Petrogenesis of Quaternary Shoshonitic Volcanism in NE Iran (Ardabil): Implication for Postcollisional Magmatism
Trachyandesites, trachytes, andesites, and pyrocalstic rocks, with shoshonitic signature, are the main Quaternary volcanic rocks in the Sabalan region (Ardabil). Plagiocalse, K-feldspar, biotite associated with clinopyroxene, and glass are the main constituents of these lavas. Plagioclases are andesine to labradorite while clinopyroxenes have augitic composition. The Sabalan volcanic rocks show enrichment in LREEs (relative to HREEs) and are characterized by enrichment in LILEs and depletion in HFSEs. Petrological observations, along with rare earth and trace elements geochemistry, suggest shoshonitic signature for Sabalan lavas. This signature highlights derivation from a subduction-related source. The Sabalan volcanic rocks are isotopically characterized by derivation from an enriched mantle source with a tendency to plot in the fields defined by island-arc basalts (IAB) and OIBs (in εNd versus 87Sr/86Sr diagram). The geochemical and isotopic characteristics of the Sabalan lavas suggest that their magma has been issued via low degree partial melting of a subduction-metasomatized continental lithospheric mantle. The formation of these lavas is related to slab steepening and breakoff in a postcollisional regime.
Cenozoic magmatism is well known from the peri-Arabic region, north of the Bitlis-Zagros Suture Zone (Figure 1), and ranges in age from Eocene to Plio-Quaternary. The timing of the eruptions and pooling the plutons mostly coincides with and postdates a series of continental collisional events in the northern Bitlis-Zagros Suture Zone region [1, 2]. The Eocene magmatism in the peri-Arabic region (both in the Urumieh-Dokhtar magmatic belt and south of the Anatolian block) is calc-alkalic to shoshonitic, which resulted from subduction of the Neotethyan Ocean beneath the central Iranian and Anatolian blocks. The Plio-Quaternary (and Quaternary) alkaline-ultrapotassic magmatism occupies most of the Turkish-Iranian high plateau (NW Iran and NE Turkey) and is mostly characterized by within-plate and/or subduction-related geochemical characteristics [3–5]. The geochemical variations of late Cenozoic lavas indicate a progression from calc-alkaline to alkaline compositions with time .
The Turkish-Iranian high plateau is bounded on the north by the Eastern Pontide arc and Lesser Caucasus magmatic belt and to the south by continental blocks including Bitlis, Puturg, Sanandaj, and Sirjan blocks. Plio-Quaternary and/or Quaternary volcanic cones and flows are scattered in this plateau (such as Mt. Ararat, Sahand, Sabalan, Nemrut, and others) and most of them indicate postcollisional shoshonitic-alkaline affinities [7–10]. The Sabalan volcano (Ardabil) is among the youngest volcanic calderas, in eastern border of the Turkish-Iranian Plateau, consisting of Miocene trachyandesites and Plio-Quaternary trachy-andesite, pumiceous andesites, and dacites with ultrapotassic-shoshonitic signatures. The aim of this study is to recognize the major, trace, REE and Sr-Nd isotopes characteristics of the Sabalan Quaternary lavas and to present a consistent tectonomagmatic model for the formation and evolution of the Sabalan volcano.
2. Regional Geology
The southwestern Sabalan region is characterized by Miocene magmatism with occurrence of calc-alkaline and high K calc-alkaline to shoshonitic lavas . Moreover, late Miocene sodic alkaline lavas are common in south of the Sabalan region . Eocene and Oligo-Miocene lavas are also distributed in NW regions of Ardabil, near Meshkin Shahr-Ahar volcanic belt. The Eocene lavas are mostly andesites, trachyandesites, trachybasalts and analcime-bearing tephrites with shoshonitic to alkaline geochemical signatures. Volcanic breccias, tuffs, and interbedded lavas, with columnar joints are common in the N-NE of the Sabalan region. Andesitic to analcime-bearing tephritic dikes crosscut the pyroclastic sequence, mainly at NE of Meshkin Shahr. Eocene granitic, monzonitic, and monzogabbroic intrusions are common in the Ahar-Meshkin Shahr magmatic belt. These plutonic rocks have shoshonitic geochemical affinity .
