University of Isfahan Petrological Journal 2228-5210 16 3 2025 09 23 Petrology and geochemistry of volcanic and subvolcanic rocks in the south of Asagi igneous complex, northwest of Zahedan, Sistan Suture Zone Petrology and geochemistry of volcanic and subvolcanic rocks in the south of Asagi igneous complex, northwest of Zahedan, Sistan Suture Zone 75 100 29880 10.22108/ijp.2025.146089.1367 FA Mohammad Bameri Professor, Department of Geology, Faculty of Basic Sciences, University of Sistan and Baluchestan, Zahedan, Iran Reyhaneh Abbasi M. Sc. Graduate, Department of Geology, Faculty of Basic Sciences, University of Sistan and Baluchestan, Zahedan, Iran Abdolreza Partabian Associate Professor, Department of Geology, Faculty of Basic Sciences, University of Sistan and Baluchestan, Zahedan, Iran Journal Article 2025 07 24 Introduction The Asagi igneous complex is located in the northwest of Zahedan city (Figure 1A) and is part of a southeast–northwest trending magmatic belt. The igneous rocks of this belt were formed during at least two distinct periods. In the first period, Oligocene igneous rocks occur as extrusive (lava and pyroclastic), intrusive, and subvolcanic bodies that are mostly potassic. In the second period, Pliocene igneous rocks appear as sodic extrusive lavas (Camp and Griffis, 1982; Piri, 2018; Boomeri et al., 2020, 2022; Moradi et al., 2016; Nazari et al., 2022). Based on these studies, the Oligocene rocks belong to alkaline, calc-alkaline, high-potassium calc-alkaline, and shoshonitic magmatic series. These rocks are attributed to subduction-related and post -collisional tectonic settings. The aim of this paper is to investigate the geology, petrography, and geochemistry of volcanic and subvolcanic rocks in the southern part of the Asagi igneous complex. It also explores the tectonic setting and magma origin. Geology The Asagi igneous complex lies within the Sistan Suture Zone (Figure 1B), situated between the Lut and Afghan blocks. This zone includes two ophiolite complexes—Neh in the west and Ratuk in the east —separated by the Sefid-Abeh sedimentary basin (Tirrul et al., 1983). The Neh and Ratuk complexes contain Upper Cretaceous ophiolites and locally metamorphosed flysch-type sedimentary rocks of Upper Cretaceous to Eocene age. These complexes are commonly intruded by Cenozoic igneous rocks. The Sefid-Abeh basin consists of clastic rocks and limestone deposited in shallow to deep marine environments, with a thickness of approximately eight kilometers (Tirrul et al., 1983). This basin also hosts numerous intrusive and extrusive igneous bodies, indicating multiple magmatic phases during the Cenozoic (Camp and Griffis, 1982; Boomeri et al., 2021; Nazari et al., 2022). Strike-slip faults have played a key role in controlling magmatism in the region (Bagheri and Damani Gol, 2020). The study area is located within the Sefid -Abeh basin, where small outcrops of ophiolitic and sedimentary rocks are intruded by larger outcrops of basic to acidic volcanic and subvolcanic rocks, most of which have undergone hydrothermal alteration (Figures 2 and 3). Research Method Twenty-four thin sections were prepared from collected samples for petrographic analysis. Thirteen relatively unaltered samples were selected for geochemical analysis of major, minor, and rare earth elements. Major and some minor elements were measured using X-ray fluorescence (XRF) at Tarbiat Modares University, while rare earth and trace elements were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) at Novin Shimiyar Laboratory in Tehran. Petrography Petrographic studies (Figure 4) reveal that the igneous rocks in the study area include andesite, andesiticbasalt, dacite, trachyte, quartz monzonite, and diorite porphyry. These rocks predominantly exhibit porphyritic textures. The main minerals are plagioclase (with or without quartz), orthoclase, hornblende, biotite, and augite. Secondary, opaque, and accessory minerals are present in most samples. Some primary minerals show disequilibrium textures, such as resorption and zoning. Secondary minerals include calcite, quartz, sericite, and clay minerals, occasionally accompanied by chlorite, epidote, orthoclase, and biotite. Opaque minerals mainly consist of pyrite, hematite, magnetite, and iron hydroxides. Geochemistry The SiO₂ content in the studied rocks ranges from 53.66 to 66.34 wt.