Geochemical zonation in the Sunda volcanic arc, and the origin of K-rich lavas

The Sunda volcanic arc provides a good example of variation in the geochemistry of lavas across an island arc. In addition to the well-known correlation between K2O/SiO2 ratios and depths to Benioff Zone in Pleistocene ? Recent lavas of Java, there are well-defined relationships for 'incompatible' elements (Rb, Cs, Ba) and light rare earth elements. Estimated primary magma compositions for individual volcanic centres of Java indicate a progressive change in the conditions of primary basaltic magma production across the arc. The proportion of partial melting of peridotitic upper mantle appears to decrease from about 25% for olivine tholeiite magmas associated with tholeiitic lava series, to 5?10% for basanite primary magmas of high-K calc-alkaline lava series. The corresponding depths of final equilibration of magmas with crystalline residues increase from about 30 km to 60 km. These variations in conditions of magma production are probably superimposed upon an increase in the content of K and incompatible elements in the magma sources. The distinctive potassium-rich, strongly undersaturated Pleistocene lavas of northern Java probably originated from magmas produced at greater depth, in mantle enriched in phlogopite. Preliminary liquidus phase relationships for a primitive leucite basanite indicate a maximum depth of mantle origin of about 80 km. Such a composition could not be derived by melting of a lherzolite mantle in the presence of H2O alone, and the probable presence of CO2 in the mantle is indicated.