The Sabalan volcano consists of both late Miocene (old) and Quaternary (young) trachyandesitic and trachytic to dacitic lavas with huge bodies of ignimbrites-density pyroclastic flows . The Sabalan magmatic activity can be divided into two stages. Stage 1 is huge masses of ignimbrites and volcanic ashes associated with interbedded andesitic to trachyandesitic lavas, erupted in early stages. After several eruptional phases, during collapse of the central domain, a caldera was formed. Stage 2 is dominant with growth of dacitic to trachyandesitic domes in the central parts of the old caldera. The Quaternary Sabalan lavas were described as high K calc-alkaline to shoshonitic by Dostal and Zebri  with highly differentiated REE patterns in chondrite-normalized diagrams. Didon and Germain  also suggested high K calc-alkaline and shoshonitic signatures for the Sabalan volcanic rocks.
Trachybasalts are minor in the Sabalan volcano and occur mostly as old, Miocene lava flows (Figure 2). They are distributed in the southwestern parts of the Sabalan caldera. These rocks grade upward into trachyandesitic lavas and occasionally are interbedded with pyroclastic rocks. Trachyandesites-porphyritic andesites occur mostly in the southern to western parts of the Sabalan caldera and are associated with pumiceous andesites. Trachytes are characterized by the presence of amphibole and biotite crystals. These rocks usually are vesicular. Quaternary porphyritic trachyandesites are mostly distributed in the central parts of the caldera and are the main constituents of the Quaternary Sabalan lavas (Figure 2). These lavas (trachyandesites) are intebedded with ignimberites. Dacites are minor in the Sabalan region and occur mostly as aphyric lavas associated with trachyandesites.
Ignimbrites constitute the major components of the pyroclastic eruptions and contain compressed fiammes with andesitic composition. Pumice and scoria flows are common. Lahar deposits, showing the latest stages of the volcanic activity, are predominant in the Sabalan region.
3. Petrography of Sabalan Quaternary Volcanic Rocks
Here, we adopted a classification based on the occurrence type of the rocks in the field, petrographical studies and mineral modes. In this way, we subdivided the rock samples into six groups.(1)Trachyandesites. These rocks are porphyritic with large (1-2 cm) K-feldspar phenocrysts with slight alteration into clay minerals. Biotite is another rock-forming mineral and occurs as euhedral to subhedral phenocrysts. Plagioclase shows zoning and sometimes alteration into clay minerals and sericite. The groundmass indicates alteration into clay and chlorite, pseudomorph phases after glass shards. Plagioclase and K-feldspar microlites occur in the groundmass. Amphibole in trachyandesites is rare while clinopyroxene in more mafic varieties is common. These rocks show hyaloporphyritic texture.(2)Dacitic Lavas. K-feldspar, plagioclase, biotite, and quartz are predominant phases of dacitic lavas. These minerals occur as small crystals within groundmass and/or as microphenocrysts. K-feldspar and plagioclase show slight alteration into sericite and clay minerals. Biotite displays brown paleochroism and is fresh. These rocks are mostly aphyric and most samples show holohyaline texture.(3)Trachytes. Trachytes have more K-feldspar than plagioclase and biotite and no quartz compared to dacites. Plagioclase phenocrysts show oscillatory to patchy zoning. K-feldspar shows slight alteration into clay minerals. Plagioclase microlites and glass shards are common in the groundmass. These rocks show intersertal to hyalo-microlitic porphyritic texture.(4)Andesites. Andesites are characterized by higher amounts of plagioclase than K-feldspar and occurrence of clinopyroxene microphenocrysts. Plagioclases show normal to oscillatory zoning. Plagioclases are less altered into clay than K-feldspar. Clinopyroxenes are subhedral to anhedral and occasionally show twining. Plagioclase microlites are dominant in the groundmass of the rocks.(5)Ignimbrites. These rocks are characterized by high amount of biotite associated with K-feldspar, plagioclase, and rarely quartz with flowed glass shards (eutaxitic texture). These rocks sometimes show layering, dark layers rich in biotite, and white layers rich in feldspar, due to the gravity flow of the crystal aggregates after explosion of the volcano. Oxidized andesitic fiammes are characteristic of some ignimbrite flows.(6)Pumice and Scoria. Andesitic scoria flows with large and abundant vesicles are common. Pumices and compressed pumices with fiammes are other rocks that accompany the pyroclastic units. Andesitic scoria flows have large amounts of glass, with rare plagioclase and clinopyroxene microphenocrysts.
