CURRENT AND FUTURE CHALLENGES IN METAMORPHIC PETROLOGY (Invited Presentation)

The petrological community has made enormous progress in understanding metamorphic processes and their impact on the geological, geochemical, and geophysical evolution of the lithosphere at the planetary scale. This research has, in turn, revealed a host of emerging scientific questions with fundamental implications for metamorphic pressure distributions as well as energy and mass transport, several of which are reviewed here.

Metamorphism at or near lithostatic pressure has been a cornerstone of petrology for many decades. In recent years, however, this prevailing wisdom has been challenged on scales ranging from individual grain contacts to outcrops to entire orogenic belts. If non-lithostatic pressures are widespread, then the traditional interpretation that pressure is directly related to depth must be reevaluated. Given the limited strength of deeply-buried rocks, extremely transient pressure excursions may be a clue to non-lithostatic behavior.

Studies of geochemical cycling have revealed that metamorphism plays a much more active role in the transfer of elements (e.g. C, S, N, and halogens) between deep reservoirs and the shallow hydrosphere and atmosphere. The more work that is done, the more important these connections are shown to be. This research has also highlighted how much still needs to be learned. Quantifying input and output fluxes in subduction zones has been and will continue to be a major research focus. Deep cycling in collisional orogenic belts has received much less attention but has the potential to be a major contributor to global fluxes. Furthermore, non-traditional modes of fluid-rock interaction, such as interface-coupled dissolution-reprecipitation, may be much more important controls on element cycling that previously recognized.

UHT and UHP metamorphic rocks are now firmly established as important components in many of the world’s orogenic belts. Yet a surprisingly diverse range of interpretations remains regarding how extreme P-T conditions are reached, and how such rocks are exhumed. In addition, the relatively poor and selective preservation of the traditional UHP minerals diamond and coesite means that new UHP indicators must be established.

Addressing these and other developing research avenues will provide exciting scientific challenges for many years to come.