USING LU–HF GARNET GEOCHRONOLOGY AND INVERSE PHASE EQUILIBRIA MODELING TO DECODE THE PROGRADE P–T–T PATH OF DEEP CRUSTAL MIGMATITES

It is generally difficult to determine the prograde P–T–t path of deep crustal migmatites due to changes in composition as a consequence of loss of melt and dissolution of the accessory minerals commonly used for U–Pb geochronology. Indeed, P–T pseudosections constructed for melt-depleted bulk rock compositions may only be used to determine peak P–T conditions and the retrograde evolution. Furthermore, monazite and zircon rarely yield information that may be reliably linked to the prograde path; these minerals most commonly record ages that relate to growth from melt trapped during cooling. In contrast, direct isotopic dating of the major metamorphic minerals, such as garnet, avoids the ambiguity involved in linking growth of accessory minerals to the time of growth or breakdown of the major minerals, provided the isotope system was not reset during the remainder of the P–T evolution or subsequently.

In this study, Lu–Hf geochronology of peritectic garnet is coupled with inverse phase equilibria modeling of open system melting to decode the prograde P–T–t path for migmatitic paragneisses and orthogneisses from the Fosdick migmatite–granite complex in West Antarctica. In inverse modeling, melt is reintegrated into the residual bulk chemical composition in a stepwise fashion along a credible prograde P–T path to generate a plausible sub-solidus composition. This procedure generates a series of P–T pseudosections that may be used to provide constraints on the prograde P–T path. Garnet from across the Fosdick complex yields Lu–Hf ages of 115–111 Ma. The melt-reintegrated pseudosections show the onset of garnet growth at ~800°C between 0.6 and 1.0 GPa up to the estimated peak P–T of 830–870°C at 0.6–0.75 GPa. Sm–Nd garnet ages of 102–99 Ma from a subset of the same samples overlap the range of U–Pb monazite ages from these rocks that are interpreted to date growth as melt crystallized during cooling from 111 to 96 Ma. We interpret the Sm–Nd garnet ages to have been modified by diffusion-controlled re-equilibration during cooling. The results of this study demonstrate that the prograde P–T–t path of deep crustal migmatites may be successfully determined by dating garnet using an isotope system that is unlikely to have been reset after growth in combination with inverse phase equilibria modeling of open system melting.