Geochemical and Sr‐Nd Isotopic Signatures of Basaltic‐Andesitic Rocks From Iheya Ridge, Middle Okinawa Trough: Insights Into Mantle Source and Fractional Crystallisation Mechanism

ABSTRACT The Okinawa Trough (OT), situated in the southeastern expanse of the East China Sea shelf, is an active back‐arc basin profoundly shaped by the subduction of the Philippine Sea Plate. Although magmatic processes across different segments of the OT have been extensively studied, key uncertainties remain regarding the composition of magma sources, the dominant mechanism governing magmatic evolution and the specific contribution of subduction‐derived components—particularly the Iheya Ridge (IR) in the middle Okinawa Trough (MOT). To address these issues, this study aims to decipher the mantle source characteristics, magmatic evolution processes and role of subduction‐related materials in the IR by analysing major and trace element compositions together with Sr‐Nd isotopic characteristics of three volcanic rock samples from the IR. The samples span a compositional range from basalt to andesite (SiO 2 = 51.55–60.07 wt%), reflecting the evolution of intermediate‐mafic magmas. Consistent trace element patterns and restricted Sr–Nd isotopic variations ( 87 Sr/ 86 Sr = 0.705043–0.707050; 143 Nd/ 144 Nd = 0.512734–0.512862) indicate derivation from a common magma source, with compositional variations attributed to fractional crystallisation. Isotopic and geochemical modelling reveal that the mantle source is a ternary mixture of Depleted Mantle (DM), Enriched Mantle Type II (EMII) and subducted sediments, with subducted sediments contributing 1.5%–4.0% and mantle‐derived melts accounting for 96.0%–98.5%. Fractional crystallisation involving plagioclase, clinopyroxene and apatite is identified as the dominant process driving magmatic differentiation, yielding intermediate magmas from basaltic parents. This study provides new constraints on mantle dynamics and magmatic evolution in young back‐arc systems, highlighting the interplay between slab‐derived inputs and extension‐driven mantle upwelling.