Paper No. 10
Presentation Time: 10:35 AM
ASSESSMENT OF RE-EQUILIBRATION MECHANISMS OF PLAG AND CPX IN HIGH-PRESSURE AMPHIBOLITES AND RETROGRADE ECLOGITES
High-pressure Gt-Cpx-Pl-Hbl-Qtz metabasic rocks commonly exhibit inclusions of Pl in Cpx. Such textures are assumed to result from exsolution of sodic Pl from an originally omphacitic Cpx, and are taken as evidence for decompression from eclogite facies conditions. Reintegration of the measured Pl and Cpx compositions often yields an omphacitic composition. However, competing equilibria among all phases in the rock occurring during retrograde cooling may negate the assumption that the measured Pl and Cpx compositions are those established by exsolution of omphacitic Cpx. Microprobe point analyses and X-ray elemental-intensity maps have been used to characterize mineral compositions and compositional zoning in a suite of metabasites from the Southern Appalachian orogen. Some Cpx-amphibolites contain two compositionally distinct generations of Pl; smaller (5-25 µm) inclusions in Cpx range from Ab 74 to Ab 79, while larger (≥ 40 µm) grains in Grt-Cpx-Pl-Hbl matrix range from Ab 83 to Ab 86, suggesting that the enclosing Cpx has prevented Pl inclusions from completely re-equilibrating with phases in the matrix. This indicates that reintegrating phases in symplectic Cpx is valid for calculating early Cpx compositions. Compositional zoning patterns in Cpx, specifically the preservation of Na-rich regions that are devoid of Pl inclusions, also supports reintegration. Cores of Cpx-Pl-Hbl symplectite contain exolved Pl and Hbl as well as Cpx with the highest Na content. Rims of Cpx-Pl-Hbl symplectite lack Pl and Hbl inclusions, contain low-Na Cpx, and are commonly surrounded by larger (≥40 µm) Pl grains. Inclusion-free, low-Na Cpx surrounded by coarse Pl may have formed due to higher diffusion rates at grain boundaries. It may be prudent to omit the inclusion-free Cpx rims from reintegration calculations, and base reintegration calculations on chemical compositions of Cpx-Pl-Hbl intergrowths that occur in sympectite cores. Compositional zoning, symplectitic intergrowths, and mineral compositions of major phases and inclusions in Cpx and Grt are being examined in a suite of eclogites and retrograded eclogites in order to better understand the exsolution process, and more accurately model P-T pathways of high-pressure metabasites.