Rb-Sr Isochron Ages for the Giants Range Granite, Northeastern Minnesota
Published:January 01, 1972
Seven whole-rock samples of two-mica quartz monzonites from the central portion of the synkinematic to late kinematic Giants Range batholith give a Rb-Sr isochron age of 2,670 ± 65 m.y. (Rb87 λβ = 1.39 × 10−11 yr−1). This compares with a 2,730 m.y. Pb207—Pb206 age for sphene. Mineral-whole-rock isochron ages range from 2,350 m.y. for epidote to 2,640 m.y. for muscovite. The 2,350 m.y. age probably is a maximum for the time of any later low-temperature event. U-Pb ages for sphene and Rb-Sr mineral-whole-rock data suggest an age of about 2,700 m.y. for the post kinematic Linden syenite.
Initial Si87/Sr86 ratios of 0.7009 ± 0.0004 and 0.7002 ± 0.0019 for the Linden syenite and the Giants Range Granite, respectively, suggest a source with a low Rb/Sr ratio.