MODERN-STYLE METAMORPHISM IN THE PALEOPROTEROZOIC: A PRODUCT OF STOCHASTICITY IN ‘TIME TO SUBDUCTION INITIATION’ WITHIN OCEAN BASINS? (Invited Presentation)

The archive of peak-metamorphic temperature/pressure (T/P) through geologic time has proven an important recorder of secular evolution in plate tectonics. Another characteristic of metamorphism that can constrain tectonic process/setting is time and length scale of metamorphism. Barrovian-type (sensu lato), biotite–garnet–staurolite-and/or-kyanite–sillimanite metamorphic isograd sequences commonly have thickness ~5–15 km, recording thermal activity over time scales ~1–10 Myr. These metamorphic isograd sequences are a typical tectonometamorphic feature of Phanerozoic orogeny. However, all pre-Ediacaran (>635 Ma) examples of the phenomenon cluster within a brief period of the Paleoproterozoic (c. 2100–1750 Ma) that also yielded the only >750 Ma examples of modern-style (particularly ‘cold’) high-P/low-T metamorphism (e.g., Ganne et al., 2012, Nature Geoscience 5, 60; Weller & St-Onge, 2017, Nature Geoscience 10, 305). The apparent production of modern-style metamorphic rocks and isograd sequences within a narrow time window of the Paleoproterozoic is difficult to reconcile with predictions of monotonic change in tectonic style in response to monotonic secular cooling of the mantle.

The life span of an ocean basin is not deterministic: compare Tuzo Wilson’s ‘Proto-Atlantic’ (the Iapetus Ocean), which underwent subduction initiation after 40–50 Myr, to the subsequent Atlantic Ocean, which has not seen subduction initiation in its >170 Myr history. Stochasticity in ‘time to subduction initiation’ within ocean basins affects the age and density of downgoing oceanic lithosphere and must thereby influence subduction zone geodynamics (i.e., hinge advance v. retreat and stress history of the overriding plate; e.g., East Pacific- v. West Pacific-style convergence). Stochasticity in the ‘survival time’ of tectonic elements may sometimes cause local tectonic responses that are atypical of a prevailing ‘stage’ in secular cooling, superimposing ‘randomness’ on a more general, monotonic tectonic evolution trend. Modern-style metamorphism at 2100–1750 Ma may record the development of exceptionally dense lithosphere within exceptionally old ocean basins, giving rise to geodynamic settings in the Paleoproterozoic that are more representative of the colder Phanerozoic than the earlier Earth.