Abstract
The eruption of shoshonitic magmas in Fiji during the Late Miocene‐Pliocene (5.5–3 Ma) from 11 main volcanic centers along three broad ENE and NNW trending lineaments coincides with well‐developed spreading in the North Fiji and Lau back‐arc basins and maximum rotation of the Fiji Platform. The most mafic shoshonitic lavas (absarokites) range from 8.4 to 15.2 wt % MgO and are variably clinopyroxene + olivine‐phyric. Fijian shoshonitic suites display a range of enrichment in large ion lithophile elements, Th, U, and P relative to rare earth elements and high field strength elements (HFSE), reflecting variable contributions by the subarc mantle source and subduction‐related components. The subarc mantle source component controls HFSE, heavy rare earth elements, to a lesser degree light rare earth elements (LREE), and most importantly the sensitivity of the mantle source with respect to subduction‐related enrichment processes, whereas Pb, K, Sr, Ba, Rb, Th, U, P2O5, and LREE are contributed by hydrous or supercritical fluid(s) and sediment melts that are added to the subarc mantle. Fijian shoshonitic suites situated ∼150 km apart display a wide range of Nb/Yb (0.3–4.3), implying that there is significant spatial heterogeneity in the sub‐Fijian mantle with respect to the ambient fertility of mantle sources independent of subduction‐related enrichment. The range in incompatible element ratios (e.g., Th/Nb, U/Nb, Ba/Th, Ba/La, P/Nd, and Ce/Pb) displayed by Fijian shoshonitic suites cannot reflect addition of the same subduction‐derived component to variably enriched subarc mantle sources. Differences in the relative amount of fluid versus sediment melt and potentially the composition of the subducted sediment are required to explain the data. The greater overall subduction‐related enrichment in Fijian shoshonites relative to calk‐alkaline and tholeiitic arc magmas is attributed to a melt generation process involving low‐degree partial melting of metsomatized subarc lithosphere in contrast to magmas associated with active or steady state arcs, where higher degrees of melting occur in response to volatile fluxing of the mantle wedge. Shoshonitic magma generation occurs in linear zones during extension of the arc lithosphere preceding arc fragmentation and establishment of back‐arc spreading centers.
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