REFINING THE PROTEROZOIC HISTORY OF NORTH AMERICA THROUGH COMBINED USE OF LU-HF GEOCRHONOLOGY AND PETROLOGIC ANALYSIS

Proterozoic rocks of northern New Mexico have been interpreted to record a polymetamorphic history that produced “Al₂SiO₅ triple point assemblages”. Here we present new microstructural observations coupled with Lu/Hf geochronology that show the triple point assemblages record a single metamorphic event between 1450 and 1400 Ma, not polymetamorphism.

Garnets collected across the kyanite, sillimanite and andalusite metamorphic zones in the Tusas and Picuris Mountains are interpreted to be intratectonic between D₂ and D₃ deformation events. Garnets overgrow inclusions that are continuous from core to rim, and curve outside the garnets into the matrix foliation. This shows that the rocks were folded twice prior to garnet growth, and then after garnet growth. Textures allow that garnet maybe early syntectonic with the D₃.

In the kyanite zone of the Tusas Mountains, garnet bearing schists have the peak metamorphic assemblage garnet+muscovite+quartz+chlorite+biotite±staurolite. Kyanite is absent from schistose rocks, but is abundant in aluminous quartzite where it occurs with muscovite+quartz+chloritiod. These mineral assemblages are consistent with temperatures near 500 ^o C and pressures near or above the Al2SiO5 triple point.

In the sillimanite zone of the southern Tusas Mountains schists contain garnet-biotite-muscovite-quartz-plagioclase with skeletal and embayed staurolite. Garnet in many cases is rimmed by sillimanite and biotite, and isolated from matrix muscovite. Kyanite is locally preserved and partially to completely replaced by sillimanite. Textures have been interpreted to indicate decompression at temperatures above the Al₂SiO₅ triple point.

In the andalusite zone of the Picuris range rocks contain garnet+muscovite+quartz+plagioclase+biotite+K-feldspar. Pressure and temperature is not well constrained in these rocks. However, nearby andalusite-corderite schists have textures consistent with isothermal heating and cooling at pressures near 3 kbar (Wingstead, 1997).

Geochronologic constraints require that metamorphism occurred between 1450 and 1400 Ma. Overlap in the Lu/Hf garnet ages suggests that the different metamorphic zones developed in a single progressive metamorphism, and do not represent superposition of distinct metamorphic events separated in time.