Microscopic study of primary quartz grains from Chilean granitic rocks of known radiogenic ages shows a significant correlation between the intensity of the undulatory extinction in this mineral and the age of the rock. In the Paleozoic rocks the undulatory extinction ranges from 31° to 35°; in Jurassic rocks it ranges from 23° to 25°; in Cretaceous rocks it ranges from 15° to 17°; and in Tertiary rocks it ranges from 12° to 13°.

The marked difference in the intensity of the undulatory extinction in quartz grains is interpreted as the result of the degree of tectonic deformation of the granitic rocks of different ages present in a single linear folded belt affected by successive orogenic movements.

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