ANALYSIS OF SPATIAL AND TEXTURAL RELATIONSHIPS AMONG ACCESSORY PHASES IN A LAYERED AMPHIBOLITE: IMPLICATIONS FOR TRACE-ELEMENT THERMOBAROMETRY AND GEOCHRONOLOGY

Systematics of Ti and Zr distribution among mineral phases in metamorphic rocks have been used as the basis for several recently-developed thermobarometers (e.g. TitaniQ, Zr-in-titanite, Zr-in­-rutile). An understanding of the growth history of Ti- and Zr- bearing minerals such as titanite, zircon, and rutile allows precise correlation of pressure and temperature with time, so long as the minerals are in equilibrium with the rock during metamorphism. The accuracy of trace-element thermobarometers is dependent upon the ability to apply constraints upon the activities of Ti and Zr during progressive metamorphism; an understanding of the spatial, textural, and temporal relationships among the Ti and Zr-bearing minerals is vital.

In this study, these relationships were evaluated for a sample of banded amphibolite gneiss collected from the Inner Piedmont of the Southern Appalachians in west-central Georgia. The gneiss consists of two primary layer types defined by their mineralogy: an amphibole + plagioclase-rich layer and epidote + quartz-rich layer. After petrographic and SEM-EDS analysis of thin-sections the following observations were made: (1) Titanite is present in both the amphibole and epidote layers; titanites in amphibole layers contain inclusions of amphibole, and titanites in epidote layers contain inclusions of epidote. (2) Zircon appears both as inclusions in titanite and within the matrix of each layer. These textures indicate that these minerals were in equilibrium with the rock during metamorphism. (3) Rutile is abundant only as inclusions within titanite. This indicates that rutile is not part of the equilibrium assemblage during metamorphism, and cannot be used to estimate peak metamorphic conditions. Now that the textural and spatial relationships of Ti- and Zr-bearing phases are well characterized, TitaniQ and Zr-in-titanite thermometry techniques will be applied. The results will aid in the reconstruction of PTt paths in this region of the Southern Appalachians.