K. Aoki, B. F. Windley, S. Maruyama & S. Omori reply: First, we thank Viete, Oliver & Wilde for their interesting and thought-provoking comments on the timing of the high-pressure granulite facies (HGR) metamorphism recorded in metamorphic rocks at Cairn Leuchan, Scotland, published by Aoki et al. (Reference Aoki, Windley, Maruyama and Omori2013). Based on new metamorphic data of garnetites and garnet-amphibolites at Cairn Leuchan and new zircon U–Pb ages of amphibolitized eclogite at Tomatin, we suggested in our publication that the HGR metamorphism was retrograde after eclogite facies before the c. 470 Ma ‘Barrovian metamorphism’. Viete, Oliver & Wilde however speculate that the HGR metamorphism at Cairn Leuchan may have occurred at c. 1000 Ma, as a result of their new U–Pb zircon age of the Cowhythe Gneiss at Portsoy and from previous studies of the geological structure and geochronology. We are grateful for this opportunity to describe, albeit in a preliminary manner, our new understanding and tectonic model of the Caledonian orogen in Scotland and western Ireland of which the Barrovian metamorphism is a key component. A reply to a comment is not the correct place to propose an entirely new paradigm for such a classic orogen, but we will present our model more fully in a future publication.

Viete, Oliver & Wilde presented a new U–Pb age of 1003±6 Ma for zircons in melt veins in the Cowhythe Gneiss near Portsoy, and suggested reasonably that this gneiss belongs to a tectonic slice of Precambrian basement which originally overlay Grampian sediments. This idea follows that of Sturt et al. (Reference Sturt, Ramsay, Pingle and Teggin1977) and Ramsay & Sturt (Reference Ramsay, Sturt, Harris, Holland and Leake1979). Viete, Oliver & Wilde then went on to correlate the Cowhythe Gneiss with the gneiss at Cairn Leuchan described by Aoki et al. (Reference Aoki, Windley, Maruyama and Omori2013). The correlation of the two gneisses is weak and unacceptable, because they are completely different. The Cowhythe Gneiss is an early term for the Cowhythe Psammite Formation which is a ‘psammitic to semipeltic schist with rare limestone and pelite bands’ (Stephenson & Gould, Reference Stephenson and Gould1995; Stephenson et al. Reference Stephenson, Mendum, Fettes, Smith, Gould, Tanner and Smith2013). In contrast, the predominant rocks at Cairn Leuchan described by Aoki et al. (Reference Aoki, Windley, Maruyama and Omori2013) are layers of amphibolite facies (AM) metavolcanic rocks up to 1 km thick (Stephenson et al. Reference Stephenson, Mendum, Fettes, Smith, Gould, Tanner and Smith2013); they contain garnet-rich layers/pods and locally contain melt veins. The amphibolitic rocks are interbedded with subordinate metasedimentary semipelitic to pelitic schists and gneisses that contain partial melt veins and diagnostic garnet and sillimanite. Most importantly, in their latest detailed analysis and synthesis, Stephenson et al. (Reference Stephenson, Mendum, Fettes, Smith, Gould, Tanner and Smith2013) concluded that the Cairn Leuchan rocks belong to the Queen's Hill Formation of the Crinan Subgroup of the Argyll Group of the Upper Dalradian, and that ‘although these gneissic rocks were once interpreted as pre-Dalradian basement, they are now assigned to the Crinan Subgroup’. They cannot therefore be correlated with the 1.0 Ga Cowhythe gneisses.

