A slightly metamorphosed igneous sequence is exposed beneath upper Eocene limestone on Eua, the southernmost of the eastern line of limestone-covered islands of the Tonga group. This igneous sequence occurs as lava, agglomerate, conglomerate, and tuff; compositions include high-alumina tholeiite, basaltic andesite, quartz gabbro, olivine basalt, and dacite tuff. Well-defined dikes of acid andesite (also pre-limestone) cut this sequence. Phenocryst mineralogy is typically highly calcic plagioclase, augite, and titanomagnetite, plus alteration products (mainly uralite, chlorite, epidote, calcite, and pyrite).

Characteristic chemical features of the Eua igneous suite include relatively high alumina, very low K2O, P2O5, Rb, Ba, and REE, and relatively low TiO2, Ni, and Cr. The Na/K, V/Ni, and K/Rb ratios are high, and Rb/Sr ratios are very low. Measured Sr87/Sr86 ratios (= initial) of a tholeiitic basalt and an acid andesite give 0.7034 and 0.7038, respectively.

The Eua igneous suite evidently represents a very early stage of island arc evolution. Geochemically, these Eua rocks are decidedly less fractionated (or more “primitive”) than the recent Tongan basaltic andesite-dacite association which constitutes the western line of active volcanic islands of Tonga. This latter suite can be classified as one of the circum-Pacific “low potash andesite suites” (or “island arc tholeiitic series”). In terms of the trace and minor element data, the Eua igneous suite is similar in many respects to ocean-floor basaltic lavas, although perhaps characterized by lower TiO2, Ni, and Cr than is typical of most ocean floor lavas.

It is concluded that the over-all chemistry and mineralogy of the Eua igneous suite suggest close similarities with the volcanic and mafic plutonic components of described ophiolite complexes; this is partly supported by the reported occurrence of peridotite and dunite from the nearshore flank of the Tonga trench. Thus, it is postulated that on Eua we have the topmost part of an underlying ophiolite complex exposed, possibly developed at the location of and at the inception of the site of initial subduction of lithosphere that has subsequently been responsible for the development of the Tonga arc system.

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