The Mesozoic diabase dikes of Liberia are tholeiites whose 87Sr/86Sr and 87Rb/86Sr ratios scatter widely on the Rb-Sr isochron diagram. The problem is attributed to differences in the initial 87Sr/86Sr ratios of these rocks which range from 0.70311 to 0.70792, assuming a uniform age of 186Ma for the dikes and using ,~(87Rb)=l.42x 10-11y -1. The range of values is similar to that observed in the Mesozoic basalt flows and dikes of other Gondwana continents.
New whole-rock K-Ar dates confirm previous conclusions that the diabase dikes in the Liberian and Pan-African age provinces of Liberia absorbed extraneous r176 after intrusion. Only the dikes in the Paynesville Sandstone have K-Ar dates that range from 117 Ma to 201 Ma and may not contain extraneous 4~ However, dikes from all three age provinces of Liberia have elevated initial 878r/86Sr ratios. These results indicate that contamination with radiogenic 87Sr occurred primarily before intrusion of the magma whereas the addition of extraneous 4~ occurred after emplacement and reflects the age and mineral composition of the country rock.
The ~180 values of the Liberian diabase range from +5.6%0 to +9.10%o and correlate positively with initial 87Sr/86Sr ratios. The data can be modeled by fractional crystallization and simultaneous assimilation of crustal rocks by the magma. However, samples containing amphibole and biotite replacing pyroxene deviate from the Sr-O isotope trajectories of the model and appear to have been depleted in 180 and enriched in 87Sr by interactions with groundwater at high temperature.