CONTRASTING WHOLE-ROCK SM-ND AND ZIRCON LU-HF SYSTEMATICS IN HIGH-GRADE PALEOARCHEAN GNEISSES, BEARTOOTH MOUNTAINS, MT/WY

U-Pb systematics and trace elements were measured in zircons from three samples of tonalitic gneiss from the eastern Beartooth Mountains (MT/WY) using the SHRIMP RG ion microprobe. U-Pb data reveal that protoliths of the gneisses crystallized at ~3.5 Ga. Common traits include SiO₂ from 66-70 wt %, K₂O/Na₂O <0.5, and smooth, chondrite-normalized whole-rock REE patterns showing moderate LREE enrichment (La_n = 20-40x) and minimal Eu anomalies. Whole-rock isotopic systematics (Sm-Nd and U-Th-Pb), however, lead to very different interpretations for the geologic history of the respective protoliths. A magnetite-bearing biotite tonalitic gneiss (3492 +/- 9 Ma) from a sequence of layered gneisses yields an initial εNd of +1. Two xenolithic magnetite-, titanite-bearing biotite tonalitic gneiss samples within the 2.8 Ga Long Lake magmatic suite have crystallization ages of 3485 +/- 7 Ma and 3484 +/- 13 Ma, and they yield initial εNd of -10 and -13, respectively. Such low initial ratios can only be explained as (1) a consequence of derivation from a very ancient enriched source or (2) that Sm/Nd increased and LREEs were depleted during post-crystallization alteration or metamorphism. Increases in Sm/Nd during high-grade metamorphism are often difficult to constrain because they require detection of a general depletion of the LREEs of only a few percent that do not produce elemental anomalies. In these ancient rocks, however, the consequent Sm-Nd model ages (Tdm) are older than the earth, which argues against the LREE abundances being pristine and in favor of post-crystallization alteration. This interpretation is supported by initial εHf values from magmatic zircons, which range from (+4 to -2 εHf) and average 0 εHf in each case, analytically indistinguishable from the εNd value of the layered gneiss (+1). The HREEs in these zircons and their Ti-based crystallization temperatures appear to be preserved, suggesting that the HREE abundances in the zircons were not affected to the same degree as the LREEs in the whole rocks, if they were affected at all. These results suggest that initial εHf values derived from individual zircons are more likely to provide accurate records of crust-mantle interaction and magma sources in ancient, high-grade metamorphic terranes than the whole-rock Sm-Nd system.