EVIDENCE FOR A CIRCA 1.4 GA METAMORPHIC EVENT FROM LU-HF GARNET GEOCHRONOLOGY IN THE TUSAS AND PICURIS MOUNTAINS, NORTHERN NEW MEXICO, USA

We performed radiogenic Lu-Hf garnet dating in Proterozoic supracrustal rocks exposed in the Tusas and Picuris mountains of northern New Mexico to constrain the timing of the Proterozoic tectonic assembly of North America. The Lu-Hf isotope system dates onset of garnet growth, and therefore of amphibolite facies metamorphism. Four garnet-bearing samples from the Vadito Group all yield well-constrained ages between 1.45 and 1.40 Ga; a garnet-biotite schist from the Vadito Group in the central Tusas (11TU4A) yields an age of 1405.3 ± 4.4 Ma (MSWD = 1.4); a garnet-sillimanite schist from the Vadito Group in the southern Tusas at Cerro Colorado (11TU9C) yields an age of 1399 ± 9 Ma (MSWD = 0.6); a garnet-staurolite schist from the Vadito Group at Cerro Colorado (11TU10C) yields an age of 1449.8 ± 5.6 Ma (MSWD = 0.46); and finally a garnet-bearing metarhyolite from the Vadito Group in the Picuris (11PIC21) yields an age of 1456 ± 16 Ma (MSWD = 3.2).

Two additional samples yield dates that are geologically consistent with the dates above, but with somewhat larger errors and MSWD values. Two garnet-biotite schists from the Vadito Group in the central Tusas (11TU4B and 11TU8A) yield ages of 1409 ± 14 Ma (MSWD = 5.1) and 1419 ± 19 Ma (MSWD = 4.4) respectively.

In total, these rocks show no evidence for earlier circa 1800 – 1600 Ma garnet growth, which is the age of Yavapai-Mazatzal orogeny in southwestern Laurentia. Based on thin section observations, garnet growth is interpreted to be intratectonic between D2 and D3 deformation events. Typical peak metamorphic mineral assemblages in the samples are staurolite, biotite, and garnet (see Hunter et al., this meeting). This assemblage is consistent with peak metamorphic temperatures between 550 and 600° C.

Lu-Hf ages show that prograde amphibolite facies metamorphism occurred in the Tusas and Picuris mountains circa 1.4 Ga. There is no evidence in these rocks for multiple Proterozoic metamorphic events in northern New Mexico. D1 and D2 fabrics developed prior to garnet growth, and therefore at lower metamorphic grade. The age of garnet growth in the Picuris and Tusas mountains provides further evidence for a widespread Mesoproterozoic prograde metamorphic event in southwestern North America.