Petrography, Geochemistry, and Magmatic Processes of Oligocene-Miocene Tuzla Volcanics, Biga Peninsula, NW Türkiye

Abstract

The Tuzla area, located in the Ayvacık district of Çanakkale (Biga Peninsula, northwestern Türkiye), hosts a Oligocene-Miocene volcanic system comprising andesitic, dacitic, rhyolitic lavas, trachyandesite, pyroclastics, and ignimbrites, and the Kestanbol Pluton. Petrographic and X-ray diffraction (XRD) analyses indicate that the altered volcanic units are dominated by porphyritic dacitic/rhyodacitic and trachyandesitic rocks, with silicification, iron oxide formation, and opacification. XRD results reveal smectite, smectite–illite/mica, illite–mica, kaolinite, cristobalite–opal, K-feldspar, plagioclase, dolomite, hematite, and quartz as the principal mineral phases. Geochemical data, including rare earth elements (REEs), suggest that fractional crystallization of primary mineral phases played a major role in controlling magmatic evolution. Chondrite-normalized REE patterns display enrichment in light REEs relative to heavy REEs, indicating derivation from a common magma source. K₂O–Na₂O and (Na₂O + K₂O)–FeOᵗ–MgO (AFM) diagrams show high-K calc-alkaline, calc-alkaline, and tholeiitic affinities, with most rhyodacite/dacite and all trachyandesite samples plotting in the tholeiitic field. Tectonic discrimination diagrams indicate formation in both volcanic arc and intraplate tectonic settings. Moderate enrichments in Ba and Sr reflect magmatic evolution and source characteristics, whereas the highest concentrations are attributed to post-magmatic fluid–rock interaction. Overall, the Tuzla volcanic rocks originated from a collision-related enriched lithospheric mantle source and subsequently evolved through fractional crystallization and assimilation processes, accompanied by crustal contamination and variable hydrothermal overprint.

1. Introduction

The Biga Peninsula in northwestern Anatolia is a geologically and economically significant region due to its complex tectonic evolution, widespread magmatism, and abundant mineral and geothermal resources. The peninsula exposes a mosaic of metamorphic, ophiolitic, and Neogene sedimentary and magmatic rocks, recording multiple tectonic events from the Palaeozoic to the Quaternary. The widespread Cenozoic volcano-plutonic units are particularly important for understanding the region’s tectono-magmatic evolution, hydrothermal systems, and potential for metallic and industrial mineral deposits [1,2].

Turkey, located within the Alpine–Himalayan orogenic belt, represents one of the key geodynamic regions where volcanic and plutonic rocks are widely exposed. Northwestern Anatolia, in particular, is characterized by collisional magmatism, well-defined crust–mantle interaction signatures, and the coexistence of magmatic and tectonic processes within a structurally complex framework [3,4,5]. In terms of the regional structural framework, the east–west–trending orogenic system of Türkiye is subdivided into four main tectonic units from north to south: the Pontides, Anatolides, Taurus, and marginal fold belt, as illustrated in Figure 1 [6]. Northwestern Türkiye is bounded by the Intra-Pontide Suture Zone to the north and the İzmir–Ankara–Erzincan Suture Zone to the south and corresponds to the Pontides (or the Sakarya Zone) [7,8]. Within this tectonic setting, the Biga Peninsula represents a key exposure of the Sakarya Zone basement, comprising the Kazdağ, Karadağ, Çamlıca, and Karabiga massifs [7,9].

The Biga Peninsula records Paleogene–Neogene magmatism in three main stages—Eocene, Oligocene, and Miocene—and is widely interpreted as being of post-collisional origin [8,11,12,13,14,15,16]. Subduction of the Neo-Tethyan oceanic lithosphere beneath the Sakarya Zone commenced in the Early Cretaceous (~105 Ma) and was followed by collision between the Sakarya and Tauride–Anatolide continental blocks during the Eocene [7,10,11,17]. The earliest magmatic products associated with this collision include Middle Eocene granitic plutons and andesitic volcanic rocks [11,18,19,20]. Early–Middle Miocene magmatism in the Biga Peninsula is characterized by widespread andesitic, dacitic, rhyolitic, and pyroclastic units, indicating intense and sustained volcanic activity [3,21]. Preceding magmatic phases during the Oligocene to Early Miocene produced granitic intrusions and compositionally related volcanic units [22,23,24,25]. Geochemical trends indicate a systematic evolution from medium-K calc-alkaline compositions in the Eocene, to high-K calc-alkaline affinities in the Oligocene, and ultimately to high-K alkaline and shoshonitic compositions during the Miocene. Na-rich alkaline volcanic products represent the final magmatic stage following the Early Miocene [3,4,15,16,26,27].

The present study investigates the petrographic, geochemical, and petrogenetic characteristics of the Oligocene-Miocene Tuzla volcanic rocks in the Ayvacık region. By providing new constraints on magma evolution and associated hydrothermal processes, this study aims to address existing gaps in the understanding of post-collisional volcanism within in this geodynamically complex area.

