Volcanic pyroclast analyses from DSDP Hole 13-123

The pyroclastic deposit drilled at site 123 is a rhyodacitic material. Major and rare-earth element abundances in these volcanic rocks suggest that they are the result of a differentiation process by fractional crystallization from a calc-alkaline magma. The vertical variation of mineralogy and chemistry of this deposit is the result of three superposed stages of weathering. The first is aerial weathering, the second is the product of alkaline brines, and the third is a hydrothermal alteration.
The K/Ar age of the pyroclastic deposit is 23.8 ± 1.2 m.y.; this age is not disturbed by the presence of phillipsite; consequently, the first two alteration stages are very early. The existence of these two stages implies that the age of opening of the Valencia Trough begins at the Oligocene-Aquitanian boundary after the eruptive stage.