You seem to be getting into an example of rather great complexity here (welcome to the real world). My short response is, many different things can be discovered by studying such a system. One can get multiple legitimate dates from such things - They can date different events in the rocks history.
My bottom line is, the evolution side might be best served by letting one of our resident (real) experts respond to this, rather than the quasi-experts (such as myself) cluttering things up with our responses.
I think that means "Let's now hear from JonF".
Edited to add to subtitle.
This message has been edited by minnemooseus, 12-10-2004 03:16 PM
I think it is possible to have other isochrons produced, other than those that determine the age of a rock.
Example: I believe if you sample a number of different igneous rocks that are of different ages, but are of a common magmatic origin, you can produce a isochron. But the isochron will determine the age of when the different magmas fractionated from a parent magma.