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.

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Composition of the Continental Crust

3.01 – Composition of the Continental Crust

Supra-subduction zone (SSZ) ophiolites have the geochemical characteristics of island arcs but the structure of oceanic crust and are thought to have formed by sea-floor spreading directly above subducted oceanic lithosphere. They differ from ‘MORB’ ophiolites not only in their geochemistry but also in the more depleted nature of their mantle sequences, the more common presence of podiform chromite deposits, and the crystallization of clinopyroxene before plagioclase which is reflected in the high abundance of wehrlite relative to troctolite in their cumulate sequences. Most of the best-preserved ophiolite complexes in orogenic belts are of this type. 

Geological reconstructions suggest that most SSZ ophiolites formed during the initial stages of subduction prior to the development of any volcanic arc. Evidence from these ophiolites suggests that the first magma to form in response to intra-oceanic subduction is boninitic in composition, derived by partial melting of hydrated oceanic lithosphere in the ‘mantle wedge’. As subduction proceeds, the magma composition changes to island-arc tholeiite, probably because the hydrated asthenosphere of the ‘mantle wedge’ eventually becomes the dominant mantle source. Other SSZ ophiolites formed in the early stages of back-arc spreading following splitting of a pre-existing arc. Nonetheless the more common mechanism for formation of SSZ ophiolites appears to have been pre-arc rather than back-arc spreading

Characteristics and tectonic significance of supra subduction zone ophiolites

Summary We review extensively the evidence and arguments bearing on the nature of Palaeotethys in relation to the age of formation, location and multiplicity of Neotethyan strands and their fate. We conclude that Palaeotethys did not die early but was only finally subducted northwards in the Tertiary along the Vardar-Intra Pontide-East Anatolian suture. Neotethyan strands must have opened into it at all times. The Adriatic promontory remained attached to Africa but rotated anti-clockwise in the mid-Tertiary. The Pontides are considered to be Eurasian and the Cimmerides are viewed as a ‘collage terrain’ formed along an oblique-convergence margin. The south Aegean, Greek and Turkish microcontinental blocks were rifted-off Gondwana in the Triassic but formation of braided Neotethyan oceanic crustal strands was essentially confined to mid-Jurassic in the Hellenides and to the Cretaceous in Turkey. We propose a new model of ophiolite genesis by asymmetrical spreading-ridge collapse in an attempt to explain both arc-like ophiolite chemistry prior to major volcanic arc edifice construction, and the synchroneity of sub-ophiolite metamorphic sole formation with Atlantic opening phases. Jurassic dispersal of Hellenide blocks had little effect in the unexpanded Turkish mosaic, but northwards Cretaceous opening of Tauride Neotethyan strands caused oblique collision deformation in the Pelagonian zone and unresolvable complexity in the Aegean. Late Cretaceous and Tertiary arc-volcanism was related in part to continuing Palaeotethyan subduction, and in part to Neotethyan destruction initiated after ridge-collapse. Diachronous collisions ensued from the Late Cretaceous onwards but significant oceanic tracts must have persisted at least to Mid-Tertiary to satisfy Africa-Eurasia separation constraints determined from Atlantic anomaly fitting. Our favoured plate evolutionary model is presented in 7 sketch-maps.

Introduction: aspects of the geological evolution of the Eastern Mediterranean

The ultrapotassic rocks: Characteristics, classification, and constraints for petrogenetic models

The ultrapotassic rocks from southeastern Spain

The ophiolites of Northern Pindos have been studied in a section close to the village of Perivoli (Grevena District) The section comprises cumulus rocks ranging from ultramafics to gabbros, overlain by dolerites (non-cumulus microgabbro) capped by thick frequently pillowed lava flows The sequence is cut by basaltic dykes While the cumulus rocks and the dolerites are mostly fresh, the lavas and dykes are strongly transformed Major and trace element (Ni, Cr, Sc, Y, Zr, Nb, Sr, Ba, Zn, Cu, V, Li) data are presented for selected samples from the sequence For some elements, the volcanic/subvolcanic rocks (flows, dykes, dolerites) exhibit wide chemical characteristics which are considered to mainly reflect variations within the parent magmas Some lavas appear to be closely comparable with the present-day ocean-floor basalts, while other flows and most of the dykes are strongly depleted in some “incompatible” elements and are similar to some rocks from immature island arcs The dolerites have transitional chemical features The Pindos lavas differ from Western Mediterranean ophiolites in that the former have lower Ti,P,Zr,Y, higher Fe tot and normally higher Ti/Zr ratio The volcanic/subvolcanic rocks from Pindos have been derived from separate magmas Some lavas were possibly produced by variable partial melting of an already depleted mantle source, while the lavas exhibiting ocean-floor affinity were probably generated by partial melting of a less depleted source The wide chemical variations of the Pindos lavas cannot be easily explained by an ocean-ridge system An “island arc-marginal basin system” could better account for the observed chemical features

The geochemistry and petrogenesis of an ophiolitic sequence from Pindos, Greece

The distribution of U has been studied in two metamorphic rock-series with a gradient of regional metamorphism. One series ranges from the lowest greenschist to amphibolite facies and the other one shows increasing metamorphic grade from amphibolite to granulite facies. Several medium and high pressure granulitic inclusions from alkali basalts were also analyzed. The abundances of U in the rocks do not appear to be affected by metamorphism below the granulite facies grade. Granulites are depleted in U in comparison with equivalent rocks of amphibolite facies grade. There are also differences in their U distribution, as the bulk of U in amphibolite facies rocks is located along the fractures and cleavage planes of ferro-magnesian minerals and in U-rich accessories, while in granulites, most of the U resides in accessory minerals. It seems that the depletion of U in granulites is due to a loss of U which is not located in accessory minerals or in the crystal structure of rock-forming minerals and may also be related to a migration of hydrous fluids, perhaps during dehydration.

Uranium in Metamorphic Rocks

Accessory minerals and δ18O and δ13C of marbles from the Mediterranean area

The lamproitic rocks of southeast Spain (8.6-6 Ma) occur mainly as dykes and plugs in the provinces of Murcia, Almeria and Albacete. Phlogopite, olivine, apatite, spinel, sanidine and glass are ubiquitous. Clinopyroxene, ortiopyroxene, K-rich amphibole, biotite, ilmenite, pseudobrookite, leucite, analcime and carbonates may be additional phases. The magmas crystallized at temperatures below ca. l200oC. Oxygen fugacity during crystallization varied from ca. WM at Calasparra to values intermediate between Mn1_"O - Mn3O4 (MnH in the text) and NNO at Jumilla and Aljorra. The /(HF)f(H2O) and

Silvio Capedri

Papers

The ultrapotassic rocks from southeastern Spain

The geochemistry and petrogenesis of an ophiolitic sequence from Pindos, Greece

Uranium in Metamorphic Rocks

Accessory minerals and δ18O and δ13C of marbles from the Mediterranean area

Petrogenesis and conditions of crystallization of Spanish lamproitic rocks