Tectonically controlled origin of three unusual rock suites in the Woodlark Basin

Tectonics(December 1986), 5(7):1145-1160

Index Terms/Descriptors: andesites; basalts; basins; block structures; convection; crust; flexure; genesis; hydrothermal alteration; igneous rocks; island arcs; magma chambers; magmas; mantle; marginal basins; mechanical properties; metasomatism; New Georgia Island; oceanic crust; Pacific Ocean; plate tectonics; sea-floor spreading; South Pacific; Southwest Pacific; structural controls; tectonophysics; thermal effects; upper mantle; volcanic rocks; West Pacific; Woodlark Basin

Latitude & LongitudeS10°00'00'' - S7°00'00'' and E155°00'00'' - E158°00'00''

Abstract:

We propose alternative mechanisms for the origin of three unusual rock suites, high-Mg andesites, NaTi basalts, and arclike rocks, that have been dredged from the Woodlark basin, southwest Pacific Ocean. We show that the high-Mg andesites and NaTi basalts are associated with an unusually cool ridge environment. The cooling is due to increased hydrothermal circulation, stimulated by an unusually high crustal permeability. The high permeability is mainly due to cracking of the Woodlark basin lithosphere as it passes over the flexural bulge in front of the subduction zone, although local processes, such as faulting in fracture zones, may also make some contribution. This increased convective cooling affects magma dynamics and chemistry at the ridge crest. The high-Mg andesites occur where the hydrothermal circulation lowers the temperatures in the upper oceanic crust, causing the magma chamber to sit in the mantle rather than in the crust and promoting the interaction of basalt magma with harzburgite. The NaTi basalts are also the indirect result of increased crustal cooling, which causes anomalously low degrees of partial melting of their depleted mantle source. The arclike rocks are caused by interaction with volatiles from lithosphere that was emplaced beneath the edge of the basin less than 6 m.y. ago by a now inactive subduction zone to the north of the currently active one. Simple thermal models indicate that this formerly subducting plate is still cold enough to retain volatiles and to remain seismically active. As the old plate loses volatiles, they rise into the mantle convection cell which feeds the Woodlark basin ridge crest. Because ridge subduction increases the likelihood of ophiolite obduction, these observations and explanations of Woodlark basin tectonics are potentially important for ophiolites.