Relict Igneous Oxygen Isotope Ratios in Rocks from the Russell Lake Allochthon, Southern Appalachians

The Russell Lake Allochthon (RLA) is an apparently thin sheet of ultramafic and mafic rocks that may have been thrust over rocks of the Carolina superterrane, Cat Square terrane, and Tugaloo terrane (Hatcher et al., 2007) of the southern Appalachians during the Alleghanian orogeny (Allard and Whitney, 1994). An oxygen isotope study conducted on coronas between olivine and plagioclase, in a metatroctolite, shows that δ¹⁸O_olivine values of 5.4 ‰ (±0.4, 1 S.D.), and δ¹⁸O_plagioclase values of 6.6 ‰ (±0.1, 1 S.D.) are consistent with high temperature equilibration. Using an experimental anorthite-water fractionation equation of Matsuhisa et al. (1989), a theoretical olivine-water fractionation equation of Zheng (1993), and a Δ_{plagioclase-olivine} value of 1.2 ‰, yields a temperature of 1068^oC for formation of these coronas. These oxygen isotope signatures and calculated temperatures are similar to those obtained for ultramafic rocks in the Oman ophiolite (Dunlop and Fouillac, 1986).

This high temperature equilibration obtained for this metatroctolite, which is indistinguishable from magmatic temperatures, is consistent igneous crystallization followed by cooling at relatively high pressures of at least 0.8 GPa, which led to the reaction of olivine and plagioclase during cooling. Furthermore, the olivines have NiO compositions that vary from 0.1 to 0.4 wt. % at nearly constant Fo contents of 73-76, which is indicative of a cumulus origin for the olivine (Takahashi, 1991). Such forsterite contents are consistent with an equilibrium melt richer in Fe compared to mid-ocean ridge basalt, pointing to a high magnesium andesitic parental magma consistent with an island arc origin for the RLA. Calcium-rich compositions of plagioclases (An_95-99) in the same metatroctolite are also consistent with an island arc origin for the RLA (e.g., Thy, 1987).