Potassic magmatism hosting economic Au deposits occurs within the Tabar–Lihir–Tanga–Feni island chain, NE Papua New Guinea. The islands are situated within a former forearc basin, and magmatism is triggered by extension post-dating subduction along the Manus-Kilinailau Trench. Current models attribute the formation of K-rich magmas to preferential melting of hydrous phases in a metasomatized mantle source in response to incipient rifting. However, the conditions of melting and magma evolution at crustal depths, which result in a wide range of lava compositions and volcanic edifices of variable types, remain poorly constrained.
This study focuses on primitive K-rich basalts, previously unreported from the region, and more Na-rich absarokites sampled from the South Tabar volcanic ridges (STR) during cruise SO299 DYNAMET (June–July 2023). Fresh lava samples were petrographically studied and mineral compositions were measured by EPMA. Representative whole-rock powders were analyzed for their major and trace element compositions and radiogenic isotope ratios.
Our results show that primitive K-rich basalts, defined as ankaramites due to their high clinopyroxene content, represent mantle-derived potassic melts that underwent minimal differentiation at crustal levels. Primary olivine-hosted melt inclusions preserve primitive, K-rich ankaramitic compositions. Olivine and clinopyroxene xenocrysts scavenged from lower-crustal cumulates record an earlier phase of igneous activity, reflecting the complex evolution of the basin’s crust.
More evolved lavas from the STR and the Tabar Group islands show progressive differentiation toward trachytic to phono-tephritic compositions, accompanied by hydrothermal activity. Melt differentiation was dominated by fractional crystallization of clinopyroxene, ultimately producing biotite-saturated melts. Lower contribution of subducted sediments to evolved rocks compared with primitive ankaramite, indicated by higher 144Nd/143Nd and lower 87Sr/86Sr, reflects either assimilation of oceanic crust or exhaustion of hydrous phases in the mantle source during flux-induces partial melting.