CORONAS IN METATROCTOLITES FROM THE RUSSELL LAKE ALLOCHTHON, SOUTHERN APPALACHIANS

The petrology of metatroctolites and metapyroxenites in northeast Georgia constrains the tectonic evolution of the Russell Lake Allochthon (RLA) – an apparently thin sheet of ultramafic and mafic rocks that were thrust over basement during the Alleghanian orogeny (Allard and Whitney, 1994). Coronas between olivine and plagioclase are common in the metatroctolites. These coronas consist of orthopyroxene and minor actinolite adjacent to coarse-grained olivine, tschermakitic amphibole, and tschermakitic amphibole – spinel symplectites adjacent to plagioclase. The actinolite is interpreted to be a retrograde phase.

The rocks display relict cumulate textures. The metatroctolites are poor in Na₂O and K₂O, as well as the relatively immobile incompatible trace elements Y and Zr. Large variations are displayed by Ni and Sr which would be concentrated in cumulate olivine and plagioclase. These chemical characteristics are consistent with cumulate protoliths for the metatroctolites.

Olivines (Mg#_82-84), orthopyroxenes (Mg#_77-80) and plagioclase (An_95-98) have very restricted compositions. Use of the Fe-Mg exchange reaction between olivine and orthopyroxene yields an estimated exchange temperature of approximately 650^oC. This relatively high temperature is consistent with the high Al₂O₃ content (14-22 wt%) of the primary amphiboles in the coronas, and amphibolite facies conditions for the initial development of the coronas.

The corona-producing reaction of plagioclase with olivine suggest that either the RLA was subjected to pressures of at least 0.8 GPa, possibly due to subduction of oceanic crust beneath an island arc in a peri-Gondwana environment in Neoproterozoic times (e.g., Dennis and Wright, 1997), or cooled at high (>0.8 GPa) pressures from magmatic temperatures. Remarkably calcic plagioclase may reflect highly depleted magmatic source rocks. Minerals consistent with greenschist facies conditions (e.g., actinolite) were formed during thrusting of the RLA onto the North American craton during the final stages of the assembly of Pangaea.