UNCOVERING THE DEEPEST ROOTS OF THE MESOPROTEROZOIC PICURIS OROGENY: PRELIMINARY U/PB MONAZITE AGES AND P-T ESTIMATES FROM THE SIERRA ESTRELLA MOUNTAINS, ARIZONA, USA

The Sierra Estrella Mountains near Phoenix, Arizona expose a remarkable, deep crustal section of Mesoproterozoic, ca. 1.4 Ga, granulite facies rock (Meijer, 2019). We present preliminary P-T estimates, and new metamorphic monazite ages from the area and propose that these rocks generally correlate with crustal xenoliths from southern Arizona, southern New Mexico and the four–corners area of the southwest. This region of high-grade metamorphism likely extends to southern Colorado and northern New Mexico and represents a belt of significant crustal thickening across the southwestern U.S. associated with the 1.5–1.36 Ga Picuris Orogeny.

All metasedimentary samples from the Sierra Estrella show evidence of partial melting. In the north end of the range, metapelites preserves Grt-Bt-Pl-Qtz-Ky-Sil with significant retrograde Ms-Chl-Ep, and Grt-Bt-Pl-Qtz-Ky-Sil-Kfs with very little retrograde overprint. Reaction textures, mineral assemblage diagrams and quartz-inclusion in garnet (QuiG) geobarometry give preliminary P-T estimates of about 800 °C and 8-10 kbar. Metasedimentary rocks from the southern third of the mountain range preserve Pl-Kfs-Qtz-Bt-Sil with some retrograde Ms. Mineral assemblage diagrams indicate P-T conditions near 800 °C and 7-8 kbar.

Monazite grains in all samples vary in length from 30 to 100 µm and show no alteration to significant replacement by coronas of apatite + allanite/epidote. Preliminary, in-situ LA-ICP-MS U/Pb monazite dates from four samples range 1411 ± 5 to 1397 ± 7 Ma and reflect the timing of partial melting. Deformation overprints crystallization of the partial melts in all samples.