CHEMICAL DATING OF MONAZITE IN THE BLACK HILLS, SOUTH DAKOTA, WITH IMPLICATIONS FOR PROTEROZOIC THERMOTECTONISM IN THE EASTERN WYOMING PROVINCE

The Laramide Black Hills uplift, South Dakota, represents the easternmost exposure of the Archean Wyoming province. Here, published geochronology has shown that ~2560-2590 Ma basement granitoids were nonconformably overlain by intracratonic rift successions deposited at ~2450-2100 and ~1970-1880 Ma, and that the entire complex was multiply deformed and metamorphosed between ~1780 and 1715 Ma. However, relatively little is known about Paleoproterozoic history: (1) between ~1880 Ma (youngest gabbro in sedimentary rift basin) and ~1780 Ma (onset of Wyoming-Superior collision and thermotectonism of basin sediments); and (2) between ~1715 Ma (intrusion of Harney Peak granite) and ~1650 Ma (oldest argon closure in metapelitic muscovite). Accordingly, monazites in four samples of basement granitoid and associated metasediment have been chemically dated in situ by electron microprobe, using a new analytical method that incorporates improved background acquisition and interference corrections. Ca. 130 spots yield monazite crystallization ages ranging from ~1650 to ~1900 Ma, and a cumulative probability plot of the ages faithfully reproduces known metamorphic pulses previously dated at ~1760 and ~1715 Ma. However, ~10% of the monazites apparently formed during an ~1810 Ma metamorphic pulse, whereas ~35% apparently formed during retrograde cooling (post-granite). Thus, the older population appears to record evidence of a cryptic Trans-Hudsonian event previously unrecognized in the Black Hills. This event is preserved only in the lowest-grade rocks, suggesting that the ~1810 Ma monazites were relatively shielded from subsequent recrystallization that overprinted higher-grade rocks closer to the granite. The younger ~1715-1650 Ma ages appear to record evidence of a retrograde hydrothermal regime that persisted for ~60-70 m.y. following granite emplacement.