to 2.5 mm and commonly exhibit zoning and twinning. The distribution of clinopyroxene and opaque minerals within the matrix, alongside plagioclase crystals, is observed. In addition, textural features such as plagioclase zoning, plagioclase twinning, and porphyritic textures are identified in the volcanic rocks (Figure 4a). In addition to the mineral assemblages described above, it is noteworthy that both plagioclase and amphibole appear in dual textural forms—as macrocrysts (phenocrysts) and as microlites within the matrix. Plagioclase phenocrysts commonly exhibit oscillatory zoning and polysynthetic twinning (Figure 4a), while microlitic plagioclase occurs in the interstitial matrix. Similarly, amphibole appears as coarse subhedral phenocrysts and also as finer needle-like crystals dispersed in the groundmass (Figure 4b,c). This coexistence suggests a multi-stage crystallisation history, likely linked to variations in pressure, temperature, and magma residence time. These observations align with the conceptual framework of Jerram and Martin [28], which highlights how crystal populations reflect dynamic processes operating within the magma plumbing system.

The pronounced zoning and twinning observed in plagioclase phenocrysts are important petrographic features that reflect fluctuations in temperature and chemical composition during crystallisation, offering insights into the cooling history and dynamic conditions of the magma [29]. The concentrations and distributions of these minerals within the matrix are essential data for assessing the magmatic differentiation processes and rock evolution. Biotite occurs as subhedral to euhedral phenocrysts with pleochroism and basal cleavage, typically comprising 15–20 vol.% of the rock (Figure 4b). Optical microscope images of clinopyroxene (10–20 vol.%) and opaque minerals (3–5 vol.%) under crossed nicols show that these phases are scattered and locally clustered, which may reflect variations in the cooling rates and mineral growth during magma solidification (Figure 4b).

Iron-titanium oxide minerals, such as magnetite or ilmenite, are typically minor components. Hornblende (15–20 vol.%) is a common amphibole mineral in the trachy-andesite samples, typically occurring as euhedral to subhedral phenocrysts (Figure 4c).

An optical microscope image under crossed nicols shows opaque minerals within the matrix and a section where biotite crystals are relatively scarce. This image is crucial for comprehending the rocks’ chemical and physical strength characteristics. Accessory minerals (2–3 vol.%) can include trace amounts of zircon, apatite, and sphene.

5.2. Whole-Rock Geochemistry

5.2.1. Major Elements

The major element compositions of the studied trachy-andesites are summarised in

Table 1. Major and trace element data for the volcanic rocks in the Ahlatlı–İspir–Erzurum region.

