| Abstract: | Uranium-lead ages were measured on 14 samples of sphene from rocks aged from 1000 to 2750 x 10(6) years. All samples gave concordant or nearly concordant ages, the maximum difference between the Pb(206)-U(238) and Pb(207)-Pb(206) ages being 10 percent. Sphene has more concordant ages than has the coexisting zircon in each of seven rocks in which they were compared. Sphene sometimes has greater ages than does coexisting biotite, although in two metamorphic rocks, in which metamorphism was sufficiently intense to cause redistribution of radiogenic strontium-87 between various mineral phases, sphene dates the time of metamorphism rather than of original crystallization of the rocks. |
