Omphacite breakdown reactions and relation to eclogite exhumation rates

Eric D. Anderson, David P. Moecher

Research output: Contribution to journalArticlepeer-review

55 Scopus citations

Abstract

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 P-T 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.

Original languageEnglish
Pages (from-to)253-277
Number of pages25
JournalContributions to Mineralogy and Petrology
Volume154
Issue number3
DOIs
StatePublished - Sep 2007

Bibliographical note

Funding Information:
Acknowledgments 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.

Keywords

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

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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