4. Geochemistry of Quaternary Lavas
4.1. Analytical Methods
For this study we selected ten fresh samples from Quaternary Sabalan volcanic rocks for major, trace, and REE elements analyzed at ALS, Canada, using ICP-AES for major elements and ICP-MS for trace and REE elements. The uncertainty (1 sigma) is ~2% for major elements and 5 to 10% for trace elements (depending on concentration). Mineral analyses have been performed at Paris VI University using Cameca SX 100. Accelerating voltage, beam current, and beam diameter for the analyses were 20 kV, 20 nA, and 3 μm, respectively.
The isotopic analyses were carried out at the Laboratório de Geologia Isotópica da Universidade de Aveiro (Portugal). For Sr-Nd isotopes, samples were dissolved with HF/HNO3 in Teflon Parr acid digestion bombs at 200°C. After evaporation of the final solution, the samples were dissolved with HCl (6 N) and dried down. The elements for analysis were purified using a conventional two-stage ion chromatography technique: (i) separation of Sr and REE elements in ion exchange column with AG8 50 W Bio-Rad cation exchange resin; (ii) purification of Nd from other lanthanide elements in columns with Ln resin (ElChroM Technologies) cation exchange resin. All reagents used in sample preparation were subboiling distilled, and water was produced by a Milli-Q Element (Millipore) apparatus. Sr was loaded on a single Ta filament with H3PO4, whereas Nd was loaded on a Ta side filament with HCl, in a triple filament arrangement. Both elements were determined using a Multi-Collector Thermal Ionisation Mass Spectrometer (TIMS) VG Sector 54. Data were obtained in dynamic mode with peak measurements at 1-2 V for 88Sr and 0.8–1.5 V for 144Nd. Sr and Nd isotopic ratios were corrected for mass fractionation relative to 88Sr/86Sr = 0.1194 and 146Nd/144Nd = 0.7219. During this study, the SRM-987 standard gave an average value of 87Sr/86Sr = 0.710263 (6) (; 95% c.l.) and the JNdi-1 standard yielded 143Nd/144Nd = 0.512098 (2) (; 95% c.l.) (143Nd/144Nd data are normalized to the La Jolla Nd standard).
4.2. Mineral Chemistry
In this section, we focus on mineral chemistry of Quaternary Sabalan trachyandesites, the predominant rocks in the region.
Plagioclases in trachyandesites are characterized by andesine to labradorite composition (37 to 65% An, Table 1). Their K2O content ranges between 0.3 and 1.4 wt% (Table 1). Their variable An content indicates spots from various bands of zoning within mineral.
Clinopyroxene of trachyandesites are characterized by augitic composition (Figure 3(a)). Enstatite (En) end-member of these clinopyroxenes ranges from 43 to 48% while their ferrosillite (Fs) component is low, varying between 10 and 15% (Table 1). TiO2 and Al2O3 range between 0.28–0.95 and 1.1–2.8 wt%, respectively, while Cr2O3 and Na2O contents show lower values of 0.1–0.7 and 0.3–0.7 wt%, respectively (Table 1). Their Ti and Na contents are different from those in alkaline rocks, characterized by high concentrations of these elements (titano-augite or aegirine-augite). In Ti + Cr versus Ca diagram of , trachyandesite clinopyroxenes have a tendency to plot in volcanic arc field (Figure 3(b)).
