Thermal Structure of the Central New England Acadian-to-Alleghenian Metamorphic High, Through Regional Correlation of Accessory-Phase Paragenesis

Rensselaer Polytechnic Institute, Troy NY

Investigators

Abstract

One of the puzzling questions in studying the evolution of Earth's crust, and the metamorphism of rocks produced during mountain-building episodes, is "what is the heat source for very high temperature metamorphism?", especially for metamorphic rocks that locally show evidence for melting, and attainment of significantly higher temperatures than the surrounding metamorphic rocks. In such high-temperature metamorphic rocks, The PI and his co-workers have discovered that the radioactive phosphate mineral monazite undergoes 3 or 4 growth reactions in a single metamorphic event. Using thermodynamic, textural, and compositional studies, the PI and co-workers link each generation of monazite with a reaction, a temperature, and a date of formation. For multiple episodes of monazite growth, the PI and co-workers extract pressure (P)-Temperature (T)-time (t)-paths. And these P-T-t paths are compared to P-T-t paths generated from numerical models for a variety of heat sources, such as pluton intrusion, asthenospheric underplating, or exothermic reactions. The work of the PI, to date, shows that high-grade metamorphism in central New England is not linked, in many cases, to adjacent granitic plutons, as the metamorphism post-dates the pluton emplacement by 40-50 million years. P-T-t paths derived from study of monazite closely match numeric P-T-t paths modeled assuming lithospheric delamination, and asthenospheric underplating, beneath New England at about 400 million years before the present. This interpretation agrees with independent geophysical evidence for lithospheric delamination beneath present-day central New England. The PI and co-workers are extending this methodology to high-grade, partially melted metamorphic rocks along the length of New England Appalachians, and are currently examining samples from SW Maine, Central New Hampshire, S New Hampshire, and Central Massachusetts to provided a more complete picture of high-grade Paleozoic (400-300 million years before present) metamorphism, and the heat sources responsible, in the Northern U.S. Appalachians. This project represents one of the first attempts to correlate accessory phase growth on an orogen-wide scale. Correlation of monazite growth events from location to location are used in a manner analogous to stratigraphic correlation. Under the assumption that monazite rims are grown at the peak of metamorphism (during melting or melt crystallization), precise monazite rim ages provide a means to track the peak of metamorphism down the length of the orogen. Lateral (E-W) variation in monazite rim ages may record collisional or extensional vergence along the orogen. A positive correlation between pressure and peak monazite age (e.g., deeper, older peak monazite) may indicate a strong mantle heating component. Integration of growth histories, thermometry, and dating of monazite on a regional scale provides a remarkably robust tool to aid in reconstructing the thermal evolution of large-scale orogens.