% (Table 1). In Harker diagrams, SiO₂ shows a negative correlation with P₂O₅, TiO₂, CaO, MgO, Fe₂O₃, and Al₂O₃, and a positive correlation with K₂O (Figure 5). These trends likely reflect magma differentiation and fractionation processes (Rollinson, 1993). The correlation between SiO₂ and Na₂O is weak, possibly due to weathering and hydrothermal alteration effects. The rocks belong to alkaline and subalkaline magmatic series (Figure 7B). Alkaline samples are potassium-rich and fall within the shoshonitic series (Figure 8), while subalkaline samples are mainly high -K calc- alkaline. Overall, the samples are classified as high -potassium calc-alkaline and shoshonitic. Geochemically, the rocks exhibit high Sr/Y (>40) and (La/Yb)N (>20) ratios, low Y (<18 ppm) and Yb (<1.9 ppm), and high Sr (>400 ppm), along with very low HFSE (Nb, Ta, Ti)—features characteristic of adakitic rocks (Castillo, 2006). Discussion and Conclusions Shoshonitic, adakitic, high -K calc-alkaline, and calc-alkaline rocks are typically associated with convergent plate boundaries, including continental margins, island arcs, and collision/post-collision zones (Morrison, 1980; Torabi, 2011). Low-K adakites are commonly found in active subduction zones. Tectonic discrimination diagrams confirm that the studied volcanic and subvolcanic rocks are related to convergent plate settings (Figure 9), specifically continental margins. Primitive mantle -normalized diagrams show enrichment in LILE relative to HFSE (Figure 10). Spider diagrams reveal positive anomalies in Cs, Pb, Th, U, and K, and negative anomalies in Rb, Nb, Ti, and P. Chondrite-normalized REE diagrams indicate enrichment in LREE over HREE, with weak negative Eu anomalies in all samples. These geochemical features are typical of high-K calc-alkaline, adakitic, and shoshonitic magmas formed in subduction-related continental margins (Wilson, 1989; Rollinson, 1993; Tatsumi and Eggins, 1995). The magmas are likely contaminated by crustal material (Wilson, 1989) and derived from an enriched mantle source (Figures 12A and B). The host rocks are garnet lherzolites enriched in phlogopite, with partial melting rates below 20 % (Figure 13). Although the geochemical signatures resemble those of magmas formed in continental margin subduction zones, considering the age, it is likely that these rocks formed in a post-collisional tectonic setting within a supra-subduction zone. Acknowledgement We thank the reviewers for their valuable comments on an earlier draft of this paper Introduction The Asagi igneous complex is located in the northwest of Zahedan city (Figure 1A) and is part of a southeast–northwest trending magmatic belt. The igneous rocks of this belt were formed during at least two distinct periods. In the first period, Oligocene igneous rocks occur as extrusive (lava and pyroclastic), intrusive, and subvolcanic bodies that are mostly potassic. In the second period, Pliocene igneous rocks appear as sodic extrusive lavas (Camp and Griffis, 1982; Piri, 2018; Boomeri et al., 2020, 2022; Moradi et al., 2016; Nazari et al., 2022). Based on these studies, the Oligocene rocks belong to alkaline, calc-alkaline, high-potassium calc-alkaline, and shoshonitic magmatic series. These rocks are attributed to subduction-related and post -collisional tectonic settings. The aim of this paper is to investigate the geology, petrography, and geochemistry of volcanic and subvolcanic rocks in the southern part of the Asagi igneous complex. It also explores the tectonic setting and magma origin. Geology The Asagi igneous complex lies within the Sistan Suture Zone (Figure 1B), situated between the Lut and Afghan blocks. This zone includes two ophiolite complexes—Neh in the west and Ratuk in the east —separated by the Sefid-Abeh sedimentary basin (Tirrul et al., 1983). The Neh and Ratuk complexes contain Upper Cretaceous ophiolites and locally metamorphosed flysch-type sedimentary rocks of Upper Cretaceous to Eocene age. These complexes are commonly intruded by Cenozoic igneous rocks. The Sefid-Abeh basin consists of clastic rocks and limestone deposited in shallow to deep marine environments, with a thickness of approximately eight kilometers (Tirrul et al., 1983). This basin also hosts numerous intrusive and extrusive igneous bodies, indicating