25 D6. Igneous and Metamorphic Petrology (Posters)

Monday, 15 April 2024
1:30 PM - 5:30 PM
Grand Ballroom

METAMORPHISM AND GRAPHITE MINERALIZATION IN THE ALABAMA GRAPHITE BELT

Paper number: 25-1
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

The Alabama Graphite Belt (AGB) is a NE-SW trending zone that stretches over 60 km parallel to regional structures in Central Alabama. High concentrations of graphite occur in saprolite and two mica schist (±garnet ±kyanite or sillimanite) of the Higgins Ferry Group. These upper amphibolite facies rocks include lenses and layers of amphibolite and garnet-rich quartzite. In bedrock schist, graphite occurs as disseminated grains and <1 cm layers parallel to regional biotite-muscovite schistosity, and along <0.5? cm cross-cutting brittle fractures. These textures indicate that carbon was mobilized during and after peak metamorphism. Potential mechanisms for graphite concentration include: mobilization into low pressure zones such as fold hinges and fractures, and extraction of partial melts leaving a graphite residue. This ongoing study will test whether partial melting is a viable mechanism of concentrating graphite layers by estimating the pressure and temperature (P-T) during regional metamorphism. A schist sample from near Weogufka Creek in the Mitchell Dam NW quadrangle contains a peak mineral assemblage of garnet, plagioclase, biotite, ilmenite, quartz, and rutile. Boudinaged and sheared veins with granitic mineralogy indicate possible melt; however, this may have been internally or externally derived. Preliminary isochemical phase diagram sections constructed with Theriak-Domino predict stability of the peak metamorphic minerals at >640°C and over 6.3 kbar. Garnet from this sample yields a Sm-Nd age of 353±3 Ma (MSWD=2.9, N=4), indistinguishable from a published age of 357±3 Ma for garnet growth in the Mad Indian Group. P-T estimates for metamorphism of Mad Indian Group rocks near Micaville are >625°C and ~9 kbar and we tentatively infer similar high pressure for the Weogufka rocks. The new P-T estimates and garnet age for regional metamorphism indicate that Higgins Ferry and Mad Indian groups form a NE trending belt of upper amphibolite facies rocks metamorphosed ca. 355 Ma. Additional work will focus on petrogenetic models for graphite mineralization in the Southern Appalachians by potentially correlating graphite mobilization to igneous (partial melting) processes.

Geological Society of America Abstracts with Programs. Vol. 56, No. 2

doi: 10.1130/abs/2024SE-398461

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