Challenges in ultrahigh-temperature (UHT) granulite metamorphism and crustal melting
Information about the evolving thermal structure of the crust during orogenesis is encoded in the microstructures, mineral assemblages and mineral compositions of igneous and metamorphic rocks. This information may be inverted through equilibrium thermodynamic methods to provide quantitative estimates of the P (pressure)–T (temperature) evolution. If P–T is linked to time (t) via well-constrained geochronology, then these data parameterize orogenesis. For near-solidus metamorphic rocks, the P–T conditions retrieved are generally consistent with estimates of heat flow into the lithosphere and average values of crustal heat production. However, UHT metamorphic rocks, those preserving evidence of T >900 ºC in the stability field of sillimanite and apparent thermal gradients >1000 °C/GPa, pose unique challenges in determining the P–T–t evolution and are more demanding to explain. Current geodynamic models of orogenesis do not typically predict such extreme apparent thermal gradients. One-dimensional numerical modeling of thermal histories suggests that a high internal concentration of heat producing elements combined with a low rate of erosion may be sufficient to power UHT metamorphism. This contrasts with popular mechanisms proposed in the literature, such as ridge subduction and slab breakoff. This session will advance our understanding of granulites and crustal melting, particularly at UHT metamorphic conditions. Thus, the focus will be on issues related to melting and melt loss, quantifying the P–T conditions and P–T paths of UHT granulites, linking geochronology to points along the P–T path in these rocks and solving the thermal conundrum that UHT metamorphism poses.
Michael Brown, Mark Caddick, Chris Clark and Johann Diener