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The Snowbird Tectonic Zone

Polymetamorphic High-Pressure Mafic Granulites in the Snowbird Tectonic Zone, Western Canadian Shield Kevin Mahan(1), Philippe Goncalves (2), Rebecca M. Flowers (1) Gregory Dumond (3) and Michael Williams (3)

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The Snowbird Tectonic Zone

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  1. Polymetamorphic High-Pressure Mafic Granulites in the Snowbird Tectonic Zone, Western Canadian Shield Kevin Mahan(1), Philippe Goncalves (2), Rebecca M. Flowers (1) Gregory Dumond (3) and Michael Williams (3) (1) Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, USA (2) Département de Géosciences, EA 2642, Univ. Franche-Comté, Besançon (3) Department of Geosciences, University of Massachusetts. Amherst, MA-01003 USA

  2. The Snowbird Tectonic Zone Modified from Tella et al., 2000

  3. East Lake Athabasca region

  4. M1 5 mm

  5. M1 assemblage: early granoblastic lenses

  6. M1 assemblage: produced from an igneous assemblage opxig + plig opxig + plig = grt1 + cpx1 + qtz 04M005

  7. M2-M3 5 mm

  8. M2 assemblage: deformation, cooling and hydration M2 : hbl2 – grt2 – pl2 – qtz2 – ilm2 +/- opx2 S2 : hbl – grt -pl grt2 hbl2 pl2 grt2

  9. M2 assemblage: deformation, cooling and hydration M2 : hbl2 – grt2 – pl2 – qtz2 – ilm2 – tit2 +/- opx2

  10. M3 assemblage: new granulite facies event (hbl2 breakdown)

  11. M3 assemblage: new granulite facies event (hbl2 breakdown) pl2 opx3cpx3pl3ilm3 hbl2 hbl2 + qtz = opx3 + cpx3 + pl3 + ilm3 + V

  12. Petrogenetic modelling combining : 1- forward modelling (phase diagram projections and sections – Perple_X06)  system TiO2-NaO-CaO-FeO-MgO-Al2O3-SiO2-H2O  updated Holland and Powell thermodynamic database  bulk composition (XRF and estimation by image analysis with mineral composition)  P-T-X diagrams to investigate the influence of envitonmental variables (P and T) and compositional variables like MH2O, XMg, A/AFM, XCa, S/SFM) 2- inverse modelling (multi-equilibrium thermobarometry – TWQ2.02)

  13. Phase relations in the CaFMASH system

  14. Phase relations in the TiNaCaFMASH system M1 assemblage

  15. Phase relations in the TiNaCaFMASH system  What is the origin of the variation between the XRF and M1 composition ? mass transfer during M2 metamorphism and deformation ?  What is the influence on phase relations ?  What is controlling the P-T conditions of orthopyroxene and ilmenite? - XMg - A/AFM

  16. Phase relations in the TiNaCaFMASH system

  17. Influence of bulk composition on opx, ilm and plagioclase stability

  18. Influence of bulk composition on opx, ilm and plagioclase stability

  19. Application to the polymetamorphic evolution of the Cora lake mafic granulite M1 assemblage: grt1 – cpx1– ilm1 – qtz1 (pl1)

  20. Application to the polymetamorphic evolution of the Cora lake mafic granulite M2 assemblage: hbl2– pl2 – grt2 – ilm2 – tit2 grt2 growth

  21. Application to the polymetamorphic evolution of the Cora lake mafic granulite Amphibole modes A retrograde PT path will produce a significant amount of amphibole But what about grt2…

  22. Application to the polymetamorphic evolution of the Cora lake mafic granulite grt modes No grt2 is produced !

  23. Application to the polymetamorphic evolution of the Cora lake mafic granulite breakdown of a metastable grt1-cpx1-qtz during a near isobaric heating path 10 Kbar T ~ 700 C Prograde grt2 crystallisation at the expense of amp2 – qtz2 - sph2 +/- pl2 (see inclusion in grt2)

  24. Application to the polymetamorphic evolution of the Cora lake mafic granulite M3 assemblage: opx3 – cpx3 – pl3 Breakdown of hbl2 into opx3 – cpx3 – pl3 M2 and M3 were developed on a single heating path.

  25. ~ 600 mm ~ 300 mm Timing of the polymetamorphic evolution This petrological interpretation requires at least two distinct metamorphic event…. M2 zircon M1 zircon

  26. Timing of the polymetamorphic evolution

  27. Conclusion Ca. 2.6 Ga: igneous crystallisation 2.55 Ga: M1 event, crustal thickening to 1.3-1.5 GPa Decompression/cooling to steady-state conditions followed by a ~600 m.y. period of deep crustal residence 1.9 Ga: Near isobaric heating (M2-M3) and mafic dyke emplacement at ~ 1.1 GPa Not discussed here 1.86 Ga: Regional exhumation to ~0.8-0.7 GPa 150-200 m.y. unroofing to near-surface conditions

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