FORMATION OF CORUNDUM-BEARING RESIDUUM ASSEMBLAGES DURING RAPID CONTACT HEATING AND EXTRACTION OF HIGH-SI MELT FROM A PELITIC XENOLITH, CORTLANDT COMPLEX, NY

Emplacement of gabbroic magmas of the Cortlandt Complex, New York induced rapid heating of pelitic schist protoliths (T up to 1200°C, P of 0.8 GPa). Xenoliths entrained within the mafic melt experienced significant melting and melt segregation processes, now preserved as a series of complex textures comprised of Si-rich veins and Al-rich, Si-poor residua (a typical assemblage is sil-spl-crn-mag-ilm/hem). Aluminous orthopyroxene selvages typically have grown along the contact between the residuum and ‘melt’ (quartz + ternary feldspar) veins. Thermodynamic modeling has been used to track the evolution of the protolith schist upon heating, accurately predicting both melt and residuum compositions for equilibrium melting simulations. Modeling indicates that melt generation was too rapid to permit substantial fractional melt loss during heating. Additional modeling of cooling of the silicate melt predicts early orthopyroxene formation, with an orthopyroxene Al content consistent with that of analyzed selvage pyroxene (8-11 wt %). We propose that this pyroxene reflects a primary melt crystallization phase rather than a reaction margin and that its position is primarily due to nucleation along the melt–residuum interface.

Close examination of the residuum (both modeled and natural) reveals a series of reactions between magnetite and aluminous spinel during heating and subsequent cooling. These reactions appear to have stabilized Al-spinel at the expense of magnetite through increase in Al₂O₃:Fe₂O₃ ratio of the oxide phase during heating. Textural evidence shows a late exsolution breakdown of the hercynite component of Al-spinel to magnetite + corundum, probably during initial stages of cooling, and that this was controlled by oxygen fugacity. Formation of corundum + magnetite by this mechanism perturbs the modal amounts of phases predicted by the model and indicates the importance of carefully accounting for changing fO₂ when thermodynamically modeling residuum formation.