BULK ROCK COMPOSITIONAL CONTROLS ON METAMORPHIC MINERAL ASSEMBLAGE STABILITY: EXAMPLES FROM THE PICURIS RANGE, NM

Multiply deformed and metamorphosed Precambrian rocks from the southern limb of the Copper Hill anticline in the Picuris Range contain widely variable mineral assemblages. For example, Grt+Bt+St+Ms+Qtz, Ms+St+Bt+Qtz, and Bt+St+Ms+And+Ky+Qtz assemblages can be found within 4 km ² of the Vadito Schist. Although these mineral assemblages could result from variation in metamorphic grade, it is unlikely to vary appreciably in such a small area. Research focuses on confirming this hypothesis and determining the relationship between mineral assemblages and bulk rock composition. Pseudosections provide a tool to constrain the relationship between the mineral assemblages and rock composition. Compositions were determined by X-ray fluorescence spectroscopy (XRF). MnNaCaKFMASH pseudosections were constructed using bulk rock compositions, THERMOCALC (V3.1), and thermodynamic data of Holland & Powell (2001). Activity models are from Holland & Powell (1998) with extension to include Mn. Sample HS-7 is a Grt+St+Bt+Ms+Ilm+Pl+Qtz schist with abundant St, Bt, and Ms. Garnet core compositional isopleths indicate conditions of initial garnet core growth in the Grt+St+Bt+Ms+Chl+Pl+Qtz assemblage field at ca. 3.0 kbar and ca. 540ºC. Mineral assemblages predicted from the pseudosection are in reasonable agreement with observed mineral assemblages. The pseudosection predicts St stability over a wide P-T range of 520-650ºC and 2-12 kbar due to the high Al₂O₃concentration (26.42 OX WT%), and Ms stability over a wide P-T range due to high K₂O concentration (6.56 OX WT%). The low CaO concentration (0.02 OX WT%) results in absence of zoisite/clinozoisite and limited stability of plagioclase. Sample 00NM01 has unusually high SiO₂+Al₂O₃(93.10 OX WT%) and the paragenesis Qtz+And+St+Ctd+Ms+Par suggests metamorphism at 500-550ºC. In this temperature range, less than 1 mode % Pl and Chl are predicted by the pseudosection; however, they were not observed in the rock. Results above show that pseudosections may prove useful for modeling mineral assemblage stability and mineral stability dependence on bulk composition. Additional samples will be modeled in the future in order to more clearly understand the relationship between mineral assemblages and bulk composition.