Bruno Dhuime

The generation and evolution of the continental crust

Abstract: 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.

Detrital zircon record and tectonic setting

ABSTRACTDetrital zircon spectra refl ect the tectonic setting of the basin in which they are deposited. Convergent plate margins are charac-terized by a large proportion of zircon ages close to the depositional age of the sediment, whereas sediments in collisional, extensional and intracratonic settings contain greater proportions with older ages that refl ect the history of the underlying basement. These differences can be resolved by plotting the distribution of the difference between the measured crystallization ages (CA) of individual zircon grains present in the sediment and the depositional age (DA) of the sedi-ment. Application of this approach to successions where the original nature of the basin and/or the link to source are no longer preserved constrains the tectonic setting in which the sediment was deposited.INTRODUCTION Detrital zircons are a minor constituent of clastic sedimentary rocks, yet their physiochemical resilience and high concentrations of certain key trace elements means that they have become an important phase in sedimentary provenance analysis and in crustal evolution studies (e.g., Cawood et al., 2007b; Hawkesworth et al., 2010). Large numbers of in situ, high precision analyses of both igneous and detrital zircons are now available, and a striking feature of the zircon record is that it clusters into peaks of crystallization ages (Condie et al., 2009). Compilations of crys-tallization ages for detrital and igneous zircons show remarkably similar patterns of peaks and troughs, although with some variation in the rela-tive amplitude of the peaks (Condie et al., 2009). This coincidence sug-gests that the sedimentary record is a valid representation of the magmatic record (Hawkesworth et al., 2010).We establish that detrital zircon spectra have distinctive age distribu-tion patterns that refl ect the tectonic setting of the basin in which they are deposited. These patterns are principally controlled by (i) the volumes of magma generated in each tectonic setting and their preservation poten-tial, (ii) the ease with which magmatic and detrital zircons of various ages and origins become incorporated into the sedimentary record, and (iii) the record of old zircons incorporated into the sediment. These in turn provide a framework that can be used to constrain the tectonic setting of sedimen-tary packages. This approach distinguishes between three tectonic settings (i.e., convergent, collisional, and extensional), and it is most sensitive when the depositional age of the sediment investigated is well constrained. Basin setting will evolve with tectonic regime; for example, arc-continent or continent-continent collision will result in the evolution of convergent and extensional basins into collisional foreland basins. Hence the three settings distinguished herein are end-members, and the zircon age patterns associ-ated with each show a spectrum of distributions that merge and overlap rather than defi ne discrete fi elds. Discriminant plots developed for igneous rock geochemistry (e.g., Pearce and Cann, 1973) or sediment framework modes (e.g., Dickinson and Suczek, 1979) often have diffuse boundaries or overlap between fi elds, but remain important approaches in understanding and constraining tectonic setting. Equally important, exceptions to simple end-member classifi cations can provide insight into subtleties of tectonic process, such as outlined below for Avalonia in eastern North America.

A Change in the Geodynamics of Continental Growth 3 Billion Years Ago

Continental Growth Spurts The appearance and persistence of continents through geologic time has influenced most processes on Earth, from the evolution of new species to the climate. The relative proportion of newly formed crust compared to reworked, or destroyed, older crust reveals which processes controlled continental growth. Based on the combined analyses of Hf-Pb and O isotopes in zircon minerals, Dhuime et al. (p. 1334) measured continuous but variable rates of new crustal production throughout Earths history. Increased rates of crustal destruction starting around 3 billion years ago coincide with the onset of subduction-drive plate tectonics, slowing down the overall rate of crustal growth. Isotopic analysis of zircons reveals the proportion of crust formed and destroyed on continents throughout Earth’s history. Models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout Earth’s history. The oxygen isotopic composition of zircons, for which uranium-lead and hafnium isotopic data provide age constraints, is a key archive of crustal reworking. We identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time. Growth of continental crust appears to have been a continuous process, albeit at variable rates. A marked decrease in the rate of crustal growth at ~3 billion years ago may be linked to the onset of subduction-driven plate tectonics.

The continental record and the generation of continental crust

Continental crust is the archive of Earth history. The spatial and temporal distribution of Earth’s record of rock units and events is heterogeneous; for example, ages of igneous crystallization, metamorphism, continental margins, mineralization, and sea water and atmospheric proxies are distributed about a series of peaks and troughs. This distribution refl ects the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly. The physiochemical resilience of zircons and their derivation largely from felsic igneous rocks means that they are important indicators of the crustal record. Furthermore, detrital zircons, which sample a range of source rocks, provide a more representative record than direct analysis of grains in igneous rocks. Analysis of detrital zircons suggests that at least ~60%–70% of the present volume of the continental crust had been generated by 3 Ga. Such estimates seek to take account of the extent to which the old crustal material is underrepresented in the sedimentary record , and they imply that there were greater volumes of continental crust in the Archean than might be inferred from the compositions of detrital zircons and sediments. The growth of continental crust was a continuous rather than an episodic process, but there was a marked decrease in the rate of crustal growth at ca. 3 Ga, which may have been linked to the onset of signifi cant crustal recycling, probably through subduction at convergent plate margins. The Hadean and Early Archean continental record is poorly preserved and characterized by a bimodal TTG (tonalites, trondhjemites, and granodiorites) and greenstone association that differs from the younger record that can be more directly related to a plate-tectonic regime. The paucity of this early record has led to competing and equivocal models invoking plate-tectonic– and mantle-plume–dominated processes. The 60%–70% of the present volume of the continental crust estimated to have been present at 3 Ga contrasts markedly with the <10% of crust of that age apparently still preserved and requires on going destruction (recycling) of crust and subconti nental mantle lithosphere back into the mantle through processes such as subduction and delamination.

