Figure F3. Lithostratigraphic section of Hole 801C basement, with Ar/Ar ages shown relative to (1) Zr compositions as a measure of magma types (note the high Zr of the alkaline magmas and the gradual increase in Zr downhole in the tholeiitic section) and (2) apparent porosity log showing the hydrothermal zone, which formed on the Jurassic ocean floor and is overlain by younger sediments and off-axis alkaline intrusions (modified from Larson et al., 1993).