A thick, extensive volcanic formation of Oligocene to early Miocene age, the Finisterre Volcanics, forms part of a Cenozoic sequence comprising the Finisterre and Adelbert Ranges of northern Papua New Guinea. The formation contains a high proportion of diverse volcaniclastic rocks and is lithologically similar to volcanic sequences described from island-arc assemblages elsewhere. The volcanic rocks are dominantly potassic basalt and low-silica andesite (48 to 56 percent SiO2) containing 1.5 to 6.5 percent K2O and having low TiO2 content typical of circumoceanic volcanic rocks. Two main groups can be recognized: abundant shoshonite and related rocks (absarokite, rare leucite trachyte) and high-K, high-Al basalt (with some high-K, low-Si andesite).
The Finisterre Volcanics are chemically similar to high-K rocks described from island arcs elsewhere in the southwest Pacific and in the Mediterranean. However, unlike some other island arcs, there is no evidence of a three-stage evolution from arc tholeiite to calc-alkalic andesite to shoshonite. The volcanic rocks probably formed in a volcanic arc that developed north of a northeastward-dipping subduction zone in response to early Tertiary plate interactions. The Finisterre volcanic magmas may have originated by partial melting of mantle material modified by slab-derived silicic melts.