The Taftan volcanic rocks: implication for the adakitic magmatism of the Makran magmatic arc

Taftan volcano, located in the southeastern Iran, is one of the active volcanic centers of the Makran magmatic arc during Neogene-Quaternary. The Makran arc is an east-west trending Neogene- Quaternary volcanic edifice (Pang et al., 2014) lying in the north of the Makran accretionary prism. Three major volcanic centers in the north of Makran from west to east are the Bazman, Taftan and Koh-i-Sultan volcanoes, respectively, form a ca. 300 km long magmatic arc. Regarding structural zones, the Taftan volcano locates in Sistan suture zone and its basement rocks include ophiolite, flysch and volcanic rocks (mostly Upper Cretaceous to Eocene). In this paper, we present petrographic and whole rock geochemical signatures of the Taftan volcanic rocks with the aims of better understanding of petrogenesis and tectonic implications of magmatism in the Makran arc.
The rock samples were crushed into centimeter-sized chips. After crushing, they handpicked in order to eliminate altered parts and remove any visible impurities. The samples were powdered by Tungsten Carbide in the sample preparation laboratory of the Iranian Mineral Processing Research Center (IMPRC). Whole-rock major and trace elements were obtained by Inductively Coupled Plasma-Emission Spectrometry (ICP-ES) using a Spectro Ciros Vision instrument and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) on Perkin Elmer Elan 6000 instruments at Acme Labs. The major elements and also Cr and Ba were determined by ICP-ES and the remaining trace elements by ICP-MS. Powdered rock samples were mixed with LiBO2/Li2B4O7 flux and fused in crucibles in a furnace. The quenched bead was dissolved in ACS grade nitric acid. Loss-on-ignition (LOI) was determined by heating a sample split to 1000°C.
Result and In the Taftan volcanics, phenocrystic assemblage consists of plagioclase clinopyroxene hornblende ± orthopyroxene ± biotite. Disequilibrium micro-textures of minerals, including sieve-texture, resorption surfaces, fine- sieve zones and regrowth bands of plagioclase and sieve- texture and opacity rim of hornblende suggest changes in physico-chemical conditions of the system (T, P, PH2O, melt chemistry). Geochemically, Taftan volcanics comprise basic to acidic compositions, although dominated by intermediate andesitic to dacitic lava and pyroclastics. Based on K2O-SiO2 diagram, the samples plot within medium- to high- K series. Except two samples showing low Mg# [100*Mg/(Mgᗭ)] (~17 and 24), the others are characterized by approximately high Mg# (> 50). In Harker variation diagrams, despite scatters of some data points, a continuous and nearly linear trends can be observed suggesting genetic relationships among the samples and melt evolutionary trend. In these diagrams some elements including MgO, TiO2, CaO, FeOT, Mg# and Cr represent negative correlation with SiO2 enhancement. These variations are consistent with fractional crystallization of ferromagnesian phases (clinopyroxene hornblende), plagioclase and Fe- Ti oxides. The volcanics display chondrite-normalized fractionated rare earth element (REE) patterns with (La/Yb)N, (Sm/Yb)N, (La/Sm)N ratios in the ranges of 9.9- 19.7, 2.4- 3.7, 3.6- 5.8, respectively. Primitive-mantle normalized multi-element diagrams are characterized by light ion lithophile element (LILE) such as Rb, K and Ba enrichment and high field strength element (HFSE) such as Ta and Nb depletion which could be interpreted as subduction zone magmatic signatures. Furthermore, geochemical characteristics of the Taftan volcanics are consistent with adakitic signatures. These include high Sr (> 645 ppm), Sr/Y (> 40) and La/Yb (>40). High Sr/Y and La/Yb ratios as well as low Y content of “adakites” has been interpreted as melt equilibration with amphibole garnet and lack of plagioclase in melt source. This mineralogy corresponds to high pressure (>10 kb) condition. Scenarios that match these requirements for adakite generation include partial melting of thickened arc or thickened post-collisional continental crust (e.g. Chung et al., 2003; Petford and Atherton, 1996; Topuz et al., 2011), partial melting of subducted slab (Defant and Drummond, 1990; Drummond and Defant, 1990), low degree partial melting of metasomatized mantle (Gao et al., 2007) or high-pressure fractional crystallization involving garnet (e.g. Macpherson et al., 2006). In the case of Taftan volcano, partial melting of subducted slab seems to be impossible because both the subduction zone and subducted slab are too old (pre- Cretaceous). Moreover, geochemistry of Taftan volcanics including trace element and isotopic characteristics are different from slab- derived adakites. Therefore, it is more realistic to consider Taftan volcanics as “adakite- like” melts generated in continental arc setting with thickened crust. Consistently, geophysical studies show thick continental crust (> 50 km) beneath Taftan volcano. In arc settings, during interaction of mantle melts with deep lower crust and as a result of complex magmatic processes such as melting-assimilation-storage-homogenization (MASH) and assimilation-fractional-crystallization (AFC), “adakitic melts” could be produced (Richards and Kerrich, 2007).
Bringing together all of petrographic and chemical evidence, here we propose following successive processes for the genesis of the Taftan volcanics:1. Partial melting of sub-continental lithospheric mantle wedge. This mantle source is reasonably more enriched than sub-oceanic depleted mantle. In addition, slab-derived materials including fluids and/or sediment melts would increase LILE content and enhancement of LILE/HFSE ratio in melt source.
2. Interaction of mantle melts with deep continental crust and assimilation processes. Melt equilibration with high pressure crustal lithology (amphibole ± garnet – plagioclase) would reinforce the adakitic signature of the melt.
3. Melt ascent and mid to upper-crustal processes including storage, refilling of magma chamber, magma mixing and fractional crystallization of plagioclase and ferromagnesian minerals.