16 Ordovician Tectonics of the Appalachians: New Ideas on an Ancient Orogen

Thursday, 19 March 2015
1:00 PM - 5:00 PM
Roberts

DIACHRONOUS METAMORPHISM BETWEEN AMPHIBOLITE AND ECLOGITE OF THE ASHE METAMORPHIC SUITE AND IMPLICATIONS FOR THE SOUTHERN APPALACHIANS

Paper number: 16-8
Program: Theme Sessions
Time: 3:00 PM – 3:20 PM

Authors

Abstract

The Bakersville, NC eclogite bodies in the southern Appalachians record peak metamorphic conditions of the Taconic orogeny at 459 +1.5/-0.6 Ma. Efforts to constrain the previous extent of eclogite-facies conditions by comparing the zircon ages, crystallization temperatures, and REE patterns of the eclogite to surrounding amphibolite bodies revealed that the amphibolites record different conditions and ages. Zircons from one amphibolite body, located only 5.5 km southwest from the eclogite, produced a LA-ICPMS age of 418 ± 8 Ma, confirmed with an ID-TIMS age of 414.5 ± 3.8 Ma. Ti-in-zircon thermometry recorded metamorphic temperatures of 650 ± 19 °C for the amphibolite. A 45 my age difference over 5.5 km indicates either a major fault between the bodies, or an amphibolite-facies Siluro-Devonian metamorphic event that is not recorded in the eclogite zircons and not previously recognized for the southern Appalachians. This event may have remained unrecognized because most of the rocks in the Ashe Metamorphic Suite are metasedimentary and contain mostly detrital zircons with 1.1 Ga cores and thin, difficult-to-date rims. Rocks with igneous protoliths (e.g. amphibolite and eclogite) can better record metamorphic events. Both the eclogite and amphibolites have a MORB geochemical signature, and the zircons have U/Th ratios <0.1, indicating they are metamorphic in origin.

The northern Appalachians have evidence of a ‘Silurian Event’, such as 430-415 Ma regional deformation and intrusive bodies in Newfoundland (Dunning et al., 1990), but little evidence for such an event exists for the southern Appalachians. In partially retrograded Bakersville eclogite, Miller et al. (2010) found titanite ages of 393.5 ± 3.5 Ma, interpreted as either Taconic cooling or as early southern Acadian metamorphism. This age would be too young to represent cooling of the 460 Ma eclogite, but not for a 415 Ma Siluro-Devonian event. The amphibolite and eclogite bodies are both adjacent to the Burnsville fault, a dextral strike-slip fault with large displacement and last motion between 377-370 Ma. One possibility is that portions of the Ashe Metamorphic Suite were transported hundreds of km southward during the Acadian orogeny, or maybe the amphibolite is evidence that this Siluro-Devonian event also affected portions of the southern Appalachians.