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Omphacite breakdown reactions and relation to eclogite exhumation rates

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Clinopyroxene + plagioclase (±Hbl ± Qtz) symplectites after omphacite are widely cited as evidence for prior eclogite-facies or high-pressure (HP) metamorphism. Precursor omphacite compositions of retrograde eclogites, used for reconstructing retrograde PT paths, are commonly estimated by reintegrating symplectite phases with the assumption that the symplectite-forming reactions were isochemical. Comparisons of broadbeam symplectite compositions to 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 a minor Ca-Eskola (CaEs) 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 at grain boundaries or internally. ABR symplectites contain coarser (∼10 μm-scale) planar lamellae and rods of Pl + Cpx II + Qtz + Hbl within clinopyroxene cores. The contrasting textures correlate with decompression and cooling rate, and degree of overstepping of the retrograde reaction (lamellar: slow, erosionally controlled exhumation with slow/low overstepping; fine-grained, grainboundary symplectite: rapid, tectonic exhumation with rapid/high overstepping). Variations in XCaEs, Xjd, and XCaTs of precursor HP omphacite are related to the symplectic mineral assemblages that result from decompression. Quartz-normative symplectities indicate quartz-producing retrograde reactions (e.g., breakdown of precursor CaEs); quartz-free symplectities (e.g., diopside + plagioclase after omphacite) indicate quartz-consuming reactions (jd, CaTs breakdown) outpaced quartz-producing reactions.

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Steve Dunn provided the Gurskøy samples for this study; Spencer Cotkin provided the Flatraket samples. The insightful reviews of Eric Essene and an anonymous referee helped resolve inconsistencies and errors. This research was supported by student research grants from Geological Society of America and the University of Kentucky Graduate School, College of Arts and Sciences, and Department of Earth and Environmental Sciences.

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Correspondence to Eric D. Anderson.

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Communicated by T. L. Grove.

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eFig. 1 X-ray map and BSE images documenting EMP broadbeam analysis areas. a) Ca-X-ray map of symplectic Lick Ridge omphacite with large square showing approximate location where 100 points were collected, each having the dimensions ∼10 × 10 μm. Smaller square with arrow represents approximate location of 9 points collected in “adjacent unreacted pyroxene”. b) Dellwood garnet granulite sample DEL05-3B2, with location of 60 × 70 μm grid covering symplectite in aegirine-augite (Agt). c) Dellwood HP-amphibolite, sample DEL03-3B, with location of 70 × 80 μm grid. d) BSE image of symplectite at omphacite grain boundaries, Flatraket, WGR Norway sample F28-C. Large rectangle represents location of symplectite analyses; small square is approximate location of adjacent unreacted pyroxene. e) BSE image of Gurskøy clogite with location of broadbeam analysis in lobate Cpx-Pl symplectite, and location of adjacent unreacted pyroxene. The small inclusions in matrix omphacite are primarily Qtz + Hbl.

eFig. 2 (supplemental) BSE images with locations of broadbeam analyses of hornblende + quartz inclusions and surrounding clinopyroxene. a) Lick Ridge eclogite (ABR); b) Dellwood HP-amphibolite (ABR); c) Dellwood garnet granulite (ABR), light gray in reintegrated area is Hbl, black phase is quartz; d) Gurskøy clogite with abundant quartz inclusions (black) in omphacite (WGR).

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Anderson, E.D., Moecher, D.P. Omphacite breakdown reactions and relation to eclogite exhumation rates. Contrib Mineral Petrol 154, 253–277 (2007).

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  • Symplectite
  • Omphacite
  • Eclogite
  • Eastern Blue Ridge
  • Western Gneiss Region
  • PT path
  • Retrograde
  • Isothermal decompression
  • Reintegration
  • Hornblende quartz exsolution
  • Ca-Eskola