MONAZITE CHEMICAL AGE DATING IN THE CLASSIC AL2SIO5 TRIPLE-POINT ROCKS, NORTHERN PICURIS MOUNTAINS, NEW MEXICO: EVIDENCE FOR ~1.4 GA REGIONAL METAMORPHISM

The northern Picuris Mountains preserve several exceptional occurrences of two and three coexisting Al₂SiO₅ polymorphs. Monazites are common within the aluminous quartzites (Ky-Sil-Ms-Cld-Plag-Qtz-Hem±St) and schists (Ky-Sil-And-Ms-Cld-Plag-Qtz-Hem) of the Ortega Formation. Chemical age determinations suggest that all three Al₂SiO₅ polymorphs formed near 1.4 Ga, contrary to some earlier models that proposed near triple-point metamorphic events with kyanite and sillimanite growth at both ~1.65 Ga and 1.4 Ga.

Within the quartzite, kyanite is replaced by sillimanite, both are aligned in S₂ and define an extension lineation. A schist layer within the quartzite contains cm-scale andalusite porphyroblasts in a muscovite-rich matrix. Inclusions of kyanite and sillimanite (both fibrolite and prismatic crystals) occur within andalusite. Sillimanite also occurs within the matrix and is intergrown with muscovite. Andalusite often pseudomorphs muscovite. Kyanite, sillimanite, and muscovite are aligned in S₂, which is overgrown by the andalusite porphyroblasts. These textures suggest that kyanite is the earliest of the three polymorphs followed by sillimanite, and finally, andalusite. Monazite crystals occur within the matrix and as inclusions within andalusite, and more rarely, kyanite.

X-ray maps (U-Th-Pb-Y) show variable zoning patterns within monazite. Many grains show relatively high U, Th and Pb concentrations in the center and lower concentrations near the rims. Other grains within the same samples show little zoning in U, Th or Pb. Y zoning is variable with some grains showing high Y rims and others showing high Y cores. Regardless of the chemical zoning patterns within a grain, no distinct age domains were measured, within the precision of the analyses. Twenty-two matrix monazite and monazite inclusions within andalusite and kyanite yielded age distributions between ~1.1 and 1.5 Ga with maximums near 1.4 Ga. Uncertainties are relatively large, but the lack of ~1.65 monazite grains or even cores strongly suggests only one amphibolite facies regional metamorphic event at ~1.4 Ga. Many monazite crystals overgrow foliated Fe-oxides suggesting the presence of pre- or syn-1.4 Ga deformational fabrics.