A survey is given of the dimensions and composition of the present continental crust. The abundances of immobile elements in sedimentary rocks are used to establish upper crustal composition. The present upper crustal composition is attributed largely to intracrustal differentiation resulting in the production of granites senso lato. Underplating of the crust by ponded basaltic magmas is probably a major source of heat for intracrustal differentiation. The contrast between the present upper crustal composition and that of the Archean upper crust is emphasized. The nature of the lower crust is examined in the light of evidence from granulites and xenoliths of lower crustal origin. It appears that the protoliths of most granulite facies exposures are more representative of upper or middle crust and that the lower crust has a much more basic composition than the exposed upper crust. There is growing consensus that the crust grows episodically, and it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons, or 2.7 Ga). There appears to be a relationship between episodes of continental growth and differentiation and supercontinental cycles, probably dating back at least to the late Archean. However, such cycles do not explain the contrast in crustal compositions between Archean and post-Archean. Mechanisms for deriving the crust from the mantle are considered, including the role of present-day plate tectonics and subduction zones. It is concluded that a somewhat different tectonic regime operated in the Archean and was responsible for the growth of much of the continental crust. Archean tonalites and trond-hjemites may have resulted from slab melting and/or from melting of the Archean mantle wedge but at low pressures and high temperatures analogous to modern boninites. In contrast, most andesites and subduction-related rocks, now the main contributors to crustal growth, are derived ultimately from the mantle wedge above subduction zones. The cause of the contrast between the processes responsible for Archean and post-Archean crustal growth is attributed to faster subduction of younger, hotter oceanic crust in the Archean (ultimately due to higher heat flow) compared with subduction of older, cooler oceanic crust in more recent times. A brief survey of the causes of continental breakup reveals that neither plume nor lithospheric stretching is a totally satisfactory explanation. Speculations are presented about crustal development before 4000 m.y. ago. The terrestrial continental crust appears to be unique compared with crusts on other planets and satellites in the solar system, ultimately a consequence of the abundant free water on the Earth.

The geochemical evolution of the continental crust

Post-collisional strongly peraluminous granites

Porphyroclasts of feldspar and other relatively rigid minerals in mylonites commonly have mantles of dynamically recrystallized material that extend as tails into the matrix. The internal shape symmetry of such porphyroclasts is usually orthorhombic or monoclinic; the orientation of the porphyroclast with respect to the foliation (external symmetry) can also be described by these symmetry classes. An identical monoclinic external symmetry of most porphyroclasts in a given sample indicates non-coaxial flow in the matrix during at least the last stages of deformation. Two classes of porphyroclast systems with monoclinic symmetry have been identified on geometrical grounds. sigma-type porphyroclasts are characterized by wedge-shaped tails of recrystallized material. Median lines of the tails lie on opposite sides of, and do not cross, a marker line drawn parallel to the mean foliation. sigma-type porphyroclasts may lie isolated in a homogeneous matrix (sigma/sub a./-type) or may be in clusters associated with shear bands or S-C mylonites (sigma/sub b/-type). delta-type porphyroclasts commonly occur in ultramylonites and have highly attenuated recrystallized tails. Median lines of the tails cross the marker line adjacent to the porphyroclast which results in an embayment of matrix material adjacent to the host grain. More complex porphyroclast systems include ellipsoidal overturned delta-types,more » complex sigma - delta types and folded porphyroclast aggregates. In all cases, the symmetry of porphyroclast aggregates with respect to the foliation can be used to accurately determine the sense of vorticity in the mylonites.« less

Porphyroclast systems as kinematic indicators

Geochemical and petrological evidence of the subduction of delaminated Adriatic continental lithosphere in the genesis of the Neogene-Quaternary magmatism of central Italy

Granite is one of the most important components of the continental crust on our Earth; it thus has been an enduring studied subject in geology. According to present knowledge, granite shows a great deal of heterogeneity in terms of its texture, structure, mineral species and geochemical compositions at different scales from small dike to large batholith. However, the reasons for these variations are not well understood although numerous interpretations have been proposed. The key point of this debate is whether granitic magma can be effectively differentiated through fractional crystallization, and, if so, what kind of crystallization occurred during the magmatic evolution. Although granitic magma has high viscosity because of its elevated SiO2 content, we agree that fractional crystallization is effectively processed during its evolution based on the evidence from field investigation, mineral species and its chemical variations, and geochemical compositions. These data indicate that crystal settling by gravitation is not the only mechanism dominating granitic differentiation. On the contrary, ﬂow segregation or dynamic sorting may be more important. Accordingly, granite can be divided into unfractionated, fractionated (including weakly fractionated and highly fractionated) and cumulated types, according to the differentiation degree. Highly fractionated granitic magmas are generally high in primary temperature or high with various volatiles during the later stage, which make the fractional crystallization much easier than the common granitic melts. In addition, effective magmatic differentiation can be also expected when the magma emplaced along a large scale of extensional structure. Highly fractionated granitic magma is easily contaminated by country rocks due to its relatively prolonged crystallization time. Thus, granites do not always reflect the characteristics of the source areas and the physical and chemical conditions of the primary magma. We proposed that highly fractionated granites are an important sign indicating compositional maturity of the continental crust, and they are also closely related to the rare-elemental (metal) mineralization of W, Sn, Nb, Ta, Li, Be, Rb, Cs, REEs, etc.

Highly fractionated granites: Recognition and research

Origin and significance of the Permian high-K calc-alkaline magmatism in the central-eastern Southern Alps, Italy

Lower crustal granite genesis connected with chemical fractionation in the continental crust of Calabria (southern Italy)

Relationships between intermediate and acidic rocks in orogenic granitoid suites: petrological, geochemical and isotopic (Sr, Nd, Pb) data from Capo Vaticano (southern Calabria, Italy)

REE distribution in the clay fraction of pelites from the southern Apennines, Italy

The Sila and Serre granitoids of Calabria were emplaced in the late Carboniferous at depths ranging from 6 to 23 km in a postcollisional extensional regime. Their fabric, which developed during and after final crystallization up to the solid state, strongly increases in intensity with emplacement depth. This relationship is attributed to the thermal history of the Calabrian basement. Cooling histories of granitoids, constrained by geological data and Rb–Sr cooling ages on micas, demonstrate that residence times of rocks at temperatures greater than those of the brittle–ductile transition vary greatly as a function of initial emplacement depth. This explains why shallow-level granitoids, that remained for about 10 Myr at temperatures above those of the brittle domain, retain their original magmatic fabric. By contrast, the strong fabrics of the deep-seated granitoids are explained by solid-state strain overprint that lasted more than 100 Myr at temperatures above those of the brittle domain.

Alfredo Caggianelli

Papers

Origin and significance of the Permian high-K calc-alkaline magmatism in the central-eastern Southern Alps, Italy

Lower crustal granite genesis connected with chemical fractionation in the continental crust of Calabria (southern Italy)

Relationships between intermediate and acidic rocks in orogenic granitoid suites: petrological, geochemical and isotopic (Sr, Nd, Pb) data from Capo Vaticano (southern Calabria, Italy)

REE distribution in the clay fraction of pelites from the southern Apennines, Italy

Thermal history vs. fabric anisotropy in granitoids emplaced at different crustal levels: an example from Calabria, southern Italy