IMPLICATIONS OF DETRITAL AND METAMORPHIC MONAZITE AGE DATA FROM PROTEROZOIC QUARTZITES AND BASEMENT IN CENTRAL WISCONSIN

Detrital zircon age data from Proterozoic quartzites in the southern Lake Superior region have helped 1) to constrain the age of quartzite deposition (post 1730 Ma) and 2) to better characterize their provenance. We report here preliminary detrital total-Pb monazite ages from the Baraboo, Hamilton Mounds and recently identified Seven Sisters quartzite (Medaris and Dott, 2005). Single spot monazite ages between 1852 and 1917 Ma in the Baraboo sequence compare well with a cluster of detrital zircons in that age range (peaks at 1850 and 1920 Ma; Van Wyck and Norman, 2004). The Hamilton Mounds quartzite contains geon 18 Penokean monazite grains with 2100-2000 Ma xenotime inclusions; the quartzite's depositional age is thus well-constrained between Penokean orogenesis and intrusion of a cross-cutting dike at 1760 Ma. In contrast, the overlying Seven Sisters quartzite contains detrital Penokean monazite grains overgrown by ca. 1775 Ma rims of monazite, confirming that it is younger than the underlying Hamilton Mounds quartzite. The absence of Archean monazite ages at this preliminary stage is consistent with a northerly provenance dominated by geon 18 & 17 widespread metamorphic events (Schneider et al., 2004). A metapelitic unit from the Waterloo quartzite yielded only minor detrital monazite ages. A majority of the total-Pb monazite analyses yielded age population peaks corresponding to geon 16 Mazatzal metamorphism and emplacement of the Wolf River batholith. The geon 14 total-Pb ages are consistent with a geon 14 Ar-Ar hornblende age from an amphibolite dike that cuts the quartzite.

Lastly, we report a 1740 Ma U-Pb ion microprobe age on monazite from a coarse-grained garnet staurolite schist near Hamburg, WI. This age documents geon 17 amphibolite facies metamorphism concurrent with 1750-1740 Ma magmatism in central Wisconsin. Although complex, the Wisconsin basement and quartzite cover rocks reveal a progressive younging and overall waning of Proterozoic metamorphism to the south (widespread geon 17 in the far north to aerially restricted geon 14 in the south).