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The continental crust is the archive of the geological history of the Earth. Only 7% of the crust is older than 2.5 Ga, and yet significantly more crust was generated before 2.5 Ga than subsequently. Zircons offer robust records of the magmatic and crust-forming events preserved in the continental crust. They yield marked peaks of ages of crystallization and of crust formation. The latter might reflect periods of high rates of crust generation, and as such be due to magmatism associated with deep-seated mantle plumes. Alternatively the peaks are artefacts of preservation, they mark the times of supercontinent formation, and magmas generated in some tectonic settings may be preferentially preserved. There is increasing evidence that depletion of the upper mantle was in response to early planetary differentiation events. Arguments in favour of large volumes of continental crust before the end of the Archaean, and the thickness of felsic and mafic crust, therefore rely on thermal models for the progressively cooling Earth. They are consistent with recent estimates that the rates of crust generation and destruction along modern subduction zones are strikingly similar. The implication is that the present volume of continental crust was established 2-3 Ga ago.

Identifying and dating large impact structures is challenging, as many of the traditional shock indicator phases can be modified by post-impact processes. Refractory accessory phases, such as zircon, while faithful recorders of shock wave passage, commonly respond with partial U–Pb age resetting during impact events. Titanite is an accessory phase with lower Pb closure temperature than many other robust chronometers, but its potential as indicator and chronometer of impact-related processes remains poorly constrained. In this study, we examined titanite grains from the Sudbury (Ontario, Canada) and Vredefort (South Africa) impact structures, combining quantitative microstructural and U–Pb dating techniques. Titanite grains from both craters host planar microstructures and microtwins that show a common twin–host disorientation relationship of 74° about . In the Vredefort impact structure, the microtwins deformed internally and developed high- and low-angle grain boundaries that resulted in the growth of neoblastic crystallites. U–Pb isotopic dating of magmatic titanite grains with deformation microtwins from the Sudbury impact structure yielded a 207Pb/206Pb age of 1851 ± 12 Ma that records either the shock heating or the crater modification stage of the impact event. The titanite grains from the Vredefort impact structure yielded primarily pre-impact ages recording the cooling of the ultra-high-temperature Ventersdorp event, but domains with microtwins or planar microstructures show evidence of U–Pb isotopic disturbance. Despite that the identified microtwins are not diagnostic of shock-metamorphic processes, our contribution demonstrates that titanite has great potential to inform studies of the terrestrial impact crater record.

U-Pb detrital zircon data show that the East Mainland Succession, the presumed correlative of the Dalradian Supergroup in the Shetland Islands, Scotland, is dominated by Mesoproterozoic and Archaean material, with some Palaeoproterozoic detritus. The data are most consistent with derivation from eastern Laurentia, although western Baltica sources cannot be excluded. A magmatic event at c. 576 Ma supplied detritus to the Clift Hills Group, and was the source of high-temperature fluids that resulted in growth of new metamorphic zircon, and altered old detrital grains within the underlying sedimentary pile. This provides a constraint on the age of the global Shuram-Wonaka event recognized within the Whiteness Group, which underlies the Clift Hills Group. The presence of common Archaean detritus is compatible with broad time-correlation of the East Mainland Succession with the Dalradian Supergroup. However, differences in age, thickness and basin evolution are consistent with deposition in separate basins along the extending Laurentian margin during supercontinent break-up and development of the Iapetus Ocean. Similarities in the detrital zircon records of the East Mainland Succession and the offshore Devonian-Carboniferous Clair Group permit derivation, at least in part, of the latter from the Shetland Islands and proximal sources on the adjacent continental shelf.

The Glenelg-Attadale Inlier is the largest basement inlier within the Caledonian orogen in NW Scotland. A Western Unit consists of trondhjemite-tonalite-granodiorite (TTG) gneisses and subordinate mafic intrusions with locally preserved mafic high-pressure granulite and eclogite assemblages. An Eastern Unit comprises TTG gneisses, Grenville-age (c. 1.1 Ga) eclogite and metasediments. U-Pb zircon ages from the Western Unit TTG gneisses are highly disturbed with Neoarchaean upper intercepts and Palaeo- to Mesoproterozoic lower intercepts, suggesting strong reworking at these times. U-Pb zircon, Sm-Nd and Lu-Hf garnet-clinopyroxene dates from the Western Unit high-pressure granulite and eclogite yield Neoarchaean (c. 2.6-2.8 Ga) and Palaeoproterozoic (c. 1.75 Ga) ages, respectively. These ages correspond to the ages of partial resetting of the TTG gneisses. The eclogite in the Western Unit may represent the high-pressure convergent margin to the lower-pressure Laxfordian events within the Lewisian Gneiss Complex, whereas the Mesoproterozoic eclogites in the Eastern Unit may represent a farther eastward Grenville-age margin. Further east, Ordovician high-pressure granulites within the Moine Supergroup may represent another, later, Grampian convergent margin. These high-pressure belts were developed sequentially towards the east and telescoped westwards from Palaeoproterozoic to Early Palaeozoic times.