Sample	E1	E2	E3	E4	E5	E6	E7	E8	E9	E10
Major oxides (wt %)
SiO2	54.76	54.73	54.69	54.71	54.42	54.56	54.53	54.42	54.57	54.68
TiO2	0.74	0.75	0.73	0.75	0.74	0.75	0.74	0.72	0.72	0.71
Al2O3	17.53	17.48	17.52	17.49	17.52	17.53	17.44	17.55	17.63	17.58
Fe2O3 a	7.21	7.52	7.42	7.54	7.58	7.44	7.42	7.38	7.46	7.32
MnO	0.18	0.18	0.18	0.18	0.18	0.18	0.18	0.18	0.18	0.18
MgO	2.86	2.81	2.80	2.83	2.95	2.94	2.89	2.92	2.93	2.91
CaO	6.32	6.37	6.34	6.31	6.35	6.36	6.37	6.39	6.17	6.23
Na2O	4.59	4.49	4.51	4.51	4.49	4.64	4.60	4.61	4.70	4.55
K2O	2.59	2.64	2.62	2.58	2.53	2.58	2.55	2.58	2.5	2.49
P2O5	0.37	0.41	0.40	0.40	0.42	0.39	0.41	0.39	0.41	0.41
LOI	2.61	2.4	2.54	2.49	2.57	2.39	2.62	2.62	2.49	2.71
Sum	99.76	99.78	99.75	99.79	99.75	99.76	99.75	99.76	99.76	99.77
Trace elements (ppm)
Sc	14	16	15	16	17	15	17	15	15	17
V	139	140	139	140	147	143	147	143	146	145
Co	18	15	16	16	20	19	19	17	18	17
Ni	15	15	13	12	17	16	17	15	19	14
Cu	81	73	91	81	93	94	90	86	91	91
Zn	75	73	81	77	90	82	90	85	85	84
Ga	19.70	20.20	20.91	20.69	20.19	19.62	19.44	19.94	20.01	20.53
Rb	76.19	76.45	78.59	73.07	71.40	70.50	70.25	72.64	68.38	70.53
Sr	720	666	755	721	760	743	741	731	725	741
Y	17.54	17.03	18.01	17.77	17.22	17.74	17.67	17.86	17.17	17.77
Zr	121	112	129	123	126	124	124	123	122	125
Nb	8	7	8	7	8	7	8	8	8	7
Cs	5.54	5.51	5.80	5.56	5.68	5.40	5.62	5.76	5.59	5.55
Ba	565	560	564	556	552	550	561	559	557	546
La	22.93	22.78	23.40	22.76	21.34	22.24	22.31	22.46	21.99	22.13
Ce	89.69	86.22	90.20	67.16	63.51	65.92	65.73	67.62	64.79	66.35
Pr	5.50	5.46	5.64	5.47	5.21	5.37	5.29	5.62	5.26	5.31
Nd	21.32	21.54	21.89	21.62	20.38	20.69	20.36	21.18	20.83	20.83
Sm	4.43	4.20	4.36	4.29	4.23	4.12	4.55	4.58	4.26	4.61
Eu	1.38	1.28	1.29	1.33	1.26	1.31	1.27	1.33	1.29	1.37
Gd	3.82	3.89	4.05	3.94	4.02	3.70	3.99	3.82	3.98	3.81
Tb	0.58	0.56	0.56	0.54	0.50	0.57	0.59	0.61	0.62	0.55
Dy	3.15	2.78	3.23	3.22	2.94	3.17	2.94	3.11	2.80	3.11
Ho	0.62	0.55	0.58	0.59	0.60	0.55	0.57	0.59	0.55	0.57
Er	1.77	1.68	1.75	1.73	1.70	1.82	1.87	1.87	1.71	1.70
Tm	0.24	0.26	0.23	0.22	0.22	0.23	0.24	0.24	0.23	0.25
Yb	1.58	1.57	1.59	1.68	1.51	1.44	1.70	1.70	1.59	1.65
Lu	0.26	0.24	0.22	0.24	0.22	0.23	0.20	0.21	0.24	0.21
Tl	0.26	0.29	0.27	0.26	0.29	0.26	0.30	0.25	0.19	0.22
Th	4.12	3.96	4.17	4.10	3.78	3.90	3.76	3.77	3.78	3.74
U	1.39	1.44	1.46	1.41	1.37	1.31	1.35	1.29	1.41	1.32
Mg#	44.00	42.53	42.77	42.64	43.53	43.90	43.55	43.94	43.75	44.05
Eu/Eu*	1.03	0.97	0.94	0.99	0.93	1.03	0.91	0.97	0.96	1.00

^a Total iron as Fe₂O₃. Mg# = [molar 100 × Mg/(Mg + Fe⁺²)]. Eu/Eu* = (Eu)_cn/[(Sm)_cn × (Gd)_cn]^0.5 (“cn” refers to chondrite-normalised values).

. The studied trachy-andesites have SiO₂ contents ranging from 54.42 to 54.76 wt.%, Al₂O₃ from 17.44 to 17.63 wt.%, Fe₂O₃ from 7.21 to 7.58 wt.%, total alkalis (Na₂O + K₂O) between 7.02 and 7.22 wt.%, and MgO between 2.80 and 2.95 wt.%. The calculated Mg# values range from 42.53 to 44.05, indicating evolved melt compositions consistent with significant fractional crystallisation [30].

According to the Total Alkali–Silica (TAS) classification diagram [31], which includes the alkali–subalkaline subdivision line proposed by Irvine and Baragar [32], the volcanic samples are classified as trachy-andesites and fall within the transitional field (Figure 5).

5.2.2. Minor and Trace Elements

The minor and trace element compositions (ppm) of the studied trachy-andesites are summarised in Table 1. The trace element compositions also show a narrow range, with Rb values ranging from 68.38 to 78.59 ppm, Y from 17.03 to 18.01 ppm, and Ba between 546 and 565 ppm. This restricted variability further supports the interpretation of a geochemically homogeneous magma series.

