Cainozoic volcanism of the Tabar, Lihir, Tanga, and Feni Islands, Papua New Guinea: geology, whole-rock analyses, and rock-forming mineral compositions.

Geological surveys have been made of these island groups constituting a dominantly alkaline volcanic chain, roughly parallel to, and NE of, the Tertiary island arc of New Ireland, in NE Papua New Guinea. The islands consist mainly of Pliocene and Pleistocene lava flows and volcaniclastic deposits, but pre-Middle Miocene volcanic rocks are known in the Tabar Islands. Quartz trachytes, the only silica-oversaturated rocks of the chain, represent the youngest extrusive rocks in each island group. Raised Pleistocene coral reefs form fringing terraces on many of the islands, and Miocene reef limestone is preserved on Simberi Island in the Tabar Islands. Present-day thermal activity is found in each of the island groups, but is best developed on Lihir and Feni Islands, where the thermal areas mostly occupy the calderas of Quaternary volcanoes. Whole-rock chemical analyses have been compiled for 116 volcanic rocks (major and trace elements and, for some rocks, REE abundances and ⁸⁷Sr/⁸⁶Sr values). Rock-forming minerals in many rocks have also been analysed by microprobe. The analysed rocks are mainly phonolitic tephrite and trachybasalt, but more mafic types are basanite, tephrite, alkali basalt, transitional basalt and ankaramite. Clinopyroxene-rich cumulate inclusions are a common feature of some lava flows.-C.N.