A matter of preservation

Differences in the preservation potential of crustal rocks may explain peaks in crustal ages previously attributed to enhanced crust formation.

When Continents Formed

Island arc rocks provide a better constraint on when the continental crust was generated. When and how the continental crust was generated remains a fundamental question in Earth sciences. It has been widely believed that the trace element–enriched continental crust and the depleted upper mantle are complementary reservoirs, and that the continental crust has grown from the depleted upper mantle (1, 2). Model ages for neodymium (Nd) and hafnium (Hf) isotopes reflect when new continental crust was generated (2), and traditionally they have been calculated for crust derived from the depleted mantle (see the figure, left panel). The implication is that the isotope composition of the depleted mantle is similar to that of new continental crustal material as it is extracted from the mantle. However, the isotope composition of island arc rocks, and hence of new continental crust, is different from that of the depleted mantle (3, 4). We argue that model ages should be calculated using the composition of new continental crust, which is generally more enriched isotopically than the depleted mantle.

From sediments to their source rocks: Hf and Nd isotopes in recent river sediments

Unraveling continental evolution from the sedimentary record requires an understanding of time-integrated erosion laws that link sediments to their source rocks, and the extent to which erosion laws vary in different erosion systems. Detrital zircons from the Frankland River (southwestern Australia) define a continental growth curve that is strikingly similar to the Nd in shales curve for the Australian continent. This suggests that the detrital zircon data can be used as a good proxy for the sedimentary record through time. The advantage is that the age distribution of the zircons allows the contributions from different source regions to be determined for any sediment sample. Using integrated Hf and U-Pb isotopes in detrital zircons, and Nd isotope ratios of bulk recent sediments along the Frankland River, the relative contributions of different source terrains have been determined and expressed through an erosion parameter K, which relates the proportions of the material from different source rocks in the sediments to the proportions of those source rocks present in the overall catchment of the sediments analyzed. The results suggest that values of K=4–6 are representative of mature river systems that sample large source areas, and that these should be used to reevaluate models of the evolution of the continental crust that were generally limited by the assumption of K. For the Gondwana supercontinent, K values of 4–6 indicate that at least 50% of the present-day volume of the continental crust was generated by the end of the Archean.

Tectonics and crustal evolution

We thank the Natural Environment Research Council (grants NE/J021822/1 and NE/K008862/1) for funding.

Not all supercontinents are created equal: Gondwana-Rodinia case study

The geologic records associated with the formation of the supercontinents Rodinia and Gondwana have markedly different seawater Sr and zircon Hf isotopic signatures. Rodinia-related (Grenville-Sveconorwegian-Sunsas) orogens display significantly less enriched crustal signatures than Gondwana-related (Pan-African) orogens. Seawater Sr isotope ratios also exhibit a more pronounced crustal signal during the span of the Gondwana supercontinent than at the time of Rodinia. Such isotopic differences are attributed to the age and nature of the continental margins involved in the collisional assembly, and specifically to the depleted mantle model ages, and hence the isotope ratios of the material weathered into the oceans. In our preferred model the isotopic signatures of Rodinia-suturing orogens reflect the closure of ocean basins with dual subduction zones verging in opposite directions, analogous to the modern Pacific basin. This would have resulted in the juxtaposition of juvenile continental and island arc terrains on both margins of the colliding plates, thus further reworking juvenile crust. Conversely, the assembly of Gondwana was accomplished primarily via a number of single-sided subduction zones that involved greater reworking of ancient cratonic lithologies within the collisional sutures. The proposed geodynamic models of the assembly of Rodinia and Gondwana provide a connection between the geodynamic configuration of supercontinent assembly and its resulting isotopic signature.

The oldest crust in the Ukrainian Shield – Eoarchaean U–Pb ages and Hf–Nd constraints from enderbites and metasediments

Abstract The oldest crust in the Ukrainian Shield occurs in the Podolian and Azov domains, which both include Eoarchaean components. U–Pb age data for Dniestr–Bug enderbites, Podolian Domain, indicate that these are c. 3.75 Ga old, and Lu–Hf isotope data indicate extraction from chondritic to mildly isotopically depleted sources with ɛHf up to c. +2. Nd model ages support their Eoarchaean age, while model ages for Dniestr–Bug metasedimentary gneisses indicate that these also include younger crustal material. Most of the Hf-age data for metasedimentary zircon from the Soroki greenstone belt, Azov Domain, reflects Eoarchaean primary crustal sources with chondritic to mildly depleted Hf isotope signatures at 3.75 Ga. A minor portion is derived from Mesoarchaean crust with a depleted ɛHf signature of c. +4 at 3.1 Ga. U–Pb zircon ages from Fedorivka greenstone belt metasediments are consistent with the Soroki age data, but also include a 2.7–2.9 Ga component. Nd whole rock model ages provide support for a younger crustal component in the latter. Both domains have been subject to Neoarchaean, c. 2.8 Ga, and Palaeoproterozoic, c. 2.0 Ga, metamorphism. The spatial distribution indicates that the Podolian and Azov domains evolved independently of each other before the amalgamation of the Ukrainian Shield.