A few years ago I took a course in the "Evolution of Desert Environments". Uranium and thorium have high melting points and as magma cools, these elements crystallize out of solution and fall to the magma chamber's depths and remelt. Radiometric dates are only accepted if they agree with what geologists already believe the age should be. The geologist may have found some fossils in Sedimentary Rocks A and discovered that they are similar to fossils found in some other rocks in the region.
Later on the magma is poor in iron, magnesium, and calcium and rich in uranium, thorium, sodium, and potassium.