GARNET SM-ND AND LU-HF AGES FROM THE PICURIS OROGENY: UNDERSTANDING THEIR MEANING USING TRACE ELEMENT ZONING

The two most common garnet geochronometers, Lu-Hf & Sm-Nd, are used to date and interpret orogenic events. However, Lu-Hf dates are often older than Sm-Nd dates for the same sample, which causes uncertainty with interpretations. The age difference is often attributed to growth zoning or diffusion of Lu, Hf, Sm, or Nd. We present EPMA and LA-ICPMS data across garnet grains and 7 new garnet Sm-Nd ages that are 50-80 m.y. younger than published Lu-Hf ages for the same/similar samples from the Picuris-Tusas Mountains, northern New Mexico.

There are two populations of Lu-Hf ages separated by the Plomo-Pecos fault: 1400 Ma north and 1450 Ma south. Sm-Nd ages (±2s) north of the fault are 1351±6 and 1355±7 Ma, while samples south are 1389±5, 1377±7, 1386±8, 1380±11, and 1391±17 Ma. Ages south of the fault are similar to published monazite ages (1380-1360 Ma). The garnet age differences between the two systems could result from 1) diffusive re-equilibration of Sm and Nd, 2) isotopic disequilibrium between garnet and other phases, or 3) core-rim sampling bias of growth zoned parent elements.

EPMA data indicate garnet has decreasing Mn and increasing Mg# from core to rim in all but the 1380±11 Ma sample. We interpret all but this sample to preserve growth zoning. We infer little diffusion because only the outer 100 um of garnet grains show an increase in Mn and the regional metamorphic temperatures reached ≤600°C. LA-ICPMS data indicate 3 types of Lu zoning: 1) high Lu core, low Lu rim, 2) low Lu core, higher Lu mantle, lower Lu rim, and 3) low Lu core and high Lu rim. Additionally, LA-ICPMS data indicate 2 types of Sm and Nd zoning: 1) decreasing Sm and Nd from core to rim and 2) increasing Sm from core to rim, unzoned Nd. Sharp trace-element zoning, growth zoning of major elements, and slower diffusion of REE and Hf than major elements preclude significant diffusion of Lu, Hf, Sm, and Nd.

We dismiss diffusion because prograde and sharp Lu and Sm zoning are preserved, and temperatures were ≤600°C. Disequilibrium cannot be fully discounted; a few samples show resorption textures and fluid alteration. However, two age populations separated by a fault make this unlikely. We favor the third option: Lu-Hf ages reflect older high Lu core bias and Sm-Nd ages reflect younger high Sm rims. In this case, garnet grew for 50-80 m.y., extending the duration of the Picuris orogeny.