UHT metamorphism in collisional belts

Ultrahigh-temperature (>900°C; UHT) metamorphism is the most extreme thermal condition that the continental crust is capable of sustaining at crustal depths of 15-55 km. Therefore, UHT metamorphic rocks serve as the window into the deep crustal processes that operated in the past, and provides opportunity to study how processes evolved over time. Records of UHT metamorphism are available from the Mesoarchean in a variety of tectonic settings, most commonly in refractory crustal rocks that are left after partial melting and melt loss. It has been clearly understood that the UHT metamorphism requires geothermal gradients that are much higher than the average continental geotherm. However, there is little consensus about the mechanisms by which UHT metamorphic conditions are achieved. Moreover, the duration of UHT metamorphism is closely linked with the type of tectonic setting and the nature of heat. The temperature and duration exert a first order control on the crustal strength and, therefore, influences its stability and evolution.

In our studies, we integrate field-based observations, microtextural studies, mineral chemistry, thermodynamic modelling and texturally-controlled in-situ monazite geochronology to characterize UHT conditions in Precambrian crustal rocks in collisional zones and determine the source of heat and duration of UHT metamorphism. By combining these, we comment on the evolution of crust over time.