Petrography, Geochemistry and Tectonic setting of Tertiary volcanic rocks in the North of Chah Gonbad (Northeast of Seh Chengi, Lut Block)

IntroductionThe Chah Gonbad area is located 125 km southwest of Birjand, 90 km southwest of Khousf, and about 40 km southwest of Khour village, between the longitudes 58º07ʹ30ʺ–58º14ʹ50ʺ and the latitudes 32º36ʹ23ʺ–32º42ʹ40ʺ. Based on the 1:100, 000 map of Seh Changi prepared by Eftekharnejad and Stöcklin (1975), the studied area is located in the northeast of Sechangi, north of the Lut Block. Access to the area is possible via the main Birjand–Khousf–Khour–Kerman road and the Khour–Chah Gonbad–Sechangi side road (Figure 1). There are various opinions about the formation of the volcanic rocks of the Lut Block (Eftekharnejad, 1972; Darvishzadeh, 1976; Jung et al., 1983; Camp and Griffis, 1982; Tirrul et al., 1983; Tarkian et al., 1983; Pang et al., 2013; Omidianfar, et al., 2018; Kalatbari Jafari et al., 2019, 2020, 2021; Fotoohi Rad et al., 2022; Yousefzadeh and Chahkandinezhad, 2023). The studied rocks range from basic/intermediate to acidic compositions. The main purpose of this research is therefore to provide detailed petrological and geochemical information on volcanic rocks in the Chah Gonbad area to constrain the tectonic setting of these rocks and to identify the geology of the Lut Block in eastern Iran.Regional GeologyThe Tertiary volcanic rocks are extensively exposed in the north of Chah Gonbad. These volcanic and pyroclastic rocks, which cover most of the region, include tuff, perlite, and ignimbrite (Paleogene), as well as rhyolite, dacite (associated pyroclastics), and andesite/ basaltic andesite (Neogene) (Figure 2).Research Method In order to carry out this research, reports, geological and topographic maps, satellite images of the region, and references related to the research topic were first prepared and reviewed. In the next step, during 8 days of fieldwork, rock sampling was carried out by examining their field relationships. In the third step, 79 thin sections were prepared, and their mineralogical and textural descriptions were identified using a polarizing Leitz microscope. Then, 10 samples with the least alteration were selected and sent to Acme Canada Laboratory for analysis of major elements by the ICP-ES method and analysis of trace elements by the ICP-MS method. GCDKit, Excel (@2007), Grapher, and ArcGIS software were used to draw the diagrams and geological map. To calculate the amounts of Fe₂O₃ and FeO, Minpet software was used following the method of Irvine and Baragar (1971).PetrographyThe Chah Gonbad area has extensive outcrops of volcanic rocks with basic/intermediate to acidic compositions. The studied rocks range from basaltic andesite to rhyolitic compositions (basaltic andesite/andesite, rhyolite, dacite, perlite, and ignimbrite), with a peak in acidic compositions. These rocks are dominated by porphyritic texture with microlitic groundmass, glomeroporphyritic, hyaloporphyritic, poikilitic, perlitic, and spherolitic textures. Plagioclase (oligoclase–andesine), sanidine, pyroxene, hornblende, biotite, and quartz are common minerals. Evidence of disequilibrium, including sieve texture, chemical zoning and resorption margins in plagioclase, opacified margins in hornblende, and rounded or embayed edges in quartz and sanidine, are observed in these rocks.Geochemistry and PetrogenesisBased on various diagrams, the samples from Chah Gonbad fall within the range of andesite/basalt, andesite, trachyandesite, dacite, trachydacite, and rhyolite. These rocks have calc-alkaline, high-potassium calc-alkaline, and shoshonitic affinities (Figure 9C). In the Co versus Th diagram (Hastie et al., 2007), which is used for volcanic rocks, the samples show consistent trends. The Sun and McDonough diagram (1989) was used to normalize trace elements to the primitive mantle (Figure 10A). The overall geochemical characteristics, including depletion in Ba, Nb, P, Ti, and Ta, enrichment in LILEs (i.e., Cs, Th, U, K, Rb) relative to HFSEs (i.e., Nb, P, Zr, Ti, Ta), negative anomalies of Nb, Ti, and Ta, and high LILE/HREE ratios in the studied rocks, are features associated with subduction zone magmas. The observed negative Nb anomaly in these samples is an indicator of continental rocks and may suggest crustal participation in magmatic processes (Rollinson, 1993). Depletion of HFSEs such as Nb, P, Ta, and Ti is a prominent feature of arc environments and may result from magma derived from subducted oceanic crust and the overlying mantle wedge, which underwent fractional crystallization, assimilation, and contamination with crustal materials (Saunders et al., 1992; Nagudi et al., 2003). To study the behavior of rare earth elements in samples from the region, a normalized spider diagram with chondrites was used (Boynton, 1984) (Figure 10B). In this diagram, LREEs show enrichment relative to HREEs. According to Winter (2010), the enrichment in LREEs indicates formation in subduction zones. The low Eu depletion in these rocks could be due to high oxygen fugacity during formation and crystallization. Based on Nb versus Zr, Ta/Yb versus Th/Yb, and Yb versus Th/Ta ratios (Figures 11A-11E), the studied rocks are located in a subduction–post -collision setting and in the active continental margin. Geochemical characteristics and tectonic discrimination diagrams suggest that these volcanic rocks presumably formed in an immature continental arc setting (Figure 12). Considering the geological setting and petrological and geochemical evidence, it can be concluded that these rocks were formed in a post-collisional zone during delamination of the continental lithosphere in the Lut Block.ConclusionThe depletion of Ti, Nb, and Ta in the rocks of the region, along with low HREE and high LREE contents, indicates magmatism in a subduction zone. These rocks belong to the active continental margin. Depletion in Ti, Nb, and Ta (TNT) and Ba enrichment in Cs, Th, U, and Rb provide evidence of the role of continental crust in magmatic processes. Low amounts of Ni (>20) and Co (0.6–24.8), Mg# values less than 40 (12–40), and Nb/Ta ratios greater than 1 (8.9–23.5) indicate the prominent role of the crust in the formation or evolution of the parent magma. Based on geochemical evidence, the volcanic rocks of the Chah Gonbad region were formed in a post-collisional zone during thinning of the continental lithosphere in the Lut Block and in an immature continental margin arc.

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