Soltan Maidan Complex (SMC) in the eastern Alborz structural zone, northern Iran: magmatic evidence for Paleotethys development

Abstract

Soltan Maidan Complex is located in the eastern Alborz zone, Iran. Thickness of this complex ranges from 436 to 1,286 m and its age is Upper Ordovician–Silurian. It is the most important and voluminous Lower Paleozoic magmatic activity in Iran. This complex consists of mafic lava flows, agglomerates, and tuffs accompanied with some sedimentary intercalations. Field geology evidence indicate both subaerial and submarine eruptions during accumulation of this complex and also occurrence of a long-time exhumation event in the study area from Upper Ordovician to Middle Devonian time. The volcanic rocks of Soltan Maidan Complex indicate a range composition of basalt to basaltic-andesite and show transitional to alkaline nature of the primary magma along with evolution via fractional crystallization process. These rocks represent fairly high degrees of partial melting (between 14–20 %) from a common garnet peridotitic mantle source in an intra-continental rift setting. Relatively high Nb and Zr contents (i.e., 21.68 and 189.63 ppm, respectively) show the generation of magma from an enriched mantle source. Also, Th/La (with an average of 0.13) and Zr/Nb (with an average of 9.11) ratios of the samples illustrate mixing between an OIB-like magma with an EMI-type mantle source. The primary magma shows lack or minor role of the crustal contamination during its ascent. According to the many evidences, Soltan Maidan magmatism has occurred in the Upper Ordovician–Silurian times in relevant to a cycle of rift-related magmatic events and breakup of the northern margin of Gondwana in the process of Paleotethys development in the north of Iran.