Lutetium-Hafnium Ages and the P-T History of High-Pressure Rocks in the Appalachians

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

Essene/Mukasa EAR-0087448 Our research is directed toward developing techniques that will address the geological history of suites of rocks that were once deeply buried during subduction of ocean crust or during subsequent continental collision. The rocks under consideration all were buried to depths of 40-60 km during an ancient mountain-building event, the Taconic Orogeny, in the Appalachian Mountains of eastern North America. This area has undergone a complex tectonic history, with three mountain-building events linked to accretion of oceanic materials or exotic terranes in the Phanerozoic: the Taconic at ca. 450 Ma, the Acadian at ca. 400 Ma, and the Alleghenian beginning at 320 Ma. Some tectonic slices contain materials that were also part of ancient North America and underwent a much older high-grade metamorphism in Grenville time about 1.1 Ga ago. In any such repeatedly deformed, buried and faulted terrane, it is often difficult to identify with certainty the timing of any given event, and robust geochronogical and thermobarometric techniques are needed to identify the growth of minerals rather than their cooling or resetting during some subsequent orogeny. Our work will apply the Lu-Hf isotopic dating technique on minerals with high blocking temperatures for these elements (garnet, clinopyroxene, rutile and zircon) that can be separated from high-pressure rocks (eclogites and garnet granulites) which, in some cases, have undergone subsequent lower grade events (amphibolite facies metamorphism). This effort will be combined with new thermobarometric techniques designed to minimize the problems encountered in high-variance assemblages found in most eclogites. These approaches will be combined with mapping, structural analysis and U-Pb geochronology being done by our collaborators to better establish the timing of the early tectonic history of thrust slices in the Appalachians.