Craig D. Storey

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.

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.

A matter of preservation

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

Some garnet microstructures: an illustration of the potential of orientation maps and misorientation analysis in microstructural studies

The microstructures of two contrasting garnet grains are mapped using automated electron backscatter diffraction. In both cases there is a very strong crystallographic preferred orientation, with measurements clustered round a single dominant orientation. Each garnet grain is divided into domains with similar orientations, limited by boundaries with misorientations of 2° or more. In both samples most of misorientation angles measured across orientation domain boundaries are significantly lower than those measured between random pairs of orientation domains. One sample is a deformed garnet that shows considerable distortion within the domains. Lines of orientation measurements within domains and across domain boundaries show small circle dispersions around rational crystallographic axes. The domain boundaries are likely to be subgrain boundaries formed by dislocation creep and recovery. The second sample is a porphyroblast in which the domains have no internal distortion and the orientation domain boundaries have random misorientation axes. These boundaries probably formed by coalescence of originally separate garnets. We suggest that misorientations across these boundaries were reduced by physical relative rotations driven by boundary energy. The data illustrate the potential of orientation maps and misorientation analysis in microstructural studies of any crystalline material.

Episodic, mafic crust formation from 4.5 to 2.8 Ga: New evidence from detrital zircons, Slave craton, Canada

The ϵHf and δ18O values in detrital zircons from the Slave craton, Canada, indicate three episodes of crust formation between ca. 4.5 and 2.8 Ga, namely at ca. 4.4–4.5 Ga, ca. 3.8 Ga, and ca. 3.4 Ga. Most of the juvenile crust appears to have been mafic in composition, and there is no clear evidence for initial granitic protocrust in the Hadean of the Slave craton. The range of initial ϵHf values in zircons increases from 3.9 to 2.8 Ga, indicating that both extraction of new material from mantle and reworking of the older crust are important for the secular evolution of the continental crust. A preliminary review of available Hf data in zircons suggests that the three episodes of crust generation may have been of global importance. The mafic crust formed in the Archean and the Hadean was then reworked for at least ~0.5–1.5 b.y., as indicated by data from the Slave craton, Gondwana, and the Limpopo Belt of Africa.

Geochemistry and U–Pb protolith ages of eclogitic rocks of the Asís Lithodeme, Piaxtla Suite, Acatlán Complex, southern Mexico: tectonothermal activity along the southern margin of the Rheic Ocean

Recent data indicating that the Piaxtla Suite (Acatlán Complex, southern Mexico) underwent eclogite-facies metamorphism and exhumation during the Devono-Carboniferous suggest an origin within the Rheic Ocean rather than the Iapetus Ocean. The Asís Lithodeme (Piaxtla Suite) consists of polydeformed metasediments and eclogitic amphibolites that are intruded by megacrystic granitoid rocks. U–Pb (zircon) data indicate that the metasediments were deposited after c. 700 Ma and before intrusion of c. 470–420 Ma quartz-augen granite. The metasedimentary rocks contain abundant Mesoproterozoic detrital zircons (c. 1050–1250 Ma) and a few zircons in the range of c. 900–992 and c. 1330–1662 Ma. Their geochemical and Sm–Nd isotopic signature is typical of rift-related, passive margin sediments derived from an ancient cratonic source, which is interpreted to be the adjacent Mesoproterozoic Oaxacan Complex. Megacrystic granites were derived by partial melting of a c. 1 Ga crustal source, similar to the Oaxacan Complex. Amphibolitic layers exhibit a continental tholeiitic geochemistry, with a c. 0.8–1.1 Ga source (TDM age), and are inferred to have originated in a rift-related environment by melting of lithospheric mantle in the Ordovician. This rifting may be related to the Early Ordovician drift of peri-Gondwanan terranes (e.g. Avalonia) from Gondwana and the origin of the Rheic Ocean.

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.

