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

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