| چکیده انگلیسی مقاله |
Introduction
The Cenozoic magmas in Urmia-Dokhtar Magmatic Belt (UDMB) show a wide range of composition. Among them, study of adakitic type magmatism of this belt (or arc) has gained momentum among the researchers on petrological studies. The origin of most adakitic magmas in Iran is attributed to the subducting oceanic lithosphere melting (Jahangiri, 2007; Sahfaii Moghadam et al., 2016; Jamshidi et al., 2018; Omrani, 2018). Identifying the spatial and temporal location and a more detailed description of the characteristics of adakitic magmatism in northwest Iran, as an important section of the UDMB, can lead to a better understanding of the history of late Cenozoic magmatism, where a wide range of normal calc-alkaline, alkaline and shoshonitic magmas are formed extensively. In this regard, the present study examines the petrology, geochemistry and tectonic environment of the adakitic volcanic rocks between Sari-Teppeh and Zonouz (north of Marand city, located in East Azarbaijan province).
Geological Background
The subduction of the Neotethys under the central Iranian plate during the Late Cretaceous to the Paleogene, followed by the collision of the Iranian and the Arabian plates (continental-continental collision), developed four structural zones in Iran. These structural zones with northwest-southeast trend are the Zagros Fold-Thrust belt, Sanandaj-Sirjan metamorphic and magmatic zone and Urmia-Dokhtar magmatic belt (Alavi, 1994; Mohajjel et al., 2003). Omrani et al. (2008) have divided the volcanic rocks of Urmia-Dokhtar magmatic belt (including the study area) into two categories: Eocene and Miocene to Plio-Quaternary. Regionally, the Miocene-Pliocene volcanic rocks are outcrops in the form of composite volcanoes and of large and small domes with the composition of andesite, latite, trachyte, dacite and rhyolite. Volcanic rocks of the studied area include andesite and dacite in gray to light gray color. Field studies of the sub-volcanic domes show their penetration into the Miocene marl, shale and sandstone sediments. Therefore, the age of Upper Miocene to Pliocene can be considered for the volcanic rocks.
Analytical Methods
Following the sampling, about 40 thin sections were prepared for petrographic studies. Fifteen samples with the least alteration are selected and analyzed for major oxides and minor elements by XRF method in the laboratory of the Geological Survey of Iran. The major elements of 6 samples out of the 15 are analyzed by ICP-OES (lithium metaborate-nitric acid dissolution method) and trace elements, including rare-earth elements (REEs), are analyzed by the ICP-MS (Four-acid digestion method) at the Zar-Azma lab (Iran).
Discussion
The studied volcanic rocks have a dominant texture of porphyry with phenocrysts of plagioclase, hornblende and occasionally biotite and quartz that make up about 50% of the rock volume. Plagioclase phenocrysts have a variety of disequilibrium textures of sieve texture, state of corrosion, and chemical zoning, as well as alteration with different intensity to sericite, calcite and chlorite is observed in them. Hornblende phenocrysts are usually euhedral to subhedral with simple twinning and opacity rims. Based on the chemical classification by Le Bas et al. (1986), the studied rocks are andesite and dacite in composition. In addition, in the K2O versus silica diagram of Peccerillo and Taylor (1976), the samples show calc-alkaline characteristics. The rare earth elements (REEs) chondrite-normalized patterns for the samples indicate a large negative slope for the light and medium rare earth elements (LREE and MREE), while a relatively lower slope is observed for the heavy rare earth elements (HREE). Unlike andesite, dacite, and rhyolite sodic series (ADRs) of normal arc, the studied rocks have high Sr/Y and LaN/YbN ratios (respectively 31.21 to 115.96 and 7.49 to 29.66). Also these rocks have low values of Y (5.2 to 10.2 ppm) and YbN (2.39 to 4.78 ppm) and in the graphs of Y versus Sr/Y and (La/Yb)N versus YbN, are all plotted within the adakite field. Sari-Tappeh adakite samples have relatively low MgO values (<4.04 wt%) and show most of the characteristics of high-silica adakite (HSA). According to the Sr versus CaO+Na2O diagram by Martin et al. (2005), all samples are plotted within the HSA field. Also, the samples have high Sr and low Y values and medium to high Sr/Y ratios, which are typical features of HSA. The studied rocks show slightly more negative Nb and Ti anomalies compared to common arc andesites and dacites. They also show higher Sr/Y and (La/Yb)N ratios and lower Y and YbN values compared to normal arc rocks. All these geochemical features can be consistent with the characteristics of adakitic rocks resulting from partial melting of the subducting oceanic lithosphere or the lower continental crust (Martin et al., 2005; Castillo, 2006). The studied rocks have high Ba/Rb (38.3-71.28) and low Rb/Sr (0.22-0.02) ratios, which are compatible with an amphibole-containing origin rather than a phlogopite-containing origin. Combining the existing geochemical characteristics shows that the melting of an amphibolite source containing garnet can form the magma that formed the studied rocks.
Conclusion
The stratigraphic age of these rocks is middle Miocene and later and they were formed in a post-collision environment. Sari-Tappeh volcanic rocks have calc-alkaline nature. They show low values of Y and Yb and high values of Sr and Sr/Y along with medium values of MgO, which is consistent with the characteristic of high silica adakites. Based on the geochemical features of this magmatism and considering the tectono-magmatic models regarding adakitic magmatism, it appears that the partial melting model of oceanic basalt under the condition of garnet-amphibolite facies during the slab breaking-off the Neotethys can appropriately elucidate the formation of the studied rocks. |