Geology, petrography, and geochemistry of the volcanic islands of Tonga

Abstract

The South Pacific Ocean is an ideal area in which to test some of the compositional implications of plate tectonics and sea-floor spreading. The island arc of Tonga marks an active zone of seismic disturbances and volcanic activity that may be related to thrusting of the oceanic crust beneath the marginal crustal plate on which the islands are constructed. This marginal plate appears to be of oceanic character in the vicinity of Tonga, and the Tongan volcanoes thus provide an opportunity to examine the nature of magmas of a typical circum-Pacific orogenic zone located within an oceanic crustal regime. Volcanic eruptions in the Tonga Islands have averaged about one every four years during the present century, and milder submarine eruptions may have escaped notice. The submarine volcanoes and volcanic islands appear to be located along en-echelon fractures that strike slightly east of the general trend of the islands. These fractures would tend to open under right-lateral shear stress in the sense of the New Zealand Alpine fault and would tend to close under compression directed perpendicular to the island arc from the Pacific side. Thus, the relative amount and direction of movement of crustal plates on either side of the island arc may control the frequency and intensity of volcanic eruptions. The active volcanic islands and sea mounts are along a submarine ridge about 200 km west of the axis of the Tonga trench. The youthful constructional volcanic topography and limited paleomagnetic data indicate a very recent age for the volcanic activity. The rocks of these islands are basaltic andesite, andesite, and dacite, with the exception of Niuafo'ou, from which only basalt has been reported. These rocks differ from most other circum-Pacific andesite-dacite suites in their very low content of alkali, especially low K2O, although similar rocks have been reported from a few other island arcs, such as the Izu peninsula of Japan, the Marianas, and the South Sandwich Islands. The chemical peculiarities are represented modally by exceptionally calcic plagioclase and near absence of alkali feldspar. Pre-late Eocene basalt and uralitized gabbro on Eua Island at the southeast end of the group may represent uplifted pre-arc sea floor. These rocks are high in Al2O3 and are especially poor in K2O. The variation in K2O and total alkalies across the Tonga arc is similar to that observed in other circum-Pacific island arcs. The volcanic rocks show moderately strong iron enrichment, the highest Fe/Mg ratios appearing in the more siliceous andesites and dark dacites. This iron enrichment should not be regarded as evidence for a tholeiitic affinity for these rocks. Trace element abundances are similar to those of many other calcalkaline island arc lavas and apparently are consistent with a genetic evolution of the lavas by fractional crystallization. Bouger gravity anomalies decrease from south to north, from a high of +243 mgal on Eua to a low of +122 mgal on Fonualei. This decrease in gravity is matched by a slight increase in potash content in lavas of otherwise comparable composition, implying a change in the composition of the deep crust or mantle from which the lavas are derived. However, the most important compositional changes along the Tonga-Kermadec-New Zealand volcanic line occur between the Kermadecs and New Zealand, and probably reflect the influence of crustal contamination beneath the North Island.