In terms of dating geologic age, basalt is an excellent marker. Using Potasium 40 or Argon dating will get you a fairly accurate date of a volcanic event.
Some volcanic horizons are indeed very useful for radiometricly dating, and for the dating of sediments between volcanic horizons. But basalt, the mafic (high iron and magnesium) end member of the types of igneous rocks is (and someone correct me if I'm wrong) not radiomentrically datable by the common methods, including such as Potasium/Argon, Uranium/Lead, and variations there of. Basalt is very low in Potasium and Uranium.
One thing I don't understand is how we know the original ratios of these elements in volcanic rocks. I understand carbon dating, because of how nitrogen is turned into carbon-14 in the upper atmosphere, but what about the other radioisotopes used to date older strata?
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