DISEQUILIBRIUM CRYSTALLIZATION OF GARNET CORES FOLLOWED BY EQUILIBRIUM RIM GROWTH: IMPLICATIONS FOR DETERMINING PRESSURE-TEMPERATURE PATHS, ALBION MOUNTAINS, IDAHO

Disequilibrium nucleation and growth of garnet may be followed by equilibrium growth that results from and records changes in pressure (P) and temperature (T). By distinguishing disequilibrium growth from equilibrium growth, equilibrium-based models can be used to construct P-T paths, and tectonic interpretations can be made.

We studied garnet from amphibolite facies pelitic schist with inclusion-rich cores and inclusion-free rims. Core zoning profiles for Fe, Mg, Mn, and Ca are nearly flat, whereas rims display normal Mn growth zoning resulting from Rayleigh fractionation. We propose a three-stage growth history with initial disequilibrium growth of the inclusion-rich core by chlorite breakdown, followed by partial consumption of garnet during a staurolite-forming reaction, and finally equilibrium growth of the rim by the breakdown of staurolite. We interpret the flat Mn zoning as evidence that the cores grew too rapidly to maintain chemical equilibrium between reactants and products with respect to Mn, whereas the rims grew slowly and in equilibrium with reactant phases. The rapid core growth was likely due to inhibited nucleation and reaction overstepping. The flatness of the Mn profile in the cores may be explained by a limited rate of intergranular Mn diffusion in conjunction with a progressive decrease in the garnet growth rate due to the decrease in reaction affinity that occurred as garnet growth rectified the overstep.

Preservation of equilibrium growth zoning is a fundamental requirement of thermodynamic models used to simulate chemical zoning in garnet. Because the cores grew out of equilibrium, P-T paths could not be extracted from them; however, P-T paths were extracted from the rims using numerical thermodynamic modeling that simulates garnet growth zoning profiles and intracrystalline diffusion. All samples in this study are from the schist of Willow Creek in the northern Albion Mountains of southern Idaho, from the hanging wall of a major Sevier-age thrust, the Basin-Elba fault. The P-T paths are distinctly different from those obtained nearby in the footwall, which suggests thrusting post-dated garnet growth in the hanging wall. The P-T paths indicate about 0.35 GPa of pressure increase from about 520 to 580 °C documenting major crustal thickening in the hinterland of the Sevier orogen.