Kent C. Condie

Chemical composition and evolution of the upper continental crust: Contrasting results from surface samples and shales

Abstract The average chemical composition of juvenile upper continental crust (UC) as a function of age is estimated from chemical analyses, geologic maps, stratigraphic sections and isotopic ages. High plutonic/supracrustal ratios in Archean UC may reflect in part a different mode of crustal formation prior to 2.5 Ga. Greenstones show an increase in felsic volcanics and graywackes in post-Archean successions, a drop almost to zero in the proportion of komatiites at the Archean-Proterozoic (A/P) boundary, and an increase in the proportion of andesites in the Phanerozoic. Compared to the Early Archean, Late and post-Archean UC are depleted in Mg, Cr, Ni and Co, and post-Archean UC is enriched in LILE (large-ion lithophile elements; K, Rb, Ba, Th, U), HFSE (high field strength elements; Ti, P, Zr, Hf, Nb, Ta, Y), and HREE (heavy rare-earth elements). Negative Eu anomalies characterize UC of all ages, although they are relatively small in Archean UC. Cratonic shales show the same secular compositional changes as UC with the following important exceptions: At the A/P boundary, only shales show a decrease in Fe, V and Sc and an increase in Na, Ca and Sr, and only UC shows a significant increase in HREE and Y. Although post-Archean UC has a larger Eu anomaly than Archean UC, it is clear that both Archean UC and Archean shales have negative Eu anomalies, and that shales give only a weak indication of an increasing Eu anomaly in UC formed after the Archean. In both UC and shales at the A/P boundary, Cr/Th, Ni/Co and Co/Th ratios decrease, whereas the Th/U ratio increases only in shales and not in UC. The lack of HREE depletion and the high Fe, V and Sc contents of Archean shales indicate these shales were not derived from sources similar in composition to exposed Archean UC. The shale sources, which are now removed by erosion, must have been composed largely of basalt (± komatiite) (60%) and granite (40%), with little contribution from HREE-depleted TTG (tonalite-trondhjemite-granodiorite), which now dominates in Archean exposures. This suggests strong vertical zonation in the Archean continents with granites intruded into greenstones at very shallow levels. Compositional changes in UC or/and shales at the A/P boundary can be explained in one of the four ways: (1) a greater amount of basalt and komatiite in Archean UC (high Cr and Ni in Archean UC and shales); (2) garnet/amphibole fractionation during production of TTG by partial melting of hydrous basaltic crust (low HREE and Y in Archean UC); (3) change in TTG magma source from descending slab or thickened crust to metasomatized mantle wedge (increase in LILE and LREE in post-Archean UC and shales); and (4) a decrease in the intensity of chemical weathering after the Archean (increase in Na, Ca and Sr in post-Archean shales).

Plate tectonics and crustal evolution

Preface Plate tectonics The Earths crust Tectonic settings Crustal evolution The core and mantel Supercontinents Atmosphere, oceans and climate Life and mass extinctions The origin of the solar system Earth systems Appendix - methodologies Index.

Episodic continental growth and supercontinents: a mantle avalanche connection?

Abstract Episodic growth of continental crust and supercontinents at 2.7, 1.9, and 1.2 Ga may be caused by superevents in the mantle as descending slabs pile up at the 660-km seismic discontinuity and then catastrophically sink into the lower mantle. Superevents, in turn, may comprise three or four events, each of 50–80 My duration, and each of which may reflect slab avalanches at different locations and times along the 660-km discontinuity. Superplume events in the late Paleozoic and Mid-Cretaceous may have been caused by minor slab avalanches as the 660-km discontinuity became more permeable to the passage of slabs with time. The total duration of a superevent cycle decreases with time reflecting the cooling of the mantle.

Geochemistry of Archean shales from the Witwatersrand Supergroup, South Africa: Source-area weathering and provenance

