PETROLOGIC STUDY OF ZONED MINERALS TO DETERMINE THE P-T HISTORY OF THE BLUE RIDGE ECLOGITES, S. APPALACHIANS

Eclogites in the Blue Ridge province provide a view into the deep levels of an Appalachian (460 Ma) subduction and collision event. Eclogite lenses, the largest of which is ~ 1 km long and 300 m thick, occur within amphibolite facies rocks. Preservation of eclogites ranges from excellent (omphacite + garnet) to poor (hornblende + plagioclase). Garnet textures, compositions, and zoning patterns vary on a small (sub-outcrop) scale. Some samples contain abundant small (< 1 mm), euhedral garnets; others contain fewer but larger (5-7 mm), anhedral garnets. Many garnets exhibit a decrease in Fe and Mn from core to rim (with a thin zone of higher Mn at the rim), and an increase in Ca and Mg, though some garnets show essentially no zoning. Typical compositions are Alm53-58 Sps2-3 Prp12-15 Grs 26-30.

Clinopyroxene is zoned, as is the surrounding hornblende. Relict cores of cpx are the most sodic (up to Di40 Jd60), and are surrounded by discontinuous rims of more calcic cpx. Zoned hornblende has more Ca-Mg-rich rims than cores. Plagioclase occurs in the matrix, as rims on garnet, and as inclusions in garnet. Texturally early plag is An16, and later plag (rim) has An contents of 22-40 mol%.

The presence of many zoned phases, partial replacement of cpx by hbl, and decomposition of garnet to plagioclase coronas or symplectitic rims of cpx + plag or hbl + plag makes thermobarometric applications challenging. Pairing of garnet core compositions with the most sodic cpx in pristine eclogite yields temperatures of 730-790 °C at 14-16 kbar, with relatively high P-T conditions also calculated for a garnet rim adjacent to hornblende: 660-700 °C, 8-9 kbar. Maximum pressures may have been higher, as even the most unaltered eclogite shows evidence for decompression at high T, and likely re-equilibration of mineral compositions. Suggestions that P-T conditions were higher than those recorded include: quartz blebs in cpx which may suggest an initial supersilicic px composition, low but detectable amounts of Na in garnet, and relict inclusions in zircon of minerals not found in the matrix.