The N-MORB-normalised multi-element diagram (Figure 6a) shows prominent enrichment in large-ion lithophile elements (LILEs: Cs, Rb, Ba) and depletion in high-field-strength elements (HFSEs: Nb, Ti, Zr). This geochemical pattern is widely accepted as a diagnostic feature of subduction-related magmatic sources, where fluid or melt derived from the subducted slab enriches the mantle wedge in LILEs while HFSEs are retained in slab residues or accessory phases [8]. The chondrite-normalised rare earth element (REE) patterns (Figure 6b) display enrichment in light REEs and nearly flat heavy REE segments with minor to no Eu anomalies (Eu/Eu* = 0.91–1.03). Discrimination, based on trace element ratios, suggests that the volcanic rocks were derived from a lithospheric mantle source, consistent with partially melting a previously metasomatised mantle domain [8].

5.3. Sr–Nd Isotope Geochemistry

The whole-rock Sr–Nd isotopic geochemistry results are given in

Table 2. Sr–Nd isotopic data and calculated TDM model ages for the volcanic rocks in the Ahlatlı–İspir–Erzurum region.

Sample	Rb a (ppm)	Sr (ppm)	87Rb/86Sr	87Sr/86Sr	±2σm b	(87Sr/86Sr)i c,d,e	εSr(T)	Sm (ppm)	Nd (ppm)	147Sm/144Nd	143Nd/144Nd	±2σm	(143Nd/144Nd)i	εNd(T) f	TDM (Ga) g
E1	76.19	720	0.306	0.706089	0.000007	0.706038	22.0	4.43	21.32	0.1256	0.512625	0.000005	0.512611	−0.1	0.91
E3	78.59	755	0.301	0.706087	0.000006	0.706037	22.0	4.36	21.89	0.1204	0.512628	0.000004	0.512615	0.0	0.85
E5	71.4	760	0.272	0.706055	0.000005	0.706010	21.6	4.23	20.38	0.1255	0.512626	0.000004	0.512612	−0.1	0.91
E7	70.25	741	0.275	0.706098	0.000005	0.706052	22.2	4.55	20.36	0.1351	0.512622	0.000004	0.512607	−0.2	1.02
E9	68.38	725	0.273	0.706103	0.000006	0.706057	22.3	4.26	20.83	0.1236	0.512618	0.000004	0.512605	−0.2	0.90

^a The concentration data are the same as in Table 1, and the ⁸⁷Rb/⁸⁶Sr and ¹⁴⁷Sm/¹⁴⁴Nd values were derived from the relations Rb/Sr × 2.8956 and Sm/Nd × 0.60456, respectively. ^b The 2σ_m values are the mean standard deviations of the measurements. ^c The initial isotopic ratios were calculated using this formula (⁸⁷Sr/⁸⁶Sr)i = (⁸⁷Sr/⁸⁶Sr) − (⁸⁷Rb/⁸⁶Sr) × (eλt − 1), (¹⁴³Nd/¹⁴⁴Nd)i = (¹⁴³Nd/¹⁴⁴Nd) − (¹⁴⁷Sm/¹⁴⁴Nd) × (eλt − 1)] [36]. ^d The decay constants used for calculating the initial isotopic ratios are after [37], (λ_Rb) = 1.393 × 10⁻¹¹/year) and [38], (λ_Sm) = 6.54 × 10⁻¹²/year). ^e The present-day chondritic uniform reservoir: ⁸⁷Rb/⁸⁶Sr = 0.0827, ⁸⁷Sr/⁸⁶Sr = 0.7045, ¹⁴⁷Sm/¹⁴⁴Nd = 0.1967, ¹⁴³Nd/¹⁴⁴Nd = 0.512638. ^f εNd_(T) = [(¹⁴³Nd/¹⁴⁴Nd)_Sample (T)/(¹⁴³Nd/¹⁴⁴Nd)_CHUR(T) − 1] × 10⁴. ^g T_DM(Ga) = 1/λ_Sm–Ndx ln{1 + [((¹⁴³Nd/¹⁴⁴Nd)_Sample − 0.51315)/((¹⁴⁷Sm/¹⁴⁴Nd)_Sample − 0.2137)]}. f_Sm/Nd = (¹⁴⁷Sm/¹⁴⁴Nd/0.1967) − 1 f_Sm/Nd: samples of value.

. The samples’ initial ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios were corrected using an age of 16.7 Ma, based on zircon U–Pb ages from Miocene felsic volcanic rocks in the Eskiköy–Doğanşehir area (Malatya), which mark the onset of post-collisional magmatism in Eastern Anatolia [35]. Although this age does not correspond to the exact sampling location and is geographically distant, it represents the regional magmatic episode and geodynamic setting during the Miocene. The volcanics display relatively homogeneous initial isotopic compositions of (⁸⁷Sr/⁸⁶Sr) ranging from 0.706010 to 0.706057 and (¹⁴³Nd/¹⁴⁴Nd)i from 0.512605 to 0.512615. The TDM model ages of the analysed volcanic samples range between 0.85 and 1.02 Ga, as shown in Table 2.