With a few exceptions, shales from the Archean Witwatersrand Supergroup (~2800 Ma) in South Africa are depleted in Na, Ca, LILE, REE and HFSE compared to Phanerozoic shales. Cr, Co and Ni are enriched in all Witwatersrand shales and Fe and Mg are high in shales from the West Rand Groups (WRG) and lower Central Rand Group (CRG). Shales from the CRG and uppermost WRG are enriched in Na, Al, LILE, REE, HFSE and transition metals relative to shales from the lower WRG. Chondrite-normalized REE patterns for all Witwatersrand shales are enriched in light-REE and exhibit small to moderate negative Eu anomalies. A positive correlation of REE and Al2O3 contents in the shales suggests that REE are contained principally in clay minerals. Relative to shales from the CRG, shales from the WRG exhibit depletions of Na, Ca and Sr, a feature probably reflecting intense chemical weathering of their source rocks. CIA indices in Witwatersrand shales are variable (chiefly 70–98), even within the same shale unit. Such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism. Compared to present-day upper continental crust, all but the Orange Grove, Roodepoort, and K8 shales appear to have been derived from continental sources depleted in LILE, REE, and HFSE and enriched in transition metals. Computer mixing models based on six relatively immobile elements (Th, Hf, Yb, La, Sc, Co) and four source rocks indicate that the relative proportions of granite, basalt and komatiite increased with time in sediment source areas at the expense of tonalite. The contributions of basalt and komatiite appear to reach a maximum during deposition of the Booysens shale, and granite during deposition of the K8 shales and possibly during deposition of the Orange Grove shales.

Evolution of the Kaapvaal Craton as viewed from geochemical and Sm-Nd isotopic analyses of intracratonic pelites

Precambrian cratonic pelites from the Kaapvaal Craton in southern Africa have similar REE patterns with relative LREE enrichment and absence of significant depletion in HREEs. They have a narrow range of 147Sm144Nd ratios with a mean value of 0.118, which is identical to the mean value of ≈450 worldwide fine-grained samples of all ages obtained by isotopic dilution analyses. This value is probably the best estimate for the upper continental crust. The Kaapvaal pelites also have distinct Cr/Th ratios, but overlap in Eu/Eu∗ ratios, suggesting that variable provenance and sedimentary recycling were important both during and after the Archean. Because the light REE budget is controlled chiefly by granitoids, which mask contributions of mafic-ultramafic components, the relatively uniform Sm/Nd ratios in sediments do not indicate a near-constant composition for the upper continental crust. Most Kaapvaal pelites have negative ϵNd(T) values, indicating important contributions of older crustal sources. Overall, there is a slight decrease of ϵNd(T) values with decreasing age, but no clear distinction is apparent at the A/P boundary at 2.5 Ga. Almost all of Kaapvaal pelites have TDM ages greater than their depositional ages but younger than 3.6 Ga, suggesting an absence of rocks older than 3.6 Ga in the Kaapvaal Craton. The debate on growth or no-growth of continents depends much on the choice of parameters in model calculations. The crucial parameters include sediment flux in subduction zones and delaminated lower crust, and the Sm/Nd ratio of continental crust. Unfortunately, the available data are ambiguous in modelling studies. Neodymium isotopic data and Sm/Nd ratios cannot be taken as a robust argument against the no-continental-growth model advocated by R. L. Armstrong (1991).

Another look at rare earth elements in shales

Linear correlation and mass balance considerations indicate that clays are more important than zircon (or other heavy minerals) in hosting both light and heavy REE in cratonic shales. Considering the uniformity of the Eu anomaly and REE distributions in cratonic shales, it seems likely that detritus is thoroughly mixed during weathering and sediment transport and that the REE patterns in shales reflect the average REE pattern of the sources, regardless of the ratio of clays to heavy minerals in the shales. Shales, suspended river loads, and near-shore marine sediments all have broadly similar REE patterns, and reported HREE depletions in the latter two groups may reflect incomplete dissolution of heavy minerals during sample preparation. REE distributions in shales and suspended river loads determined by INNA are similar to those in average upper continent crust, and REE distributions in shale averages NASC and PAAS seem to be reasonably representative of the average composition of post-Archean upper continental crust.