Detrital Zircon Data from the Eastern Mixteca Terrane, Southern Mexico: Evidence for an Ordovician—Mississippian Continental Rise and a Permo-Triassic Clastic Wedge Adjacent to Oaxaquia

The eastern part of the Mixteca terrane of southern Mexico is underlain by the Petlalcingo Group (part of the Acatlán Complex), and has been interpreted as either a Lower Paleozoic passive margin, or a trench/forearc sequence deposited in either the Iapetus or Rheic oceans. The group, from bottom to top, consists of: (1) the Magdalena Migmatite protolith (metapsammites, metapelites, calsilicates, and marbles), which grades up into (2) the meta-psammitic Chazumba Formation; overthrust by (3) the Cosoltepec Formation (phyllites and quartzites with minor mafic meta-volcanic horizons). The group is unconformably overlain by the Pennsylvanian—Middle Permian Tecomate Formation, which is overthrust by the ∼288 Ma Totoltepec pluton and unconformably overlain by Middle Jurassic rocks. In contrast to previous inferences that the protoliths of the units (1) to (3) were early Paleozoic in age, detrital zircon LA-ICPMS ages combined with published data constrain depositional ages as follows: (i) Magdalena Migmatite protolith: post-303 Ma-pre-171 Ma (Permian—Early Jurassic); (ii) Chazumba Formation: post—239 Ma—pre-174 Ma (Middle Triassic—Early Jurassic); and (iii) Cosoltepec Formation: post—455 Ma—pre-310 Ma (uppermost Ordovician-Mississippian). Given the different ages and depositional environments of the Cosoltepec Formation versus the Chazumba Formation and Magdalena protolith, we recommend redefining the Chazumba and Magdalena as lithodemes grouped in the Petlalcingo Suite and excluding the Cosoltepec Formation. Detrital zircons in all three units show a population peak at ∼850-1200 Ma, suggesting derivation from the adjacent ∼1 Ga Oaxacan Complex. A ∼470-640 Ma peak is limited to the Cosoltepec Formation whose source may be found in ∼470 Ma plutons in the Acatlan Complex, beneath the Yucatan Peninsula, and in the Brasiliano orogens of South America. The inferred turbiditic protolith of the Chazumba Formation and Magdalena protolith suggests that it represents a clastic wedge deposited in front of S-verging Permo-Triassic thrusts on the western margin of Pangea. The mainly oceanic affinity of the basalts in the Cosoltepec Formation suggests deposition of sedimentary protoliths in a continental rise fringing Oaxaquia. These data are more consistent with deposition of the Cosoltepec Formation in the Rheic Ocean than in the Iapetus Ocean.

Impact melt sheet zircons and their implications for the Hadean crust

Impacts may have been important mechanisms of crustal redistribution and differentiation, particularly during intense postaccretionary bombardment between 4.5 Ga and 3.9 Ga ago. Evidence of crustal processes during this period is largely provided by detrital zircons from the Yilgarn craton, Australia. Trace element compositions, crystallization temperatures, and inclusion populations of these ancient zircons have been taken as evidence for predominantly granitic source magmas, implying widespread felsic continental crust on the early Earth. However, there is little knowledge of zircons formed in impact melt sheets, a potential source for the Hadean zircons. Here we present Ti thermometry, trace elements, and inclusion populations of zircons from the 1.85 Ga Sudbury impact melt sheet (Ontario, Canada). Our results demonstrate that large variations in zircon crystallization temperature and composition will be an inevitable consequence of the evolution of such magmatic systems. We also show that zircons in mafic rocks crystallize in residual liquids of granitic composition, producing inclusion assemblages that are remarkably similar to those reported for the ancient Yilgarn grains. Thus, we conclude that the trace element compositions and inclusion populations of the Hadean zircons are consistent with crystallization from more mafic melts than previously recognized, although high crystallization temperature distributions of Sudbury zircons indicate that impact melt sheets were not a dominant source for the grains older than 3.9 Ga.

Grenvillian age decompression of eclogites in the Glenelg-Attadale Inlier, NW Scotland

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.