The Geophysical Significance of Oceanic Plagiogranite

Rocks ranging in composition from trondhjemite to diorite (plagiogranites) have been recovered from ocean ridges and are common constituents of ophiolites. Velocities and densities of diorite and trondhjemite from the MidAtlantic Ridge are shown to differ significantly from similar properties of metadolerite and gabbro. Compressional (V p) and shear (V s.>velocities of plagiogranites are relatively low (V p =4.78-5.91 km/s at 1 kbar, Vs = 2.81-3.37 km/s at 1 kbar), as are densities (2.57-2.64 g/cm3) and Poisson's ratios (0.24-0.27). These data lend strong support to the probable existence of a low-velocity/density zone within layer 3 of the oceanic crust. Based on observations in ophiolites, it is postulated that this zone can be up to I km in thickness and is laterally discontinuous.