Adirondack Thermal History

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

The Adirondack Mountains, New York are an outlier of the Grenville Province that stretches from Texas to Labrador. This terrain represents the deeply eroded roots of a vast mountain belt that existed 1200 to 1000 million years ago. Like the Himalaya today, these mountains were subjected to intense heating and pressure, followed by erosion, allowing rocks formed deep in the Earth?s crust to cool and rise to the surface, accounting for the topography, ore deposits, and recreational potential of the region. This study will apply newly developed techniques of geochemistry to determine the thermal and fluid history that formed these rocks. This study will test the results of a pilot study in a range of rock types and localities in the Adirondacks. That first study proposed that surprisingly rapid cooling and uplift occurred in the NW Adirondacks along a major mylonite zone that extends over 300 km (Carthage-Colton, Bonamici et al. 2011). The new results will test this model, constrain the peak metamorphic temperatures, and assess the significance of the concentric ?bulls-eye? pattern of isotherms that has classically been interpreted to result from domical uplift. Cooling and uplift rates will be combined with on-going studies of U-Pb geochronology to test competing theories for uplift of the Adirondack Highlands at the end of the Ottawan Orogeny (~1050 Ma). This project will develop and apply a new approach to studies of oxygen isotope thermometry and speedometry in high-grade metamorphic rocks. Novel capabilities for accurate and precise in situ analysis of oxygen isotope ratios from small 1-10 μm spots will be employed to analyze mineral inclusions that are armored by refractory host minerals and to quantify intracrystalline diffusion gradients that formed during cooling. These profiles will be modeled with updated code for the Fast Grain Boundary diffusion model. Samples will be selected primarily from granulite facies rocks. Garnet-quartz thermometry will be tested in detail across the Adirondacks with a series of outcrop tests for self-consistency, and other mineral systems will be tested locally. Garnet + quartz are common in a range of orthogneisses and metasediments, allowing the same pressure independent thermometer to be applied across the terrane. Temperatures will be calculated for both quartz inclusions in garnet, and in quartz surrounded by garnet coronas, and these T?s will be compared to those from garnets in quartzite, and to other geothermometry. Oxygen isotope profiles will be measured in single crystals of titanite as well as garnet, and selected zircons and pyroxenes.