T25 Microprobe Monazite Geochronology: New Developments and Applications

Sunday, 27 October 2002
8:00 AM - 12:00 PM

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

Paper number: T25-9
Program: Technical Sessions
Time: 10:00 AM – 10:15 AM

Authors

Abstract

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.