Abstract
Continental crust buried during collisional orogeny typically records pressures of 3 GPa or lower; however, pressures much higher than this are recorded locally, which would suggest burial to mantle depths. Deep continental subduction is not observed in active orogens and should be hindered by the positive buoyancy of sialic crust relative to the mantle. Non-lithostatic pressure caused by mechanical contrasts between rock types provides an alternative explanation for extreme pressures recorded in buried continental crust; however, its occurrence and significance in natural systems is debated. Mechanical pressure heterogeneities were proposed specifically to explain extreme pressures of c. 5.5 GPa obtained in enstatite eclogite veins in the archetypal subducted continental terrane, the Western Gneiss Complex (WGC) in Norway. In this study, we use Lu–Hf garnet geochronology to test when, and thus, in what part of the burial cycle of the WGC the enstatite eclogite assemblages actually equilibrated. The results show that equilibration occurred at c. 393 Ma, which is much later than the typical ages obtained from ‘normal’ eclogites in the WGC and represents a time when the terrane was already at crustal depths (< 2.5 GPa). Finite element modeling of mechanical pressure distribution demonstrates that late extreme pressure excursions are feasible for the given rock system and could explain the seemingly spurious conditions recorded in these unusual rocks. The recognition of non-lithostatic ultrahigh-pressure in deeply buried continental crust allows crucial simplification of models for continental subduction and validates the importance of rock thermo-mechanics in interpreting observations from collision zones.
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Acknowledgements
We thank K. Gordon, V. Lai, and E. Czech for their assistance with MC-ICPMS, LA-ICPMS, and EPMA analyses, respectively, E. Kooijman, D. Spengler, and H.L.M. Van Roermund for many fruitful discussions in the field, and J. Scoates and D. Weis for their comments and suggestions. Constructive criticisms and suggestions by J. Ganguly and two anonymous reviewers helped improve this manuscript and we gratefully acknowledge editor O. Müntener for his careful and considerate editorial handling. Financial support is provided by the Multi-disciplinary Applied Geochemistry Network travel fund, a Natural Sciences and Engineering Research Council of Canada (NSERC)—Alexander Graham Bell Canada Graduate Scholarship (CGSD3-475186-2015) to J.A.C., an NSERC Discovery Grant (RGPIN-2015-04080) and a Canadian Foundation for Innovation and British Columbia Knowledge Development Fund infrastructure grant (joint project 229814) to M.A.S. All data needed to evaluate the conclusions of this paper are present in the paper or in the Supplementary materials.
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Cutts, J.A., Smit, M.A. & Vrijmoed, J.C. Evidence for non-lithostatic pressure in subducted continental crust. Contrib Mineral Petrol 175, 3 (2020). https://doi.org/10.1007/s00410-019-1633-z
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DOI: https://doi.org/10.1007/s00410-019-1633-z
Keywords
- Eclogite
- Lu–Hf
- Garnet
- Ultrahigh-pressure
- Non-lithostatic
- Western Gneiss Complex