2. Geological Setting

The study area is located within the Tuzla volcanic–geothermal system near Ayvacık in the Biga Peninsula (Çanakkale, northwestern Türkiye), where Oligocene-Miocene volcanic and plutonic units are widely exposed. This region forms part of the Sakarya Zone, bounded by the Intra-Pontid Suture to the north and the İzmir–Ankara–Erzincan Suture to the south (Figure 1). The geological evolution of the area reflects prolonged tectonic processes, including continental collisions, oceanic basin closure, magmatic arc development, and post-collisional extension since the Paleozoic [28,29,30]. The basement of the peninsula is represented by the amphibolite-facies Kazdağ Kazdağ Group, which comprises, from bottom to top, the Fındıklı, Tozlu, Sarıkız, and Sutüven formations [29]. These metamorphic units are unconformably overlain by Paleozoic recrystallized limestones, which are, in turn, covered by Miocene rhyolitic tuffs and agglomerates. Tectonically overlying the basement are the Permo–Triassic Karakaya Complex—composed of partially metamorphosed and strongly deformed clastic and volcanic rocks belonging to the Nilüfer, Hodul, Orhanlar, and Çal units—and the Cretaceous Çetmi ophiolitic mélange [9]. To the north of the Sakarya Zone, the Ezine Zone is exposed and comprises the Ezine Group, the Çamlıca metamorphics, and the tectonically emplaced Denizgören Ophiolite. This zone is separated from the Sakarya Zone by the Çetmi Mélange. The Çamlıca metamorphics contain high-pressure/low-temperature eclogitic relics, indicating Maastrichtian metamorphism (65–69 Ma) [31]. The Ezine Group consists mainly of Permo–Triassic carbonates and epicontinental sedimentary rocks metamorphosed under greenschist-facies conditions [32]. The Çetmi Mélange, composed of spilite, greywacke, shale, serpentinite, and radiolarite, represents an ophiolitic unit emplaced during the Late Cretaceous and transported southward during the Paleocene collision [33,34]. The Denizgören ophiolite, consisting predominantly of partially serpentinized harzburgites, tectonically overlies both the Çamlıca metamorphics and Ezine Group [9].

Eocene magmatism in the region is represented by granitic plutons and their volcanic equivalents [14,35]. During the Early Miocene, post-collisional magmatic activity produced high-K calc-alkaline to shoshonitic, I-type plutonic rocks (e.g., the Kestanbol Pluton: 21.5 ± 1.6 Ma [36]; 22.3 ± 0.2 Ma and 22.8 ± 0.2 Ma [35]) as well as coeval calc-alkaline to shoshonitic volcanic units [3,36,37,38]. This magmatic episode is widely interpreted as a consequence of post-collisional continental extension [3,38,39,40]. Miocene magmatic units in the study area include the Tuzla Volcanics and the quartz monzonite–monzogranite Kestanbol Pluton [23]. Volcanic activity commenced with the eruption of rhyolitic tuffs and agglomerates, followed by trachytic–trachyandesitic lava flows, and culminated in extensive rhyolitic tuff and ignimbrite deposition [2]. ⁴⁰Ar-³⁹Ar ages obtained of biotite (21.22 ± 0.09 Ma) and leucite (22.21 ± 0.07 Ma) crystals indicate that tephriphonolite dyke emplacement was coeval with the intrusion of the Kestanbol Pluton during the Early Miocene (21.5 ± 1.6, 22.8 ± 0.2 Ma) [41].

The Tuzla Volcanics, representing the most widespread volcanic unit of the Oligocene-Miocene, consist predominantly of hydrothermally altered porphyritic andesites, basaltic–andesitic lavas, and associated pyroclastic deposits. These units are exposed around Babadere, Naldöken, Taşboğaz, Çamköy and Kestanelik (Figure 2) and are intruded at depth by the Kestanbol Pluton. The volcanic succession is subdivided into three principal units: (i) Oligocene-Early Miocene felsic lava flows (rhyolite–dacite), characterized by massive, hard, and brittle flows locally associated with flow breccias. These rocks display colors ranging from grey to greenish and reddish-brown, and well-developed joint and fracture systems are common. (ii) Lava–pyroclastic alternations composed of well-bedded pumice and crystal tuffs, volcanic blocks, and bombs. These units are commonly affected by hydrothermal alteration, resulting in yellow, light brown, or cream-colored hues (Figure 3a–c). (iii) Later-stage andesitic–trachyandesitic lava flows, locally intercalated with flow breccias, lahar deposits, and lacustrine sandstones [38].

K-Ar dating of rhyolitic tuffs yields an age 26.5 ± 1.1 Ma (Late Oligocene) [30], whereas andesitic and trachyandesitic units were dated at 21.2 ± 0.9 Ma using the K–Ar method and 22.72 ± 0.19 Ma and 22.97 ± 0.23 Ma based on SHRIMP U–Pb zircon analysis [15]. Volcanic activity was largely controlled by NE–SW to NW–SE trending fault zones, which also governed in localized subsidence and episodic lacustrine sedimentation.

The Late Miocene volcanic succession exposed south of the Kestanbol Pluton constitutes the most extensive ignimbrite sequence in the region. These ignimbrites are exposed north of the Tuzla and Kestanelik and are predominantly pyroclastic in origin. According to [38], the ignimbrite succession displays a tripartite architecture comprising a basal surge unit, a welded ignimbrite, and an overlying unwelded pumice-rich tuff. Columnar jointing is common in the upper levels, whereas progressive welding in the lower to middle parts produces distinct petrographic zonation. The lower part of the succession begins with latite and andesite lava flows, which are intruded by basaltic and andesitic dykes. The most widespread ignimbrite unit attains a thickness of approximately 40–45 m and contains intermediate volcanic lithic fragments as well as granitic clasts. The upper part of the succession consists of alternating ignimbrite and andesite layers, forming the most extensive pyroclastic sequence in the area. Locally, late-stage rhyolite domes (Tuzla and Ortatepe) developed with compositions similar to those of the Ayvacık volcanics. U–Pb zircon ages obtained from the ignimbrites cluster around the Early Miocene age of ~22.3 ± 1.2 Ma [38].

The ignimbrite succession represents the late stage of regional volcanism and comprises Middle–Late Miocene ignimbrites, pumice-rich ignimbrites, tuffs, and agglomerates exposed around Taşağıl, Tabaklar, and Yukarıköy (Figure 2). The youngest volcanic products in the area are Late Miocene alkaline basalts. All volcanic and plutonic units are unconformably overlain by the Upper Miocene–Pliocene Bayramiç Formation, which consists composed of continental conglomerates, mudstones, marls, and lacustrine limestones [29,43].

