Eocene high-K magmatism and rear-arc mantle evolution in NE Iran: temporal-geochemical perspectives

The Cenozoic Iranian arc records episodic magmatism during northeastward subduction of Neo-Tethyan oceanic lithosphere. Here we present 13 new whole-rock major- and trace-element analyses, eight whole-rock Sr-Nd-Pb isotope analyses, and zircon U-Pb-Hf data from two monzonitic samples collected approximately 30 km northwest of Roshtkhar. Zircon U–Pb geochronology resolves a ca. 49 Ma intrusive episode, placing these monzonites among the earliest well-dated occurrences in the Toroud–Sabzevar belt. We compare this local dataset with published Eocene suites from Toroud, Biarjmand, Sangan, Kashmar, and Roshtkhar to evaluate temporal and source relationships across the northeast Iran rear-arc (NEIRA). The studied monzonites have high-K to shoshonitic compositions, light-rare-earth-element enrichment, negative Nb-Ta-Ti anomalies, initial 87Sr/86Sr of 0.7048–0.7056, εNd(t) of −1.7 to +0.9, and zircon εHf(t) of +3.0 to +6.2. Together, these characteristics are consistent with derivation froman enriched, subduction-modified peridotitic mantle source. Elevated Th/Ba ratios and Nd–Pb isotope compositions consistent with sediment-derived end-members indicate that source metasomatism involved sediment-melt contributions, with fluid-mobile Ba enrichment playing a secondary role. Rb/Sr and Ba/Rb systematics support involvement of a K-bearing mantle phase such as phlogopite, but they are non-unique and may also reflect feldspar or biotite fractionation. The uniformity of Sr-Nd isotope ratios across the narrow sampled SiO2 range reflects a differentiation history dominated by fractional crystallization, with any earlier crustal interaction predating the sampled compositional interval. Regional Eu-anomaly patterns document temporal variation in feldspar stability and crystallization conditions across the ca. 50–38 Ma NEIRA record. Placed within the established spatial and tectonic framework, the ca. 49 Ma monzonites represent a local expression of high-flux, slab-retreat-related magmatism consistent with the wider Eocene NEIRA record.