4.2.3. Glass Shards
Glass shards in the groundmass of trachyandesites have minor traces of alteration and then were selected for EMP analysis. Their SiO2 content is variable, from 66.7 to 75.7 wt% (Table 1), higher than SiO2 content of whole rocks (57.6–65.4 wt%). The K2O content of these glasses is between 3.6 and 8.1 wt%, with low content of MgO (0.003–0.07 wt%) and TiO2 (0.2–1.1 wt%).
4.3. Whole Rock Chemistry
The Sabalan lavas have slight alteration, characterized by low content of LOI values (1.1–1.9 wt%, Table 2), except sample S08-7 with 3.2 wt% LOI. The lavas are intermediate to acidic in composition, with SiO2 content ranging from 57.6 to 65.4 wt% (Table 2). In total alkalis versus SiO2 diagram of , the Sabalan lavas (bulk rock) show tendency to plot in trachyandesitic to trachydacitic fields, similar to other shoshonitic lavas from NW Iran (Figure 4). These lavas tend to plot in trachydacite and rhyolite domains based on the composition of the glass shards of the rock groundmass. K2O and TiO2 contents of the Sabalan lavas vary between 2.5–4.8 and 0.7–0.9 wt% (Table 2). In K2O against SiO2 diagram , the lavas show affinity to high-K calc-alkaline and shoshonitic rocks. This geochemical signature is common for other postcollisional rocks from NW Iran and NE Turkey  (Figure 5). In chondrite-normalized REE diagram (Figure 6), Sabalan lavas show a fractioned REE trend, with enrichment in LREEs relative to HREEs (La(n)/Yb(n) = 18.8–20.4). Some samples show slight depletion in Eu, indicating plagioclase fractionation.
The Sabalan lavas show conspicuous depletion in Nb, Ta, Ti, and P (e.g., Nb(n)/La(n) = 0.5-0.6) with positive anomalies in most LIL elements including Rb, Th, U, and Pb (e.g., Th(n)/La(n) = 1.4–2.9 and U(n)/La(n) = 1.5–3.2) (Figure 6). These rare earth and trace elements patterns are mostly consistent with shoshonitic signature of these rocks, indicating magmatism along an active continental margin.
4.4. Nd-Sr Isotopes
Trachyandesitic samples have uniform initial (t = 2 Ma) 87Sr/86Sr values (0.704-0.705) with εNd ranging from ~+1 to +1.8. In the initial 87Sr/86Sr versus εNd variation diagram (Figure 7), the Sabalan volcanic rocks are characterized by derivation from an enriched mantle source, and all samples show tendency to plot in a field defined by island-arc (IAB) and ocean-island (OIB) basalts. The samples mostly show affinity to plot near the Ghoshchi alkali gabbros (unpublished data), Suffi-abad I-type granites , and Eastern Pontides adakites  (Figure 7). The Sabalan shoshonitic lavas have higher εNd than Quaternary Salmas lavas (unpublished data), indicating mantle source of the Sabalan lavas slightly depleted compared to that of Salmas lavas.
