The youngest rocks from an old arc and the oldest rocks from a juvenile one: The memoirs of a SW Pacific subduction zone.

摘要

The Fiji-Tonga-Kermadec volcanic arc system has been active for at least 50 My, resulting from westward subduction of the Pacific Plate beneath the Australian Plate. Volcanism during its initial and final stages is investigated using new high-resolution measurements of major and trace elements and isotope ratios of Sr, Nd, Hf and Pb. Parts I and II focus on lavas associated with juvenile rifting in the modern Havre Trough back-arc. Part III covers the diversity of lavas during arc volcanism following subduction initiation, as recorded in Eocene to Miocene rocks from Viti Levu, Fiji, and 'Eua, Tonga representing First- and Second-Arc stages, separated by a hiatus associated with South Fiji Basin back-arc spreading.;Lavas associated with Havre Trough back-arc extension behind the active Kermadec arc are geochemically diverse. The back-arc is characterized by along-arc segmentation into compositional and morphologically distinct Arc and Basin regimes. Trace element and isotopic constraints are used to distinguish mantle and slab-derived components in the source of lavas from both regimes. All have an enriched mantle component similar to "C" with respect to Pb isotopes, but with higher Nd isotopes indicating an ancient recycled crustal source with little sediment. The same enriched mantle component is present in earlier lavas from the South Fiji Basin back-arc. Compared with the Basin regime, Arc-regime lavas have more slab-derived material, added up to ∼80 km behind the volcanic front in the form of a hydrous melt of sediment. In contrast, Basin-regime slab-derived components are more often near-solidus fluids carrying material from sediment and altered oceanic crust. Calculated slab-derived components are consistent with a model of spatially constrained sediment melting in the presence of varying proportions of residual accessory phases (zircon, rutile, monazite). Thus, variability in slab-derived components may reflect along-arc thermal anomalies in the mantle wedge. In this way, subduction components can vary even when the subducted material remains constant.;The Fiji First Arc is characterized by eruptions of closely associated lavas of dissimilar melt types, ranging from tholeiitic to calcalkaline and including MORB-like and boninite-like end-members. The diversity requires more complex melting during First Arc than previously thought. They have the same enriched mantle component as is present in later South Fiji Basin and Havre Trough back-arc lavas. They suggest a mostly fluid slab-derived component from altered oceanic crust +/- volcaniclastic sediment, with little addition of melt-mobile elements Nd and Hf. Second Arc lavas are more depleted and less geochemically diverse than those of the First Arc, possibly reflecting mantle preconditioning by either First Arc or South Fiji Basin melting.