ULTRA-FAST HIGH-PRESSURE METAMORPHISM: A POTENTIAL PETROLOGIC TEST FOR DEVIATIONS FROM LITHOSTATIC PRESSURE

The conventional wisdom holds that metamorphic reactions take place under lithostatic pressure so that the thermodynamic pressure, reflected by the mineral assemblage, is directly correlated with depth. The rocks are assumed to be weak and thus cannot sustain large pressure variations (Green, 2005). Nevertheless, lateral stress variations and significant pressure deviations from lithostatic are predicted in several recent geodynamic simulations (summarized in Gerya, 2015). In this study, we present the first tightly-constrained petrological observation of metamorphism under a tectonic overpressure (Taconic orogen, northwest Connecticut, USA). Eclogite in a Taconic thrust slice records a pressure-temperature (P-T) anomaly of 9–14.5 kbar, 700–750 °C, superimposed on the regional greenschist-amphibolite facies background (450–550 °C, 4–8 kbar; e.g., Hames et al. 1991). The peak conditions and P-T path are inferred from pseudosections based on the mineral assemblage and mineral compositions_. Sharp compositional zones are preserved in garnets from the eclogite and suggest extremely short time scales. Remarkably, the diffusion modeling for garnet zones indicates that the post-peak-T compression lasted only 500 years. The pressure peak was followed be a decompression process lasting <500 years. If the pressure increase (~5 kbar) were due to the burial along a lithostatic pressure gradient, the descent speed would exceed 30 m/yr, defying any observed or modeled subduction rates. Overpressure in response to the deformation and/or heating in the orogen, and subsequent overpressure release, are the only possible explanations for the ultra-fast metamorphic process. The short-lived event also requires a transient and local heat source like strain heating that imposed a thermal anomaly on the regional lower-grade background, consistent with the geologic setting in a thrust zone. Garnets preserving sharp chemical zones in high-pressure terranes may be recorders of ultra-fast high-pressure metamorphism indicative of transient but significant deviations from lithostatic pressures.