The Glenelg-Attadale Inlier is the largest basement inlier within the Caledonian Moine nappe of NW Scotland. In the eastern part of the inlier amphibolite-facies retrogression of the eclogites is associated with tectonic fabrics, and P-T estimates indicate significant decompression (c. 20 km). Previous Sm-Nd mineral-whole-rock dates indicated that peak eclogite-facies metamorphism occurred around c. 1.08 Ga, which was correlated with the Grenvillian orogeny. However, the middle REE enrichment of the analysed garnets suggests the influence of apatite inclusions. It is therefore likely that the interpretation of the c. 1.08 Ga age is complex, possibly reflecting re-equilibration at lower temperatures. Sampled eclogites contain zircon in a number of distinct textural forms that are mainly associated with pargasite and plagioclase, and are part of the retrograde amphibolite-facies assemblages. Titanite extensively replaces rutile, and is clearly associated with the retrograde amphibolite-facies event. A second textural type of titanite forms anhedral grains with plagioclase and pargasite, which is developed where the retrograde amphibolite-facies assemblage overprints the eclogite mineralogy. U-Pb dating has yielded the following ages: zircon age of 995±8 Ma, and variably discordant rutile ages between 416 and 480 Ma. U-Pb and Pb-Pb isochrons on titanite and plagioclase/quartz separates yielded ages of 971±65 Ma and 945±57 Ma, respectively, in agreement with the zircon age. Analysed zircons and titanites are texturally part of the amphibolite-facies assemblage. The new zircon age demonstrates that amphibolite-facies metamorphism during exhumation occurred at 995±8 Ma; the titanites could have closed with respect to Pb at this time or alternatively at some time between c. 1000 and 900 Ma. These data clearly demonstrate that parts of the Scottish basement underwent major thick-skinned tectonics during the Grenvillian orogeny. Rutile is part of the eclogite-facies paragenesis, and yet has young ages; these data are best explained by reheating producing near-total Pb loss related to emplacement of the late- to post-tectonic Ratagain Granite Complex at c. 425 Ma, at the end of the Caledonian orogeny.

Peak and retrograde P–T conditions of Grenville-age eclogites from the Glenelg–Attadale Inlier of the northwest Highlands of Scotland are presented. Peak conditions are estimated as c. 20 kbar and 750–780°C, in broad agreement with previous work. The eclogites subsequently followed a steep decompression path to c. 13 kbar and 650–700°C during amphibolite facies retrogression. Peak eclogite facies metamorphism occurred > 1080 Ma and retrogression at c. 995 Ma, suggesting fairly sluggish uplift rates of < 0.3 km/Ma and cooling rates of < 1.25°C/Ma, when compared with other parts of the Grenville orogeny and/or modern orogens. However, current poor constraints on the timing of peak metamorphism mean that these rates cannot be used to interpret the geodynamic evolution of this part of the orogen. The P–T–t data, together with petrology and the field relationships between the basement rocks of the Glenelg–Attadale Inlier and the overlying Moine Supergroup, mean that it is difficult to support the currently held view that an unconformable relationship exists between the two. It is suggested that more data are required in order to re-interpret the Neoproterozic tectonic evolution of the northwest Highlands of Scotland.

This chapter contains sections titled: Abstract Introduction The Composition and Differentiation of the Continental Crust The Zircon Archive The Igneous and Sedimentary Records Crustal Growth Processes from the Igneous Record Using Zircons The Lachlan Case Study The Continental Record ‐ Peaks of Crust Generation or a Function of Preservation? The Composition of the Early Proto Continental Crust The Sedimentary Record and Erosion Models Summary Acknowledgements Further Reading References

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