COMPARISON OF SM-ND AND LU-HF GARNET AGES II: AMPHIBOLITE FACIES ROCKS, PICURIS-TUSAS MOUNTAINS, NEW MEXICO

Garnet geochronology constrains the timing of garnet-growth and tectonic processes such as timescales of orogeny. Two isotopic systems are commonly employed: Sm-Nd and Lu-Hf. However, the 2 systems seldom yield the same age, presenting a problem for interpretations. Diffusion of parent and/or daughter elements and zoning of the parent element have been inferred as causes for the age discrepancies. In low to medium grade rocks with compositional zoning which is attributable to growth, diffusion cannot explain age differences. In these rocks, primary parent element zoning in garnet can skew ages toward the grain segments with highest concentrations. In the case of bulk garnet grain sampling, long durations of garnet growth or multiple thermal events leading to large core-rim age differences must also be considered. This study applies Sm-Nd garnet geochronology to pelites from the Picuris and Tusas Mountains in northern New Mexico for direct comparison with previously published Lu-Hf results.

We compare 2 samples of garnet-staurolite-biotite-muscovite schist, ca. 1 km apart in the Picuris Mountains: the Lu-Hf garnet age is 1400±3.4 Ma (Aronoff et al., 2016) and our new Sm-Nd age is 1351±6.2 Ma (5 points, MSWD: 1.07, probability: 0.36). Both ages are bulk multi-grain aliquots with no separation of core vs. rim. Garnet was separated by grain size for Sm-Nd, but not for the Lu-Hf analysis. The 50 m.y. gap is difficult to explain without knowledge of Sm, Nd, Lu, and Hf zoning in the garnet, which is currently in progress. However, prograde Mn zoning is preserved in the garnet, indicating that REE zoning is likely preserved. Additionally, peak metamorphic temperatures did not exceed 600°C. Together, these data discount diffusion as a cause for the 50 m.y. age difference. Therefore, we propose that the age difference is a result of 1) growth zoning of parent elements, 2) core-rim sampling bias of garnet with large core-rim age differences (either from long duration of growth or two growth events), or 3) disequilibrium of garnet with surrounding minerals. The Sm-Nd age presented here is the first of 8 planned ages. Seven additional samples are currently in progress and future results will determine if Sm-Nd ages are consistently younger than the Lu-Hf ages for this region in New Mexico and which of the aforementioned processes is the cause.