3. Materials and Methods

This study is based on volcanic samples collected from the field, including six samples (K-1, K-2 code) from the Kestanelik area and eight samples (T-1, T-3 code) from the Tuzla area. An integrated approach was applied to comprehensively characterize the studied rocks. Mineralogical and petrographic characteristics were investigated using standard thin section petrography and X-ray diffraction (XRD; n = 9). Major and trace element geochemical compositions were determined for all 14 samples. The combined results of these analyses were used to constrain magma source characteristics, fractional crystallization and assimilation processes, and the tectonic setting of the volcanic rocks.

3.1. Mineralogical–Petrographic Analysis

Fourteen representative thin sections were prepared from volcanic rock samples. The samples, cut to dimensions of 5 × 5 × 5 cm, were processed at the Mineralogy–Petrography Laboratory of the Department of Mineral Analyses and Technology, General Directorate of Mineral Research and Exploration (MTA). Petrographic examinations were conducted using a Leica BX–51 polarizing microscope (Leica Microsystems, Wetzlar, Germany) at magnifications of 4 × 10 and 10 × 10 in the “Water–Rock and Mineral Analysis Laboratory” of the Department of Geological Engineering, Süleyman Demirel University (SDU). Petrographic observations focused on rock classification, textural characteristics, and mineral assemblages. Photomicrographs were acquired using a 5.1-megapixel digital camera (Lumenera Corporation, Ottawa, ON, Canada) mounted on the microscope.

Nine powder samples were analyzed by X-ray diffraction (XRD) at the Mineralogy-Petrography Unit of the Department of Mineral Analysis and Technology, General Directorate of Mineral Research and Exploration (MTA). The analyses were performed using a Shimadzu XRD-6000 diffractometer (Shimadzu Corporation, Kyoto, Japan) with CuKα radiation and a Ni filter. Clay mineral assemblages were examined in detail using XRD patterns obtained under scanning conditions (2–70° 2θ), as well as oriented mounts analyzed under air-dried (2–30° 2θ), ethylene glycol–treated (2–30° 2θ), and heat-treated conditions.

3.2. Geochemical Analysis

Nine representative rock samples were pulverized to <200 mesh at Activation Laboratories Ltd. (Actlabs, Canada) for whole-rock chemical analysis. Major oxides and trace elements concentrations were determined using inductively coupled plasma–emission spectrometry (ICP-ES). For rare earth element (REE) analyses, the samples were digested in dilute nitric acid and subsequently fused with a lithium metaborate/tetraborate flux to ensure complete dissolution. The resulting solutions were analyzed by inductively coupled plasma–mass spectrometry (ICP-MS).

4. Results

4.1. Petrographic Characteristics

4.1.1. Optical Microscopy

A total of 14 thin sections were prepared from the volcanic rocks collected in the study area. Petrographic analyses were performed under plane-polarized and cross-polarized light using a polarizing microscope, and representative photomicrographs were obtained. The principal phenocryst phases in the rhyodacite/dacite and andesite/trachyandesite samples are plagioclase, quartz, and biotite, accompanied by subordinate amphibole and opaque minerals (Figure 4). In the rhyodacite/dacite thin sections (Figure 4a,b,d,e), plagioclase crystals commonly exhibit polysynthetic twinning and zoning, whereas biotite shows pronounced opacification (Figure 4a). Figure 4b displays subhedral plagioclase with well-developed twinning, euhedral to subhedral quartz, and opaque minerals, together with hexagonal microcrystalline quartz and minor relict feldspar grains. The groundmass is predominantly glassy and exhibits intense staining related to Fe–Ti oxide–bearing phases associated with plagioclase, quartz, and biotite. Silicification and opacification are also widely observed (Figure 4c). Microcrystalline quartz and quartz veins of variable thickness and orientation occur as products of silica-rich fluids. The introduction of calcium together with silica locally resulted in calcite formation. Iron-rich zones display strong iron oxide enrichment and pervasive opacification, with euhedral opaque minerals occurring both within these zones and along quartz veins (Figure 4c). Microcrystalline quartz and quartz veins of variable thicknesses and orientations occur within silica-rich solutions. The introduction of calcium alongside silica led to the formation of calcite. Iron-rich areas exhibit intense iron oxide enrichment and widespread opacification, with euhedral opaque minerals present both in iron-rich zones and within the quartz veins (Figure 4c). In Figure 4d, anhedral quartz, amphibole, and biotite crystals are observed, whereas Figure 4e shows biotite flakes with a spherulitic texture. These spherulitic domains may represent early-formed components that were subsequently remobilized and reintroduced into the magma. Figure 4f illustrates sanidine crystals within trachyandesite/andesite displaying a vesicular texture, where some crystals are well preserved while others show varying degrees of alteration.

4.1.2. X-Ray Diffraction (XRD) Analysis

X-ray diffraction (XRD) analyses of volcanic rock samples collected from surface units in the Tuzla field reveal well-defined clay mineral assemblages. Representative diffraction patterns and corresponding analytical results are shown in Figure 5. The samples are dominated by smectite, smectite–illite/mica interstratifications, illite/mica, and kaolinite. Analysis of the clay fraction indicates that illite exhibits basal reflections with d(001) spacings ranging from 9.95 to 10.33 Å, d(002) spacings from 4.36 to 5.70 Å, and d(003) spacings from 2.45 to 2.58 Å. Smectite displays a wider range of basal spacings, with d(001) values between 11.16 and 14.94 Å, d(002) values from 3.47 to 3.57 Å, and d(003) values from 1.49 to 1.54 Å. Air-dried (AD), ethylene glycol–treated (EG), and heat-treated preparations at 350 and 550 °C show that the 001 and 002 reflections of smectite remain largely stable, whereas the 003 reflection disappears following these treatments. In long-scan analyses of samples in which smectite is not readily apparent, the characteristic smectite d(001) peak becomes evident after AD and EG treatments, confirming the presence of minor or poorly crystallized smectite layers. In some samples, EG treatment results in smectite expansion up to 21.77 Å, indicating well-developed swelling behavior. Heating at 350 and 550 °C leads to a progressive decrease in the d(001) spacing, consistent with the structural collapse of smectite layers.

