From what I read, the intrusive igneous originated from the hot mantel of the planet's core. I would assume that plate tectonics would be one cause of the rise of the intrusive igneous from the mantel to above the fossil. How else does it end up above the fossil sediment being studied?
It does not matter where the igneous material originated.
It did not flow up through cracks in the crust and slide in between the fossil layer and the layer above it. That would be intrusive and that is not the kind of layer we are talking about that is used to date the fossil.
Picture a situation where there has been a volcanic eruption and a layer of ash covers everything, on the surface, the top layer.
Many, many years later the area where that ash layer is, is flooded and becomes the bottom of a lagoon in a shallow sea. Animals that live in the sea die and some of them become buried in sediment and eventually become fossils.
Many, many years later the sea dries up and the fossil layer are on the surface, the top layers.
There is another volcanic eruption and this time a layer of lava covers the fossil layer and it is now the top layer.
We can date the ash layer that is under the fossil layer. And we can date the lava layer that is on top of the fossil layer, so we know that the age of the fossils has to be between the age of the ash layer and the age of the lava layer.
Do you understand what I am saying?
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