FIFTY YEARS AFTER THE REVOLUTION: METAMORPHISM AND PLATE TECTONICS (Invited Presentation)

Plate tectonics provides a context for contemporary metamorphism in relation to different tectonic settings. Following the magical year of 1968, when the elements of plate tectonics were finally pulled together following Wilson’s 1965 insight, the relationship between blueschist-type metamorphism and subduction was recognized by Ernst, and subsequently confirmed by multiple studies. In parallel, the asymmetry of subduction, whereby the downgoing slab depresses isotherms creating low dT/dP along the slab whereas magma generation in the mantle wedge above leads to high dT/dP in the overriding plate, was modeled by Oxburgh and Turcotte, providing an explanation for the formation of Miyashiro’s paired metamorphic belts, although, as Brown has argued, juxtaposition of these belts may be due to strike-slip translation along the trench.

The 1980s saw conditions of crustal metamorphism extended to Ts and Ps almost beyond imagination. Following the discovery in 1980 by Ellis and others that large areas of lower crustal rocks in Antarctica record T >1000 °C, more than 50 localities globally have been shown to record T >900 °C. Within 4 years, the exciting realm of ultrahigh-pressure (UHP) metamorphism was uncovered through the identification by Chopin of coesite in pyrope schists of the Dora Maira massif (Western Alps) and confirmation of its occurrence in eclogite from Norway. More surprising still has been the discovery of microdiamonds in several UHP terranes and evidence of former stishovite in deeply subducted metasediments. Studies of UHP metamorphism and advances in microanalysis have shed light on how subduction recycles materials through the mantle.

During the Precambrian, mantle Tp was higher than today and the geodynamic regime may have been different. De Roever (pre-1968) and Ernst and Grambling (post-1968) were first to discuss secular change in metamorphism, but the quantitative data on which arguments were based were poor. Thermobarometry was in its infancy and fully quantitative P–T phase diagrams for chemical systems approaching the complexity of natural rocks still lay in the future. It was more than 30 years before sufficient reliable P–T and age data were available to allow a full analysis of secular change by Brown, which enabled him to contribute to the debate about when plate tectonics emerged on Earth.