Olivine-hosted melt inclusions in Pliocene–Quaternary lavas from the Qorveh–Bijar volcanic belt, western Iran: implications for source lithology and cooling history

Abstract

Silicate melt inclusions (SMIs) are small droplets of magma that become trapped in minerals during crystal growth. SMIs in olivine crystals can provide critical information on the range of melt compositions and processes that occur during melt generation, evolution, transport, and eruption. The Pliocene-Quaternary volcanic rocks in the Qorveh-Bijar volcanic belt of western Iran show porphyritic and microlithic textures, with olivine and clinopyroxene being the dominant minerals. Magnesian olivines in these volcanic rocks contain primary SMIs. The composition and characteristic of olivine-hosted SMI of these rocks are investigated to constrain the source lithology for mafic volcanism. Bulk compositions of the SMIs overlap those of their host rocks and extend to higher CaO/Al 2 O 3 values. The estimated entrapment pressures and temperatures of the studied SMIs are 9.1-10.3 kbar and 1220-1355°C. The calculated mafic parental melt contains 42.36 wt.% SiO 2 , low total alkalis (3.22 wt.%), and high MgO (16.1 wt.%). Exploratory calculations using pMELTS show that this parental composition underwent variable degrees of fractional crystallization, as reflected by the variable compositions of the SMIs. Several lines of evidence including pyroxene xenocrysts and high FeO/MnO, FC3MS (FeO/CaO -3*MgO/SiO2), and Zn/Fe ratios (14-21), suggest that a metasomatized pyroxenitic source contributed to the genesis of the parental melt. Amphibole in the SMIs indicates a high volatile content in the parental melt, which we conclude was generated from a metasomatized lithospheric mantle source. The pyroxenite source also contained garnet. Our geochemical results lead us to propose a new petrogenetic model. Specifically, we infer that a dense and unstable portion of the lithosphere underwent localized laminar detachment and downward flow, i.e. lithospheric drip. This drip underwent volatileenhanced partial melting during descent through the underlying hot asthenosphere and generated the studied volcanic rocks.

Tectonic settings of the Plio–Quaternary volcanism in Iran from multidimensional and multielement solutions

“…This drip underwent volatile enhanced partial melting during descent through the underlying hot asthenosphere and generated the volcanic rocks. A CE setting would be consistent with the hot mantle as well as with the extensive fault system documented by Salehi et al (2019).…”

“…As mentioned above, area A5, located in the Sanandaj‐Sirjan zone close to the suture zone, showed a Col setting, whereas the isolated Bijar area (A6), also located in the same zone, has already evolved to a CE setting. Salehi et al (2019) proposed that the dense and unstable portion of the lithosphere underwent localized laminar detachment and downward flow, that is, lithospheric drip. This drip underwent volatile enhanced partial melting during descent through the underlying hot asthenosphere and generated the volcanic rocks.…”

“…Therefore, it is not unusual that in 14 papers (Ahmadzadeh et al, 2010, 2011; Alishahi et al, 2013; Amel et al, 2008, 2009; Biabangard et al, 2015; Ghasempour et al, 2014; Jalili et al, 2015; Khodami et al, 2009; Noghreyan et al, 2011; Pirooj et al, 2014; Salehi et al, 2019; ShahbaziShiran & Shafaei Moghadam, 2014; Sherafat et al, 2012), the authors used “Plio‐Quaternary,”Pliocene‐Quaternary” or “Pliocene to Quaternary” (P–Q) in the title, without presenting any age data. Similarly, in four papers (Amel & Akbarzadeh Laleh, 2017; Fazelihagh et al, 2017; Khezerlou et al, 2017; Pirmohammadi et al, 2011), the authors mentioned “Pliocene to Quaternary” in the abstract, without presenting or citing any age data.…”

“…Most authors (Ahmadzadeh, Jahangiri, Lentz, & Mojtahedi, 2010; Amel & Akbarzadeh Laleh, 2017; Amel et al, 2009; Amel, Moayyed, Ameri, Abedini, & Moazzen, 2008; Biabangard, Shahraki, & Boomari, 2015; Fahim Guilany, Darvishzadeh, Sheikhzakariaee, & VosoughiAbedini, 2019; Jalili, Amel, Hajaliogli, & Moayyed, 2015; Khezerlou, Amel, Gregoire, Moayyed, & Jahangiri, 2017; Khodami, Noghreyan, & Davoudian, 2009; Khosroshahi, 2015; Mahmoudi & Khajehei, 2019; Mojarrad & Souri, 2019; Noghreyan, Khodami, Davoudian, & Shabanian, 2011; Pirmohammadi et al, 2011; Pirooj, Jahangiri, Amel, & Kamali, 2014; Salehi, Torkian, & Furman, 2019; ShahbaziShiran & Shafaei Moghadam, 2014; Sherafat, Yavuz, Noorbehesht, & Yýldýrým, 2012) used the total alkalis vs. silica (TAS) classification diagram, but, in none of the papers, the rock classification was achieved by strictly following the International Union of Geological Sciences (IUGS) recommendations (Le Bas, 2000; Le Bas, Le Maitre, Streckeisen, & Zanettin, 1986; Le Maitre et al, 2002). According to the IUGS, unless both Fe oxidation varieties were measured, the total Fe must first be split into FeO and Fe 2 O 3 and then all major oxides be adjusted to sum up to 100% on an anhydrous basis, which can be easily achieved through a suitable computer program, such as IgRoCS (Verma & Rivera‐Gómez, 2013).…”

“…For tectonic inferences, none of the authors, except Amel and Akbarzadeh Laleh (2017) who employed the multidimensional diagrams of Agrawal, Guevara, and Verma (2008), used the recent developments of multidimensional techniques that have been available for about 15 years (Verma, 2020a, 2020b; Verma, Guevara, & Agrawal, 2006; Verma, Pandarinath, Verma, & Agrawal, 2013). Conventional bivariate (Gorton & Schandl, 2000; Muller, Rock, & Groves, 1992; Pearce, 1982; Pearce & Cann, 1973) and ternary (Muller & Groves, 1997; Pearce & Norry, 1979) diagrams were used by most researchers for deciphering the tectonic setting of the areas they studied (Ahmadzadeh et al, 2010; Alishahi et al, 2013; Amel et al, 2008, 2009; Biabangard et al, 2015; Fahim Guilany et al, 2019; Fazelihagh, Amel, & Jahangiri, 2017; Mojarrad & Souri, 2019; Pirooj et al, 2014; Salehi et al, 2019; Taheri et al, 2017). Other workers (Khezerlou et al, 2017) did not provide any constraints on the possible tectonic setting of their study area.…”