MELTING OF METAPELITIC XENOLITHS IN MAFIC MAGMA, MELT REMOVAL, AND RESIDUUM FORMATION AT 0.8 GPA

“Emery” deposits in the Cortlandt Complex, New York are Al-rich, Si-depleted rocks that formed by partial melting of pelitic schist xenoliths rapidly heated (~100 years) in olivine gabbro magma to T ≥ 1200°C at P of 0.8 GPa. Rapid heating produced disequilibrium decomposition of the hydrous minerals in the schist and induced significant fluid-present melting, soon followed by fluid absent melting, resulting in subdivision of originally homogeneous protolith into two fractionated products: peraluminous siliceous melt and reactive residual aluminous matrix (sil-spl-crn-mag-ilm/hem). Simple modeling suggests initial melting approximately at the crn- and H₂O-saturated eutectic in the Qtz-Plg-Kfs ternary system. Following depletion of Plg from the residuum, the melt composition tracked along the Qtz-Kfs cotectic, ending with dissolution of SiO₂ into the melt phase after depletion of all Kfs component from the matrix. The final melt composition, as represented by trapped microveins in the xenoliths, was highly siliceous, with >90 wt% SiO₂ and 80+ modal% qtz. Secondary silicate phases (sapphirine, Al-orthopyroxene, garnet, and sillimanite) were produced in aluminous residuum where aSiO₂ was raised adjacent to siliceous microveins. Using the phase equilibria modeling program Perple_X, a series of computer experiments thermodynamically modeled the partial melting of the protolith schist and concurrent generation of non-melted residual material in order to determine the petrogenesis of the residual aluminous matrix. Batch and fractional melt removal experiments were conducted at a constant pressure (0.8 GPa) using an averaged bulk composition of the schist protolith. Preliminary results show that, at temperatures in excess of 1200°C, the batch melt removal method produces a siliceous melt compositionally similar to the microveins and a residual aluminous assemblage closely resembling the emery. The continuous melt removal method, although producing a siliceous melt, forms a residual material assemblage with silicate phases not present in the natural emery composition. This work is the first step in determining the approach to equilibrium for the set of silication reactions that took place between the siliceous melt and the aluminous restite.