The Sabalan volcanic rocks are characterized by a fractionated REE pattern, with high LREE/HREE ratio, resembling shoshonitic rocks from active continental margins and/or postcollisional magmatism after closure of oceanic basins. Depletion in high field strength elements and enrichment in large lithophile elements are conspicuous. Such shoshonitic volcanism is characteristic of the Turkish-Iranian high plateau [2, 24]. The glass shards in the groundmass of these rocks are characterized by more fractionated nature, with higher SiO2 and K2O contents, indicating quenching products of more-evolved melt after fractionation of mafic minerals. Trachyandesite clinopyroxenes have a tendency to plot at volcanic arc rocks field in Ti + Cr versus Ca diagram, indicating an arc-related environment for the formation of Sabalan volcanic rocks. These characteristics show that these clinopyroxenes are crystallized from an arc-related calc-alkaline magma. The Sabalan lavas are similar to Goshchi alkali gabbros both of them are characterized by derivation from an isotopically enriched mantle source (OIB mantle source). They are clearly different from crustal contaminated, postcollisional Islamic peninsula shoshonites and ultrapotassic rocks. When compared to other Iranian postcollisional Plio-Quaternary lavas, like the Salmas basalts, the Sabalan rocks show higher εNd values, clearly indicating a slight depleted mantle source for the Sabalan lavas and/or contamination of the Salmas lavas with crustal materials during their ascent to the surface. The interaction with crustal rocks and/or derivation from a subduction-contaminated source is also clear from Th/Yb versus Ta/Yb diagram  for the Sabalan lavas (Figure 8). In this diagram, the Sabalan lavas show higher Th/Yb ratio, far from within plate mantle trend, resembling those lavas that erupted above an active continental margin. Higher Th content could have also resulted from interaction of ascending lavas with crustal rocks, via assimilation-fractional crystallization (AFC) processes. The Sabalan lavas seem to show contamination with high Th (and high 87Sr/86Sr) crustal rocks during their ascent to the surface beneath the volcano. This could be inferred from rare, partially dissolved crustal xenoliths in the Sabalan lavas, which can be evidence for crust contamination. In order to evaluate the mantle-melting regime of the Sabalan lavas, we used the Sm/Yb versus La/Sm diagram . The Sabalan lavas are relatively similar to Salmas Quaternary basalts, characterized by low degree of partial melting (<0.1%) of a mantle source containing clinopyroxene-garnet lherzolites (Figure 9). Because the Sabalan lavas are characterized by higher La/Sm ratio, it seems that their mantle source contains more modal garnet than model source in Figure 9 and/or contains other minor phases including phlogopite and/or apatite.
5.2. Geodynamic Setting
Plio-Quaternary (to Quaternary) volcanism in the Peri-Arabian region (in Turkish-Iranian high plateau) postdates the subduction of Neotethyan lithosphere since ~13 Ma . These volcanic rocks become more alkaline with time and towards the south [2, 4, 5, 27]. However, most of volcanic rocks, like the Salmas and/or Maku Quaternary lavas, still show a SSZ geochemical signature including enrichment in LILE and/or depletion in HFSE, despite the lack of subduction system at that time. Geochemical and geodynamic evidence also shows a subduction setting for the genesis of Sabalan lavas: fractionated REEs patterns (high LREEs/HREEs ratio), enrichment in Th, Ba, Rb, U, Pb, and depletion in Nb, Ta, and Ti. Such geochemical characteristics are common in subduction-related environments . These observations and isotopic characteristics of the lavas suggest that their magmas were probably derived from partial melting of a subduction-metasomatized continental lithospheric mantle in the garnet/spinel lherzolite field . Widespread volcanism across the Turkish-Iranian plateau throughout the late Cenozoic until Plio-Quaternary is strictly ascribed to postcollisional mantle upwelling during slab steeping and breakoff beneath a subduction-accretion complex [4, 28].
The Sabalan Quaternary volcanism includes a sequence of trachyandesites, latites, trachytes with huge bodies of ignimbrites, and pyroclastic rocks. Plagioclase with andesine to labradorite and clinopyroxene with augitic composition are predominant phases in these volcanic rocks. Lavas show shoshonitic geochemical signature with a fractioned REEs trend, enrichment in LREEs relative to HREEs, and depletion in HFSEs. The Sabalan volcanic rocks are isotopically characterized by derivation from an enriched mantle source (with/without crustal influence). All samples show tendency to plot in a field defined by island-arc (IAB) and ocean-island (OIB) basalts, in terms of their εNd value and 87Sr/86Sr ratio. The lavas are characterized by higher Th/Yb ratio, resembling those lavas that erupted above an active continental margin with geochemical evolution via assimilation-fractional crystallization (AFC) process. The geochemical and isotopic signatures of the Sabalan lavas suggest that their melts have been issued via low degree partial melting of a subduction-metasomatized continental lithospheric mantle during Neotethyan slab steeping and break-off beneath the Turkish-Iranian high plateau.
Conflict of Interests
The author declare that there is no conflict of interests. Hereby I confirm that I have not any type of conflict of interests in this paper.
This study was supported by a research Grant from University of Mohagheghe Ardabili.
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