Discussion
Which one of the following methods is best suited for dating highly deformed and metamorphosed Archaean gneissic rocks containing multiple age components ?
Answer: B
The best method is single zircon dating.
A strongly deformed and metamorphosed gneiss may contain zircons, or zones within one zircon, of several ages: inherited cores, magmatic growth and metamorphic rims. U-Pb dating of individual grains, or of spots within a grain by ion microprobe (SHRIMP) or laser ablation, can resolve each of those components, and zircon keeps its U-Pb record through high-grade metamorphism.
Whole rock U-Pb and Pb-Pb dating mix all the components in the sample and give a single, often meaningless, average or a disturbed isochron. Carbon-14 dating works only on organic material younger than about 50,000 years and is useless for Archaean rocks.
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