4.2. Geochemistry

Fourteen volcanic rock samples collected from surface exposures in the Tuzla field were analyzed for major oxides, trace elements, and rare earth elements (REEs). The analytical results are summarized in

Table 1. Major oxide (wt.%), trace and rare earth elements (ppm) concentrations of Tuzla volcanic rocks samples (Key notes: DL, detection limit; LOI, lost on ignition).

		Rhyodacite/Dacite						Trachyandesite
Major Oxides (wt.%)	DL	T-1a-2	T-1e	T-3b	K-1a	K-1b	K-2d	K-2e	T-1b	T-1c	T-1g	T-3a	T-3c	K-2a	K-2b
SiO2	0.01	65.39	51.80	69.89	73.36	67.47	64.36	69.38	70.27	76.53	58.23	66.20	71.18	72.37	72.79
Al2O3	0.01	11.62	13.21	16.29	10.32	14.66	14.67	17.68	14.07	12.50	17.89	13.21	12.63	12.63	15.84
Fe2O3	0.04	1.25	4.28	0.76	3.66	2.99	9.47	3.21	3.46	2.52	4.28	6.53	1.23	4.73	1.68
MgO	0.01	0.02	1.53	0.15	2.26	1.21	0.64	0.79	0.18	0.04	0.26	0.22	0.15	0.20	0.43
CaO	0.01	0.26	11.15	0.72	1.23	0.99	0.07	0.19	0.72	0.37	0.99	0.81	0.99	0.33	0.13
Na2O	0.01	0.67	2.05	1.43	0.10	0.23	0.07	0.06	1.56	0.35	1.39	1.58	1.01	0.10	0.12
K2O	0.01	1.93	1.16	0.81	3.62	7.05	3.15	3.34	094	0.13	1.33	0.91	0.74	3.26	4.35
TiO2	0.01	0.555	0.481	0.740	0.993	0.433	0.453	0.447	0.585	0.446	0.800	0.795	0.581	0.939	0.865
P2O5	0.01	0.79	0.15	0.27	0.66	0.16	0.30	0.17	0.13	0.07	0.27	0.17	0.14	0.16	0.13
MnO	0.01	0.010	0.031	0.030	0.066	0.052	0.014	0.016	0.019	0.012	0.022	0.014	0.021	0.008	0.007
LOI	−5.1	14.72	12.87	9.58	3.85	5.49	7.18	5.60	9.01	6.65	12.67	8.92	9.03	5.78	4.26
Total	0.01	97.23	98.69	100.7	100.1	100.7	100.4	100.9	101.00	99.62	98.12	99.37	97.69	100.5	100.6
Trace element (ppm)
Ba	2	1287	701	233	357	987	763	535	1192	13,060	264	1292	20,050	5089	1003
Cs	0.1	0.9	46.2	7.7	2.6	5.3	8.6	11.4	25.1	3.5	23.3	26.8	11.7	4.5	8.2
Ni	20	<20	50	<20	130	20	<20	50	30	30	50	<20	40	<20	<20
Co	1	3	11	3	22	8	7	13	10	2	6	3	14	9	1
V	5	87	92	137	143	70	58	55	60	40	90	151	66	133	148
Cu	10	20	20	60	60	20	50	60	60	10	40	20	20	30	30
Zn	30	<30	60	<30	50	50	110	90	100	30	70	<30	40	<30	<30
Pb	5	203	34	905	74	36	29	41	92	26	52	85	59	37	40
Rb	1	11	59	42	174	311	146	170	68	10	92	79	49	121	190
Sr	2	5320	797	967	202	211	54	20	581	722	572	939	1042	539	306
Y	0.5	27.8	17.6	23.8	28.1	13.0	16.5	16.1	16.3	21.8	13.1	20.4	15.2	21.0	19.0
Zr	1	198	141	310	351	167	173	164	302	292	216	294	385	313	289
Nb	0.2	10.9	8.7	15.1	16.2	7.9	8.5	7.4	12.9	15.6	17.7	14.9	10.3	18.1	15.3
Th	0.05	50.9	11.4	57.4	53.1	15.8	22.6	16.8	34.2	35.2	35.4	41.5	33.8	40.9	32.2
W	0.5	6.2	1.8	1.8	8.7	4.8	13.8	3.8	9.1	3.9	5.8	1.3	7.7	9.4	11.2
Hf	0.1	3.8	4.7	8.0	10.9	4.9	6.0	4.8	8.9	7.9	6.7	7.9	11.1	9.3	11.3
U	0.01	10.60	3.19	59.00	14.60	5.12	4.33	4.91	6.96	1.24	4.93	5.70	9.76	12.10	14.40
Ta	0.01	1.23	0.60	1.67	1.87	0.70	0.74	0.68	1.38	1.17	1.42	1.58	1.36	2.14	1.65
Sb	0.2	5.8	7.9	9.6	1.9	1.8	3.8	0.8	13.2	3.1	11.3	13.0	22.0	0.8	2.7
Rare Earth Element (ppm)
La	0.05	96.9	34.5	91.8	85.6	40.6	39.2	37.9	52.2	63.8	75.2	68.7	49.1	80.1	78.8
Ce	0.05	184.0	67.9	17.0	167.0	76.5	72.9	72.8	94.3	114.0	142.0	128.0	91.0	147.0	143.0
Pr	0.01	20.0	7.9	18.6	21.0	8.7	8.3	8.5	10.8	12.5	15.8	13.8	10.3	17.0	17.5
Nd	0.05	72.7	28.3	64.2	77.0	30.9	28.4	29.4	36.4	42.8	46.0	47.7	36.9	61.3	62.7
Sm	0.01	14.40	5.44	11.50	13.70	5.20	5.02	5.71	6.06	7.22	5.10	9.02	6.20	8.80	9.45
Eu	0.005	3.77	1.16	2.20	2.53	1.03	0.85	1.06	1.39	1.22	0.82	1.93	1.21	1.28	1.57
Gd	0.01	11.10	4.05	6.59	9.18	3.69	4.11	4.90	4.14	4.99	2.93	5.84	4.28	4.00	4.88
Tb	0.01	1.29	0.57	0.85	1.27	0.45	0.56	0.68	0.51	0.72	0.42	0.74	0.55	0.64	0.71
Dy	0.01	5.70	3.16	4.60	5.63	2.42	2.97	3.22	2.92	3.78	2.33	3.98	2.72	3.87	3.68
Ho	0.01	0.89	0.61	0.83	0.99	0.44	0.52	0.55	0.56	0.77	0.44	0.73	0.52	0.76	0.63
Er	0.01	2.24	1.71	2.33	2.49	1.29	1.52	1.54	1.59	2.10	1.38	2.16	1.47	2.01	1.78
Tm	0.005	0.312	0.229	0.344	0.342	0.185	0.206	0.223	0.223	0.292	0.202	0.328	0.228	0.274	0.246
Yb	0.01	2.00	1.42	2.20	2.17	1.20	1.42	1.40	1.59	1.87	1.53	2.07	1.64	1.59	1.54
Lu	0.002	0.303	0.240	0.343	0.363	0.186	0.244	0.239	0.274	0.330	0.261	0.316	0.260	0.250	0.253

