Ankaramite-Like Dikes and Picrite Basalts from Mesozoic Ashin Ophiolite Assemblage, Central-East Iranian Microcontinent: Implications for Petrogenesis and Neotethyan Tectonics

The Ashin ophiolite assemblage is located in the northwest corner of the Central East Iranian Microcontinent (CEIM). It represents the Neotethyan suture zone formed during the Late Cretaceous to Early Paleogene closure of the Eastern Neotethys Ocean. The crustal sequence comprises plutonic and volcanic rocks dominated by suprasubduction zone (SSZ) arc-type geochemical signatures, accompanied by minor oceanic island basalt (OIB)-like pillow lavas. Based on whole-rock geochemistry and mineral chemistry, the Ashin mafic rocks can be divided into two distinct geochemical groups. Group-1, which includes high-Mg mafic dikes and associated picrite lava flows showing island arc tholeiitic (IAT) affinity. Geochemical characteristics of cumulate mafic dikes are consistent with primitive arc melts displaying ankaramitic affinity. These rocks are characterized by low SiO2 (39.45–50.86 wt %), very low alkali contents (Na2O + K2O = 0.31–1.86 wt %), low TiO2 (0.22–0.60 wt %), high Mg# (56–83 wt %), high Ni and Cr concentrations, and CaO/Al2O3 ratios >1. Mineralogical features include abundant high-Ca clinopyroxene and anorthitic plagioclase, locally showing cumulate textures. Trace element petrogenetic modeling suggests that primitive arc melts were derived from partial melting of the depleted mantle wedge metasomatized by slab-derived fluids or sediment melts, supporting formation in an SSZ setting during subduction initiation. In contrast, Group-2 pillow basalts display calc-alkaline and OIB-like affinities and were likely derived from a relatively subduction component–free mantle source. These pillow basalts show enrichment in large ion lithophile elements (LILE: La, Ce, Rb, Ba) and depletion in high field strength elements (HFSE: Nb, Zr, Th). Group-1 samples display low chondrite-normalized La/Yb ratios (LaN/YbN = 0.07–0.88) and moderate Th/Yb ratios (0.02–0.24), plotting above the mantle array and confirming arc-related signatures. In contrast, Group-2 pillow basalts display Nb–Ta enrichment and high LaN/YbN ratios (8.48–9.65), suggesting contribution from plume-influenced mantle domains. The coexistence of primitive high-Mg ankaramitic-affinity melts and picritic compositions in the Ashin ophiolite likely records early-stage arc magmatism characterized by high-temperature mantle and enhanced fluid-fluxed melting during subduction initiation. These data support a geodynamic model in which the Ashin ophiolite formed within a nascent intraoceanic arc basin during the early closure of the Eastern Neotethys, recording the transition from primitive proto-arc magmatism toward more mature arc development.