SUBMARINE IGNIMBRITES IN UNIT V, IODP SITE U1437: EVIDENCE FOR ARC-FRONT EXPLOSIVE VOLCANISM DURING A REAR-ARC HIATUS

International Ocean Drilling Program Expedition 350 drilled into the volcaniclastic succession of the Enpo-Manji Basin in the rear-arc of the Izu-Bonin arc, Western Pacific. Unit V (~7.5 - 9 Ma) is the oldest stratigraphic unit composed of the fine-grained, volcaniclastic deposits typical of the younger parts of the succession. The preceding and following units are locally derived from the adjacent back-arc seamounts, in contrast to Unit V that is, at least in part, geochemically fingerprinted to the arc-front, ~80 km to the east. Unit V is characterized by 10 - 50 cm-thick, graded, density current deposits intercalated with thicker, more massive tuffaceous mudstone layers. The density current deposits exhibit erosional basal contacts, and are composed of fresh to weakly altered, sand-sized, volcanic glass shards (tuff) and fiamme. Fiamme are interpreted to be pumices flattened during compaction and diagenesis, and not welding. The coarsest, freshest tuff at the base of some deposits contain stylolites parallel to the horizontal fabric of the fiamme. Each deposit is reverse coarse-tail graded with fiamme increasing in size upwards, with no significant change in matrix grain size. Based on these observations, we have interpreted these deposits are individual submarine ignimbrites rather than turbidites because of the overwhelming dominance of juvenile volcanic material and near total absence of pelagic sediment, rock fragments, and microfossils. It is these same submarine ignimbrites that are geochemically fingerprinted to the distal arc-front, strongly suggesting a direct sediment pathway from the (up-channel) arc-front, westward along the axis of the Enpo-Manji Basin. This implies that the Enpo-Manji Basin and the surrounding Manji and Enpo seamount chains were well-established bathymetric features stretching eastward to the arc by <9 Ma. Furthermore, Unit V represents a ~1.5 Ma hiatus in proximal volcanism during the ~17 - 3 Ma back-arc seamount volcanic phase.