Partial melting and decompression of the Thor-Odin dome, Shuswap metamorphic core complex, Canadian Cordillera

Britt H. Norlander, Donna L Whitney, Christian Teyssier, Olivier Vanderhaeghe

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The Thor-Odin dome region of the Shuswap metamorphic core complex, British Columbia, contains migmatitic rocks exhumed from the deep mid-crust of the Cordilleran orogen. Extensive partial melting occurred during decompression of the structurally deepest rocks, and this decompression path is particularly well recorded by mafic boudins of silica-undersaturated, aluminous rocks. These mafic boudins contain the high-temperature assemblages gedrite - cordierite + spinel + corundum + kyanite/sillimanite ± sapphirine ± högbomite and gedrite + cordierite + spinel + corundum + kyanite/sillimanite + garnet ± staurstaurolite (relict) ± anorthite. The boundins are interlayered with migmatic metapelitic gneiss and orthogneiss in this region. The mineral assemblages and reaction textures in these rocks record decompression from the kyanite zone (P>8-10 kbar) to the sillimanite-cordierite zone (P<5 kbar) at T ~ 750 °C, with maximum recorded temperatures of ~ 800 °C. Evidence for high-temperature decompression includes the partial replacement of garnet by cordierite, the partial to complete replacement of kyanite by corundum + cordierite + spinel (hercynite) ± sapphirine ± högbomite symplectite, and the replacement of some kyanite grains by sillimanite. Kyanite partially replaced by sillimanite, and sillimanite with coronas of cordierite ± spinel are also observed in the associated metapelitic rocks. Partial melt from the sorrounding migmatitic gneisses has invaded the mafic boudins. Cordierite reaction rims occur where minerals in the boudins interacted with leucocratic melt. When combined with existing structural and geochronologic data from migmatites and leucogranites in the region, these petrologic constraints suggest that high-temperature decompression was coeval with partial melting in the Thor-Odin dome. These data are used to evaluate the relationship between partial melting of the mid-crust and localized exhumation of deep, hot rocks by extensional and diapiric processes.

Original languageEnglish (US)
Pages (from-to)103-125
Number of pages23
Issue number3-4
StatePublished - 2002

Bibliographical note

Funding Information:
We thank B. Evans, J. Stout, C. Kopf, A. Fayon, and J.-P. Burg for helpful discussions. This research is supported by NSF grant EAR-9814669 to CT and DLW. Constructive reviews by J. Cheney, E. Ghent, and S. Paterson helped substantially to improve this manuscript.


  • Decompression
  • Geothermobarometry
  • Metamorphic core complex
  • Omineca belt
  • Partial melting
  • Petrology


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