Potassium-rich volcanic rocks of the Muriah complex, Java, Indonesia: Products of multiple magma sources?

The extinct Pleistocene volcano Muriah, situated behind the main Pleistocene—Recent Sunda magmatic arc in north-central Java, has erupted at least two contrasted groups of lavas. One group forms a well-defined compositional series (Anhydrous Series) from leucite basanite to tephritic phonolite, with olivine and tschermakitic clinopyroxene the main phenocrysts. The other group, the “Hydrous Series”, includes compositionally variable tephrites and high-K andesites with common plagioclase, biotite and amphibole. Lavas of the Anhydrous Series are much richer in LIL trace elements than the most potassic lavas of neighbouring active volcanoes, but relative HFS element enrichment is less pronounced. REE patterns have almost constant slopes from La (250–600 times chondrites) to Yb (5–10 times chondrites), while those of lavas of active centres are less light-enriched, and show flattening in the heavy REE. Anhydrous Series initial ⁸⁷Sr/⁸⁶Sr ratios (0.7043–0.7046) are lower than those of active centres (0.7047–0.7053). Hydrous Series lavas are intermediate in all these geochemical characteristics.The most mafic A-series leucite basanite, with Mg/(Mg + Fe²⁺) 0.69, 140 ppm Ni and 620 ppm Cr was probably derived from the primary magma for the series by fractionation of only 5 wt.% olivine. Its REE pattern suggests derivation from a garnet-bearing source. Experiments on this basanite, with up to 10% olivine and 20% orthopyroxene added, and in the presence of H₂O and H₂O/CO₂ mixtures, have shown that for all but very high magma water contents, the olivine and garnet liquidus fields are widely separated by fields of phlogopite and clinopyroxene. There is no liquidus field of orthopyroxene. Hence, if magma production involved an equilibrium melting process alone, the most probable sources are of garnet-bearing phlogopite clinopyroxenite type. Alternatively, this magma may represent the end-product of interaction between a low-K basanite magma from a garnet lherzolite source in the asthenosphere and a phlogopite-bearing lherzolite zone in the lower lithosphere. Its production was probably related to crustal doming and extension superimposed on the dominant subduction regime. Hydrous Series magmas may have resulted from mixing between Anhydrous Series magmas and high-K calc-alkaline basaltic to andesitic magmas more directly related to subduction processes.