Precambrian clastic sedimentation systems

Abstract The unique and evolving nature of the Precambrian geological environment in many ways was responsible for significant differences between Precambrian clastic sedimentary deposits and their Phanerozoic-modern equivalents. Some form of plate tectonics, with rapid microplate collisions and concomitant volcanic activity, is inferred to have led to the formation of greenstone belts. Explosive volcanism promoted common gravity-flow deposits within terrestrial greenstone settings, with braided alluvial, wave/storm-related and tidal coastline sediments also being preserved. Late Archaean accretion of greenstone terranes led to emergence of proto-cratons, where cratonic and rift sedimentary assemblages developed, and these became widespread in the Proterozoic as cratonic plates stabilised. Carbonate deposition was restricted by the paucity of stable Archaean terranes. An Early Precambrian atmosphere characterised by greenhouse gases, including CO2, in conjunction with a faster rotation of the Earth and reduced albedo, provide a solution to the faint young Sun paradox. As emergent continental crust developed, volcanic additions of CO2 became balanced by withdrawal due to weathering and a developing Palaeoproterozoic microbial biomass. The reduction in CO2, and the photosynthetic production of O2, led to aerobic conditions probably being achieved by about 2 Ga. Oceanic growth was allied to atmospheric development, with approximately 90% of current ocean volume being reached by about 4 Ga. Warm Archaean and warm, moist Palaeoproterozoic palaeoclimates appear to have become more arid after about 2.3 Ga. The 2.4–2.3 Ga Huronian glaciation event was probably related to continental growth, supercontinent assembly and weathering-related CO2 reduction. Despite many analogous features among both Precambrian and younger sedimentary deposits, there appear to be major differences as well. Two pertinent examples are rare unequivocal aeolian deposits prior to about 1.8 Ga and an apparent scarcity of Precambrian foreshore deposits, particularly those related to barrier island systems. The significance of these differences is hard to evaluate, particularly with the reduced palaeoenvironmental resolution because of the absence of invertebrate and plant fossils within Precambrian successions. The latter factor also poses difficulties for the discrimination of Precambrian lacustrine and shallow marine deposits. The temporal distribution of aeolian deposits probably reflects a number of possible factors, including few exposed late Archaean–Palaeoproterozoic cratonic areas, extensive pre-vegetative fluvial systems, Precambrian supercontinents and a different atmosphere. Alternatively, the scarcity of aeolian deposits prior to 1.8 Ga may merely reflect non-recognition or non-preservation. Precambrian shallow marine environments may have been subjected to more uniform circulation systems than those interpreted from the Phanerozoic-modern rock record, and Precambrian shelves probably were broad with gentle seaward slopes, in contrast to the narrow, steep shelves mostly observed in present settings. Poorly confined Precambrian tidal channels formed sheet sandstones, easily confused with fluvial or offshore sand sheets. Epeiric seas were possibly more prevalent in the Precambrian, but active tectonism as proto-continents emerged and amalgamated to form early supercontinents, in conjunction with a lack of sufficient chronological data in the rock record, make it difficult to resolve the relative importance of eustatic and tectonic influences in forming epeiric embayments and seaways. Other differences in Precambrian palaeoenvironments are more easily reconstructed. Ancient delta plain channels were probably braided, and much thicker preserved delta successions in the Precambrian are compatible with the inferred more active tectonic conditions. Pre-vegetational alluvial channel systems were almost certainly braided as well. Common fluvial quartz arenites are ascribed to differences in weathering processes, which probably changed significantly through the Precambrian, or to sediment recycling. Although Precambrian glacigenic environments were probably the least different from younger equivalents, their genesis appears to reflect a complex interplay of factors unique to the Precambrian Earth. These include emergence and amalgamation of proto-continents, the early CO2-rich atmosphere, the development of stromatolitic carbonate platforms, early weathering, faster rotation of the Earth and the possible role of changes in the inclination of the Earths axis.

Behavior of rare earth elements in a paleoweathering profile on granodiorite in the Front Range, Colorado, USA

