Geochemical insights and petrogenetic processes of ophiolitic fragments from Avajiq and Silvana: implications for Neo-Tethyan Evolution in northwest Iran

Abstract

The Avajiq and Silvana ophiolites are part of the Late Cretaceous Neo-Tethyan ophiolitic belt located in northwestern Iran, between the Sevan-Akera suture to the north and the Bitlis-Zagros suture in the southwest. The Avajiq ophiolite formed between the South Armenian Block (SAB) with a Gondwanan origin to the north and the Eastern Anatolian Plateau (EAP) to the south and west. In contrast, the Silvana ophiolite is situated at the juncture of the EAP and the Sanandaj-Sirjan Zone (SaSiZ). The mantle sequences of both ophiolites are composed of harzburgites with various degrees of refertilization. The compositions of olivine (Fo: 90-90.8), spinel (Mg#: 0.49 to 0.69 and Cr#: 0.25 to 0.59), orthopyroxene (Mg#: 0.90-0.96), and clinopyroxene (Mg#: 0.91-0.94), along with the bulk geochemistry of highly serpentinized ultramafic rocks, were evaluated to distinguish the tectonic settings of both ophiolites. The spinel composition indicates that the harzburgites represent mantle residuum formed after moderate to high degrees of partial melting (16-19% for the Avajiq and 10-17% for the Silvana). This is supported by the low bulk rock content of incompatible elements such as Ti and heavy rare earth elements (REE). Harzburgites generally display U-shaped chondrite-normalized REE patterns, with elevated light REE (LaN/SmN = 3.5-4), reflecting early partial melting events and multiple episodes of depletion, modified by subsequent melt/fluid-dominated metasomatism. The observed moderate to significant enrichment in fluid-mobile elements, such as U, Pb, Ba, and Sr, in the harzburgites are linked to serpentinization/refertilization resulting from fluid/melt-rock interactions. Field relationship and geochemical comparisons with other Neo-Tethyan ophiolites suggest that most samples from the Silvana ophiolite along with those from the Zagros ophiolites, predominantly exhibit abyssal characteristics, although some may be more closely related to fore-arc settings. In contrast, the Avajiq fore-arc ophiolite is associated with the Khoy-Maku back-arc basin, situated far from the southern and northern Neo-Tethyan sutures.