Petrologic and tectonic mechanisms for the formation of submarine calderas in the Okinawa Trough: implications for large on-land silicic magma systems

Abstract:

Okinawa Trough is located in a young and actively spreading back-arc basin that extends for 1200 km behind the Ryukyu arc-trench system. Many active hydrothermal fields have been recognized in this area, which are often accompanied by massive sub-seafloor volcanogenic sulfide deposits. The majority of deposits have been discovered in association with calderas. The Okinawa Trough is characterized by bimodal magmatism, similar to other rifting regions. These volcanic rocks represent a compositional gap between SiO₂ = 56-66 wt%. Many authors have assumed that silicic melts form due to the fractionation of basaltic magmas. However, detailed mechanisms for caldera-forming eruptions have not yet been well-documented. In addition, the caldera depressions in the Okinawa Trough do not always have typical morphologies, and instead are ellipsoidal or rectangular shaped.

The SIP CK16-05 scientific drilling project was recently conducted using the D/V Chikyu, which directly penetrated the caldera floor in the Okinawa Trough. Drilling observations included highly-altered intracaldera ignimbrites in deeper portions of the holes. The existence of an active basalt-hosted hydrothermal field in the Okinawa Trough suggests the presence of hot basaltic rocks at shallow crustal depth, with recharged seawater reaching this depth. In addition, the composition of silicic rocks just after the compositional gap (SiO₂ = 67 wt%) is very similar to the minimum melt composition of a granitic system and the experimental partial melt of hydrous basalt. Therefore, the origin of silicic magma can be reasonably explained by re-melting of hydrothermally-altered basaltic rocks at upper crustal depths. Basaltic lavas are significantly vesiculated, suggesting a volatile-rich magma. It is expected that such volatile elements and elements leached from intense basalt-seawater hydrothermal alteration prior to partial melting are transferred to the silicic partial melt. Furthermore, these leaching products lower magma viscosity by enhancing the depolymerization of the melt. The viscous felsic magma and regional extensional tectonic regime in the rift zone produce relatively small and ellipsoidal-rectangular and irregularly shaped subsidence calderas, compared to the on-land ‘typical’ large subsidence calderas.