Metamorphic zircons such as those at Cairn Leuchan, that grow during partial melting, generally show low luminescence and weak or no zoning (e.g. Oliver et al. Reference Oliver, Bodorkos, Nemchin, Kinny and Watt1999; Foster, Schafer & Fanning Reference Foster, Schafer and Fanning2001; Söderlund et al. Reference Söderlund, Möller, Andersson, Johansson and Whitehouse2002). Viete, Oliver & Wilde however state that zircons in the Cowhythe Gneiss have prominent oscillatory zoning, but that feature is unique to igneous zircons (e.g. Corfu et al. Reference Corfu, Hanchar, Hoskin, Kinny, Hanchar and Hoskin2003). Further, the analysed zircon on the left of Figure 2b appears to have an outermost low-luminescence darker rim that cuts the oscillatory zoning. In light of these relations, it is possible that the analysed domains that yielded a mean age of 1003±6 Ma were not formed by metamorphism/migmatization but belong to an inherited detrital domain; they are therefore comparable with other zircons that yielded 3000–1300 Ma ages. There is therefore a possibility that the age of 1003±6 Ma does not reflect the time of the metamorphism/migmatization, but that of the upper age limit of deposition of the pelitic protolith. The analysed zircons are not metamorphic formed during partial melting but are detrital; in this case the determined age does not represent the time of Grenville migmatization, but rather the upper age limit of deposition of the pelitic protolith. Also, because the nearby Inzie Head gneisses and the Ellon gneisses have a Rb–Sr isochron age of 691±39 Ma and 724±120 Ma, respectively (Sturt et al. Reference Sturt, Ramsay, Pingle and Teggin1977), it is more likely that the Cowhythe Gneiss was metamorphosed during Neoproterozoic time rather than with the Leuchan metavolcanic rocks during Ordovician time. More zircon dating of these pre-Dalradian gneisses is clearly needed.

Vorhies, Ague & Schmitt (Reference Vorhies, Ague and Schmitt2013) carefully conducted high-resolution U–Pb spot secondary ion mass spectrometry (SIMS) dating of zircons from metamorphic rocks at Cairn Leuchan, and determined that the granulite-amphibolite facies metamorphism there occurred at c. 472±5 Ma. Accordingly, it seems probable that the time between the HGR and AM metamorphic events was very short; the metamorphic ages therefore become congruent within the error bars of analysis.

We now come to the perennial problem of the source of the heat for Barrovian metamorphism and crustal melting, as eloquently discussed by e.g. Jamieson et al. (Reference Jamieson, Beaumont, Fullsack, Lee, Treloar and O'Brien1998), Johnson & Strachan (Reference Johnson and Strachan2006), Lyubetskaya & Ague (Reference Lyubetskaya and Ague2010), Viete, Forster & Lister (Reference Viete, Forster and Lister2011) and Viete et al. (Reference Viete, Forster and Lister2011; Reference Viete, Oliver, Fraser, Forster and Lister2013). Despite the fact that this discussion has not involved the more extreme problems of the heat source of ultrahigh-temperature metamorphism and of eclogite facies metamorphism of many other orogens, the Barrovian heat source still remains an enigma. The main model that has dominated thinking about the tectonic development of the Caledonian orogen has considered thrust-generated crustal thickening, internal radioactive heating from the base of over-thickened crust, conduction during thermal relaxation and crustal exhumation caused by erosion (e.g. England & Thompson, Reference England and Thompson1984). However, Lyubetskaya & Ague (Reference Lyubetskaya and Ague2010) pointed out that overthrusting of continental crust with average values of basal heating and radiogenic heat production cannot produce the conductive heat exchange required for the P–T–t paths characteristic of typical Barrovian metamorphism, or the required timescale of at least 50 Ma (Viete, Forster & Lister Reference Viete, Forster and Lister2011), because the short life span of the Caledonian Barrovian metamorphism of 18–12 Ma is well established (e.g. Dewey, Reference Dewey2005; Oliver et al. Reference Oliver, Chen, Buchwaldt and Hegner2000; Lyubetskaya & Ague, Reference Lyubetskaya and Ague2010). We agree with Viete, Oliver & Wilde that the lack of rocks in the Grampian Terrane enriched in heat-producing elements, combined with a model of thrust-generated doubling of crustal thickness, would require a timescale of 30–40 Ma to produce the necessary temperature increase; we also agree that this is not possible given the short life span required for the Barrovian metamorphism. All these considerations lead us to agree with Viete et al. (Reference Viete, Oliver, Fraser, Forster and Lister2013) that the model of thrusting to create such a thickened crust, and responsive deep crustal heating and isostatic uplift and thermal relaxation, is totally untenable for the Scottish Caledonides. However, Viete et al. (Reference Viete, Oliver, Fraser, Forster and Lister2013) concluded that the Barrovian metamorphism formed ‘as a result of advection of heat from the lower crust and/or mantle’. Because we do not believe that the Barrovian metamorphism was a mere ‘transient phase of crustal thermal equilibrium’, we propose an alternative and more viable model to explains the whole Caledonian orogenesis: the extrusion of a major wedge of hot deep eclogite which was exhumed up a subduction channel several tens of kilometres thick, similar to that summarized by Agard et al. (Reference Agard, Yamato, Jolivet and Burov2009) and Jamieson et al. (Reference Jamieson, Unsworth, Harris, Rosenberg and Schulmann2011) for many orogens worldwide. Such a wedge extrusion model is supported by the presence of originally eclogite facies rocks at Tomatin and Cairn Leuchan, which are (as predicted) within the central highest-grade sillimanite-kyanite Barrovian zone of the extruded wedge. Further, since such a high-temperature source obviates the need to find that long-sought-for enigmatic extra heat source in the Caledonides (‘where's the heat?’, Jamieson et al. Reference Jamieson, Beaumont, Fullsack, Lee, Treloar and O'Brien1998; ‘the missing heat problem of Barrovian metamorphism’, Lyubetskaya & Ague, Reference Lyubetskaya and Ague2009) which has given rise to such disparate (and in our opinion, unnecessary) proposals such as: mantle heat advection associated with gabbro intrusions (Viete et al. Reference Viete, Richards, Lister, Oliver, Banks, Law, Butler, Holdsworth, Krabbendam and Strachan2010); sheeted magmas concentrated on shear zones (Viete et al. Reference Viete, Forster and Lister2011); heat dissipation from underlying mid-crustal shear zones during Grampian extension (Viete, Forster & Lister Reference Viete, Forster and Lister2011); metamorphic thermal reactions (Lyubetskaya & Ague, Reference Lyubetskaya and Ague2010); elevated radiogenic heat production (Huerta, Royden & Hodges, Reference Huerta, Royden and Hodges1998); back-arc setting (Johnson & Strachan, Reference Johnson and Strachan2006; Viete, Oliver & Wilde); and large-scale contact metamorphism (Viete et al. Reference Viete, Oliver, Fraser, Forster and Lister2013).

