Reconciling Garnet Lu–Hf and Sm–Nd and Monazite U–Pb Ages for a Prolonged Metamorphic Event, Northern New Mexico

Abstract

AbstractGarnet Sm–Nd and Lu–Hf isotopic data are increasingly used in tandem to characterize tectono-metamorphic events and their duration because the two chronometers may yield a progression from older Lu–Hf to younger Sm–Nd ages. In this contribution, we utilize garnet compositional zoning and monazite U–Pb ages to evaluate explanations for differences in garnet Sm–Nd and Lu–Hf ages for low- to mid-amphibolite facies rocks from the Picuris and Tusas Mountains, New Mexico. Ten Sm–Nd ages, eight Lu–Hf garnet ages (6 previously published), and four matrix monazite U–Pb dates span 1422–1370 Ma, 1456–1399 Ma, and 1377–1356 Ma, respectively. The weighted means of garnet ages are 1398 ± 11 Ma for Sm–Nd (N = 10/10) and 1402 ± 4 Ma for Lu–Hf (N = 6/8); however, Sm–Nd and Lu–Hf ages from three samples differ by 29, 34, and 55 my with differences between 2σ age envelopes of 12, 9, and 38 my., respectively. In these samples, primary Lu and Sm zoning in garnet, diffusion modification of zoning, or disequilibrium is sufficient to cause older Lu–Hf ages and younger Sm–Nd ages. Alternatively, two of the three Lu–Hf ages, c. 1450 Ma, may reflect the onset of a prolonged regional event or an earlier thermal event, and the three c. 1370 Ma garnet Sm–Nd ages may reflect diffusional re-equilibration. These three c. 1370 Ma ages are indistinguishable from the monazite ages, and garnet have trace element zoning consistent with diffusional alteration of growth zoning in Sm and Nd but not Lu and Hf. Garnet from the other seven samples have prograde zoning with little to no evidence for diffusional relaxation of zoning in these elements. We conclude that the majority of garnet preserve primary compositions and indistinguishable garnet ages of c. 1400 Ma, indicating a c. 22 my garnet growth history based on weighted mean. The two oldest c. 1450 Lu–Hf and three youngest c. 1370 Sm–Nd garnet ages require an earlier thermal history and modification of garnet composition, respectively. Results of this study redefine the youngest extent of amphibolite facies metamorphism during the Picuris orogeny to at least c. 1370 Ma.