Abstract

The Uchee belt of the southern Appalachian Piedmont, in west-central Georgia, is a Barrovian metamorphic facies-series terrane that is characterized by a steeper metamorphic gradient than the adjacent Inner Piedmont rocks in Alabama. As a result of bulk compositional differences, amphibolites from the Uchee belt developed high- and low-variance subsets of the assemblage calcium amphibole + plagioclase + quartz + epidote + titanite ± garnet ± magnetite ± ilmenite ± rutile ± calcite ± diopside ± biotite. Plagioclase compositions change from An24 to An88 as a result of continuous reactions during prograde (M1) metamorphism. Amphiboles from symplectic rocks are typically zoned from tschermakitic cores [Mg/(Mg + Fe) = 59–77] to actinolite rims [Mg/(Mg + Fe) = 80–90]. Symplectites of epidote + quartz commonly occur around plagioclase and amphibole in garnet-free high-variance rocks, suggesting that the symplectites may have formed after (M1) metamorphism by the reaction 0.43 An50 + 1.00 Di [Mg/(Mg + Fe) = 55] + 0.33 H2O + 0.14 O2 → 0.26 Mg-Hbl + 0.17 Ep (Ps21) + 0.82 Qtz. Another reaction, 23.97 An50 + 0.28 Mg-Hbl + 4.70 Ep (Ps21) + 1.3 H2O → 20.24 An38 + 0.17 Tr + 7.ll Ep (Ps15) + 1.00 Qtz + 0.86 O2 describes the mantling of amphibole by symplectite, the zoning in plagioclase to albitic compositions, zoning in amphibole to tremolite, and the depletion of Fe in symplectite epidote.

Thermobarometric studies indicate a maximum pressure of 10 ± 1 kbar and a temperature of 750 ± 30 °C for the prograde (M1) metamorphism followed by retrograde (M2) metamorphism at ~524 ± 50 °C and 6 kbar recorded by the symplectites. Modeling of symplectite-producing reactions suggests that the stable assemblage at upper amphibolite facies conditions was clinopyroxene + hornblende + plagioclase, which gave way to epidote symplectite and tremolite at lower amphibolite or upper greenschist facies conditions. The low equilibration temperature for symplectite, which is supported by low-pressure estimates calculated from reaction relationships, suggests that the symplectite formed during erosion and uplift along a path of low dP/dT immediately following the maximum P-T conditions. The composition of fluids during metamorphism of the Uchee belt, estimated from devolatilization equilibria, suggests the fluids were heterogeneous during (M1) metamorphism but were dominantly H2O rich during retrograde (M2) metamorphism. The textural and compositional characteristics of retrograded amphibolites suggest that H2O-rich fluids were important during progress of the symplectite-producing reactions.

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