6. Discussion

6.1. Petrogenesis and Source Characteristics

Major and trace elements analyses are reported in Table 1. Whole-rock major and trace element geochemistry has been widely used to understand the evolution of studied rocks and the tectonic history of various geological units and sequences [30,39].

The observed geochemical homogeneity among the İspir–Ahlatlı trachy-andesites may be attributed to the sustained storage of parental magmas under relatively uniform thermal conditions over an extended period. This scenario is supported by the thermodynamic modelling of Lino et al. [40], who demonstrated that basaltic magma reservoirs subjected to stable thermal regimes could evolve toward compositionally homogeneous eruptive magmas through continuous crystallisation-differentiation processes. Such thermal conditions may have facilitated the homogenisation of intermediate magmas before an eruption, especially in regions with moderate to high geothermal gradients and prolonged magma residence times. In contrast to the typical bimodal volcanic suites commonly observed in post-collisional extensional regimes, the İspir–Ahlatlı volcanic rocks are dominated by intermediate compositions without associated coeval basaltic or felsic products. This suggests a distinct tectono-thermal setting. As Lino et al. [40] revealed, extensive lithospheric delamination can elevate the geothermal gradient above 20 °C/km, enhancing the partial melting of the mafic lower crust and sublithospheric mantle. Under such conditions, intermediate volcanic rocks may form directly or evolve from basaltic precursors through fractional crystallisation and limited crustal assimilation. In the İspir–Ahlatlı region, the observed trachy-andesitic compositions likely reflect such thermal conditions, implying that elevated geothermal flux and crust–mantle interaction promoted the development and eruption of intermediate magmas during the post-collisional evolution of the Eastern Pontides.

On the silica versus total alkali diagram [31], the volcanic rocks are classified as trachy-andesite (Figure 5). The volcanic rocks have an intermediate composition. The N-MORB-normalised trace element patterns of the samples exhibit broadly similar trends, characterised by enrichment in large-ion lithophile elements (e.g., Cs, Rb, and Ba) and depletion in high-field-strength elements (e.g., P, Zr, Ti, and Y) (Figure 6a). Enrichments in highly soluble elements like Cs and Rb relative to HFSEs (Nb and Ti) indicate a significant role of subduction-zone fluids and/or melts produced by the dehydration and partial melt of subducted sediments and slab [41,42].

The chondrite-normalised rare earth element (REE) patterns of the volcanic rocks (Figure 6b) show similar distributions. The light rare earth elements (LREE; LaN/YbN = 8.85–10.41) are generally more enriched relative to the heavy rare earth element (HREE; DyN/YbN = 1.12–1.43) enrichment in the volcanic rocks (Table 1). The volcanic rocks exhibit negligible europium (Eu) anomalies, with Eu/Eu* values ranging from 0.91 to 1.03. Lower Nb/La ratios (approximately < 0.5) point to a lithospheric mantle source [39]. Conversely, high Nb/La ratios (Nb/La > 1) indicate an OIB-like asthenospheric mantle source or a mixing of lithospheric and asthenospheric-derived material (Nb/La ≈ 0.5 to 1) [41]. Figure 7a shows the La/Yb versus Nb/La diagram adapted from [43], on which the studied volcanics have low Nb/La ratios (0.31–0.37), indicating a lithospheric mantle source. We suggest that a subduction-modified lithospheric mantle contributes to generating the trachy-andesite under study. Recent geochemical interpretations have suggested that enriched mid-ocean ridge basalts (E-MORB) may serve as diagnostic indicators of tectonic settings involving lithospheric modification or mantle source enrichment. According to Xia and Li [44], while E-MORB signatures are typically associated with oceanic domains, their trace element patterns can also reflect subduction-modified mantle sources in continental regions, especially when subduction-related components persist over geological timescales. In our study, the trace element spider diagrams reveal moderately enriched incompatible element patterns (e.g., enrichment in LILEs such as Ba and Rb, depletion in Nb and Ti), which resemble E-MORB-type affinities. Therefore, the suggestion of an E-MORB-like component does not imply a purely oceanic ridge origin but rather reflects enrichment processes in a post-collisional extensional environment, possibly involving metasomatised lithospheric mantle components retained from earlier subduction episodes.

