This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.

/pdf/composition-of-the-continental-crust-2hxz49vsod.pdf

Composition of the Continental Crust

Simple thermal models based on the creation and cooling of the lithosphere can account for the observed subsidence of the ocean floor and the measured decreased in heat flow with age. In well-sedimented areas, where there is little loss of heat due to hydrothermal circulation, the surface heat flow decays uniformly from values in excess of 6 µcal/cm² s (250 mW/m²), for crust younger than 4 Ma (4 m.y. B.P.), to close to 1.1 µcal/cm² s (46 mW/m²) through crust between 120 and 140 Ma. After 200 Ma the heat flow is predicted to reach an equilibrium value of 0.9 µcal/cm² s (38 mW/m²). The surface heat flow on continents is controlled by many phenomena. On the time scale of geological periods the most important of these are the last orogenic event, the distribution of heat-producing elements, and erosion. To better understand the effects of age, each continent is separated into four provinces on the basis of radiometric dates. Reflecting the preponderance of Precambrian crust, two of these provinces cover the Archean to the middle Proterozoic, and the third covers the late Proterozoic to the Mesozoic. The mean heat flow decreases from a value of 1.84 µcal/cm² s (77 mW/m²) for the youngest province to a constant value of 1.1 µcal/cm² s (46 mW/m²) after 800 Ma. The nonradiogenic component of the surface heat flow decays to a constant value of between 0.65 and 0.5 µcal/cm² s (25 and 21 mW/m²) within 200–400 Ma. Using theoretical models, we compute the heat loss through the oceans to be 727 × 1010 cal/s (30.4 × 1012 W). The comparison between the theoretical and measured values allows an estimate of 241 × 1010 cal/s (10.1 × 1012 W) for the heat lost owing to hydrothermal circulation. We show that the heat flow through the marginal basins follows the same relation as that for crust created at a midocean spreading center. These basins have a corresponding heat loss of 71 × 1010 cal/s (3.0 × 1012 W). The heat loss through the continents is calculated from the observations and is 208 × 1010 cal/s (8.8 × 1012 W). Our estimate of the value for the shelves is 67 × 1010 cal/s (2.8 × 1012 W). The total heat loss of the earth is 1002 × 1010 cal/s (42.0 × 1012 W), of which 70% is through the deep oceans and marginal basins and 30% through the continents and continental shelves. The creation of lithosphere accounts for just under 90% of the heat lost through the oceans and hence about 60% of the worldwide heat loss. Convective processes, which include plate creation and orogeny on continents, dissipate two thirds of the heat lost by the earth. Conduction through the lithosphere is responsible for 20%, and the rest is lost by the radioactive decay of the continental and oceanic crust.

We place bounds of between 0.6 and 0.9 µcal/cm² s (25 and 38 mW/m²) for the mantle heat flow beneath an ocean at equilibrium and between 0.40 and 0.75 µcal/cm² s (17 and 31 mW/m²) for the heat flow beneath an old stable continent. The computed range of geotherms for an equilibrium ocean overlaps the range of stable continental geotherms below a depth of 100 km. The mantle heat flow beneath a continent decays with a thermal time constant similar to that of the oceanic lithosphere. The continental basins subside with the same time constant. These observations are evidence that there is no detectable difference between the thermal structure of an equilibrium ocean and that of an old continent. Thus the concept of the lithosphere as a combination of a mechanical and a thermal boundary layer can be applied to both oceans and continents. We evaluate the constraints placed on models based on this concept by seismological observations. In the absence of compelling evidence to the contrary we favor these models because they provide a single explanation for the thermal structure of the lithosphere beneath an equilibrium ocean and a stable continent.

