On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

Abstract

a b s t r a c tUltrahigh temperature (UHT) metamorphism is the most thermally extreme form of regional crustal metamorphism, with temperatures exceeding 900 C. UHT crustal metamorphism is recognised in more than 50 localities globally in the metamorphic rock record and is accepted as 'normal' in the spectrum of regional crustal processes. UHT metamorphism is typically identified on the basis of diagnostic mineral assemblages such as sapphirine þ quartz, orthopyroxene þ sillimanite AE quartz and osumilite in MgeAlrich rock compositions, now usually coupled with pseudosection-based thermobarometry using internally-consistent thermodynamic data sets and/or Al-in-Orthopyroxene and ternary feldspar thermobarometry. Significant progress in the understanding of regional UHT metamorphism in recent years includes: (1) development of a ferric iron activityecomposition thermodynamic model for sapphirine, allowing phase diagram calculations for oxidised rock compositions; (2) quantification of UHT conditions via trace element thermometry, with Zr-in-rutile more commonly recording higher temperatures than Ti-in-zircon. Rutile is likely to be stable at peak UHT conditions whereas zircon may only grow as UHT rocks are cooling. In addition, the extent to which Zr diffuses out of rutile is controlled by chemical communication with zircon; (3) more fully recognising and utilising temperature-dependent thermal properties of the crust, and the possible range of heat sources causing metamorphism in geodynamic modelling studies; (4) recognising that crust partially melted either in a previous event or earlier in a long-duration event has greater capacity than fertile, unmelted crust to achieve UHT conditions due to the heat energy consumed by partial melting reactions; (5) more strongly linking UePb geochronological data from zircon and monazite to PeT points or path segments through using Y þ REE partitioning between accessory and major phases, as well as phase diagrams incorporating Zr and REE; and (6) improved insight into the settings and factors responsible for UHT metamorphism via geodynamic forward models. These models suggest that regional UHT metamorphism is, principally, geodynamically related to subduction, coupled with elevated crustal radiogenic heat generation rates.Ó 2014, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Pan–African granulites of Madagascar and southern India: Gondwana assembly and parallels with modern Tibet

“…Cryptic 0.63-0.61 Ga ages in these rocks can be linked to initial collision along the Mozambique suture, but it is unclear why peak thermal gradients of 25-35°C km −1 were generated at 0.58-0.53 Ga, which is >70 Myr after the Mozambique collision but before the proposed Malagasy collision. Many rocks in southern Madagascar and southern India record peak T > 900°C, making them examples of ultra high-temperature (UHT) metamorphism (Harley, 1998(Harley, , 2008Kelsey, 2008;Kelsey and Hand, 2015). Two main mechanisms have been proposed to account for UHT conditions in the crust: advection of mantle heat by lithosphere extension and magmatism, and crustal heat production by radioactive decay, with other possibilities unlikely to be significant on a regional scale .…”

“…Kelsey and Hand (2015) argued that UHT events with durations >40 Myr are driven by radiogenic heat, although they misclassified Madagascar and southern India as UHT domains of short duration. Radiogenic heating is promoted by high concentrations of heat-producing elements and thick continental crust, and is maximised in long-lived orogens but reduced by erosion .…”

A matter of time: The importance of the duration of UHT metamorphism

“…Largely as a result of the developments encapsulated in the reviews by Harley and co-workers (Harley 1998a(Harley , 1998b(Harley , 2004(Harley , 2008Clark et al, 2011) and Kelsey and co-workers (Kelsey, 2008;Kelsey and Hand, 2015) it is now accepted that granulite, and in particular UHT, metamorphism plays a fundamental role in the development and stabilisation of continents through accretionary and collisional orogenesis, as well as being of importance in extensional environments. For example, with over 60 UHT areas or localities now recognised globally , which formed at various times from the mid Archaean through to the Cenozoic (Brown, 2006), it is clear that understanding this type of extreme metamorphism is critical to constraining the large-scale tectonic processes affecting the deep crust and lithosphere throughout Earth history.…”

“…The application of calculated phase diagrams to quantify the P-T conditions of granulites and UHT rocks (G-UHT metamorphism) has been described and evaluated in some detail in recent reviews (Kelsey, 2008;Kelsey and Hand, 2015), and it is now largely accepted that this approach (i.e., the pseudosection methodology based on the pioneering theoretical work of B.J. Hensen) -when used carefully with due regard for system complexity, melt production and transfer and domainal equilibrium -provides important insights into granulite and UHT P-T conditions and histories for many rock compositions.…”

Melt Reintegration Modelling: Testing against a Subsolidus Reference Assemblage

“…However, it is well established that residual bulk compositions are not appropriate for modelling the prograde evolution of a rock [12,18,51]. The melting and melt loss that has occurred in the field area would have altered the composition of the rocks so that the subsolidus part of the pseudosection does not represent real prograde assemblages.…”