Petrogenesis and Tectonic Setting of the Young Volcanic Adakitic Rocks in Darehzereshk’s Hassan abad (SW Yazd)

Petrogenesis of continental magmatic arc is more complicated than oceanic arcs. Because some various parameters affect the composition of magmatism in the congenital margin, such as continental crust contamination, mixing with crustal magmas and the deformation phenomena from primary mantle magma (Martin et al., 2005; Eyuboglu et al., 2018; Pawley and Holloway, 1993). From a historical point of view, adakite is initially known as a special rock on the Adak island, described by Kay, (1978), called by Mackenzie as andesite. The unusual geochemical features of these rocks are: (SiO2 ≥ 56 wt%، Al2O3 ≥ 15 wt%، K2O/Na2O < 0.5 wt%، Sr > 400 ppm، Y ≤ 18 ppm و Nb < 10 ppm). The low content of HREE due to the presence of hornblende or garnet in the adakites melting source. Negative anomalies of Nb and Ti are also the properties of the magmas drifted from subduction zones. Totally, it can be said that the abundance of the major and rare elements in volcanic rocks of the Darehzereshk’s Hassanabad correlate with the average chemical composition of adakites in other parts of the world. According to the Martin and Moyn (2003) classification, the study adakites are classified as high silicic adakites (HSA).

From the study area was collected more than 100 samples. From 40 samples were created tine-section microscopic. petrography has done by Zeiss research microscopy in Kharazmi microscope lab. In order to study the geochemical characteristics of the igneous rocks in the area, 35 samples of outcrops, granodioritic intrusive rocks, dykes, and xenoliths were selected and 8 of them were analyzed by ICP-MS at Zarazma company. XRF analysis has done in the Laboratory of Damghan University. The results are presented in Table, 1. EPMA  data got in the Iran Minerals Processing Research Center.

The Darehzereshk’s Hassanabad volcanic complex located in the microcontinental Iran block, along the Urmia-Dakhtar volcanic belt and 80 kilometers west of Yazd city (Fig,1 A,B). After field geology and sample collection, petrography and XRD, ICP/MS and EPMA analyses were done. Based on collected data, the volcanic rocks in study area include diorite-andesitic dikes and intrusive body, trachyte, dacite and rhyolite lava and domes. In the outcropes and specimens, these rocks are bright, light-gray to dark gray color. Andesite-diorite dikes are subvolcanic (Fig. A3), and often show the microlitic porphyry and glomero porphyry texture. In the dacite lava, plagioclase crystals are often characterized by zoning and unstable core (Fig. C3). Also the amphiboles present in the microscopic sections of these samples are sometimes chemically zoned (Fig. D3). the plagioclase crystals mostly have reaction margin (Fig. E3). In rhyolite and rhyodacite, 5 to 15 percent quartz is present and shows corrosion gulf (Fig. F3).
The whole rock chemical composition of volcanic rocks and dikes in the classification diagrams (e.g. Le Bas et al, 1986) plotted in the andesite, dacite and trachyte areas (Fig. A4). Sub-volcanic rocks included intrusive and xenoliths plotted in the range of granodiorite and gabbro areas. These rocks with the sub-alkaline nature located in the calc-alkaline magma series. The contained rare light earth elements (LREE) are enriched and heavy rare earth elemental (HREE) depleted (Tab-1). In addition, there is a negative anomaly of Nb and Ti (Fig-D4), which is one of the indices of the calc-alkaline adikatic magma in subduction zone. The continuous pattern of the major and rare elements of the samples in the Harker diagrams (Harker, 1909) indicates the same origin and the role of crystalline differentiation in the magma evaluation (Rolinson, 1983). Other geological evidence, such as the enrichment of large ion lithophile elements such as K, Rb, Th, U, also approved this opinion (Fig. D4). Based on the Sr/Y ratio against Y (Defant and Drummond, 1990), all volcanic samples are the adakite (Fig. A5). In the SiO2/MgO diagrams against K/Rb (Fig. B5) (Avdeiko and Bergal, 2015) and the Sr diagram against CaO+Na2O (Castillo, 2012), these rocks are located in the high-silicic adakite (HAS) (Fig. C5). On the other hand, the xenoliths that are present in the volcanic rocks originated from melts due to metasomatism of the mantel wedge by the influence of oceanic crust-derived fluids, which indicates the deeper sources than relative to the adakits (Tatsumi et al. 1986). The results of EPMA from the plagioclase show the end members of these crystals in between (An 47-26) (Fig. A6). The amphibole crystals analyses illustrated calcific compound (Fig. B6) and are within the range of ferro-hastingsite composition (Fig. C6). Also, amphiboles composition indicates the low-to-moderate ƒO2 pressure conditions that are associated with the mantel sources. Thermo-barometric calculations based on the chemical composition of plagioclase and alkali-feldspar crystals (Putritka, 2008), show the magma generates at a temperature between 895° C and 1080° C and a pressure of 10 to 12 kbar. This property illustrated crustal- mantel boundary at a depth of 30 to 36 km.

Based on petrography and geochemistry data, igneous rocks in the Hassan bad are divided into two group. The first group is associated with continental arc environments and is part of the intrusive body and xenoliths. This rocks crystalized under the 7.5 to 8.8 Kabr pressure. The second group, which includes the adakits of the region under study, is mainly silica-saturated rocks that originate from enrichment fluids from magma at a relatively high depth (30-36 km depp and 985 to 1080 °C) indicating metasomatic activity in the lower crust and upper mantle in the western part of the Yazd block.