. The rhyodacite/dacite samples show SiO₂ contents ranging from 51.80 to 73.36 wt.%, Al₂O₃ from 10.32 to 17.68 wt.%, Fe₂O₃ from 0.76 to 9.47 wt.%, MgO from 0.02 to 2.26 wt.%, CaO from 0.07 to 11.15 wt.%, K₂O from 0.81 to 7.05 wt.%, and Na₂O from 0.06 to 2.05 wt.%. These major-element compositions indicate pronounced differentiation and a wide range of compositional variability. The systematic decrease in MgO and CaO with increasing SiO₂ reflects progressive magmatic evolution, primarily controlled by fractional crystallization of mafic minerals and plagioclase. Locally elevated Fe₂O₃ contents, reaching up to 9.47 wt.% in some samples, suggest variable redox conditions and secondary enrichment related to hydrothermal alteration. The trachyandesite samples are characterized by SiO₂ contents between 58.23 and 76.53 wt.%, Al₂O₃ ranging from 12.63 to 17.89 wt.%, and Fe₂O₃ contents from 1.23 to 6.53 wt.%. Alkali contents vary between 0.13 and 4.35 wt.% for K₂O and between 0.10 and 1.58 wt.% for Na₂O, whereas the remaining major oxides generally occur at concentrations below 1 wt.%. Sample K-2c, collected from a locality marked by pronounced iron-oxide enrichment, exhibits a relatively high Fe₂O₃ content, reflecting localized oxidation processes. The upper range of SiO₂ values in the trachyandesite samples overlaps with that of the rhyodacite/dacite group, indicating transitional compositions and/or advanced stages of magmatic differentiation. Overall, the volcanic rocks of the Tuzla field display a broad range of silica contents, varying from 51.80 to 76.53 wt.% SiO₂, with an average value of 67.80 wt.%. Their relatively high SiO₂ and Al₂O₃ contents, together with generally low concentrations of other major oxides (typically < 1 wt.%), indicate that these rocks represent silica-rich and highly evolved volcanic products. Variations in K₂O and Na₂O contents reflect differential behavior of alkali elements during magma evolution and post-magmatic processes. In particular, low Na₂O values in some samples are likely attributable to plagioclase fractionation and/or sodium loss during hydrothermal alteration, whereas locally elevated K₂O contents suggest potassic enrichment related to crustal input or fluid–rock interaction. The geochemical characteristics of sample K-2c further highlight the influence of localized oxidation and hydrothermal processes within the Tuzla field.

Trace-element data indicate that Sr concentrations range from 20 to 5320 ppm in the rhyodacite/dacite samples and from 306 to 1042 ppm in the trachyandesites, whereas Ba contents vary from 233 to 1287 ppm in the rhyodacite/dacite samples and from 264 to 20,050 ppm in the trachyandesites (Table 1). Moderate Sr concentrations may reflect limited plagioclase fractionation and/or local plagioclase accumulation during magma evolution, whereas lower Sr values are consistent with Sr depletion resulting from plagioclase removal during advanced stages of magmatic differentiation. In contrast, the extremely high Ba contents—particularly values reaching up to 20,050 ppm in some trachyandesite samples—cannot be satisfactorily explained by primary magmatic processes alone (Table 1). Such anomalously high Ba concentrations strongly suggest the involvement of secondary processes, most likely related to hydrothermal alteration and the precipitation of Ba-bearing minerals, such as barite, within fractures, veins, or pore spaces. Similar secondary Ba enrichment has been widely documented in geothermal and hydrothermal systems, where circulating fluids mobilize and redeposit large-ion lithophile elements (LILEs).

In the Zr/TiO₂–Nb/Y discrimination diagram of [44], which employs major and selected trace-element concentrations for volcanic rock classification, the studied samples plot predominantly within the rhyodacite/dacite and trachyandesite fields (Figure 6a). According to the K₂O–SiO₂ diagram of [45], the rhyodacite/dacite samples are mainly distributed within the calc-alkaline field, whereas most of the trachyandesite samples fall within the tholeiitic field (Figure 6b). In the AFM ternary diagram of [46], the majority of the rhyodacite/dacite samples plot within the tholeiitic field, with only two samples extending into the calc-alkaline field, while all trachyandesite samples are restricted to the tholeiitic domain (Figure 6c). Overall, these geochemical classifications indicate that the magmatic evolution of the Tuzla volcanic rocks reflects a complex differentiation history involving both tholeiitic and calc-alkaline evolutionary trends.

