Thermodynamic modeling of crustal melting using xenolith analogs from the Cortlandt Complex, New York, USA

Abstract

Emplacement of gabbroic magmas of the Cortlandt Complex, New York, induced rapid (less than 5 years) heating of pelitic schist protoliths (up to ~1200°C at ~0.9 GPa). Xenoliths entrained within the mafic melt experienced significant melting and melt-segregation, now represented as a series of complex fabrics and structures comprising Si-rich veins and an Al-rich, Si-poor residuum (a typical assemblage is spinel-magnetite-ilmeno-hematite-sillimanite ± sapphirine ± corundum). Subtle microscopic textures in the residuum include corundum-magnetite symplectites, which are interpreted to be a result of oxidative breakdown of the hercynite component in spinel during cooling. Aluminous orthopyroxene selvages in veins have typically grown along the contact between the residuum and quartzofeldspathic 'melt'. Hybrid monzonorite and monzodiorite crop out near the xenoliths and are interpreted to represent assimilation by the mafic magma of some of the partial melt produced from the pelitic xenoliths. Equilibrium-melting and batch-melting thermodynamic models track the evolution of the pelitic schist, its partial melt upon heating, and the residuum from melting and melt extraction. We introduce a 'filter-pressing' cooling calculation to simulate the crystallization of the quartzofeldspathic veins. Modeling results yield the following: (1) an initial partial melt that, when mixed with the estimated composition of the mafic melt, produces a hybrid igneous rock consistent with the monzonorite found near the xenolith; (2) a high-T melt that upon 'filter-pressing' crystallization produces a mineral assemblage that texturally and compositionally corresponds to the quartzofeldspathic veinlets retained in the samples within xenolith interiors; (3) a residual material that, when oxidized, resembles the aluminous assemblages in the residuum. Modeling of crystallization of the high-T melt predicts early orthopyroxene formation, with the Al content of orthopyroxene consistent with that of analyzed selvage pyroxene. We propose that this pyroxene reflects a primary melt crystallization phase rather than reaction-rim margins of the veins against residual matrix.