GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 108-12
Presentation Time: 11:15 AM


KELLY, Eric D. and ASHLEY, Kyle T., The Jackson School of Geosciences, The Unversity of Texas at Austin, Austin, TX 78712,

We applied thermodynamic modeling and quartz-in-garnet (QuiG) Raman barometry to constrain a pressure-temperature (P-T) path from a graphitic garnet-bearing metapelite collected from the Rinconada Fm in the Picuris Mountains (PM), New Mexico. Contributions by Hollister to advances in garnet chemical zoning and aluminosilicate triple-point conditions are routinely exploited in rocks from the PM. Here, we address challenges of chemical disequilibrium and produce a P-T path to improve the framework of the Picuris Orogeny and to enhance further studies of metamorphism.

Using local matrix (Qz-Pl-Bt-Ilm) surrounding a garnet with the highest core XMn, we calculated a bulk composition and a phase diagram (Theriak-Domino, MnNCKFMASHT, aH2O=0.5–0.7) to fit garnet crystallization constraints: (1) low nucleation T (~450 °C) as predicted for high XMn in garnet core; (2) protracted crystallization (to ~530 ± 25 °C) as required to produce trace-element zoning patterns; (3) biotite stability midway through garnet growth as shown by inclusions only found in garnet mantles and rims; and (4) loss of chlorite midway through garnet growth and less than ~530 °C as suggested by lack of chlorite in rock, peak garnet rim T (530 ± 25 °C), and slowed rate of garnet crystallization (change in reactants) in numerical simulations of this sample. Garnet core isopleths of XFe and XMn have tightly spaced, nearly isothermal contours, and are least affected by Ca disequilibrium due to strong Mn partitioning and high XFe. Isopleths bracket core composition to 430–460 °C over H2O activity range. Ca disequilibrium recorded in garnet zoning has impeded classical barometry in PM rocks, especially for early and middle portions of garnet crystallization. However, preliminary QuiG barometry on quartz inclusions within garnet mantles yields increase and decrease in P of 1–2 kbar along radial traverse. Without core and rim results, the full range of P changes cannot be assessed, but should be larger.

We propose a prograde P-T path spanning garnet crystallization beginning from 430–460 °C and 3–4 kbar, rising by 1–2 kbar (or possibly 2–3 kbar) during increasing T, and ending near the triple point of 530 °C and 4 kbar. This path will enhance investigations of kinetics of metamorphism and provide new constraints on the Picuris Orogeny.