A DEEPER LOOK AT THE MESOPROTEROZOIC CA. 1.4 GA PICURIS OROGENY IN ARIZONA, USA: OBSERVATIONS FROM GRANULITE FACIES METAMORPHIC ROCKS IN THE SIERRA ESTRELLA MOUNTAINS

Meijer (2019) reported what is likely the first known occurrence of Mesoproterozoic granulite facies rocks in Arizona. To better constrain the conditions of metamorphism, metapelitic rocks were collected from the north end of the Sierra Estrella mountain range and are characterized by Grt-Bt-Pl-Qtz-Ky-Sil with significant retrograde Ms-Chl-Ep (sample R), and Grt-Bt-Pl-Qtz-Ky-Sil-Kfs with very little retrograde overprint (sample W). Both samples contain mm-scale Grt with melt inclusion textures, aligned Sil fibers, and Ky inclusions. Garnet in both samples show significant resorption and fracturing. For sample R, the matrix contains abundant Ky and a few grains of prismatic Sil; Ky-Bt-Pl fills in Grt fractures and partially rims Grt. Sample W shows distinct leucosomes plus Sil replacing Ky and apparent melt films of Pl and Kfs in the matrix. Deformation overprints crystallization of the partial melts.

Preliminary thermodynamic modeling of these two samples was performed with Theriak–Domino and the H&P ’98 data set. The Sil inclusions within Grt suggest Sil growth possibly during subsolidus St-breakdown or Ms-melting in both samples. The presence of Ky bearing melt textures in both samples may reflect prograde growth of Ky from Ms- or Bt-melting suggesting minimum P­–T conditions > 7.5 kbar and > 700 °C. The replacement of Grt by Ky-Bt-Pl indicates melt crystallization at P > 7.5 kbar for sample R. A significant amount of retrograde Ms in sample R implies in-situ melt crystallization during retrograde cooling.

In sample W, the preservation of Kfs in the matrix and the lack of retrograde Ms, suggests cooling followed melt extraction at P above Crd stability (5-6 kbar). The preserved peak T assemblage yields modeled P­–T constraints of 780–870 °C at 7.5–9 kbar with retrogression into the Sil+melt field.

Previously reported zircon ages for the area suggest partial melting near 1.4 Ga (Meijer, 2019). The timing and P-T conditions of metamorphism in the Sierra Estrella mountains are similar to high-grade metamorphic rocks in northern New Mexico and southern Colorado; these rocks likely represent the southwestern extension of the Picuris orogenic belt.