On the significance of short-duration regional metamorphism

Abstract

Short-duration regional metamorphism is a recently observed and poorly understood phenomenon in metamorphic geology. In this review, it is defined as metamorphism on time scales that limit length scales (of the associated thermal anomaly) to significantly less than the thickness of the orogenic crust (<10 myr) or subducted oceanic lithosphere (<5 myr). Without appealing to exceptional heat sources, thermal models have been unable to account for peak metamorphic temperature during collisional orogenesis and subduction. This observation, combined with restricted time scales for regional metamorphism, suggests that metamorphic facies series can record atypical and transient thermal conditions (related to punctuated and localized heat advection and/or production), rather than normal, ambient conditions for the tectonic setting to which they are allied. Highprecision geochronology can resolve short-duration metamorphic estimates of 1 -10 myr. However, diffusion geospeedometry typically yields extremely short metamorphic durations (<1 myr); tools in metamorphic geology may have matured to the point that the discipline is beginning to recognize episodicity and criticality in deep processes. New, very high-precision petrochronology techniques offer great potential to probe the veracity of extremely short metamorphic durations being obtained from diffusion geospeedometry. Benchmarking of these new very high-precision petrochronology techniques must become a priority for metamorphic geology.Received 26 May 2016; revised 2 October 2016; accepted 27 October 2016Early interpretations of the geological record leant heavily on catastrophism. As geology matured as a science, uniformitarianism came to supersede catastrophism. Uniformitarianism offers a more sophisticated, logical framework for interpretation of the rock record within the context of current and observable processes. However, the grand success of the uniformitarian philosophy has also resulted in a propensity toward interpretation of deep (and thus unobservable) geological phenomena in terms of slowly evolving or steady-state processes. A notable example of this preference for gradualism in deep processes has been ideas for the origins of regional metamorphism and metamorphic facies series, and implications for the nature of tectonism. Barrow (1893Barrow ( , 1912 was the first to map regional metamorphism on the basis of diagnostic mineral assemblages. His Barrovian metamorphic sequence, exposed in NE Scotland, comprises a series of 'isograds' marking the first appearance of chlorite, biotite, garnet, staurolite, kyanite and sillimanite in pelitic lithologies, in the direction of increasing metamorphic grade. Regional metamorphism recorded by Barrovian-type isograds defines intermediate pressure/temperature (P/T ) conditions (575 -1250°C GPa Barrow (1893) originally proposed that his Barrovian metamorphism resulted from magmatic advection of heat from depth. Following development of the theory of plate tectonics, numerous models were proposed for the ori...

Prolonged (>100 Ma) ultrahigh temperature metamorphism in the Napier Complex, East Antarctica: A petrochronological investigation of Earth's hottest crust

“…While it is possible that the earlier stages of UHT metamorphism recognized by Harley et al (2001) reflect one or more short pulses of localized heating (Pownall, Hall, Armstrong, & Forster, 2014) and do not reflect UHT conditions being achieved across the terrane at this time, the absence of any magmatism that can be directly linked to this early stage argues against this scenario. However, it needs to be acknowledged that such small pulses of heating at the start of an orogeny could be important in achieving the regional-scale peak conditions observed (Pownall et al, 2014;Viete & Lister, 2017). It would be very difficult to recognize these localized, short-duration heating events in the current rock record as they would be significantly modified and obscured by regional-scale UHT event that follows.…”

LinT, a simplified approach to oxygen-isotope thermometry and speedometry of high-grade rocks: An example from ultrahigh-temperature gneisses of southern Madagascar

“…In more mineralogically complex samples, LinT could be used to calculate different instantaneous dT/dt for each mineral T c , defining dT/dt as a function of T. Alternatively, in rocks with effectively binary mineral assemblages in which one mineral is much more abundant than the other (such as minor magnetite in a marble), dT/ dt can be evaluated as a function of T by measuring δ 18 O of the minor phases as a function of grain size (Sharp, 1991). Although dT/dt values inverted from LinT temperatures are imprecise, the uncertainties are similar in magnitude to those obtained by cation diffusion chronometry (e.g., Spear and Parrish, 1996); dT/dt values inverted from LinT temperatures will be most useful for identifying periods of anomalously fast cooling relative to the time-integrated average (e.g., Spear and Parrish, 1996) or "short-duration" metamorphic events (Viete and Lister, 2016).…”

The Potential for Metamorphic Thermal Pulses to Develop During Compaction‐Driven Fluid Flow

“…With the advances in high‐precision geochronology, short‐duration regional metamorphism on the order of 1–10 Ma can be resolved. In addition, geospeedometry based on intracrystalline diffusion is able to constrain extremely short‐duration thermal events <1 Ma (Ague & Baxter, ; Viete & Lister, ), enabling resolution of episodicity during regional metamorphism. The current model establishes that the short time scales of channelized compaction‐driven fluid flow make it a plausible mechanism for generating transient thermal excursions superimposed on long‐term conduction‐dominated regional metamorphism.…”

“…Overthickened continental crust may store such fluids, and dehydrate within time scales of order $10 4 years in the case of rapid intermittent reactions (e.g., Figure 5), but such scenarios might not be Geochemistry, Geophysics, Geosystems 10.1002/2017GC007269 commonplace. Nonetheless, rapid intermittent dehydration likely occurs in some metamorphic regimes (Dragovic et al, 2015;and references in Ague [2014] and Viete and Lister [2017]). For example, for dehydration reactions that are univariant or narrowly divariant in pressure-temperature space (e.g., Baxter & Caddick, 2013), small variations in pressure or temperature can easily cross reaction boundaries to generate large quantities of fluids along normal geotherms.…”

“…Geochemistry, Geophysics, Geosystems 10.1002/2017GC007269 constrain extremely short-duration thermal events <1 Ma (Ague & Baxter, 2007;Viete & Lister, 2017), enabling resolution of episodicity during regional metamorphism. The current model establishes that the short time scales of channelized compaction-driven fluid flow make it a plausible mechanism for generating transient thermal excursions superimposed on long-term conduction-dominated regional metamorphism.…”