Paleozoic and Precambrian Igneous Rocks

Igneous rocks older than Mesozoic are somewhat difficult to work on as the corrections for in situ radioactive decay of U and Th are large and the values of U/Pb and Th/Pb often have undergone a recent 

Some K-feldspars in which model lead ages are in good agreement with accepted ages 

Feldspars with at least one ratio reasonably unaffected by metamorphism 

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