Petrology, Geochemistry and tectonic setting of basalts from Bojan area, Northeast of Neyshabur

Abstract

Introduction Geological events from Precambrian to Quaternary have played a very important role in producing magmatic rocks in Iran. However, magmatic rocks with the ages of Precambrian, early Cambrian and specially Tertiary are much more frequent (Aghanabati, 2004). In contrast, magmatism and plutonism in Iran with Paleozoic age considered to be very rare. Volcanic rocks with the age of Ordovician-Silurian have been reported from some restricted areas of Iran such as Soltan Meydan near Shahroud (Derakhshi and Ghasemi, 2015) and Abyaneh near Kashan (Ayati et al., 2011). Volcanic and volcanoclastic rocks from the North of Neyshabour extend with a linear trend from Garineh to Bojan. In the lack of geochronological data and on the basis of stratigraphical evidence, the age of Ordovician-Silurian has been suggested for these rocks. A detailed study on petrology and geochemistry of north Neyshabour volcanic and volcanoclastic rocks can help us reconstruct the evolution of Iran bed rock during the early Paleozoic Era. The aim of this study includes petrology and major and trace element geochemistry to present critical keys to obtain some knowledge about tectono-magmatic situation of Iran during the early Paleozoic Era, especially in the Binaloud structural zone. Material and methods This study was carried out in two parts including field and laboratory works. Sampling and structural studies were carried out during field work. The petrographic studies were performed on 45thin and polished thin sections. Geological map for the study area was also prepared. Whole-rock chemical analysis of 7 samples for major, minor, trace and rare earth elements were performed at the ACME Laboratory in Canada, by using the 4AB1 method using ICP-MS and the major oxides of six basalt samples were analyzed by X-ray fluorescence (XRF) at the Zarazma Laboratory. Results and discussion The study area is located in the Northeast of Iran 15 Km NE of Neyshabour city and 7 Km NE of Bojan village. The Bojan area consists of Paleozoic sedimentary rocks (Limestone, sandstone, Dolomite) and volcanic-volcanoclastic rocks (basalts, andesit-basalts, andesite, trachyte, agglomerate and tuff). Petroghraphic studies showed that major minerals in Bojan basalts are plagioclase, pyroxene and olivine and secondary and accessory minerals are apatite, ilmenite, magnetite, chlorite, calcite and epidote. The texture of the Basalts is porphyritic, glomeroporphyric and trachytic. Based on geochemical data, the TAS diagram shows that the Basalts fall within the fields of tephrite to trachyte and belong to alkaline series with sodic nature. MgO# is varied from 20.13 to 38.62 which can be interpreted on the basis of crystal differentiation in magma chamber. The low value of compatible elements such as nickel and descending trend of MgO versus SiO2 can clearly be explained in terms of olivine fractionation. In addition, the nearly constant ratios of incompatible elements such as Nb/Zr, Ce/Zr, La/Zr, and Rb/Zr in rocks with different SiO2 content can reveal the importance of primitive magma differentiation. In contrast, these ratios suggest that crustal assimilation plays no important role in changing primitive magma composition. Enrichment of LREE compared with HREE in the studied basalts can be explained by low degrees of partial melting. Chondrite-normalized REE patterns for basalts from the Bojan area show a very similar pattern with those from transitional - mildly alkalic basalts from the Eastern branch of the East African Rift. Spider diagram patterns for Bojan basalts normalized according to Thompson (1982) and Sun and Mc Donough (1989) show a clear enrichment of all trace elements compared with those from chondorite and primitive mantel. On the basis of tectonic setting discrimination diagrams the study area basalts fall in within plate alkaline domain. According to petrographical and geochemical data of Bojan volcanic rocks it can be concluded that magmatism in the Bojan area has been formed as a result of a cycle of within plate rifting when the Palaeo-Tethys Ocean started to open during the Ordovician-Silurian time. Acknowledgments The Research Foundation of Ferdowsi university of Mashhad, Iran, supported this study (Project 3/50860, 08/09/ 2019). We thank the university authorities for funding. The authors also would like to thank Professor Gültekin Topuz at the Eurasian Institute of Earth Sciences, Istanbul Technical University (Turkey) for his kind support and valuable comments.

Keywords

• mineralogy
• geochemistry
• basalt
• alkaline
• bojan
• binaloud