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Detrital grains from a metaquartzite of the Highland Group preserve premetamorphic U-Pb ages of between 3.17 and 2.4 Ga and indicate derivation of the sediment from an unidentified Archean source terrain.The Pb-loss patterns of these zircons and the other samples suggest severe disturbance at ca 1100 Ma ago, which the authors attribute to high-grade regional metamorphism.
Reading the record of materials the Earth provides, and particularly the record of plate tectonics, is critical, explains UMass geologist Michael Williams, because the phenomenon governs much of our understanding of issues such as the development of Earth’s crust, the evolution of climate throughout Earth’s history, and how human beings and different species of animals spread across the planet.
"If we can read the past, we have a better chance to understand the future," he said.
Second, different parts of a rock may have different ages because rocks typically record a progression of events which may involve melting, deformation, or reheating within the Earth’s crust.
Thus, dating a whole rock leaves scientists with an average age, rather than pinpointing the ages of specific events in a rock’s history.
"It’s the difference between having a single snapshot, or a lengthy, detailed videotape of a series of events.