Evidence for an inverted metamorphic gradient associated with a Precambrian suture, southern Wyoming


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Abstract An inverted metamorphic gradient associated with the northern mylonite zone of the Cheyenne belt, a deeply eroded Precambrian suture in southern Wyoming, has been documented within metasedimentary rocks of the Early Proterozoic Snowy Pass Supergroup. Metamorphic grade in the steeply dipping supracrustal sequence increases from the chlorite through the biotite, garnet, and staurolite zones both stratigraphically and structurally upward toward the northern mylonite zone. A minimum temperature increase of approximately 100° C over a km‐wide zone is required for this transition. Parallelism of inverted isograds with the trace of the northern mylonite zone implies a genetic relationship between deformation associated with that zone and the inverted metamorphic gradient within the Snowy Pass Supergroup. Field evidence together with microstructural and petrofabric analysis indicate northward thrusting of amphibolite‐grade rocks over rocks of the Snowy Pass Supergroup along the northern mylonite zone. Mineral equilibria and garnet‐biotite geothermometry on synkinematic mineral assemblages within the Snowy Pass metasedimentary rocks indicate deformation at minimum temperatures of 480° C and pressures of 350–400 MPa (3°5–4°0 kbar). This implies tectonic burial or upper plate thickness of 13–15 km. The narrow character of metamorphic zonation and microtextures within the Snowy Pass Supergroup which indicate late synkine‐matic growth of garnet and staurolite, preclude rotation of pre‐existing isograds by folding as a mechanism for development of the inverted gradient. Conductive transport of heat from the upper into the lower plate across the originally low‐angle thrust is insufficient to produce the necessary temperatures in the lower plate. Shear heating is considered insufficient to produce the observed metamorphic transition unless high shear stresses are postulated. Up‐dip advection of metamorphic fluids is a feasible, but unproven, mechanism for heat transport. The possibility that rapid uplift due to stacking of several thrust sheets may have played a role in preserving the inverted metamorphic gradient cannot be evaluated at present.

Original languageEnglish (US)
Pages (from-to)41-63
Number of pages23
JournalJournal of Metamorphic Geology
Issue number1
StatePublished - Jan 1988


  • Key‐words: inverted metamorphic gradient
  • Precambrian suture
  • metapelites
  • thermal aspects
  • thrust systems

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology


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