The heat flow through oceanic and continental crust and the heat loss of the Earth

https://www.uvm.edu/~lewebb/papers/Hacker%20et%20al%201998.pdf

U/Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling–Dabie Orogen, China

Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie–Sulu orogen in China: implications for geodynamics and fluid regime

[1] High-pressure metamorphic rocks provide evidence that in subduction zones material can return from depths of more than 100 km to the surface. The pressure-temperature paths recorded by these rocks are variable, mostly revealing cooling during decompression, while the time constraints are generally narrow and indicate that the exhumation rates can be on the order of plate velocities. As such, subduction cannot be considered as a single pass process; instead, return flow of a considerable portion of crustal and upper mantle material must be accounted for. Our numerical simulations provide insight into the self-organizing large-scale flow patterns and temperature field of subduction zones, primarily controlled by rheology, phase transformations, fluid budget, and heat transfer, which are all interrelated. They show the development of a subduction channel with forced return flow of low-viscosity material and progressive widening by hydration of the mantle wedge. The large-scale structures and the array of pressure-temperature paths obtained by these simulations favorably compare to the record of natural rocks and the structure of high-pressure metamorphic areas.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2002TC001406

Exhumation of high-pressure metamorphic rocks in a subduction channel: A numerical simulation

http://www.ig.uit.no/hardrock/index_files/pdfs/Cuthbert_Carswell_Ravna_Wain%20WGR2000.pdf

Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides

Metamorphic evolution, mineral chemistry and thermobarometry of schists and orthogneisses hosting ultra-high pressure eclogites in the Dabieshan of central China

A review of currently available information relevant to the Basal Gneiss Complex (BGC) of Western South Norway, combined with the authors’own observations, leads to the following conclusions.

1. Most of the BGC consists of Proterozoic crystalline rocks and probably subordinate Lower Palaeozoic cover.

2. The last major deformation of these rocks was during the Caledonian orogeny and involved large-scale thrusting, recumbent folding and doming. The structural development of the BGC is closely tied in with that of the Caledonian allochthon.

3. The whole eclogite-bearing part of the BGC has suffered a high pressure metamorphism with conditions of between 550°C, 12.5 kbar (Sunnfjord) and about 750°C, 20 kbar (More og Romsdal) at the metamorphic climax.

4. This metamorphism was of Caledonian age, probably rather early in the Caledonian tectonic history of the BGC and is considered to have been a rather transient event.

By setting these conclusions in a framework provided by geophysical evidence for the deep structure of the crust in southern Norway we have constructed a geotectonic model to explain the recorded metamorphic history of the BGC. It is suggested that considerable crustal thickening was caused by imbrication of the Baltic plate margin during continental collision with the Greenland plate. This resulted in high pressure metamorphism in the resulting nappe stack. Progradation of the suture caused underthrusting of the Baltic foreland below the eclogite-bearing terrain causing it to emerge at the Earth's surface, aided by tectonic stripping and erosion.

Application of isostacy equations to the model shows that eclogites can be formed by in-situ metamorphism in crustal rocks and reappear at the land surface above a normal thickness of crust in a single orogenic episode of approximately 65-70 Ma duration.

A tectonic model for the metamorphic evolution of the Basal Gneiss Complex, Western South Norway

Ultrahigh-pressure eclogites are metabasic rocks that have equilibrated at pressures within the coesite P-T stability field. This paper reviews their occurrences and petrographic characteristics in exhumed ultrahigh-pressure metamorphic terrenes within certain major Phanerozoic continental plate—collision belts. Unequivocal identification of ultrahigh-pressure eclogites depends on the presence of relict coesite or of polycrystalline quartz pseudomorphs after earlier coesite. However, preservation of such distinctive petrographic evidence is at best sporadic, leading to serious problems over whether or not particular eclogite samples have experienced “peak” pressures consistent with coesite stability. Accordingly, we also review here other petrographic, mineral-assemblage, and mineral-chemistry features that may aid in the identification of ultrahigh-pressure eclogites. In addition, we discuss the quantitative evaluation of P-T conditions for formation based on various reaction equilibria, including the do...

Petrographic Characteristics and Metamorphic Evolution of Ultrahigh-Pressure Eclogites in Plate-Collision Belts

Lower‐Crustal Granulites and Eclogites from Lesotho, Southern Africa

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