multiple magmatic phases during the Cenozoic (Camp and Griffis, 1982; Boomeri et al., 2021; Nazari et al., 2022). Strike-slip faults have played a key role in controlling magmatism in the region (Bagheri and Damani Gol, 2020). The study area is located within the Sefid -Abeh basin, where small outcrops of ophiolitic and sedimentary rocks are intruded by larger outcrops of basic to acidic volcanic and subvolcanic rocks, most of which have undergone hydrothermal alteration (Figures 2 and 3). Research Method Twenty-four thin sections were prepared from collected samples for petrographic analysis. Thirteen relatively unaltered samples were selected for geochemical analysis of major, minor, and rare earth elements. Major and some minor elements were measured using X-ray fluorescence (XRF) at Tarbiat Modares University, while rare earth and trace elements were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) at Novin Shimiyar Laboratory in Tehran. Petrography Petrographic studies (Figure 4) reveal that the igneous rocks in the study area include andesite, andesiticbasalt, dacite, trachyte, quartz monzonite, and diorite porphyry. These rocks predominantly exhibit porphyritic textures. The main minerals are plagioclase (with or without quartz), orthoclase, hornblende, biotite, and augite. Secondary, opaque, and accessory minerals are present in most samples. Some primary minerals show disequilibrium textures, such as resorption and zoning. Secondary minerals include calcite, quartz, sericite, and clay minerals, occasionally accompanied by chlorite, epidote, orthoclase, and biotite. Opaque minerals mainly consist of pyrite, hematite, magnetite, and iron hydroxides. Geochemistry The SiO₂ content in the studied rocks ranges from 53.66 to 66.34 wt.% (Table 1). In Harker diagrams, SiO₂ shows a negative correlation with P₂O₅, TiO₂, CaO, MgO, Fe₂O₃, and Al₂O₃, and a positive correlation with K₂O (Figure 5). These trends likely reflect magma differentiation and fractionation processes (Rollinson, 1993). The correlation between SiO₂ and Na₂O is weak, possibly due to weathering and hydrothermal alteration effects. The rocks belong to alkaline and subalkaline magmatic series (Figure 7B). Alkaline samples are potassium-rich and fall within the shoshonitic series (Figure 8), while subalkaline samples are mainly high -K calc- alkaline. Overall, the samples are classified as high -potassium calc-alkaline and shoshonitic. Geochemically, the rocks exhibit high Sr/Y (>40) and (La/Yb)N (>20) ratios, low Y (<18 ppm) and Yb (<1.9 ppm), and high Sr (>400 ppm), along with very low HFSE (Nb, Ta, Ti)—features characteristic of adakitic rocks (Castillo, 2006). Discussion and Conclusions Shoshonitic, adakitic, high -K calc-alkaline, and calc-alkaline rocks are typically associated with convergent plate boundaries, including continental margins, island arcs, and collision/post-collision zones (Morrison, 1980; Torabi, 2011). Low-K adakites are commonly found in active subduction zones. Tectonic discrimination diagrams confirm that the studied volcanic and subvolcanic rocks are related to convergent plate settings (Figure 9), specifically continental margins. Primitive mantle -normalized diagrams show enrichment in LILE relative to HFSE (Figure 10). Spider diagrams reveal positive anomalies in Cs, Pb, Th, U, and K, and negative anomalies in Rb, Nb, Ti, and P. Chondrite-normalized REE diagrams indicate enrichment in LREE over HREE, with weak negative Eu anomalies in all samples. These geochemical features are typical of high-K calc-alkaline, adakitic, and shoshonitic magmas formed in subduction-related continental margins (Wilson, 1989; Rollinson, 1993; Tatsumi and Eggins, 1995). The magmas are likely contaminated by crustal material (Wilson, 1989) and derived from an enriched mantle source (Figures 12A and B). The host rocks are garnet lherzolites enriched in phlogopite, with partial melting rates below 20 % (Figure 13). Although the geochemical signatures resemble those of magmas formed in continental margin subduction zones, considering the age, it is likely that these rocks formed in a post-collisional tectonic setting within a supra-subduction zone. Acknowledgement We thank the reviewers for their valuable comments on an earlier draft of this paper

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