Chondrite-normalized rare earth element (REE) spider diagrams of the studied volcanic rocks [47] are shown in Figure 7. The rhyodacite/dacite and trachyandesite samples exhibit pronounced enrichment in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), with La_N/Sm_N ratios ranging from 3.93 to 9.28 and La_N/Yb_N ratios between 16.38 and 34.50. These values indicate broadly similar REE distribution patterns among the investigated rock types (Figure 7a). Such LREE enrichment relative to HREEs is a typical geochemical characteristic of calc-alkaline volcanic suites [48,49].

The observed REE patterns indicate that the studied volcanic rocks evolved through differentiation of parental magma(s) derived from a common source. The volcanic rocks display slight negative Eu anomalies superimposed on LREE-enriched patterns (Figure 7a). The average Eu/Eu* values range from 0.57 to 0.91 for the rhyodacite/dacite samples and from 0.62 to 0.85 for the trachyandesites. These negative Eu anomalies are interpreted to result from plagioclase fractionation and may also reflect assimilation of Eu-depleted continental crustal material.

The studied volcanic rocks show enrichment in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs) in N-MORB–normalized trace element patterns (Figure 7b). They are characterized by enrichment in large ion lithophile elements (LILEs), such as K, Pb, and Th, together with depletion in Rb and Sr. High field strength elements (HFSEs), including Nb, Zr, Ti, P, and Yb, are also depleted. In addition, Ba displays both positive and negative anomalies (Figure 7b).

5. Discussion

5.1. Tectonic Setting

To constrain the tectonic setting of the Tuzla volcanics, several widely used geochemical discrimination diagrams were applied [50,51]. In the Th/Ta versus Yb diagram, most samples plot within the oceanic arc field, while a limited number extend toward the active continental margin domain (Figure 8a). Likewise, the Th/Yb versus La/Th diagram places all samples within the oceanic arc field (Figure 8b). For comparison, Oligocene–Miocene calc-alkaline volcanic rocks from the Büyükbostancı–Çiçekpınar villages (Balıkesir, NW Turkey) plot within the active continental margin field in the Th/Ta versus Yb diagram, reflecting a different tectonomagmatic setting [52]. Ref. [53] showed that the Gökçeada volcanic rocks fall within the oceanic arc field on the same diagram and display a vertically elongated distribution indicative of subduction-related enrichment, a pattern closely comparable to that observed in the Tuzla volcanics. Overall, these geochemical signatures indicate that the Tuzla volcanic rocks were predominantly generated in an oceanic arc–related tectonic setting, with a subordinate contribution from active continental margin processes. This interpretation is consistent with other Oligocene–Miocene magmatic suites in NW Türkiye and with comparable subduction-related volcanic sequences documented in comparable tectonic settings worldwide.

The Tuzla volcanic rocks display mixed geochemical affinities, as they are distributed across both the volcanic arc and intraplate fields in the Rb/10–Hf–Ta*3 and Rb/30–Hf–Ta*3 triangular diagrams (Figure 9a,b) [54]. The rhyodacite/dacite samples predominantly plot within the volcanic arc field, whereas the trachyandesites mainly fall within the intraplate field, indicating contributions from both arc-related and intraplate magmatic processes. For comparison, the Yürekli volcanic rocks, which are spatially adjacent to the Tuzla volcanics, are mainly composed of andesite–trachyandesite–dacite, with minor basaltic andesite. In the Rb/10–Hf–Ta*3 diagram, these samples plot largely within the volcanic arc field, whereas in the Rb/30–Hf–Ta*3 diagram they are distributed between the pre-collisional and post-collisional fields, with only a few samples plotting in the volcanic arc field [55]. This comparison highlights the differences in magmatic evolution and source contributions between the Tuzla and Yürekli volcanic suites.

In particular, because Rb is a large-ion lithophile element that is highly mobile and sensitive to hydrothermal fluids, the distributions observed in these discrimination diagrams may reflect not only primary magmatic signatures but also the effects of hydrothermal alteration and fluid–rock interactions. The dispersion of samples across both the volcanic arc and intraplate fields in the Rb/10–Hf–Ta*3 diagram suggests a multi-source magmatic system; however, it also implies that the positions of some samples may have been secondarily modified by Rb enrichment or depletion. In this context, the overlap between the arc and intraplate fields observed in the Rb/10–Hf–Ta*3 diagram can be partly attributed to the influence of alteration on Rb distribution. By contrast, the clearer separation observed between rock types in the Rb/30–Hf–Ta*3 diagram indicates that this diagram more effectively preserves the primary magmatic signal, as it is less sensitive to Rb mobility. The predominance of rhyodacite/dacite samples within the volcanic arc field suggests that their characteristic geochemical signatures—reflecting contributions from subduction-related metasomatized mantle sources and crustal components—have been largely retained despite hydrothermal overprinting. Conversely, the concentration of trachyandesite samples within the intraplate field implies derivation from enriched lithospheric mantle sources and indicates a post-arc/intraplate magmatic affinity.

