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

Abstract

The duration of granulite (G) and ultra-high temperature (UHT) conditions in regional metamorphism is critical to arguments regarding the tectonic settings of granulites and their relationships to Supercontinent assembly. Analysis of zircon geochronology integrated with trace element (REE) constraints on the timing of zircon growth or modification and evidence for metamorphic temperatures from Ti-in-zircon reveals that zircon can form episodically at or near the metamorphic peak in long-lived UHT granulites. This reflects the segregation, transfer and stagnation or trapping of melts which leads to local melt-rock interactions that promotes zircon crystallisation. In contrast, although rutile can retain UHT information in short duration ('fast') G-UHT terrains it is afflicted by Zr loss in long duration ('slow') terrains and yields temperatures significantly lower than those preserved by zircons formed in the same G-UHT events. An assessment of the age-duration evidence from several well-documented G-UHT terrains reveals that most are 'slow', having durations of UHT, Δt 900 , in the range 30-100 Myr. Some UHT terrains previously considered to be short-lived (Δt 900 < 10 Myr) have longer durations of UHT in the light of recent geochronology and hence are also classed as 'slow'. Of the models proposed to account for UHT metamorphism, the 'large hot orogen' (LHO) model for collisional orogeny provides the best setting for the formation of such 'slow' UHT granulites. LHO models can account not only for the P-T paths, which can range from UHT with near-isothermal decompression (UHT-ITD) through to decompression-cooling and UHT followed by near-isobaric cooling (UHT-IBC), but also for residence times under UHT conditions. The Napier Complex, the Earth's premier UHT terrain, probably formed as trapped deep crust in the hot underbelly of a late-Archaean LHO. Shorter duration UHT and near-UHT granulites also exist, mostly with Δt 900 less than 10 Myr. A number of these are likely to have formed as a consequence of severe lithospheric thinning and crustal extension accompanied by voluminous magmatism, which could occur in arc and back-arc settings affected by subduction roll-back or, as advocated by previous workers, continental arcs undergoing extension. However, attaining long-lived UHT conditions in these settings is unlikely unless the crust has inherently high radioactive heat production in addition to the transient heat added during extension and magmatism.

Two successive phases of ultrahigh temperature metamorphism in Rogaland, S. Norway: Evidence from Y‐in‐monazite thermometry

“…Modelling shows that the most likely scenario is that protracted UHT (> 10 Ma) conditions are achieved by radiogenic self‐heating of an overthickened crust over a long time‐scale (> 30 Ma). Such a scenario is, however, precluded here because of the low pressure recorded by the Rogaland UHT zone (6–4 kbar), in contrast to the 8–12 kbar predicted for the shallowest UHT rocks in the numerical models investigating crustal thickening (Harley, ; Horton, Hacker, Kylander‐Clark, Holder, & Jöns, ; Jamieson, Beaumont, Warren, & Nguyen, ). In Rogaland, therefore, the first‐order heat source is most likely to have been the underlying mantle.…”

“…In the last decade, the determination of temperature–time paths in UHT terranes mostly relied on zircon–rutile U–Pb geochronology coupled to Zr–Ti thermometry (Baldwin & Brown, ; Harley, ; Harley & Nandakumar, ; Kelsey & Hand, ; Pape, Mezger, & Robyr, ). Monazite is not used to its full potential because of the theoretical view that it entirely dissolves in the silicate melt at peak temperature conditions and subsequently grows closer to the solidus than zircon on the down temperature path (Kelsey, Clark, & Hand, ; Yakymchuk & Brown, ).…”

“…The synthesis presented by Kelsey and Hand () nevertheless highlights that, for a single terrane, the time‐scale of UHT metamorphism interpreted by different authors are widely different (e.g., Anápolis–Itauçu complex, Brazil: Baldwin & Brown, ; compared to Della Giustina, Pimentel, Ferreira Filho, & de Hollanda, ). These complications arise mainly from contrasting interpretations of zircon and monazite U–Pb geochronological data relative to P – T conditions retrieved from metamorphic phase equilibria modelling (Harley, ; Kelsey & Hand, ).…”

“…These phases are recognized by a local (rock‐scale) geochemical signature, which are indirectly related to P – T –fluid conditions in the frame of broader tectono‐thermal cycle(s). It is, however, not necessarily clear from the geochronological and trace‐element data sets how long it takes for a particular rock to reach UHT and how long it remained in the UHT window (>900°C) defined as Δ t 900 , as opposed to residence in the granulite facies window (>800°C), defined as Δ t 800 (Harley, ).…”

On the significance of short-duration regional metamorphism

“…zircon, monazite, rutile) experience dissolution-precipitation and post-crystalline resetting at UHT conditions (e.g. Kelsey 2008;Kelsey & Hand 2015;Harley 2016), adding great complexity to efforts to constrain time scales for UHT metamorphism. Durations and drivers for UHT metamorphism are beyond the scope of this review, and the interested reader is referred to the detailed reviews of Kelsey & Hand (2015) and Harley (2016).…”