| Abstract: | The late Tertiary-Recent collision of the d'Entrecasteaux Zone with the Vanuatu (New Hebrides) Island Arc appears to have been associated with a shift to generally more potassic compositions. Glass shards analysed from Quaternary volcanic ashes recovered from ODP Leg 134 drill sites follow distinct shoshonitic and calc-alkaline trends, consistent with those previously recognised onshore. In the Aoba Basin (Site 832) a marked compositional change is recognised at 37 m below sea floor (mbsf). From estimates of sedimentation rates this is equivalent to an age of about 100 ka and may be associated with the development of the Central Basin volcanoes of Santa Maria, Aoba and Ambrym on fracture zones transverse to the island arc. Deeper than 37 mbsf differentiated compositions fall on a medium-K calc-alkaline trend but the associated basaltic glasses are strongly potassic. Conversely, above this level the differentiated glasses follow a shoshonitic trend but the basaltic glasses are medium-K calc-alkaline. Separate sources are envisaged, associated with subduction and rifting. Episodic subduction of the d'Entrecasteaux Zone exercises tectonic control over the release of magma from different depths. In contrast, shoshonitic glasses are sparsely represented in the Vanuatu forearc where relatively low-K calc-alkaline compositions probably correspond to the ashes in the lower part of the Aoba Basin sequence. This is consistent with their age (> 100 ka) estimated from lower sedimentation rates at these sites. The scarcity of younger shoshonitic ashes at the forearc sites is attributed to the influence of high-level westerly winds transporting ash from the Central Chain volcanoes eastwards across the North Fiji Basin. |
