Early rifting of an oceanic island arc (Fiji) produced shoshonitic to tholeiitic basalts

Abstract

Shoshonitic volcanism began in Fiji within 50 km of and 0.5 m.y. after formation of a transverse rift which broke the oceanic Vitiaz Arc across strike. Shoshonitic basalts erupted from 10 volcanoes along three lineaments for up to 225 km away from the rift along strike, from 5.5 to 3.0 Ma, during Fiji's “incipient rifting stage.” Three shoshonitic suites are distinguished based on differentiation‐normalized alkali contents, but K‐Rb‐Ba‐Sr, P‐rare earth element (REE), and Ti‐Zr‐Hf enrichments occur independent of each other. Enrichments are extremes of typical arc geochemical signatures, and are superimposed on high field strength elements and HREE concentrations and ratios like those of arc tholeiites. Medium‐ and high‐K calcalkaline volcanics are interspersed with shoshonites. Still farther from the transverse rift, coeval tholeiitic basalts erupted along the volcanic front. Decreasing large ion lithophile element enrichments from shoshonitic through calcalkaline to tholeiitic are attributed to increased percent fusion away from the transverse rift. All three suites are basalt‐dominated despite thickened crust, due to regional tension associated with fragmentation of the oceanic arc.