Omphacite breakdown reactions in relation to eclogite exhumation rates

Category Petrology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Moecher, David; Anderson, Eric
Holding Date 04 October 2008

Clinopyroxene + plagioclase (+/- hornblende+ quartz) symplectites after omphacite or sodic clinopyroxene are widely cited as evidence for prior eclogite-facies or high-pressure (HP) metamorphism. Precursor omphacite compositions of retrograde eclogites, used for reconstructing retrograde P-T paths, are commonly estimated by reintegrating symplectite phases assuming that symplectite-forming reactions were isochemical. Comparisons of bulk symplectite compositions obtained via broadbeam microprobe analysis with adjacent unreacted pyroxene from various symplectites after clinopyroxene from the Appalachian Blue Ridge (ABR) and Western Gneiss Region (WGR) suggest that the symplectite-forming reactions are largely isochemical. Endmember calculations based on reintegrated symplectite compositions from the ABR and WGR suggest that minor Ca-Eskola component (XCaEs = 0.04 - 0.15) was present in precursor HP clinopyroxene. WGR symplectites consist of fine-grained (~1 μm-scale), vermicular intergrowths of Pl + Cpx II + Hbl that occur internally or at grain boundaries. ABR symplectites contain coarser (~10-μm-scale) planar lamellae and rods of Pl + Cpx II + Qtz + Hbl occurring within originally low-Na omphacite cores. The contrasting textures correlate with decompression and cooling rate, and rate and amount of overstepping of the retrograde reaction: lamellar = slow [~100 bar/m.y.], erosionally controlled exhumation with low overstepping [~ -0.3 kJ/my]; symplectites = rapid [~600 bar/m.y.], tectonic exhumation (2-3 mm/a) with large degree of overstepping [~ -16.0 kJ/my]. Symplectite textures in decompressed HP and UHP rocks are the fingerprint of rapid eduction or uplift and significant over-stepping of a strongly pressure dependent reaction along an isothermal decompression path. Lack of significant cooling through the isothermal interval (800-600 °C) maintains favorable reaction kinetics for omphacite breakdown, exsolution, and annealing of the retrograde assemblage. In many cases decompression assemblages show no SPO and developed essentially statically, without the influence of strain-induced recrystallization.