EPISODES OF MONAZITE GROWTH FROM THE HOMESTAKE IRON FORMATION, LEAD, SOUTH DAKOTA, U.S.A

Several generations of Proterozoic monazite from the Homestake Iron Formation in the Black Hills reveal different textures and correspond to distinct tectonometamorphic events. Monazite was investigated from samples taken across the Homestake mine, now the site of the Deep Underground Science and Engineering Laboratory (DUSEL), which represent a range of metamorphic conditions. Monazite was identified with petrographic and scanning electron microscopy, then X-ray chemical maps were generated, and the resulting identified domains were U-Th-Pb dated with the Ultrachron electron microprobe. Abundant grains are associated with ~1775 Ma Yavapai collision and the ~1750 Ma southern Trans-Hudson Orogen (THO) events. In one thin section, these ages occur in small grains with high-Y cores and low-Y rims, or large complex grains displaying irregular Th patterns. A reaction from allanite to monazite is preserved in garnet, yielding an age of 1757 ± 30 Ma. Another complex grain contains an older, ~1850 Ma high-Y core within a low-Y D2-age rim. Textures become well-zoned in Th and Y in monazite from another section, which also gives these D1 and D2 ages, along with a younger ~1670 Ma age. This age is repeated in another grain and may correspond to regional D5 deformation. Monazite from a lower-grade, more westerly sample contains such low-Th cores that are undatable at present, but some display zonation patterns within cores which may indicate more than one episode of growth. Thin, higher-Th rims allow dating and yield ~1300 and ~1200 Ma ages that are uniquely young for Black Hills monazite. These may be due to formation in the lower-temperature stability range of monazite that occurred during the slow cooling of the Black Hills.