LU-HF GARNET GEOCHRONOLOGY AND MICROSTRUCTURAL ANALYSIS CONSTRAIN THE TIMING OF CRUSTAL ASSEMBLY IN SOUTHWESTERN NORTH AMERICA

We integrated Lu-Hf garnet dating and microstructural analysis in Proterozoic rocks of northern New Mexico to constrain the timing of the Proterozoic tectonic assembly of North America. The Lu-Hf system dates the onset of garnet growth, and therefore of amphibolite facies metamorphism. When combined with microstructural studies, these metamorphic growth ages fill an important gap in knowledge between igneous zircon crystalization ages and argon cooling ages.

Four garnet-bearing samples from the Vadito Group of the Tusas and Picuris Mountains all yield well-constrained ages between 1.45 and 1.40 Ga; a garnet-biotite schist from the central Tusas (11TU4A) yields an age of 1405.3 ± 4.4 Ma (MSWD = 1.4); a garnet-sillimanite schist from the southern Tusas (11TU9C) yields an age of 1399 ± 9 Ma (MSWD = 0.6); a garnet-staurolite schist from the southern Tusas (11TU10C) yields an age of 1449.8 ± 5.6 Ma (MSWD = 0.46); and finally a garnet-bearing metarhyolite from 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 central Tusas (11TU4B and 11TU8A) yield ages of 1409 ± 14 Ma (MSWD = 5.1) and 1419 ± 19 Ma (MSWD = 4.4) respectively.

Based on thin section observations, garnet growth is interpreted to be intratectonic between D2 and D3 deformation events. Peak metamorphic conditions vary across the study area, but are consistent with amphibolite to upper amphibolite facies conditions (see Hunter et al., this meeting). However, peak metamorphic temperatures are interpreted to be lower than the closure temperatures for the Lu-Hf isotopic system.

These rocks show no evidence for earlier circa 1800 – 1600 Ma garnet growth, which is the timing of the Yavapai and Mazatzal orogenies in southwestern Laurentia. Lu-Hf ages show that amphibolite facies metamorphism occurred in the Tusas and Picuris mountains circa 1.4 Ga. 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 Mesoproterozoic metamorphic event in southwestern Laurentia. This integrated study refines our understanding of the timing of assembly of the North American continent.