U-PB AGES OF ZIRCON, MONAZITE, AND XENOTIME IN THE HARNEY PEAK GRANITE, BLACK HILLS, SD: IMPLICATIONS FOR THE TIMING AND DURATION OF SYN- TO POST-OROGENIC MAGMATISM

The eastern margin of the Archean Wyoming craton (Black Hills, SD) records a complex history of Precambrian thermotectonism and magmatism associated with convergence, rifting, and supercontinent cycles. Establishing the timing and duration of the Harney Peak magmatic event would constrain events associated with the terminal Black Hills orogeny. Accordingly, uranium-bearing accessory minerals from western (A) and west-central (B) sample sites within the Harney Peak granite (HPG) have been spot-dated by U-Pb ion-microprobe methods. Magmatic zircon, monazite, and xenotime have yielded precise ²⁰⁷Pb/²⁰⁶Pb weighted-mean ages of 1716.8 ± 9.3 Ma (2σ, MSWD = 0.95, n = 5, location A); 1703.2 ± 2.2 Ma (2σ, MSWD = 2.1, n = 9, locations A and B); and 1695.9 ± 2.9 Ma (2σ, MSWD = 1.8, n = 6, location B), respectively. These ages are consistent with microtextural occurrences of monazite and xenotime as overgrowths on zircon. Also, diffusional Pb closure temperatures for these minerals exceed the known maximum temperatures of HPG magmatism. Therefore, we interpret our results as mineral-growth ages indicating that the southern Black Hills HPG event lasted for 23 ⁺¹⁰ _-12 Myr (or ~9-33 Myr), beginning at ~1715 Ma and ending at ~1695 Ma. This ~1715-1695 Ma interval of magmatism agrees with published ages of metamorphism, magmatism, and post-magmatic cooling in several ways. First, it neatly dovetails with local ~1750-1715 Ma ²⁰⁷Pb/²⁰⁶Pb ages of pre- to syn-HPG metamorphism and with ~1690-1300 Ma ⁴⁰Ar/³⁹Ar ages of post-HPG cooling. Second, it accounts for U-Pb magmatic ages of 1713 ± 10 Ma (monazite at Whitewood Peak) and 1718 ± 22 Ma (zircon at Crook Mountain) also evident in northern Black Hills granites. Third, the ~23 Myr interval of HPG magmatism inferred here agrees within error with a ~14 Myr interval documented for the Laramide Tuolumne granitoids (CA), suggesting that the general duration of culminating magmatism in collisional orogens is on the order of ~10-20 Myr.