The investigated volcanic assemblage consistently follows a Zr/Nb vs. Y/Nb trend (Figure 7b), suggesting that fractionation crystallisation plays an essential role in magma evolution. In the (La/Sm)N versus (Gd/Yb)N diagram (Figure 7b), the studied volcanic rocks originated from a spinel–garnet transition zone. This diagram is used to determine the involvement of spinel or garnet in the mantle source. These data suggest that partial melting occurred at depths corresponding to the spinel–garnet transition, estimated to be between 80 and 90 km [45].

On the (⁸⁷Sr/⁸⁶Sr)i versus (¹⁴³Nd/¹⁴⁴Nd)i diagram, the volcanic rocks have high (⁸⁷Sr/⁸⁶Sr)i and low (¹⁴³Nd/¹⁴⁴Nd)i ratios, indicating a lithospheric mantle source (Figure 7d). The values of (⁸⁷Sr/⁸⁶Sr)i range from 0.706010 to 0.706057, (¹⁴³Nd/¹⁴⁴Nd)i from 0.512605 to 0.512615, and εNd(T) from −0.23 to −0.03. These volcanic rocks display geochemical characteristics that are indicative of a subduction-modified lithospheric mantle source, as reflected in their enrichment in large-ion lithophile elements (LILEs; Ba = 580–830 ppm, Rb = 52–65 ppm) and depletion in high-field-strength elements (Table 1, HFSEs; Nb = 4.1–5.3 ppm, Ta = 0.3–0.4 ppm, TiO₂ = 0.83–1.02 wt.%). Low La/Yb ratios (6.1–7.3) also indicate a melt derivation from the mantle. The TDM (Depleted Mantle Model) ages of the volcanic samples range from 0.85 to 1.02 Ga (Table 2), suggesting derivation from a lithospheric mantle source that experienced enrichment events during the Neoproterozoic. These ages are consistent with metasomatic processes that predate the Miocene magmatism, possibly related to earlier subduction episodes. The TDM values, together with the near-chondritic εNd (16.7 Ma) values (–0.2 to 0.0), support a model in which the parental magmas were derived from a subduction-modified lithospheric mantle [34].

The trace element and Sr–Nd isotope data suggest a magma source predominantly derived from the subduction-modified lithospheric mantle. Although primitive basaltic rocks are not exposed in the İspir–Ahlatlı area, the compositional features of the trachy-andesites imply derivation from a basaltic parental magma through fractional crystallisation (FC) or, to a lesser extent, assimilation–fractional crystallisation (AFC) processes. Regional analogues for such parental magmas include mafic volcanic rocks observed in other parts of the Eastern Pontides (e.g., Ilıca, Kandilli) and early Miocene mafic to intermediate volcanic sequences of Central and Western Anatolia [20,46,47]. Özdamar et al. [46] attributed the calc-alkaline sequences of the Galatya Volcanic Province to a subduction-enriched mantle source, while Özkul et al. [47] reported an isotopically distinct crustal contribution in the Kütahya volcanics. In the Bohemian Massif, Ulrych et al. [48] emphasised the controlling role of AFC in the petrogenesis of alkaline volcanics. Similarly, Kaygusuz et al. [20] showed that the basaltic–trachy–andesitic rocks of the Erzurum–Kandilli region originated from a metasomatised lithospheric mantle containing spinel + garnet, where the AFC component was <10% and magma evolution was primarily driven by FC and magma mixing.

Lino et al. [49] demonstrated, through major and trace element modelling and isotopic constraints, that intermediate Ediacaran volcanics in the Campo Alegre–Guaratubinha Basin (Brazil) were derived directly from coeval basalts of subduction-modified mantle origin without requiring significant crustal input. Their results showed that both basalts and trachy-andesites evolved along a common liquid line of descent, consistent with their field relationships and spatial continuity. This model closely aligns with the results for the İspir–Ahlatlı samples, highlighting the compositional inheritance from a mantle-derived parent magma. Additionally, the presence of clinopyroxene and plagioclase phenocrysts may suggest a basaltic parental magma for the İspir–Ahlatlı trachy-andesites. The evolution toward intermediate compositions likely occurred through fractional crystallisation processes under specific tectono-thermal conditions. As emphasised by Lino et al. [49], high crustal accretion rates and elevated geothermal gradients can significantly influence the development of intermediate magmatic products, even in arc or post-collisional settings. These dynamic parameters may facilitate the upward migration and differentiation of basaltic melts, contributing to the formation of trachy-andesitic compositions. The geochemical and petrographic features observed in our samples are consistent with this interpretation.

