Figure 3. The locations of isotopically (radiometrically) dated rock units discussed in text.

Sample number	Rock unit	Age (Ma)	Age-dating technique	Reference
1	Jd	200	40Ar/39Ar	Kunk and others (1992).
2	Zcr	564+-9	U/Pb	Aleinikoff and others (1995).
3	Zrd	571.5+-4.7	U/Pb	Aleinikoff and others (1995).
4	Ybg	1055+-5	U/Pb	Aleinikoff and others (1993).
5	Ygt	1070	U/Pb	Aleinikoff and others (1993).
6	Yhg	1110	U/Pb	Aleinikoff and others (1993).

U.S. Geological Survey, U.S. Department of the Interior
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