Lithology and tectonomagmatic setting of volcanic and plutonic rocks from the Lak area, southwest Buin Zahra, NW Iran

The Urumieh-Dokhtar magmatic arc (UDMA) of Iran with a length of about 1700 km and a width of approximately 150 km is mainly composed of Tertiary volcanic rocks and acidic to basic intrusions (Berberian, 1981; Emami et al., 1992; Darvishzadeh, 2003; Ghorbani, 2003). The UDMA has been attributed to the subduction of the Neotethys oceanic crust under the Iranian plate, which occurred from the Triassic to the Eocene (Asiabanha et al., 2012; Pang et al., 2013). The UDMA extending NW-SE includes a large volume of Cenozoic magmatism, especially in the Eocene (Chiu et al., 2013; Kananian et al., 2014). The purpose of this research is to identify the petrographic and geochemical characteristics of the volcanic and intrusive rocks of the Lak area in NW Iran and also to determine the tectonomagmatic setting of these rocks.

Geology:

The Lak area is situated 36 km southwest of Buin Zahra, in the northern part of the UDMA and the western part of the Central Iran zone (Aghanabati, 2006). The UDMA is an Andean magmatic arc with a NW-SE trend, which formed by the oblique subduction of the Neotethys oceanic plate under the central Iranian plate (Shearman et al., 1976; Berberian and King, 1981; Agard et al., 2011; Gohari et al., 2022). One of the remarkable features of the UDMA is the emplacement of intrusive masses due to late Eocene and early Oligocene pressure phase (Pyrenean tectonic phase) in volcanic-sedimentary sequences of Eocene age (Delavari et al., 2017). The main outcrops of the study area include Eocene volcanic and volcano-sedimentary rocks consisting of andesite, andesitic basalt, basalt, dacite, and rhyodacite. The volcano-sedimentary rocks of the Lak area comprise alternating lava flows (basalt to andesite) and pyroclastic materials (various types of tuff and agglomerate).

The studies carried out in the Lak area include field and laboratory parts. During the field studies, a geological map with a scale of 1:5000 in an area of 20 square kilometers was prepared. Forty-five samples of volcanic rocks and intrusive masses for preparation of thin sections and petrographic studies (20 samples), measurement of main oxides by XRF method (14 samples), and analyzing minor and rare earth element contents by ICP-MS method (17 samples) were collected and sent to the relevant laboratories for analysis.

Petrography:

In the Lak area and its surroundings, volcanic and intrusive rocks are exposed. These rocks mainly include andesite lavas, basaltic andesite, basalt and tuff, and pyroclastic deposits of lower Eocene age and intrusive masses and dykes with the composition of dacite, rhyodacite, microdiorite, and gabbro-diorite of upper Eocene-Oligocene age, which were injected into the Eocene volcanic rocks (Firouzbakht et al., 2018). Geochemistry and tectonomagmatic setting of the volcanic and intrusive rocks The amount of SiO2 in the rocks under study varies from 42 to 71% and on K2O versus SiO2 diagram, those are in the range of andesite, basaltic andesite, basalt, dacite, and rhyodacite. The range of K2O changes in these rocks as well as intrusive rocks is relatively wide so the investigated samples of volcanic and intrusive units are located in different groups of low- and medium-potassium rocks. According to the geochemical characteristics and also taking into account the temporal and spatial location of the volcanic rocks of the Lak area, it seems that these rocks are related to the magmatism caused by the subduction of the Neo-Tethys oceanic crust under the central Iranian plate and have been originated in a (magmatic arc) environment.

Possible origin of magma:

The widespread distribution of plutonic rocks in the study area, mineralogical similarity between these plutons and the volcanics as well as very similar chemical compositions of these two rock types in different geochemical and tectono-magmatic discrimination diagrams suggest that the plutonic and the volcanic rocks may have originated from the same source.The volcanic and plutonic rocks of the area are very similar to calc-alkaline lavas based on major and trace element geochemical data. The Al2O3 content of these rocks is high, but they are low in Mg#, their Zr/Y ratio is greater than 3, similar to the volcanic rocks of continental arcs (Pearce and Norry, 1979). In the diagram of normalized trace elements relative to the enriched mid-ocean ridge basalts (E-MORB), Ti (except basalts), Nb, P, and Rb have negative anomalies, but Pb and K show positive anomalies, pointing to magmatic rocks from a subduction zone (Morata and Aguirre, 2003).Magmas forming volcanic rocks in subduction zones usually originate from mantle wedges, fluids, and hydrous melts derived from subducting oceanic crust. The samples from the Lak area have high K2O contents and the downward trend in MgO values of the volcanic rocks of the area indicating that the magma originated from the mantle wedge (Gourgaud and Vincent, 2003).