The volcanic and geochemical evolution of a trachydacite-dominated island arc centre : Efate Island Group, Vanuatu Arc, SW Pacific

The Efate Island Group is a trachydacite-dominated volcanic centre in the Vanuatu island arc, southwest Pacific. This centre is part of the Central Chain of islands that form the active Vanuatu arc, and is related to current subduction of the Australian Plate beneath the Pacific Plate. The volcanic succession at Efate comprises a bimodal association of voluminous trachydacitic volcaniclastic deposits, and younger basaltic lavas and pyroclastic deposits associated with small volume volcanic cones. This association is the record of a major explosive caldera eruption that occurred in the Vanuatu arc about one million years ago. The oldest known deposits at Efate are the ~1 Ma, semi-consolidated, trachydacitic pumice breccias and shard-rich sand and silt beds of the Efate Pumice Formation. The base of the formation is not exposed, but it has a total thickness of about 500 m in the centre of Efate Island and a total bulk volume of approximately 85 km\\(^3\\). The lowermost unit of the Efate Pumice Formation is dominated by thick to very thick beds of massive and stratified pumice breccia that are variably interbedded with fine-grained shard-rich sand and silt. These beds are referred to as the Efate Pumice Breccias. The fades characteristics of the Efate Pumice Breccias indicate deposition principally by water-supported turbidity currents and debris flows in deep marine environments. The nature of the clast population, coupled with fades characteristics, further suggest that the submarine volcaniclastic mass-flows were directly generated by concurrent explosive magmatic eruptions from a volcanic caldera. A sharp, conformable surface separates the Efate Pumice Breccias from the overlying Rentabau Tuffs. These are characterised by very regularly bedded and well-sorted, fine-grained shard-rich sand and silt fades. Beds are typically massive, to locally stratified and cross-stratified, and convolute bedded intervals occur throughout the sequence. The clast population is generally >95 % volcaniclastic, but fossil foraminifera are a ubiquitous and important non-volcanic component. The Rentabau Tuffs are interpreted to represent a change in explosive activity from magmatic to hydromagmatic fragmentation of the trachydacite magmas due to flooding of vent areas by seawater. Deposition of the Rentabau Tuffs occurred by the passage of steady turbidity currents generated directly from explosive eruptions, by post-eruptive remobilisation of unconsolidated material, and by fallout from the eruption column. Vent locations and environments for the eruption that generated the Efate Pumice Formation remain unknown. However, sparse paleocurrent indicators and grain-size variations point to a source to the north of Efate. Environments are more difficult to interpret but the restricted nature of the Efate Pumice Formation and the lack of evidence of subaerially produced components in all beds, indicate that vents were probably submarine, although the eruption column may have breached the sea surface. Following eruption of the Efate Pumice Formation, late stage basaltic magmas were erupted initially from submarine vents, and continued eruptions produced small volume (3-6 km\\(^3\\)) emergent volcanoes, mainly consisting of stratified lavas with minor pyroclastic deposits and volcaniclastic breccias and conglomerates. This bimodal association is related to the successive emptying of a strongly compositionally zoned, shallow crustal magma chamber. The trachydacite magmas that were erupted to form the Efate Pumice Formation were generated by prolonged fractional crystallisation involving plagioclase, olivine, clinopyroxene and Ti-magnetite, from, basaltic parental magmas that were continually supplied from the mantle wedge. The Basalt Volcanoes Formation lavas and pyroclastic fades were eventually erupted from the core and lower parts of this magma chamber. The Basalt Volcanoes Formation lavas can be divided into three different suites on the basis of their temporal and geochemical variability. Overall these lavas have transitional tholeiitic/ calcalkaline affinities, anomalously high Sr, appreciably elevated P and strikingly low Zr contents relative to 'normal' sou.them Vanuatu arc basalts. Sr-Nd isotopes of the Basalt Volcanoes Formation lavas are akin to central, collision-affected Vanuatu arc lavas, and Pb isotopes are transitional between the central and 'normal' southern Vanuatu arc rocks, in keeping with the position of the Efate Island Group between these segments of the Vanuatu arc. Parental basaltic magmas were generated by partial melting of a mantle wedge source beneath Efate. It is hypothesised that this mantle wedge source had been previously metasomatised by ephemeral carbonatite melts that· elevated levels of Sr and P in the subsequent partial melts, but not the high field strength elements such as Zr and Nb. The generation of the carbonatite melts may be related to variations in local tectonics where the detachment of the subducting slab and subduction of a seamount is occurring and has probably persisted for at least ~3 Ma. In addition, the freezing of ascending primitive melts related to an earlier west-dipping subduction regime, may have caused local areas of enrichment notably in K, and Rb, within the mantle wedge source of the Efate basalts. These features are reflected in the unusual geochemical signature of the Basalt Volcanoes Formation lavas, relative to the central and 'normal' southern parts of the Vanuatu arc.