Post-collisional potassic magmatism with elevated Rb concentrations in the Muria back-arc, Central Java: constraints from Rb-Hf-Ta systematics and mantle source interactions

Abstract

The Muria volcanic complex (MVC), located in the rear-arc of Central Java, Indonesia, represents an unusual potassic-alkaline magmatic system within an otherwise calc-alkaline subduction-dominated arc. This study integrates compiled geochemical datasets, including major oxides, trace elements, rare earth elements (REEs), and isotopic data, with particular emphasis on Rb-Hf-Ta systematics, to constrain magma sources and tectonic settings. We emphasize Rb-Hf-Ta because these elements are reliable and widely used geochemical tracers for investigating and distinguishing tectonic environments, mantle sources, degrees of magmatic alkalinity, magmatic evolution, crustal development, magmatic-hydrothermal processes, and the involvement of sedimentary materials in magmatic systems. Geochemical results show that MVC rocks range from high-Mg# tholeiitic basalt to evolved-tholeiitic and high-alumina calc-alkaline compositions, indicating interaction of multiple magma sources. They are characterized by high K2O (2.17–9.09%), elevated Rb (100–947 ppm), high La/Yb (>20), Nb/Y (>6), Ta/Hf (>0.24), Nb/Zr (>0.1), variable Ta anomalies (δTa = 1.06–0.22), and Nb/La ratios of 1.12–0.16. Trace element spidergrams display strong Ti depletion, Nb-Ta anomalies, and minimal Eu anomalies (δEu = 0.90–0.99), consistent with enriched alkaline mantle sources. Isotopic compositions (87Sr/86Sr = 0.70418–0.70473; 143Nd/144Nd = 0.51257–0.51275) further support an asthenospheric mantle contribution. Comparative analysis with other Central Java volcanoes (CJV) indicates that Muria magmatism is distinct from typical subduction-related arc systems and is best explained by post-collisional tectonics involving upwelling enriched mantle, slab-derived fluids, sedimentary input, and oceanic-continental material mixing. We further propose that the MVC reflects post-collisional fragments beneath northern Central Java, formed during the Cretaceous collision between East Java-West Sulawesi (EJWS) block and the southeastern Sundaland margin, and later exhumed during the Pleistocene through tectonic uplift along inherited structural lineaments. Overall, the Rb-Hf-Ta systematic approach provides new constraints on the magma sources and tectonic setting of the MVC and improves our understanding of post-collisional magmatism in continental back-arc regions.