Furthermore, Asan et al. [50] reported that the AFC influence was relatively minor (~15%) in the Konya Volcanic Field. These examples illustrate that although the extent of AFC may vary, similar petrogenetic mechanisms can operate in diverse post-collisional settings.

In summary, the İspir–Ahlatlı volcanics represent intermediate magmatic products derived from a subduction-enriched lithospheric mantle, primarily evolving via fractional crystallisation. Future investigations should integrate multi-isotopic (Sr–Nd–Pb–Hf) analyses and detailed mineral-phase modelling to elucidate the magmatic evolution and source characteristics.

Figure 7. (a) La/Yb vs. Nb/La diagram [43], (b) Zr/Nb vs. Y/Nb diagram (vectors taken from [51]), (c) (La/Sm)N vs. (Gd/Yb)N diagram [52], and (d) (⁸⁷Sr/⁸⁶Sr)i vs. (¹⁴³Nd/¹⁴⁴Nd)i isotope correlation diagram [36].

Figure 7. (a) La/Yb vs. Nb/La diagram [43], (b) Zr/Nb vs. Y/Nb diagram (vectors taken from [51]), (c) (La/Sm)N vs. (Gd/Yb)N diagram [52], and (d) (⁸⁷Sr/⁸⁶Sr)i vs. (¹⁴³Nd/¹⁴⁴Nd)i isotope correlation diagram [36].

6.2. Tectonic Setting

The Neogene–Quaternary volcanism of Anatolia developed in a post-collisional setting that was marked by lithospheric extension following the closure of the Neo-Tethys [53,54].

In Central Anatolia, the intraplate extension is associated with lithospheric delamination, producing high-K volcanic rocks derived from a subduction-modified mantle source [46]. Volcanic centres such as Elmadağ, Hasandağ, Melendiz, and Acıgöl formed in a post-collisional setting that was influenced by lithospheric thinning and asthenospheric upwelling after the closure of the Neotethys Ocean. Central Anatolian volcanism evolved during a tectonic transition from collisional to extensional regimes beginning in the Miocene, driven by slab retreat and lithospheric thinning. This geodynamic shift led to the emplacement of voluminous volcanic products throughout the region [55]. Volcanism in the Central Anatolian Volcanic Province (CAVP), including the Tepekoy Volcanic Complex, developed in a post-collisional transtensional tectonic regime. This setting was characterised by slab rollback, asthenospheric upwelling, and decompression melting of a fluid-enriched lithospheric mantle, leading to the generation of hybrid magmas with both subduction-modified and intraplate signatures [56].

Göçmengil et al. [57] focused on post-collisional trachytic volcanism that developed during the Middle Eocene (~41–40 Ma) along both the northern (Almus) and southern (Yıldızeli) segments of the İzmir–Ankara–Erzincan Suture Zone (IAESZ). ⁴⁰Ar/³⁹Ar dating of sanidine phenocrysts indicates synchronous emplacement of trachytic lavas. Geochemical and Sr–Nd isotopic data suggest derivation from a metasomatised lithospheric mantle source, with limited crustal assimilation, through fractional crystallisation of a basaltic to trachy-andesitic parental magma. These findings support the interpretation that regional-scale lithospheric delamination or removal acted as a key geodynamic driver of the post-collisional volcanism in both tectonic blocks.

In Western Anatolia, slab rollback of the African plate beneath the Aegean domain induced significant extension and a geochemical transition from arc-like to more alkaline volcanism [47,58,59]. Neogene volcanism is predominantly linked to extensional tectonics, which is associated with the development of the Gediz and Büyük Menderes graben systems. These formed due to lithospheric thinning and asthenospheric upwelling during post-orogenic extension. Volcanic centres such as Kula, Simav, and Uşak–Güre exhibit a compositional spectrum, ranging from alkaline basalts to mildly tholeiitic and high-K calc-alkaline suites. Post-collisional volcanism in northwestern Anatolia, notably in the Biga Peninsula and Central Sakarya regions, reflects a complex geodynamic evolution involving slab rollback and breakoff following the closure of the İzmir–Ankara and Vardar oceans. From the Eocene to the Miocene, volcanism developed under a transitional tectonic regime marked by crustal accretion, lithospheric extension, and subduction-related mantle metasomatism. Slab rollback dominated in the west, while slab breakoff and asthenospheric upwelling prevailed in the east [60]. Uzel et al. [61] conducted a high-resolution geochronological study on Miocene volcanic rocks in western Anatolia, focusing on the İzmir–Balıkesir Transfer Zone (IBTZ). Using ⁴⁰Ar/³⁹Ar dating, they identified a pronounced magmatic hiatus during the Langhian (15.97–13.82 Ma), which they attributed to tectonic reorganisation caused by accelerated rollback of the subducted African slab. Their results emphasise that both the spatial and temporal evolution of Miocene volcanism are closely linked to extensional tectonics and the formation of metamorphic core complexes, which are driven by slab-induced mantle dynamics.

