A RETROSPECTIVE VIEW OF THE P-T PATH REVOLUTION IN METAMORPHIC PETROLOGY (Invited Presentation)

I will present an overview of early efforts to reconstruct pressure-temperature (P-T) paths, and the ways in which these paths changed the field of metamorphic petrology. I had the great good fortune to start my career just in time to help develop some of the methodologies of reconstructing P-T paths, and to see how these paths made metamorphic petrology newly relevant in the geosciences. Given the emphasis of the MGPV Distinguished Geological Career Award on field-based contributions, I will focus in particular on examples in which field data fundamentally changed the tectonic interpretation of metamorphic P-T data. I will return to my favorite field area, the Tauern Window of Austria/Italy, to show how a mismatch between petrologic data and early thermal models led to new fieldwork, and then to a new interpretation involving large-scale, orogen-parallel extension in one of the type localities of crustal thickening. Additional field and microscopic analysis showed that embrittlement of the crust began at elevated temperatures and depths, and that a succession of metamorphic reactions resulted in repeated rheological cycling at mid-crustal depths. All of these processes were a dynamic response to large-scale plate motions. Construction of P-T±t±d paths is now a standard methodology, facilitated in part by the ease of constructing pseudosections using thermodynamic modeling software packages. P-T paths have helped to broaden the scope of metamorphic petrology and make it a truly cross-disciplinary field. Metamorphic P-T data, coupled with field observations, are essential to understanding feedback effects between physical and chemical processes at depth in the earth’s crust, and then placing these processes in a geodynamic context. I hope that future students will continue to acquire the field and petrographic skills necessary to complement and go beyond the thermodynamic models.