Petrogenesis of the 5–3 Ma Rakiraki Volcanics (Nakavadra River, Fiji): insights from geochemistry and pyroxene chemistry

The rocks of the Rakiraki volcano along the Nakavadra River in Fiji are being studied to understand the complex magmatic differentiation processes associated with them. In the River section, basalts and basaltic breccias are intruded by trachybasalt dikes. The presence of felsic and mafic magmatic enclaves with chilled margins, as well as chilled margins at the trachybasalt dike in contact with basalt lava, suggests magma mixing and mingling. Plagioclase and the host trachybasalt dike rock show similar rare earth element patterns, although plagioclase has lower concentrations and a positive Eu anomaly. Variable anorthite contents in plagioclase indicate crystallization at different depths or possible magma replenishments. This concept is supported by the clinopyroxene composition, which exhibits Mg- and Cr-rich ‘mantled zones’ relative to its core and rim. An initial pulse of mantle-derived basaltic magma likely accumulated at the MOHO level before rising and ponding at crustal depths, crystallizing clinopyroxene, and plagioclase. Later, basaltic magma enriched in Cr, Mg, and Fe replenished the chamber at crustal levels, ultimately leading to eruption. The melt fractionated from the basalt intruded as trachybasalt dikes. Both enriched and depleted patterns of whole-rock rare-earth elements suggest that enriched and depleted melts contributed to the origin of the Rakiraki volcanics. Thus, crystal fractionation with periodic replenishment of basaltic magma contributed to the formation of the rocks along the Nakavadra River, producing rocks that display both LREE-depleted MORB- and enriched arc-type trace-element characteristics.