Mineralogy and whole-rock geochemistry of alkali basalts from the Bashtin area, west Sabzevar, Iran: Implications for mantle source variations

This study examines post-collisional alkali basalts in the Bashtin area, west of the Sabzevar Range, focusing on their whole-rock geochemistry and mineral composition. The alkali basalts are classified into three groups: two lava groups and one group of feeder dyke basalts, the latter affected by crystal accumulation processes. Group 2 basalts show significantly elevated values of FCKANTMS (>0.3), FCMS (0.45–0.60), and Mn/Fe (72–80) compared to group 1, which has lower values for these parameters (FCKANTMS <0.035, FCMS 0.02–0.28, and Mn/Fe 42–61). Conversely, group 1 basalts display higher Co/Fe (3.3–5.9), Ni/Co (5.3–6.7), and Mn/Zn (14–18) ratios than group 2 (Co/Fe 2.6–2.9, Ni/Co 1.5–3.7, and Mn/Zn 6–6.5). These geochemical differences suggest that group 1 arises from a peridotite-dominated source, while group 2 derives from a pyroxenite-dominated lithology.

Mineral compositions were analyzed using Electron Probe Microanalysis (EPMA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). In group 1, olivine crystals contain higher MgO, MnO, and SiO₂ but lower FeO and NiO than those in group 2. The clinopyroxenes in group 2 exhibited a more fractionated REE pattern, with La_N/Yb_N (3.5–17.4) and higher LREE/HREE (17.2–47) and MREE/HREE (6.2–7.8) ratios than those in group 1, which had La_N/Yb_N (2.3–3.0), MREE/HREE (3.8–4.6), and LREE/HREE (10.4–13). Although the feeder dyke's overall composition does not directly reflect the liquid composition, minerals composition suggest that the primary magma likely originated from a pyroxenite-dominated source, underscoring the importance of mineral geochemistry in assessing the origins of mafic rocks, particularly cumulates, where whole-rock compositions are not representative of the primary magma.