INSIGHTS INTO THE HOTTEST ROCKS OF THE PICURIS OROGENY: USING EQUILIBRIUM MINERAL ASSEMBLAGE DIAGRAMS TO UNDERSTAND P-T CONDITIONS AND PARTIAL MELTING OF CA. 1.4 GA MESOPROTEROZOIC METAMORPHIC ROCKS IN THE SOUTHERN SIERRA ESTRELLA MOUNTAINS, AZ

The Sierra Estrella Mountains near Phoenix, Arizona is a 32 km-long fault bounded uplift within the Basin and Range Province with a sequence of plutonic and supracrustal rocks that record ca. 1.4 Ga regional metamorphism and deformation associated with the Mesoproterozoic Picuris Orogeny.

Preliminary P-T estimates for these rocks record some of the highest metamorphic temperatures and pressures in the southwestern US. A sample of a migmatitic biotite-amphibolite in the southern end of the range preserves a Hbl-Pl-Bt-Ilm-Qtz assemblage with no retrograde overprint. Leucosomes crosscut a well-developed foliation defined by aligned Hbl, Pl, and Bt. Evidence of partial melting comes from schlieren and net-textured leucosomes seen in outcrop; based on leucosome volume percent, it is estimated that this sample produced about 5% melt. To better understand melting reactions and conditions of metamorphism, new P-T estimates were made from an equilibrium mineral assemblage diagram, which was created with Theriak-Domino v. 2023.03.06 using the ds62-mb50-v07 dataset for metabasalts with bulk composition data obtained via electron microprobe quantitative mapping of a thin section from the sample. Preliminary results indicate H2O-absent melting occurring between approximately 700-770 °C and 5.5-7.5 kbar.

These results are lower than previous P-T estimates from the northern part of the range, which yielded metamorphic conditions between 780-870 °C and 7.5-9 kbar for granulites with Grt-Bt-Pl-Qtz-Ky-Sil-Kfs assemblages. Previous work has also produced in-situ, LA-ICP-MS monazite dates of approximately 1.4 Ga from multiple samples across the range.

This 1.4 Ga metamorphism is consistent with crustal thickening and regional metamorphism associated with the Picuris Orogeny in the southwestern United States, which is thought to represent the southwestern portion of a larger belt of crustal thickening, metamorphism, and deformation that extends northeast across the U.S. and into eastern Canada.