ION MICROPROBE (SHRIMP U-PB) STUDY OF ZIRCON GEOCHRONOLOGY IN ARCHEAN BASEMENT LITHOLOGIES, BLACK HILLS, SOUTH DAKOTA

Zircons from basement rocks of the eastern Wyoming province (Black Hills, South Dakota, U.S.A.) have been analyzed by ion microprobe (SHRIMP) methods in order to determine precise ages of Late Archean events. In the northern Black Hills (NBH) near Nemo, the Little Elk granitic gneiss (LEG) intrudes biotite-feldspar gneiss (BFG). Both lithologies are metaluminous in composition and exhibit a pervasive NNW-SSE fabric; vestiges of an earlier NNE-SSW fabric occur only in the BFG. Zircons in the LEG and BFG exhibit uniform morphologies and textures, including double terminations and oscillatory zoning, and uniform Th/U ratios of ca. 0.6 ± 0.3 - all indicating a magmatic origin - and yield equivalent ²⁰⁷Pb/²⁰⁶Pb ages of 2559 ± 3 Ma and 2560 ± 4 Ma, respectively (upper-intercept, U/Pb concordia, ± 2s). Patches of BSE-unzoned zircon commonly truncate zoned zircon of the same age in the same crystal, suggesting that deuteric fluids caused local recrystallization during magmatic cooling. In one LEG zircon, an inherited core (Th/U » 1.3) yields a concordant ²⁰⁷Pb/²⁰⁶Pb age of 2894 ± 3 Ma, confirming the published Nd T_DM model age of 2910 Ma for the LEG source rocks. Highly discordant zircon overgrowths (Th/U » 0.2 ± 0.1) in the LEG and BFG yield an upper-intercept age of ca. 2050 ± 100 Ma, which broadly correlates with the Early Proterozoic age of voluminous gabbroic magmatism nearby; the lower intercept is consistent with near-zero-age Pb-loss, possibly related to Tertiary magmatism and associated fluids. In the southern Black Hills (SBH), magmatic zircons (Th/U » 0.08 ± 0.03) from peraluminous granitoid at Bear Mountain (BMG) yield an upper-intercept age of 2588 ± 6 Ma, thereby constraining a minimum age for biotite-plagioclase schist intruded by the BMG. Basement zircons included in this study lack any U/Pb-isotopic signature of the Wyoming-Superior convergence that imparted ca. 1715-1770 Ma (late Trans-Hudson) mineral ages on overlying supracrustal rocks. Taken together, these data indicate that Late Archean basement rocks in the Black Hills formed during separate tectonomagmatic pulses at ca. 2590 Ma (S-type granitoid, SBH) and ca. 2560 Ma (I-type granitoids, NBH), probably in response to convergent plate-tectonic processes such as previously inferred regionally during that time interval (e.g., SE Wyoming and Hearne provinces).