GEOLOGY, MINERALOGY AND MAGMA EVOLUTION OF GUNUNG-SLAMET VOLCANO, JAVA, INDONESIA

Gunung Slamet, Central Java, is a large stratovolcano within the Sunda magmatic are of Indonesia. The volcanic edifice includes products of two large overlapping Quaternary stratocones. Basaltic andesites and a ndesites, with rare basalts, dominate the western region of the comple x, known as Slamet Tua (old); and basalts and basaltic andesites compo se the eastern cone, called Slamet Muda (young). On the basis of strat igraphy, trace-element content, Zr/Nb, Zr/K and Sr-87/Sr-86 ratios, Sl amet lavas can be broadly categorized as relating to high abundance ma gma (HAM) and low abundance magma (LAM) types. The Tua and Lebaksiu se quences generally comprise the LAM group, and are older, more salic an d have higher Sr-87/Sr-86 ratios than those of HAM. LAM andesites cont ain some amphibole, but HAM andesites do not. Models involving randomi zed magma replenishment, tapping and fractionation (RTF) were develope d to explain the geochemical features of both LAM and HAM rock groups. The salic lavas of the LAM suite can be generated if fractionation wa s dominant relative to replenishment and tapping in LAM magma chambers . Conversely, magma chambers with a high proportion of replenishment a nd tapping relative to fractionation can produce dominantly mafic lava s, such as those of the HAM suite. Concave-upward heavy-rare-earth ele ment (HREE) patterns for LAM andesites are probably due to significant amphibole fractionation; HAM andesites display flat HREE patterns and do not require amphibole fractionation from parental basalts. The hig h TiO2 contents of HAM basalts and basaltic andesites (relative to tho se of ''average'' are rocks) are due to either suppressed crystallizat ion-or minor accumulation-of Ti-magnetite, in conjunction with RTF pro cesses.