Post-collisional alkaline lamprophyre magmatism in northern Iran: Implications from whole-rock geochemistry and mineral compositions

Abstract

The Shanderman lamprophyre dykes crop out in the western part of the Alborz Mountains (Talesh). These rocks are classified as camptonites, composed of primary olivine, Ti-rich diopside, kaersutite, biotite, plagioclase, K–feldspar, and minor Ti–rich spinels, magnetite, pentlandite–pyrrhotite/chalcopyrite, and powellite–scheelite. Secondary analcime–wairakite, serpentines, and prehnite are common minor minerals within the studied rocks. Olivine, Ti-rich diopside, spinel, and amphibole show distinct chemical zoning. Spinels display a core-to-rim decrease in Cr₂O₃, MgO, and Al₂O₃ concentrations and an increase in TiO₂ and FeO^T (total Fe as FeO), reflecting the oxidation state increase due to hydrothermal fluid influx. Low SiO₂ contents (<42 wt%), high MgO (12.44 to 13.98 wt%), and Fe₂O₃^T (12.76 to 13.43 wt%), Cr (318–537 μg/g) and Ni (231–327 μg/g) contents indicate the ultrabasic nature of the rocks. The samples show potassic character (2.1–2.8 wt% K₂O), along with elevated LREE and LILE, and also exhibit minor positive Eu anomalies (Eu/Eu* = 1.09 to 1.20). Olivine–spinel geothermometry indicates a maximum crystallization temperature of 1227 °C (ave. 988 °C ± 65 °C). Exsolution of pentlandite–pyrrhotite/chalcopyrite solid solutions occurred during magma cooling and crystallization. At lower temperatures, analcime–wairakite and prehnite partially replaced plagioclases. The geochemical modeling of the rocks indicates the Shanderman lamprophyre magmas were derived from low-grade melting (<5%) of amphibole–bearing garnet lherzolite source without or with very few phlogopites. The primary magma of Shanderman lamprophyres was derived from a depth of ~135 km by partial melting of a metasomatized mantle source in a post-collisional environment.