Structure of the melanesian arcs and correlation with distribution of magma types

Abstract

Chemical data on Cenozoic lavas (29 new analyses) from Melanesia indicate a zonal arrangement of lava types in the New Guinea-New Britain arc. Tholeiitic rocks occur on the oceanic side of New Guinea (Manam, Karkar), and north of New Britain. Calc-alkaline rocks occur on the East Papuan coast (Mt. Lamington, Mt. Victory). The shoshonitic rock association of the New Guinea highlands (Mt. Hagen, Mt. Giluwe) and East Papua probably represents the equivalent of the alkali-basalt association, or a further zone of magma variation across island arcs. Zonation is not distinct within the Solomon Island Arc. Lavas of the New Georgia Group show tholeiitic as well as calc-alkaline affinities, and rocks from Bougainville and Guadalcanal are calc-alkaline. The equivalent of the alkali-basalt association has not been found in the Solomons. Seismic data are reviewed. The position of the seismic zone may be correlated with the chemistry of the rocks and in the New Guinea Arc the plane dips towards the continent. In the Solomon Arc (Bougainville section) the seismic zone dips steeply towards the continent: in the southern section (New Georgia-Guadalcanal) it is almost vertical.