Untangling differentiation in arc lavas: Constraints from unusual minor and trace element variations at Salak Volcano, Indonesia

Volcanic rocks from Salak Volcano in West Java display intriguing minor and trace element geochemical variations with silica. TiO₂ and P₂O₅ contents, Y, HFSE and REE concentrations are abnormally rich in the Central Vent Group (CVG) lavas (e.g. Y = 32-69 ppm; Yb, 3-6.5 ppm) and display striking positive correlations with SiO₂. This contrasts with rocks erupted at side vents (SVG) on the eastern and western flanks of Salak and with rock suites of most other Javan volcanoes where these elements remain relatively constant with increasing SiO₂. Modelling of major and trace element data indicate that low pressure fractional crystallisation exerts strong control on the composition of CVG lavas. HFSE and HREE data are inconsistent with magma mixing, and can be explained by incompatible behaviour during fractionation of plagioclase, clinopyroxene and Fe-Ti oxide ± orthopyroxene and olivine. The observed variations in K/Rb and Ba/Th ratios and correlation of ⁸⁷Sr/⁸⁶Sr with indices of differentiation necessitate assimilation of a low K/Rb, low Ba/Th, Sr-rich contaminant with ⁸⁷Sr/⁸⁶Sr of ~ 0.7048 during fractional crystallisation. For the eastern flank SVG, deep fractionation of a phase in which HFSE and HREE are compatible (e.g. amphibole) is implicated. By extension, this is also suggested to occur beneath the majority of Javan volcanoes. Radiogenic isotope ratios (Sr-Hf-Nd) of Salak lavas are similar to other Javan lavas. SVG rocks erupted from the eastern flank vent have significantly more primitive Hf-Nd isotope ratios than other Salak volcanic rocks and may represent the least contaminated (by arc crust or subduction input) Salak lavas.