Viete et al. (Reference Viete, Oliver, Fraser, Forster and Lister2013) stated that there is no evidence of metamorphism and magmatism before c. 473 Ma in the first 15 Ma of the Grampian orogeny which could have caused the HGR metamorphism (e.g. Chew et al. Reference Chew, Daly, Page and Kennedy2003). Firstly, we do not believe that isotopic dating in the Caledonides is so abundant and tightly constrained with minuscule errors that it is possible to be so definitive in stating that HGR metamorphism could only take place during a specific 13 Ma window (Viete, Oliver & Wilde). The timing of the metamorphism in an orogen is dependent on many factors, not least the chosen tectonic model; the wrong model could yield different timing. The Grampian orogeny was caused by a brief arc–continent collision at c. 475–465 Ma (Friedrich et al. Reference Friedrich, Bowring, Martin and Hodges1999a , b; Dewey, Reference Dewey2005), and was diachronous from c. 480–465 Ma in Scotland (or 488–475 Ma according to Viete, Oliver & Wilde) to 470–460 Ma in western Ireland (Oliver, Reference Oliver2001). At Tomatin in east Scotland retrogressed eclogites have zircon ages of 485±37 Ma and 446±32 Ma (Aoki et al. Reference Aoki, Windley, Maruyama and Omori2013). In the Naver nappe in Sutherland, almost-high-pressure sillimanite-grade metamorphism of Moine rocks took place at c. 470–460 Ma (Kinny et al. Reference Kinny, Friend, Strachan, Watt and Burns1999) under conditions of c. 11–12 kbar and 650–700 ºC (Friend, Jones & Burns, Reference Friend, Jones and Burns2000). On Achill, NW Ireland blueschist facies metamorphism took place at c. 460 Ma (Chew et al. Reference Chew, Daly, Page and Kennedy2003) and, as expected, was overprinted by medium-pressure Barrovian assemblages (Yardley, Barber & Gray, 1987). Only a model of a westwards-moving extruded wedge can account for such a diachronous westwards-younging metamorphism, which adequately includes the HP metamorphism at Tomatin and Cairn Leuchan.

Models such as crustal heating by heat conduction from the mantle, contact metamorphism or a back-arc setting are totally inadequate because they cannot explain the available geological evidence. When considering the thermal evolution of the Tauern Window, we agree with Smye et al. (Reference Smye, Bickle, Holland, Parrish and Condon2011) that synthrust-heating by rapid exhumation and emplacement of a hot eclogite wedge, overlooked by previous thermal models, can best resolve the Barrovian conundrum; this plausibly explains the tectono-metamorphic evolution of the Eastern Alps as well as the Grampian Caledonides. Full details and documentation of our Caledonian model will be provided in a later publication.