Early Cenozoic magmatic heterogeneity in the Alborz rear-arc: Spatial variation from enriched to juvenile signatures

Across-arc geochemical variations in igneous rocks are common in magmatic belts that form at convergent margins, but the geological processes responsible are unclear. To investigate this, we acquired new whole-rock major and trace element data, coupled with Sr–Nd–Pb isotopic ratios, zircon Hf isotopes, and UPb geochronology for three intermediate and felsic intrusions in the Baghu, Chalu and Gandi regions of the Moallemen magmatic complex, NE Alborz rear-arc. These were compared to data from well-exposed Early Cenozoic Alborz rear-arc igneous rocks in northeastern and northwestern Iran. The Chalu intrusions are mainly monzonite, quartz-monzonite, whereas the Baghu and Gandi intrusions are granodiorite and granite, respectively. UPb zircon crystallization ages of 49.9 ± 0.74 and 46.3 ± 0.82 Ma for the Chalu intrusions indicate that they are slightly older than the Baghu granodiorite (41.2 ± 2.3 Ma) and Gandi granite (42.2 ± 0.99 Ma). The Chalu monzonite and quartz-monzonite rocks display relatively higher LILE/HFSE (Ba/Th: 60–167) but similar LILE/LREE (Ba/La: 17–21) values to those of the Baghu and Gandi intrusions (Ba/Th: 27–54; Ba/La: up to 25). All three units show the same Sr–Nd–Pb isotopic compositions, having same radiogenic Sr (⁸⁷Sr/⁸⁶Sr, 0.70400–0.70425) and Pb (²⁰⁶Pb/²⁰⁴Pb, 18.50–18.53; ²⁰⁷Pb/²⁰⁴Pb, 15.58–15.59; ²⁰⁸Pb/²⁰⁴Pb, 38.56–38.63), Nd (¹⁴³Nd/¹⁴⁴Nd, 0.51272–0.51295) and zircon Hf (+7.2 to +11.4) isotopic compositions. Modeling of SrNd isotopes suggests that these magmas were generated by the interaction of mantle-derived melts with lower continental crust through a series of assimilation-fractional crystallization (AFC) processes during ascent in the NE Alborz rear-arc. Published major and trace element data, bulk rock εNd(t) and zircon εHf(t) isotope data across the Alborz rear-arc show that melts of enriched lithospheric mantle and subducted slab-derived experienced less crustal interaction in the central and NW Alborz rear-arc than in the NE Alborz rear-arc. This variation reflects differences in subduction dynamics, crustal thickness, and mantle wedge processes along the Alborz rear-arc, indicating the utility of magmatic complexes for deciphering ancient tectonic processes. This work also settles a long-standing debate about the geodynamic evolution of the Alborz rear-arc, showing that a compressive to extensional tectonic regime existed during the Arabia–Eurasia collision.