5.2. Magma Source

The studied volcanic rocks are characterized by pronounced enrichment in large ion lithophile elements (LILEs; Rb, Ba, and K) and light rare earth elements (LREEs) relative to high field strength elements (HFSEs; Nb, Ti, Zr, Hf, and Y) and heavy rare earth elements (HREEs) (Figure 8b). This geochemical signature indicates a metasomatized mantle source enriched in LILEs by a subduction-related component. [55]. The presence of negative Nb, P, Zr, and Ti anomalies, together with LREE enrichment relative to HREEs, further supports a subduction-related continental arc affinity for the parental magmas [51,56,57,58,59]. Elevated Th/Yb ratios, coupled with enrichment in Ce, Th, and other LILE elements (Sr, K, Ba, Rb), suggest that the parental magmas were derived predominantly from the lithospheric mantle [60,61,62,63,64]. Moreover, the marked depletion of Nb relative to LILEs implies that the mantle source region was metasomatized by fluids and/or melts released from the subducting slab or associated sediments, reinforcing a subduction-modified mantle origin for the magmas [51,61,65]. Comparable trace element characteristics have also been reported for volcanic rocks from the Ezine–Gülpınar–Ayvacık region in western Anatolia, Türkiye [3], indicating similar magmatic processes and tectonomagmatic settings.

High-field strength elements (HFSEs), such as Nb, are commonly depleted relative to light rare earth elements (LREEs; e.g., La) in the lithospheric mantle. Consequently, high Nb/La ratios (>1) are generally interpreted to indicate an asthenospheric mantle source, whereas low Nb/La ratios (<0.5) are characteristic of a lithospheric mantle origin [66,67]. In the investigated rhyodacite/dacite samples, Nb/La ratios range from 0.11 to 0.25, while La/Yb ratios vary between 24.30 and 48.45. The trachyandesite samples yield Nb/La ratios of 0.19–0.25 and La/Yb ratios of 32.83–51.17. The combination of low Nb/La and high La/Yb ratios indicates that the parental magmas were predominantly derived from a lithospheric mantle source (Figure 10). For comparison, Lower Miocene Şapçı volcanic rocks, which exhibit geochemical characteristics similar to those of the Tuzla volcanics [68], plot within the same field on the Nb/La versus La/Yb diagram. In contrast, Mesozoic volcanic rocks from central Lebanon (Middle East) fall within the HIMU–OIB field on the same diagram [69], reflecting a significant contribution from an asthenospheric mantle source

The Nb/Y and Th/Y ratios are widely used geochemical parameters for distinguishing between crustal contamination and homogenization of magma sources. The Nb/Y versus Th/Y discrimination diagram [51] (Figure 11) is commonly applied to differentiate between subduction-enriched and melt-enriched magmatic systems. In the present study, the volcanic rock samples display relatively elevated Th/Y ratios. This geochemical trend points to the influence of assimilation and fractional crystallization processes and further suggests that the mantle source was metasomatized by fluids released from the subducting slab.

In the Ta/Yb–Th/Yb discrimination diagram (Figure 12), the studied volcanic rocks are characterized by elevated Th/Yb ratios and define a trend parallel to the mantle array. This distribution indicates the combined influence of subduction-related enrichment and a decrease in the degree of partial melting. Such a pattern suggests that the parental magmas were derived from a mantle source enriched by fluids released during subduction-related metasomatism and subsequently generated under relatively low degrees of partial melting. In other words, the melts originated from a mantle source that had been previously modified by subduction-derived fluids and later underwent partial melting at reduced extents [71,72]. The Ta/Yb and Th/Yb ratios are widely used to evaluate mantle source characteristics and the extent of crustal contamination, as Th is more sensitive to crustal processes than Yb and Ta [73]. The position of the samples on the diagram further implies that assimilation–fractional crystallization (AFC) processes played a significant role in the magmatic evolution of the studied volcanic rocks. A comparable influence of fractional crystallization has also been documented for Miocene–Pliocene volcanic rocks from central Armenia [74].

In the La/Nb versus Ba/Nb discrimination diagram (Figure 13), the studied volcanic rocks cluster within a field proximal to arc-related volcanic compositions. During subduction, light rare earth elements (LREEs) and large ion lithophile elements (LILEs; Ba, Sr, K, and Rb) are transported upward by fluids released from the subducting slab [61,62]. The elevated La/Nb and Ba/Nb ratios observed in these volcanic rocks therefore indicate that their mantle source was significantly influenced by subduction-related enrichment. For comparison, the Taşlıyayla volcanic rocks (Çaykara, Trabzon, NE Turkey) also plot within the arc volcanic field on the La/Nb–Ba/Nb diagram [76], exhibiting geochemical characteristics closely comparable to those of the Tuzla volcanics.

Although no isotope analyses were performed in the present study, the magma source characteristics of Miocene volcanic rocks in western Anatolia have been well constrained by previous isotopic investigations. Sr–Nd–Pb isotope data indicate that Miocene calc-alkaline to shoshonitic volcanic rocks were derived from a heterogeneous and enriched lithospheric mantle source that was variably modified by subduction-related components and subjected to limited crustal assimilation [3,80,81]. Elevated ⁸⁷Sr/⁸⁶Sr ratios and negative εNd values are generally interpreted to reflect enriched mantle domains rather than extensive upper-crustal contamination, whereas Pb isotope systematics point to mixing between depleted asthenospheric mantle and enriched mantle components. Furthermore, zircon Lu–Hf and δ18^{18}18O isotope data from felsic volcanic and plutonic rocks indicate predominantly mantle-derived magmas with subordinate crustal input during magma evolution [35,82]. Taken together, these regional isotopic constraints support a multi-component mantle source for Oligocene-Miocene magmatism in Western Anatolia related to post-collisional lithospheric thinning and asthenospheric upwelling.

5.3. Fractional Crystallization and Assimilation- Fractional Crystallization

The studied volcanic rocks are characterized by enrichment in light rare earth elements (LREEs) together with elevated La/Yb ratios, suggesting that fractional crystallization of hornblende played a significant role in controlling the relative depletion or enrichment of heavy rare earth elements (HREEs) [83]. Likewise, the presence or absence of Eu anomalies appears to be largely governed by the extent of plagioclase equilibration and/or fractionation within the magma [84]. These geochemical features indicate that the investigated volcanic rocks likely evolved from parental magmas derived from a common source. Comparative evaluation with the Yürekli volcanic rocks of northwest Türkiye [55] and the Early Miocene Şapçı volcanic rocks [61] further supports derivation from a similar magmatic source and a broadly comparable evolutionary history.