In Eastern Anatolia, the collision of the Arabian and Eurasian plates led to crustal thickening, followed by slab breakoff, and asthenospheric upwelling, which generated a broad range of volcanic products from mafic to felsic compositions [53]. Ertürk et al. [35] documented a comparable example of post-collisional magmatism in the SW Malatya region, where U–Pb zircon dating indicates the onset of felsic volcanism around 16.7 Ma. The dacitic to rhyolitic rocks show LILE enrichment, HFSE depletion, and negative εNd values, consistent with a subduction-modified lithospheric mantle source. Moderate crustal assimilation during magma evolution supports emplacement in a post-collisional extensional setting. These findings reinforce the interpretation that the Ahlatlı–İspir volcanics formed after the main collisional phase in a similar tectonomagmatic context. Aktağ et al. [62] investigated the Miocene mafic volcanics of the Tunceli region, revealing derivation from a subduction-modified lithospheric mantle with minimal crustal contamination. The basalts are enriched in LILEs, depleted in HFSEs, and geochemically consistent with melts formed near the spinel–garnet transition zone. These features point to magma generation in a post-collisional extensional regime involving lithospheric thinning and asthenospheric upwelling. A comparable geodynamic setting is inferred for the İspir–Ahlatlı trachy-andesites.

The Eastern Pontides represent a long-lived magmatic arc along the southern margin of the Eurasian plate. Subduction of the Neo-Tethyan oceanic lithosphere during the Mesozoic–early Cenozoic produced extensive arc-related magmatism, which later transitioned into post-collisional intrusions during the Eocene as subduction ceased and slab breakoff or delamination occurred [20,24]. The Anatolian volcanic record reflects the interplay between post-collisional tectonic relaxation, evolving mantle dynamics, and inherited subduction-related geochemical signatures across distinct geodynamic provinces. Geochemical data suggest that the studied trachy-andesites were derived from magmas that interacted with both the continental crust and components of the upper mantle [52]. Sr–Nd isotope data support the inference that these rocks are linked to an igneous process commencing in the upper mantle, although they are notably enriched with crustal components [18].

The tectonic regimes of the Neogene–Quaternary period provide important insights into the magmatic productivity and geodynamic evolution of the Eastern Pontides [9,15,16,17,18,21,23,24,25,51]. During this interval, a shift from compressional to extensional tectonics marked the post-collisional stage of the orogenic cycle. This transition facilitated crustal thinning and promoted asthenospheric upwelling, enhancing conditions for partial melting in the lithospheric and sublithospheric mantles. As a result, a wide range of magma compositions emerged—from basaltic to more evolved products—consistent with complex mantle–crust interactions [15,16,20].

The İspir–Ahlatlı trachy-andesites display geochemical and isotopic characteristics similar to those of other Neogene volcanic provinces in Türkiye. Their transitional composition, enrichment in LILEs, depletion in HFSEs, and moderate negative Eu anomalies are comparable to volcanic rocks from the Eastern Anatolian Volcanic Province (EAVP). In that region, particularly around Erzurum, Erzincan, and Van, Miocene to Quaternary volcanism is attributed to slab breakoff and asthenospheric upwelling following the collision between the Arabian and Eurasian plates [7]. These processes generated magmas derived from a subduction-modified lithospheric mantle.

