On Ultra-High Temperature Metamorphism in the Mid-Lower Crust

Abstract

The Cortlandt Complex in New York is a composite intrusion of six mafic plutons and contains pelitic xenoliths that experienced extensive interaction with Mg-rich basaltic melt. The complex is an excellent natural example of ultra-high temperature (UHT) metamorphic processes and country rock-magma interaction due to mappable units of hybrid igneous rocks and the presence of large, partially melted, politic "emery" xenoliths. Previous attempts to understand the formation of the UHT xenoliths in the Cortlandt have provided the petrologic foundation for more rigorous thermodynamic modeling to determine the petrogenesis of these materials and to ultimately contribute to the understanding of UHT metamorphism in the Earth's crust.

This work focuses on the development of hybrid monzonorites and emery at Salt Hill, located in the southeasternmost edge of the Cortlandt Complex. First, a thermobarometric study focuses on the P-T conditions of the country rock into which the Complex intruded. Pelitic schists from contact aureoles around a nearby pluton chemically and chronologically related to the complex, record high-P (~ 0.9 GPa, ~ 32 km depth) crustal conditions during pluton emplacement. This is interpreted to reflect loading due to the emplacement of Taconic allochthons during the waning stages of regional metamorphism before emplacement of the plutons. The second study uses thermodynamic heating calculations of pelitic schist to determine the production of norite and emery. Modeling results produce (i) an initial melt that produces a monzonorite composition when mixed with a mafic melt, (ii) a high-T melt that is texturally and compositionally homologous with quartzofeldspathic veins retained in the emery, and (iii) a residual mineral assemblage that, when oxidized, closely resembles the emery assemblage.

Finally, focus is given to understanding the relationship between norite and emery and reflection on the mineralogy and structure of the lower crust-mantle boundary. Density calculations of the emery estimate values comparable to mantle densities, implying that rare exposure of UHT assemblages may be due to the fact the material stays at lower crustal (upper mantle?) depths. Therefore, the less-rare norite and other hybrid igneous rock occurrences may be the traces of deep, unexposed, UHT metamorphic assemblages.