ZONED GARNETS FROM THE ANAKEESTA FORMATION, GREAT SMOKY MOUNTAINS NATIONAL PARK, USA

Understanding the tectonothermal setting for metamorphism in areas of multiple orogenies is often difficult due to obfuscation of prior alteration events by future ones, but can be overcome by outlining changes in chemical composition preserved in zoned minerals with P-T paths. This case study used microprobe analysis to interpret garnets from the polymetamorphic Anakeesta Formation. The Anakeesta Formation consists of slate, phyllite, and metasandstone that formed from carbonaceous and sulfidic sediments deposited in oxygen-poor conditions between turbidite episodes during the Neoproterozoic Era. It is commonly found in the central region of Great Smoky Mountains National Park. In the zone of collection, samples did not exceed the Barrovian garnet isograd, as indicated by the presence of albite and absence of aluminosilicates.

The garnets studied from this formation were found to be rich in iron and manganese. Their core and rim compositions were determined to be approximately Alm₄₃sp₄₄py_6.5gro₆and_0.5 and Alm₄₈sp₃₄py₉gro₈and₁, respectively. Samples from the formation showed increases of almandine and pyrope as well as deceases of grossular when evaluated from core to rim, which plateaued near the edge of the crystal. These changes indicated prograde metamorphism that reached a maximum pressure and temperature before remaining in that condition for a period of time.