HOW DO WE CHARACTERIZE A REGIONAL METAMORPHIC EVENT? (Invited Presentation)

Metamorphic archives of the pressure–temperature (P–T) conditions produced by tectonism track evolution in crustal P–T environments and tectonic processes through Earth history. A broader view of metamorphism and its tectonic associations may also consider the evolution in P–T–deformation that individual metamorphic rocks experienced (their P–T–d–time path). An even broader view may consider how P–T–d–t records vary in space (e.g., across a metamorphic field gradient); how the rates, tempo and/or timing of a metamorphic event vary depending on from where it is viewed.

Metamorphic complexes characterized by field gradients preserve different P–T–d–t histories in higher-grade zones relative to lower-grade zones, reflecting the thermal driver(s) and/or tectonic context for metamorphism. However, most metamorphic complexes are too old (and uncertainties on dates too large) for the most geologically precise petrochronology techniques to decipher such spatiotemporal variation. Direct comparison of multiple petrologic and deformation characteristics of regional metamorphic complexes provides a pathway toward interrogation of secular change in regional metamorphism.

We discuss some examples of comparative petrology/petrochronology that may be used (in combination) to investigate secular change in orogenic regional metamorphism. These include: (1) comparison of the timing of index mineral crystallization relative to deformation across Barrovian-type metamorphic zones; (2) direct comparison of length scales of Mn diffusion in garnet from the location of the sillimanite-in isograd—a proxy for thermal duration/episodicity at a position relatively proximal to the metamorphic heat source, reflecting the dynamics of heating; and (3) direct comparison of Raman spectra of carbonaceous material in the lowest-grade metasediments that record diffusively reset white mica ⁴⁰Ar/³⁹Ar dates—a proxy for thermal duration at a position relatively distal from the metamorphic heat source, reflecting regional metamorphic duration. Comparing these (and more) features, in parallel, offers a multi-dimensional look at secular change in orogenic metamorphism and its thermal drivers/tectonic context.