| چکیده انگلیسی مقاله |
Introduction The Makran volcanic arc in southern Iran is one of the few active ocean-continent subduction systems in the Alp-Himalaya orogen (Priestley et al., 2021). It includes four volcanic fields: Shahsavaran (SVF), Bazman (BVF), west of Khash (KVF), and Taftan (TVF) (Biabangard and Moradian, 2008; Pang et al., 2014; Firouzkouhi et al., 2017a; Ghalamghash et al., 2022; Delavari et al., 2022) (Figure 1). In this work, we investigate the influence of crustal assimilation in the genesis of fractionated rocks of SVF, BVF and TVF, focusing on insights gained from Pb isotope ratios. We have also used Pb isotope contents of 5 basaltic samples represented by Saadat and Stern (2011) to distinguish the enrichment of the parental magmas in the source from crustal contamination during magma ascend and in the magma chamber. Regional Geology The SVF and KVF are mainly composed of basaltic andesite and basaltic lava flows, with minor andesitic rocks in SVF. In contrast, the BVF and TVF consist primarily of dacitic and andesitic rocks. SVF volcanic centers evolved from shields of thin basaltic lavas to composite cones of andesite and dacite with significant pyroclastic material (Figure 2). BVF features a stratovolcano of andesitic and dacitic lava flows, pyroclastic rocks, and scattered monogenic cinder cones around Bazman volcano (Figure 2). KVF is characterized by small-volume cinder cones, some with multiple volcanic phases. TVF includes the Taftan stratovolcano, with alternating andesitic and dacitic lava flows, pyroclastics, and minor basaltic flows (Moinevaziri, 1985; Biabangard and Moradian, 2008; Saadat and Stern, 2011). Geochemistry The major element concentrations of Makran arc volcanic rocks are shown in Table 1. Using the IUGS TAS classification (Figure 4A), the rocks are classified as andesite and dacite, falling in the sub-alkaline field. High Al2O3 and CaO levels classify them as calc-alkaline, supported by the AFM diagram (Figure 4B). K2O contents align with medium-high K calc-alkaline magmas (Figure 4C). Chondrite-normalized REE patterns and NMORB-normalized spidergrams (Figure 5) for andesitic and dacitic samples from SVF, BVF, and TVF show enrichment in LILEs relative to HFSEs and LREEs, with LREEs enriched over HREEs, typical of island arc and continental-margin magmatism. TVF andesites are more enriched than BVF dacites and SVF andesites. Eu/Eu* values range from 0.73 to 1.15, with averages of 0.82 (SVF), 1.01 (BVF), and 0.89 (TVF), suggesting plagioclase fractionation, particularly in SVF and TVF. The average 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios in the studied rocks range from 38.51421 to 39.09866, 15.58849 to 15.68117, and 18.45061 to 18.82136, respectively. For TVF andesites, the averages are 39.03451, 15.67155, and 18.77488; for BVF dacites, 38.70995, 15.62541, and 18.56748; and for SVF andesites, 38.83091, 15.64839, and 18.63444. While SVF and BVF samples show no systematic isotopic variation with fractionation, TVF rocks exhibit a positive correlation between Pb isotopes and SiO2, K2O, and Eu/Eu* (Figure 6). TVF rocks have the highest Pb isotopic ratios among all samples. Discussion Elevated major elements variations with increasing SiO2 are consistent with crystallization of pyroxene, plagioclase, Hornblende, and somehow magnetite and ilmenite (Figure 7). Variation of Sr, Zr, and Th with fractionation trends suggest Pl fractionation, magma mixing and crustal contamination, respectively (Figure 8). Nb/Ta ratio is also negatively correlated with markers of fractional crystallization such as elevated SiO2 and K2O (Figure 8D). Decreasing Nb/Ta with increasing fractionation reflect amphibole and biotite (Muntener et al., 2018) fractionation and also, could be a clue to the role of lower crust in the magma evolution (Tan et al., 2022). AFC (DePaolo, 1981) and mixing models using Th/La and K/Rb (Figure 9) show the contribution of Makran granitic rocks, lower crust, and upper crust to magma evolution. Trace element AFC models and Pb isotope ratios, which correlate positively with SiO2 and K2O (Figure 6B), suggest crustal assimilation or mantle wedge fluid interactions. Pb isotopic modeling indicates up to 15% crustal contribution in andesitic and dacitic rocks, best fitting a mix of upper and minor lower crust. Taftan volcano samples show the highest crustal assimilation, while Bazman and Shahsavaran samples display less (2-8%) assimilation. The Pb isotope signatures of the andesites and dacites likely reflect a mix of crustal assimilation and an enriched mantle wedge. In addition to subduction-related fluids, Pb isotopic features may originate from the lower crust through subduction erosion. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios in MVA basaltic samples (Saadat and Stern, 2011) suggest an enriched EMII-type mantle source. If parental basaltic magmas were unaffected by crustal assimilation during ascent, excess Pb in fractionated rocks likely results from crustal assimilation. Pb isotope ratios of 5 basaltic samples from SVF and KVF (Saadat and Stern, 2011) (Figure 1) fit best with 1-3% crustal contribution in mixing models (Figure 10). These basaltic samples are considered parental magmas for SVF and KVF andesites. Thus, crustal assimilation in fractionated rocks (andesite and dacite) is estimated at up to 12% in TVF, 7% in SVF, and 2% (±1%) in BVF. Geochemical and isotopic data indicate the contaminant is a mix of upper crust (possibly Tethyan flysch) and minor lower crust. Acknowledgment The authors are deeply grateful to the reviewers of the Petrological Journal for their valuable feedback and contributions to improving the manuscript. |