2 Igneous/Metamorphic Petrology and Geochemistry I

Sunday, 14 March 2010
8:00 AM - 12:05 PM
International E

A COMPARISON OF MAJOR- AND TRACE-ELEMENT THERMOMETRY TECHNIQUES APPLIED TO A BANDED AMPHIBOLITE, WEST-CENTRAL GEORGIA

Paper number: 2-7
Program: Technical Sessions
Time: 9:45 AM – 10:05 AM

Authors

Abstract

Quantitative pressure-temperature-time (P-T-t) paths are a useful method for constraining changes in metamorphic conditions during orogenesis and testing models of tectonic evolution. However, these types of data are not yet well-constrained for portions of the Southern Appalachians. This study focuses on a sample of banded amphibolite gneiss collected from the Ropes Creek Metabasalt, part of the Inner Piedmont of the Southern Appalachians in west-central Georgia. Texturally, the gneiss consists of alternating amphibole + plagioclase and epidote + quartz layers. Previous estimates of metamorphic conditions were based on conventional hornblende-plagioclase major-element exchange. While the technique returned plausible results (620-660º C and 6-8 kbar), errors are large, diffusion could cause continued exchange during retrograde metamorphism, and these results do not provide any temporal context. Without this context, there is no way to accurately link pressures and temperatures with specific moments in time, making it difficult to produce reliable P-T-t paths. However, application of recently-developed trace-element and accessory phase geothermobarometers, when combined with geochronological analysis, provides the opportunity to precisely correlate pressure-temperature conditions with time, if the growth history of the accessory phases is known.

In the sample being studied, titanite and zircon are found as inclusions within phases and within the matrix of both primary layers. These textures indicate that these minerals were in equilibrium with the rock during metamorphism. Rutile is present only as inclusions within the titanite, which indicates that it is not part of the equilibrium assemblage; however, it may provide information on conditions during early stages of metamorphism. Understanding the growth history of Ti- and Zr- bearing minerals permits application of appropriate trace-element thermobarometers; in this case, TitaniQ (D. Wark, 2006) and Zr-in-titanite (L. Hayden, 2008). Results from these analyses will be compared to data obtained from the conventional hornblende-plagioclase major-element exchange.