EVALUATING THERMODYNAMIC MODELS FOR AMPHIBOLITES: PERPLE_X MODELING OF VADITO GROUP ROCKS FROM CERRO COLORADO, NEW MEXICO

The Vadito Group in Cerro Colorado (NM) consists of Proterozoic interlayered schists, quartzites and amphibolites. Two pressure-temperature paths for the metapelites have been proposed: isobaric heating to peak conditions of 3-4 kbar and 650°C (Bishop, 1997); or, decompression from 6-4 kbar at temperatures between 550-600°C (Barnhart et al., 2012). The spatial relationships between interlayered metapelites that have well constrained P-T estimates, and amphibolites in Cerro Colorado permits exploration of parameters controlling accuracy of thermodynamic modeling of amphibolites.

Vadito Group amphibolites assemblages are Hb + Pl + Mt ± Qz ± Ep ± Gt ± Bt ± Chl ± Tur. Hornblende compositions range from magnesiohornblende to tschermakite, plagioclase is An_34-97, and garnet is Pyp_6-14Alm_40-60Sps_20-30Grs_5-21. Textural observations of disequilibrium include Mn-enriched rims and serrated edges on garnet, chloritization of biotite, sericitization of plagioclase, and isolated dravite porphyroblasts.

Textural evidence, XRF bulk rock data, and major element mineral analyses were used to establish system compositions to model P-T conditions in Perple_X. Amphibolite temperatures from thermodynamic models range between 575°C and 700°C with pressures up to 6.5 kbar; most of these temperatures are within range of estimates from metapelites, but some are up to 100°C higher than estimates from Barnhart et al. (2012) for metapelites. Garnet-bearing amphibolite samples constrain pressures to between 5.5 and 6.5 kbar, while biotite garnet amphibolite have pressures between 5.75 and 6.3 kbar, which is consistent with peak pressures prior to decompression proposed by Barnhart et al. (2012).

Iterative modeling of different system components, including H₂O and Fe³⁺ produced models that converged upon P-T estimates that were more consistent with metapelites and less likely to over-estimate temperature. Initial H₂O was set to 2 wt%, but when adjusted for individual samples to reflect water saturation based on mineral compositions and abundances (not over saturation) P-T estimates were 575-650°C at pressures of 4-6 kbar for model rock H₂O contents between 1.3 and 2 wt%. Fe³⁺/Fe^tot ratios between 0.17-0.42 for amphibole, which are consistent with empirical estimates of Fe³⁺ abundances in amphibole, also resulted in P-T estimates consistent with those of Barnhart et al. (2012).