Ultramafic blocks from the ocean floor southwest of Australia

Samples dredged from the ocean floor near the junction of the Naturaliste Fracture Zone and the Diamantina Zone, 300 km southwest of Cape Leeuwin, Western Australia, contain 3- to 10-cm blocks of Cr-spinel lherzolites and Al-spinel-olivine clinopyroxenites. Both assemblages show textural evidence for deformation and recrystallization, with extensive development of kaersutite in one clinopyroxenite. The blocks are enclosed by clay and Fe-Mn oxide-rich fragmental material, which also contains a diverse detrital mineral suite and microfossils.Diopsides in the lherzolites are Na- and Cr-rich, with marked similarities to those of lherzolite nodules in alkaline basaltic suites. The Al- and Ti-rich diopsides of the clinopyroxenites are more magnesium than those of common pyroxene-rich nodules. The lherzolites are interpreted as upper mantle residues, while the clinopyroxenites probably represent partly recrystallized cumulates from high-temperature basaltic magmas traversing the lherzolite mantle. Both are thought to have been derived from an ultramafic body emplaced into the shallow crust near the Cretaceous/early Tertiary continent-ocean boundary off Australia.