The Miocene basaltic trachy-andesite volcanics of the Ahlatlı–İspir region (NE Türkiye, Eastern Pontides) occurred in a geodynamic context shaped by the final stages of Neo-Tethyan closure. A key question is whether these magmas formed during the collisional (syn-orogenic) stage or in a post-collisional extensional setting. The regional geological timeline suggests that subduction of the northern Neo-Tethys culminated in continental collision by the Late Paleogene (Eocene), followed by lithospheric thickening and eventual slab breakoff [63]. The studied volcanics (~Miocene in age) erupted after this collision when the Eastern Pontides experienced crustal extension and magmatism without active subduction. Previous work classifies Miocene volcanism in the Eastern Pontides as part of a post-collisional calc-alkaline phase [20].

The enrichment in LILEs and depletion in HFSEs (e.g., high Cs, Rb, Ba, and low Nb, Ti) is typical of subduction-modified sources. However, such arc-like signatures can persist in magmas well after subduction ends due to metasomatised lithospheric mantle sources [8]. Analogous cases in the literature show that volcanism can span both collisional and post-collisional stages. For example, Lino et al. [47] describe an Ediacaran basin in Brazil with two volcanic stages: an earlier syn-orogenic rift phase and a later post-orogenic stage. The differing geochemical signatures reflect the tectonic transition from active orogenesis to post-collisional extension.

By comparison, the Ahlatlı–İspir volcanic suite likely represents magmatism that followed the main collisional event in Eastern Anatolia. The magmas were sourced from a mantle modified by prior subduction, while the eruptive setting was extensional. These volcanics are best understood as part of the post-collisional phase of the Eastern Pontides, similar to other post-collisional volcanic suites within the Tethyan orogenic belt.

An alternative view considers “collisional magmatism” to include slab breakoff and orogenic collapse processes. From that perspective, these Miocene magmas might be described as late-collisional. Nevertheless, their timing (several million years after the Neo-Tethys closure) and association with extensional tectonics support a post-collisional classification. This interpretation aligns with regional studies documenting extension-related magmatism in Türkiye during the Early–Middle Miocene [20,24,46,47].

From a global perspective, the petrogenetic features of the İspir–Ahlatlı trachy-andesites are consistent with post-collisional volcanic suites in other orogenic belts. For instance, the Carpathian–Pannonian region in Central Europe displays similar calc-alkaline to shoshonitic volcanic rocks, produced by the upwelling of an enriched mantle following subduction rollback and lithospheric removal [1]. Similarly, trachy-andesitic suites from the northern margin of the Tarim Basin in NW China show overlapping La/Yb, Zr/Nb, and Nb/La ratios with the İspir samples, indicating partial melting of a metasomatised mantle wedge influenced by earlier subduction processes [14]. These global analogues reinforce the view that post-collisional magmatism commonly involves the reactivation of subduction-modified mantle sources and yields comparable geochemical and isotopic signatures.

Understanding the petrogenesis of trachy-andesites in the İspir region provides valuable insights into the regional geological framework. It highlights the magmatic evolution of the Pontides and the geodynamic processes that followed the continental collision. Post-Eocene magmatism in the Eastern Pontides reflects the influence of an extensional tectonic regime that continued into the Late Miocene and Quaternary [64]. The tectonic evolution of the Eastern Pontides is closely associated with the generation of trachy-andesitic magmas, contributing to a broader understanding of post-collisional tectono-magmatic processes in the region.

7. Conclusions

This study presents new petrographic, geochemical, and Sr–Nd isotopic data on Miocene trachy-andesitic volcanic rocks from the Ahlatlı–İspir region in the Eastern Pontides of Türkiye, offering important insights into their petrogenesis and tectonic significance. Petrographic studies reveal porphyritic textures dominated by plagioclase, biotite, amphibole, and clinopyroxene, which occur mainly as phenocrysts, with finer-grained plagioclase and opaque minerals in the groundmass, indicating fractional crystallisation.

Geochemical data confirm a transitional affinity with enrichment in LILEs (e.g., Rb, Ba) and depletion in HFSEs (e.g., Nb, Ti), consistent with a subduction-modified lithospheric mantle source. The REE patterns display significant LREE enrichment (LaN/YbN = 8.85–10.41) and minor Eu anomalies.

These results collectively indicate that the trachy-andesites formed during a post-collisional extensional regime following the closure of the Neo-Tethys Ocean. The new dataset presented here strengthens the interpretation that magmatism in the Eastern Pontides was driven by decompression melting of a subduction-modified mantle.

These trachy-andesitic rocks exhibit geochemical and isotopic characteristics similar to other Miocene volcanic units within the Eastern Anatolian Volcanic Province (EAVP), supporting a common origin from a subduction-modified lithospheric mantle in a post-collisional extensional setting.