Extending our understanding of Ultrahigh temperature crustal metamorphism

Abstract

Rapid developments in the scope and quality of internally consistent thermodynamic data sets arising from the incorporation of new experimental data and solid solution models for minerals and melts involved in ultrahigh temperature (UHT) metamorphism now place petrologists in an excellent position to evaluate the detailed mineral assemblage evolutions of many UHT rocks and place these in quantitative frameworks using P T diagrams and pseudosection approaches. These developments will soon be enhanced by extension of the methods to systems at high f o2 , enabling a wider range of critical mineral assemblages to be considered and their UHT P T evolutions, at present largely qualitatively constrained, to be determined quantitatively. Cordierite provides a useful monitor of the role and composition of any fluids and melts attending UHT metamorphism at <1000ºC. Such fluids or melts remaining in communication with the mineral assemblages following UHT have a significant effect on preservation of the metamorphic record and may also strongly influence the ages recorded by geochronological systems, even to the extent of producing widespread post UHT metamorphic zircon. Modelling of UHT metamorphism and P T paths, whether IBC, ITD or hybrid ITD IBC, in terms of specific and testable tectonic settings and processes is still problematic, with those models that can successfully duplicate the extraordinary temperatures attained in some UHT terrains generally being geologically unreasonable. Successful modelling of the thermal conditions and evolution of UHT terrains, constrained by quantitative P T t paths, is an important objective for future research that will inform our understanding of this intriguing aspect of the behaviour of the Earth system.

Speculations on the formation of cratons and cratonic basins

“…Fig. 8b shows that the ultrahigh temperature assemblages discussed by Harley (1989Harley ( , 2004 can be produced in the same way, especially if the shortening occurred in the Archaean or Early Proterozoic, when the rate of crustal heat generation was greater than it is now.…”

“…(b) shows the same curves for shortening occurring 3 Ga ago. The pink dots show P, T estimates for ultrahigh temperature metamorphic terranes from Harley (1989Harley ( , 2004. Allen and Armitage (2012).…”

New constraints on the metamorphic evolution of the Eastern Ghats Belt, India, based on relict composite inclusions in garnet from ultrahigh‐temperature sapphirine granulites

“…Orthopyroxene in this study contains >10 wt% Al 2 O 3 in the core and gives temperature of 10008C when plotted in the sapphirine þ quartz þ orthopyroxene field of the Al-isopleth diagram of Harley (2004). This assemblage has a distinct stability field towards high temperature in the FMAS petrogenetic grid.…”

“…The Al content of orthopyroxene in association with sapphirine and quartz has now been used as a reliable sensor for UHT metamorphism (Harley and Motoyoshi, 2000;Hollis and Harley, 2003;Harley, 2004) Al zoning in orthopyroxene has been experimentally calibrated in MAS system (Anastasiou and Seifert, 1972;Arima and Onuma, 1977;Hollis and Harley, 2003) as well as theoretically modelled in the more complex FMAS system (Hensen and Harley, 1990). Harley and Motoyoshi (2000) estimated temperatures in excess of 11008C in the Napier Complex, where orthopyroxene contains 12.5 wt% Al 2 O 3 , using this assemblage.…”

Confirmation of ultrahigh-temperature metapelitic granulite in the Altay orogen and its geological significance

“…UHT metamorphism is a type of very high temperature granulite facies metamorphism, where crustal rocks experienced peak metamorphic temperature in excess of 900°C, generally characterized by indicative mineral assemblages such as orthopyroxene?sillimanite?quartz, sapphirine?quartz, and osumilite?quartz, while orthopyroxene shows a high-Al feature (Al 2 O 3 [8.0 wt%) [1][2][3]. To date, there have been over 30 localities of UHT granulites reported from around the world, e.g.…”