The interaction of mantle-derived magmas with crustal material during ascent, and the resulting contamination, provides important constraints on the source characteristics of volcanic rocks [73]. Previous studies have shown that fractional crystallization and assimilation–fractional crystallization (FC–AFC) processes play a fundamental role in the petrogenesis of intermediate calc-alkaline volcanic rocks formed in subduction-zone settings [85,86,87]. Trace element and rare earth element (REE) abundances, as well as their ratios, represent key geochemical indicators of the mineral phases removed from parental magmas during partial melting and subsequent evolutionary processes, including fractional crystallization, assimilation, and crustal contamination [87]. Accordingly, assimilation of crustal material is regarded as a major geodynamic process that significantly modifies and redistributes the trace element compositions of mantle-derived magmas [63,64].

Chondrite-normalized REE patterns and N-MORB–normalized multi-element diagrams of the studied volcanic rocks are presented in Figure 8. The samples are characterized by enrichment in large ion lithophile elements (LILEs; U, Th, Rb, and K), light rare earth elements (LREEs), and particularly Pb, whereas high field strength elements (HFSEs; Nb and Ti) are depleted. The weakly developed Eu anomalies observed in the samples (Figure 8a) indicate crystallization and/or fractionation of plagioclase and K-feldspar, while pronounced P depletions in the multi-element patterns (Figure 8b) point to apatite fractionation. The presence of marked negative Nb anomalies coupled with positive Pb anomalies (Figure 8b) is a typical geochemical feature reflecting continental crustal influence.

Within the calc-alkaline series, Ref. [88] used the CaO–Y diagram to distinguish between J- and L-type trends, corresponding to relative Y depletion and enrichment, respectively, compared to the standard calc-alkaline trend. In the CaO–Y diagram (Figure 14), nearly all of the studied samples plot on the Y-depleted side of the calc-alkaline reference line, defining a J-type trend. This distribution emphasizes the significant role of hornblende and plagioclase fractionation in the petrogenesis of the volcanic rocks. Similar mineralogical controls have been reported for Upper Oligocene–Lower Miocene volcanic rocks from the Karakavak and Çukurhüseyin villages (Balıkesir Province, northwestern Türkiye) [89], as well as for Early Eocene volcanic rocks from the Artvin region in, northeastern Türkiye [90].

The relationship between fractional crystallization, source metasomatism, and/or crustal assimilation associated with subduction-derived fluids can be effectively evaluated using the Rb/Y versus Nb/Y diagram [73]. In this diagram, the studied volcanic rocks define a predominantly vertical trend and plot within the field characteristic of subduction-zone enrichment and/or crustal contamination (Figure 15). In contrast, intraplate magmatic enrichment is typically expressed by a positive correlation between Rb and Nb, corresponding to Nb/Y values of approximately 1 [91]. In the Tuzla volcanic suite, Nb/Y ratios range from 0.39 to 0.63 in the rhyodacite/dacite samples and from 0.68 to 1.35 in the trachyandesites, reflecting relative HFSE depletion that is characteristic of subduction-related magmatism. Comparable geochemical trends indicative of subduction-related enrichment or crustal contamination have also been documented in Late Cretaceous high-K volcanic rocks from the Central Pontides orogenic belt [92] and in Eocene–Oligocene volcanic units from the Momen Abad, eastern Iran [93].

6. Conclusions

Mineralogical, petrographic, and geochemical investigations of the Oligocene-Miocene volcanic rocks exposed in the Tuzla field, many of which record varying degrees of hydrothermal alteration, provide important constraints on the tectonic setting, magma source characteristics, and the roles of fractional crystallization and assimilation processes. The main conclusions can be summarized as follows:

• The Tuzla area hosts a complex Miocene volcanic system composed of andesitic, dacitic, and rhyolitic lavas, trachyandesites, pyroclastic deposits, ignimbrites, and the Kestanbol Pluton.
• Petrographic observations and X-ray diffraction (XRD) analyses indicate that the volcanic units are dominated by porphyritic dacitic–rhyodacitic and trachyandesitic rocks, which have been variably affected by pervasive silicification, iron oxide formation, and mineral opacification related to hydrothermal alteration.
• Geochemical data, particularly rare earth element (REE) systematics, demonstrate that fractional crystallization of primary mineral phases played a major role in the magmatic evolution of the Tuzla volcanics. Chondrite-normalized REE patterns show enrichment in light REEs relative to heavy REEs and broadly similar distribution trends, suggesting derivation from a common parental magma source.
• K₂O–Na₂O and AFM diagrams indicate high-K calc-alkaline, calc-alkaline, and tholeiitic affinities. Most rhyodacite/dacite samples and all trachyandesites plot within the tholeiitic field, reflecting compositional diversity within the volcanic suite.
• Tectonic discrimination diagrams collectively suggest that the Tuzla volcanic rocks were generated in both volcanic arc–related and intraplate magmatic settings, reflecting a complex tectonomagmatic evolution.
• Although moderate enrichments in Ba and Sr may partly reflect magma source characteristics and differentiation processes, the highest Ba and Sr concentrations are interpreted to result from post-magmatic hydrothermal fluid–rock interaction rather than primary magmatic compositions.
• The Tuzla volcanic suite includes rocks that preserve primary magmatic geochemical signatures as well as samples that have experienced varying degrees of hydrothermal overprinting. Overall, the volcanics were derived from a collision-related, enriched lithospheric mantle source and subsequently evolved through fractional crystallization and assimilation processes, accompanied by crustal contamination.