The Russell Lake Allochthon, southern Appalachians
Chaumba, Jefferson Brighton
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Small, isolated, and metamorphosed mafic and ultramafic bodies that outcrop in the Carolina superterrane in east and central Georgia were investigated in this study. Allard and Whitney (1994) mapped these bodies, called the Russell Lake Allochthon (RLA), and hypothesized that they form part of a thrust sheet of oceanic affinity obducted in Alleghanian times. Key outcrops were mapped, sampled, and, where possible, trenched to expose the contacts with their country rocks. The contacts are horizontal, generally sheared, with granitic gneisses and metadacites underlying the RLA. Bulk-rock geochemical data of the RLA have MgO contents ranging from 5 to >35 wt. %, and CaO contents ranging from 2 to > 17 wt. %. These MgO and CaO ranges are greater the restricted ranges of these elements occurring in magmas, and support a cumulate origin for most of the rocks from the RLA. Anhydrous MgO contents correlate negatively with other major element oxides like CaO, and correlate positively with compatible trace elements like Ni which range from 25-1100 ppm. Mineral compositional data show the plagioclases to be remarkably calcic (An99-93), and the olivines to be highly evolved (Fo84-72). Rock with similar plagioclase and olivine compositions occur in island arc settings. Amphiboles in the RLA are zoned, with pargasitic hornblende cores consistent with amphibolite facies metamorphic conditions rimmed by tremolite/actinolite which is consistent with greenschist metamorphic conditions. One body where oxygen isotopes were performed on olivine and olivine mineral separates retains relict oxygen isotope ratios whereas two other bodies show evidence of open system oxygen isotope exchange with an 18O-enriched fluid at temperatures of approximately 500oC. Sm-Nd isotope data yield an age with a large error (258 ± 115 Ma, εNd = +2.8), interpreted to be the Alleghanian deformation event. Neodymium isotope values from two bodies show a wide variation in εNd (300 Ma) values from -5.85 to +3.20, within the range of ocean island arcs. A model for the formation of the RLA is presented where numerous island arcs first collided with each other, producing high-grade metamorphism, followed by thrusting of these bodies onto the Carolina superterrane and the associated retrograde metamorphism.