Geochemistry, geothermobarometry and tectonic setting of basaltic rocks of the north of Serow (NW-Urmia)

The basic rocks in the Serowarea(northwest of Urmia) are exposed to young alluvium (Quaternary). These rocks are part of the metamorphic Sanandaj-Sirjan belt. The study area is composed of various igneous, metamorphic and sedimentary rocks and the age range is from Precambrian to the present era.

In order to perform a chemistry study of northern Serowlavas, 10 rock samples were analyzed by the ICP-MS method in Bureau Veritas laboratory in Canada. Also, olivine, orthopyroxene, clinopyroxene and plagioclase minerals were analyzed at the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS) by the JEOL JXA-81 microprobe electron analyzer.

Field studies and petrographic observations indicate that the rocks of the Serow area can be divided into alkaline basalt, trachy- basalt and trachy- andesite. One of the major textures in these rocks is porphyry texture and to a lesser extent trachyte texture. Plagioclase, orthopyroxene, clinopyroxene, olivine and amphibole are the minerals that make up these rocks. Calcite mineral is the most important and main secondary mineral in these rocks, which often fills the cavities and gives them amygdale tissue.The results of the chemical analysis of the studied rocks show that their composition is basalt to trachy- andesite-basalt and they are alkaline and within plate basalts type. Examination of rare earth elements in these rocks show that the studied samples have little differentiation in rare earth elements, especially in HREEs. But, LILEs are more differentiated and enriched. Therefore, in the normalized pattern, they show a decreasing trend from Ba to Yb. Enrichment in LILEs and LREEs compared to HFSEs and HREEs, presence of negative anomalies in Ta, Nb, P and Ti elements along with enrichment of Rb and Ba elements can be due to the role of fluids in subduction zones. The ratios of Smn/Lan, Lan/Ybn  and Smn/Ybn can be used as evidence of low melting rate and the presence of garnet in the residual melting. The La/Sm versus Sm/Yb diagram has been used to determine the degree of melting of the source rock, showing the degree of melting of 1% of the source rock with spinel peridotite composition.The results of electron microprobe analysis of olivine mineral show that the average value of Fo in alkaline basalts is 85.77 (Fo 85.77) and in andesite – basalts it is 82.29 (Fo 82.29). Clinopyroxene mineral has diopside composition in alkaline basalt rocks and diopside augite composition in basaltic andesite rocks. Orthopyroxenes ranges from En 56.84-57.03, Wo3.14-3.33 and Fs 39.36-39.54 .The composition of plagioclase in alkali and andesitic basalts is in the labradorite range.

The basic rocks in the Serow are consistent of olivine + clinopyroxene + plagioclase ± orthopyroxene ± amphibole. The results of the geochemical analysis reveal the alkaline nature and intra plate setting and volcanic arcs of these rocks. Pyroxene mineral chemistry shows relatively high oxygen pressure conditions and a water content of 10% during the crystallization of clinopyroxenes. Based on the calculations and geothermobarometric diagrams, the formation temperature of clinopyroxene in alkali basalts and andesitic basalts is 1000 to 1250˚C and their estimated pressure is less than 5 Kbar. From the tectonic point of view, it seems that the formation of these rocks is associated with processes following the complete closure of the Neo-Tethys Ocean and the continental collision.