Zircon dating metamorphic rocks
Another sample, this one from sandstone deposited by a large river in northern Scotland, must have been derived from continental rocks whose ages are represented by those determined for the individually dated sand grains.In this case, the continent from which the sand was derived has moved away as a result of continental drift, but it can be identified by the ages measured.
Given the two related uranium–lead parent–daughter systems, it is possible to determine both the time of the initial, or primary, rock-forming event and the time of a major reheating, or secondary, event.
The Khao Pret granite gives U-Pb zircon concordia age of 67.5 ± 1.3 Ma, which represents the timing of zircon crystallization from the granitic melt and accompanied sillimanite-grade contact metamorphism against surrounding metapelites and gneisses.
Metamorphic rocks in the Doi Inthanon area also share the similar plutono-metamorphic history with the Khanom and the Hub-Kapong to Pran Buri areas.
In one sample it was determined that a sandstone that underlies most of the province of Nova Scotia in Canada was probably originally deposited off the coast of North Africa and thrust over the continent before the opening of the Atlantic Ocean.
This follows because the ages observed occur in North Africa, whereas those common in North America are absent.
The upper intercept will denote the timing of the primary rock-forming event, while the lower intercept will denote the timing of the reheating event.