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Julian Cohen

Estimating the Pressure and Temperature of Metamorphism of ca. 1.4 Ga Metapelitic Rocks from the Sierra Estrella Range, Phoenix, Arizona USA


Julian Cohen ’21


Faculty Mentor(s):

Christopher Daniel, Geology

Funding Source:



Meijer (2019) reported what is likely the first known occurrence of Mesoproterozoic granulite facies rocks in Arizona. However, the temperatures and pressures of metamorphism are unknown. 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, and Grt-Bt-Pl-Qtz-Ky-Sil-Kfs with very little retrograde overprint.
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. A significant amount of retrograde Ms in sample implies in-situ melt crystallization during retrograde cooling.
To further constrain pressures of metamorphism, Raman spectroscopy was employed to measure peak positions of quartz inclusions in garnet (QuiG). The results of this work show peak shifts ranging from 0 to -0.1 cm-1. This equates to metamorphic pressures of 8-9 kbar at about 750-800 °C.
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.

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