A Paleoweathering profile on the Boulder granodiorite in northern Colorado provides an opportunity to trace the behavior of REEs from parent rock, through a weathering profile, into unconformably overlying Permian sediments. With progressive upward weathering of the granodiorite, Na2O, CaO, SiO2, TaHf, CoTh, CrSc, CrTh, ZrHf, LaSc, ZrY, and LaTh decrease; Al2O3 and Fe2O3T increase; and TiO2, MgO, K2O, P2O5, Rb, Zr, Sc, Cr, Co Hf, Nb, Ta, Y, Th, U, REE, TiNb, and ZrNb increase to maximum values and then either level off or decrease. LREE enrichment is less in the weathering profile than in the parent granodiorite and although the parent does not have an Eu anomaly (or only a slight positive anomaly), all samples from the weathering profile and overlying sediments have significant negative Eu anomalies. This observation is especially important in that it shows conclusively that a negative Eu anomaly can be produced during chemical weathering of granitoids. We suggest these Eu anomalies are due to relative enrichment of the other REEs and partial loss of Eu during the breakdown of plagioclase. The Boulder weathering profile also has a very minor negative Ce anomaly that is within error of a Ce anomaly in the parent. In the unweathered parent, >50% of the REE are contained in sphene, and in the case of La, also in allanite. From 10–20% of the REE are contained in apatite and biotite (± hornblende), and from 7–10% of the HREEs are in zircon. With exception of Eu, for which feldspars contribute about 8%, negligible amounts of REEs occur in the feldspars. In weathered samples, >75% of the REEs are contained in clay minerals. The crossover between sphene and clay control of REEs occurs over a distance of 1 m near the contact with fresh rock. Except for their small negative Eu anomalies, the clay minerals have REE patterns very similar to those of the parent rock. Isocon plots suggest apparent enrichments of many elements in the Boulder weathering profile result from losses of Na, Ca, and Si during plagioclase weathering. In addition, variable amounts of Sr, Eu, Ta, Nb, P, and Ba were lost during weathering. Although ThU, ZrY, ThSc, ZrHf, LuHf, and TiZr may have been transferred relatively unchanged from granodiorite parent to the bulk weathering profile, most other element ratios and REE distributions were significantly changed during weathering. This observation implies that caution needs to be exercised when using REE patterns and element ratios to trace sediment provenance. The fact that most element ratios and REE distributions also differ between Fountain sediments and the bulk weathering profile may be related to one or a combination of four factors, listed in order of probable decreasing importance: contribution of other sources to the Fountain sediments, sorting of minerals during sediment deposition, remobilization of elements during diagenesis, and leaching of elements by water flow through the upper meter of the weathering profile.

Geochemical changes in baslts and andesites across the Archean-Proterozoic boundary: Identification and significance

Abstract To identify accurately changes in rock composition across the Archean-Proterozoic boundary, it is necessary to compare rocks from similar lithologic associations to constrain the effect of tectonic setting. Most basalts and andesites from the greenstone association (volcanic-dominated submarine supracrustal rocks) possess a geochemical subduction-zone component similar to their counterparts from modern arc systems. Basalts with island arc geochemical affinities dominate in Archean greenstones while those with calc-alkaline affinities are most abundant in Proterozoic greenstones. Basalts with MORB or oceanic within-plate geochemical characteristics are rare in Precambrian greenstones of all ages. Preserved early Archean greenstone basalts (⩾3500 Ma) reflect mantle sources less depleted than late Archean greenstone basalts (2500–3500 Ma). Proterozoic greenstone basalts are derived from relatively enriched mantle sources compared to all Archean sources, a feature which may be due to recycling of continental sediments into the mantle following rapid late Archean continental growth. Precambrian andesites are geochemically similar to andesites from modern arcs, but Archean andesites are unique in that they are depleted in heavy REE and Y. Results are consistent with Archean andesite production by partial melting of descending mafic crust with amphibole/garnet remaining in the residue, while Proterozoic (and younger) andesites are produced by fractional crystallization of basalts.

Precambrian superplumes and supercontinents: a record in black shales, carbon isotopes, and paleoclimates?

Prominent maxima in black shale abundance and in black shale/total shale ratio occur at 2.0‐1.7 Ga, with less prominent peaks in the Late Neoproterozoic (800‐600 Ma) and in the Late Archean (2.7‐2.5 Ga). Peaks in chemical index of alteration (CIA) of shales at the same times suggest corresponding warm paleoclimates. The peaks in CIA and black shale abundance are correlated in time at a 94% confidence level. The black shale and CIA peaks may reflect the combined effects of mantle superplume events and supercontinent formation at 2.7 and 1.9 Ga. Mantle superplume events may have introduced large amounts of CO2 into the atmosphere‐ocean system, increasing depositional rates of carbon and increasing global warming. Increased black shale deposition may reflect some combination of: (1) increased oceanic hydrothermal fluxes (introducing nutrients); (2) anoxia on continental shelves; and (3) disrupted ocean currents. The apparent absence of carbon isotope anomalies at these times reflects an increase in the deposition and burial rate of both reduced and oxidized carbon. Peaks in black shale abundance at ! 2.1 Ga and 800‐600 Ma correlate with peaks in ! 13 C in marine carbonates, increases in atmospheric oxygen, and with high CIA values in shales. These are all consistent with higher rates of organic carbon burial in black shales at these times. These peaks may record the breakup of supercontinents at 2.2‐2.0 Ga and again at 800‐600 Ma, which resulted in increased numbers of partially closed marine basins, disruption of ocean currents, and increased hydrothermal vents at ocean ridges, all of which led to widespread anoxia.