205 Petrology: Metamorphic/Igneous (Posters)

Tuesday, 21 October 2014
9:00 AM - 6:30 PM
Exhibition Hall C

DETERMINING THE EXTENT OF ECLOGITE FACIES METAMORPHISM IN THE EASTERN BLUE RIDGE THROUGH GEOCHEMICAL ANALYSIS OF SURROUNDING AMPHIBOLITE BODIES

Paper number: 205-1
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Eclogite is rare in the Southern Appalachians and the best-preserved bodies occur near Bakersville, NC. It remains unclear how these high-pressure rocks were emplaced into the regional amphibolite-facies rocks of the Ashe Metamorphic Suite (AMS) in the Eastern Blue Ridge. Two possible mechanisms are the mélange back-flow model, where blocks of eclogite are plucked from the descending oceanic slab and returned to shallow depths by the upward-flowing mélange, and a buoyancy model, where the eclogite is embedded within more-felsic crust that also was subducted to eclogite-facies depths but maintained its buoyancy. With the buoyant-crust model we would expect to see regional eclogite-facies metamorphism, whereas with the back-flow model we would not. To test which model is more likely, we sampled amphibolite bodies within the AMS that are petrologically indistinguishable from retrograded eclogite. These bodies are 1.7 kilometers southeast and 5.5 kilometers southwest from the eclogite. We examined zircons from the amphibolites because they resist retrogression and record important geochemical data. For our study, we measured the REE content, the Ti concentration for Ti-in-zircon thermometry, and the U-Pb radiometric age using the LASS-ICPMS at UC Santa Barbara. A time-temperature path from these analyses allowed us to compare the metamorphic history of the eclogite and amphibolite.

Zircons from the eclogite and the amphibolites are metamorphic in origin based on their Th/U ratios of <0.01. The eclogite zircons crystallized at 460±3.2 Ma and have low HREE concentrations, indicating they grew in the presence of garnet, as expected for eclogite. The amphibolite zircons have high HREE concentrations, and a circa 440 Ma age. Ti-in-zircon thermometry shows the amphibolite zircons crystallized at 640±24°C and the eclogite zircons crystalized at 645±15°C, suggesting they grew under similar temperatures, but at different times. These differences show that the amphibolites are not retrograded eclogite and that there was not regional eclogite facies metamorphism over this part of the AMS. By constraining the possible extent of eclogite facies conditions to within the distance of the amphibolite bodies, we propose that the eclogite bodies were emplaced as exotic blocks, rather than as part of a regional eclogite terrane.