TEMPORAL CHANGES IN ARC MAGMA GEOCHEMISTRY, NORTHERN SULAWESI, INDONESIA

Late Miocene to recent subduction-related volcanics from the Sangihe A re in northern Sulawesi show a pattern of geochemical change through t ime. The oldest of these suites have compositions that indicate that t he mantle source underlying the are has experienced a previous event o f melt extraction, causing a relative depletion in incompatible elemen ts. The geochemical signature of the subduction zone component in thes e volcanics implies early domination by a fluid, which was mainly deri ved from altered MORE. The geochemical and isotopic signature of the m odem lavas, especially those from volcanic centres located furthest aw ay from the trench, are different from those of the older lavas and in dicate that the subduction zone component is now dominated by a melt o f sedimentary origin. The change from a fluid-dominated to a melt-domi nated subduction zone component may be related to the collision betwee n the Halmahera and Sangihe arcs. These systematic changes appear to b e superimposed upon quite variable parent magna compositions reflectin g variation in mantle source composition. (C) 1998 Elsevier Science B. V. All rights reserved.