MIOCENE ADAKITES FROM NORTHWEST BORNEO AN D THEIR RELATION TO THE SUBDUCTION OF THE PROTO SOUTH CHINA SEA

This study deals with eight samples from the Tertiary Sintang Intrusiv es of northwest Borneo. Six samples are granodiorites, two of which ar e dated back to 18.3 and 19.2 Ma, yielding chemical characteristics of slab melts (adakites). The two youngest rocks (16.5 and 16.7 Ma-old), are calc-alkaline dacites: their mantle source was previously metasom atized by adakitic magmas prior to melting. The Sintang adakites are c oeval with the early stage of subduction of the Proto South China Sea Basin, at least in the westernmost half of the basin, according to rec ent models. Their genesis would therefore result from the high thermal regime linked to incipient subduction of oceanic crust. Later calc-al kaline magmatism would represent a subsequent stage of the closure of the Proto South China Sea Basin.