Pleistocene to Recent stratovolcanoes in the Highlands of Papua New Guinea are made up of calc-alkaline to shoshonitic lava, tuff, agglomerate, ash, and lahar deposits. The volcanic rocks are characterized by high and variable Al, high K and total alkalis, and low Fe, Mg, and Ca. There is a continuous variation between high-K calc-alkaline, low-Si high-K calc-alkaline, and shoshonitic rocks. The elements V, Rb, Sr, Zn, Nb, and Ba are high relative to general andesitic abundances, particularly in the shoshonites. The Highlands volcanic rocks originated either in the base of thick sialic crust which is undergoing stabilization after major orogeny and uplift, or more probably, in eclogite sinking through the underlying mantle. Variation in content of K and other “incompatible” elements was either inherited from the source rocks in the base of the crust, or was produced by zone refining in a thick upper mantle zone containing interstitial fluid rich in these elements. Further variation, mainly in Fe/Mg, Si, and total alkalis, was caused by minor low-pressure crystal fractionation involving olivine, clinopyroxene, and, to a lesser extent, calcic plagioclase.