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

Granites may be subdivided according to their intrusive settings into four main groups—ocean ridge granites (ORG), volcanic arc granites (VAG), within plate granites (WPG) and collision granites (COLG)—and the granites within each group may be further subdivided according to their precise settings and petrological characteristics. Using a data bank containing over 600 high quality trace element analyses of granites from known settings, it can be demonstrated using ORG-normalized geochemical patterns and element-SiO2 plots that most of these granite groups exhibit distinctive trace element characteristics. Discrimination of ORG, VAG, WPG and syn-COLG is most effective in Rb-Y-Nb and Rb-Yb-Ta space, particularly on projections of Y-Nb, Yb-Ta, Rb-(Y + Nb) and Rb—(Yb + Ta). Discrimination boundaries, though drawn empirically, can be shown by geochemical modelling to have a theoretical basis in the different petrogenetic histories of the various granite groups. Post-collision granites present the main problem of tectonic classification, since their characteristics depend on the thickness and composition of the lithosphere involved in the collision event and on the precise timing and location of magmatism. Provided they are coupled with a consideration of geological constraints, however, studies of trace element compositions in granites can clearly help in theelucidation of post-Archaean tectonic settings.

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Trace element discrimination diagrams for the tectonic interpretation of granitic rocks

A review of the geologic history of the Himalayan-Tibetan orogen suggests that at least 1400 km of north-south shortening has been absorbed by the orogen since the onset of the Indo-Asian collision at about 70 Ma. Significant crustal shortening, which leads to eventual construction of the Cenozoic Tibetan plateau, began more or less synchronously in the Eocene (50–40 Ma) in the Tethyan Himalaya in the south, and in the Kunlun Shan and the Qilian Shan some 1000–1400 km in the north. The Paleozoic and Mesozoic tectonic histories in the Himalayan-Tibetan orogen exerted a strong control over the Cenozoic strain history and strain distribution. The presence of widespread Triassic flysch complex in the Songpan-Ganzi-Hoh Xil and the Qiangtang terranes can be spatially correlated with Cenozoic volcanism and thrusting in central Tibet. The marked difference in seismic properties of the crust and the upper mantle between southern and central Tibet is a manifestation of both Mesozoic and Cenozoic tectonics. The form...

Geologic Evolution of the Himalayan-Tibetan Orogen

New analyses of 131 samples of A-type (alkaline or anorogenic) granites substantiate previously recognized chemical features, namely high SiO2, Na2O+K2O, Fe/Mg, Ga/Al, Zr, Nb, Ga, Y and Ce, and low CaO and Sr. Good discrimination can be obtained between A-type granites and most orogenic granites (M-, I and S-types) on plots employing Ga/Al, various major element ratios and Y, Ce, Nb and Zr. These discrimination diagrams are thought to be relatively insensitive to moderate degrees of alteration. A-type granites generally do not exhibit evidence of being strongly differentiated, and within individual suites can show a transition from strongly alkaline varieties toward subalkaline compositions. Highly fractionated, felsic I- and S-type granites can have Ga/Al ratios and some major and trace element values which overlap those of typical A-type granites. A-type granites probably result mainly from partial melting of F and/or Cl enriched dry, granulitic residue remaining in the lower crust after extraction of an orogenic granite. Such melts are only moderately and locally modified by metasomatism or crystal fractionation. A-type melts occurred world-wide throughout geological time in a variety of tectonic settings and do not necessarily indicate an anorogenic or rifting environment.

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A-type granites: geochemical characteristics, discrimination and petrogenesis

This geochemical classification of granitic rocks is based upon three INTRODUCTION variables. These are FeO/(FeO + MgO) = Fe-number [or Although granitoids are the most abundant rock types FeO/(FeO + MgO) = Fe∗], the modified alkali–lime index in the continental crust, no single classification scheme (MALI) (Na2O + K2O – CaO) and the aluminum saturation has achieved widespread use. Part of the problem in index (ASI) [Al/(Ca – 1·67P + Na + K)]. The Fe-number granite classification is that the same mineral assemblage, (or Fe∗) distinguishes ferroan granitoids, which manifest strong iron quartz and feldspars with a variety of ferromagnesian enrichment, from magnesian granitoids, which do not. The ferroan minerals, can be achieved by a number of processes. and magnesian granitoids can further be classified into alkalic, Granitoids can form from differentiation of any hyalkali–calcic, calc-alkalic, and calcic on the basis of the MALI persthene-normative melt and from partial melting of and subdivided on the basis of the ASI into peraluminous, metamany rock types. Furthermore, granitic melts may be luminous or peralkaline. Because alkalic rocks are not likely to be derived solely from crustal components, may form from peraluminous and calcic and calc-alkalic rocks are not likely to be evolved mantle-derived melts, or may be a mixture peralkaline, this classification leads to 16 possible groups of granitic of crustal and mantle-derived melts. Because of this rocks. In this classification most Cordilleran granitoids are magnesian complexity, petrologists have relied upon geochemical and calc-alkalic or calcic; both metaluminous and peraluminous classifications to distinguish between various types of types are present. A-type granitoids are ferroan alkali–calcic, although granitoids. Approximately 20 different schemes have evolved over the past 30 years [see Barbarin (1990, 1999) some are ferroan alkalic. Most are metaluminous although some are for a summary thereof]. Most of these schemes are either peraluminous. Caledonian post-orogenic granites are predominantly genetic or tectonic in nature. This paper is an attempt magnesian alkali–calcic. Those with <70 wt % SiO2 are domto present a non-genetic, non-tectonic geochemical clasinantly metaluminous, whereas more silica-rich varieties are comsification scheme that incorporates the best qualities of monly peraluminous. Peraluminous leucogranites may be either the previous schemes, and to explain the petrologic magnesian or ferroan and have a MALI that ranges from calcic to processes that makes this scheme work. alkalic.

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A geochemical classification for granitic rocks

This paper reports the results of a systematic geochemical study of intermediate and acid intrusive rocks from a number of continent-continent collision zones of Phanerozoic age. Four groups of intrusions can be recognized, each associated with a particular stage in the tectonic evolution of a collision zone. 

Pre-collision calc-alkaline (volcanic-arc) intrusions which are mostly derived from mantle modified by a subduction component and which are characterized by selective enrichments in LIL elements. 

Syn-collision peraluminous intrusions (leucogranites) which may be derived from the hydrated bases of continental thrust sheets and which are characterized by high Rb/Zr and Ta/Nb and low K/Rb ratios. 

Late or post-collision calc-alkaline intrusions which may be derived from a mantle source but undergo extensive crustal contamination and can only be distinguished from volcanic-arc intrusions by their higher ratios of Ta/Hf and Ta/Zr. 

Post-collision alkaline intrusions which may be derived from mantle lithosphere beneath the collision zones and which carry high concentrations of both LIL and HFS elements. 

The geochemical evolution of crustal melts within groups (ii) and (iii) can be viewed in terms of the dehydration reactions, volatile transfer and transient geothermal gradients that result from thrust tectonics in the zone of collision.

Geochemical characteristics of collision-zone magmatism

Potassic volcanism has been widespread and semi-continuous on the Tibetan plateau since ~13 Ma, post-dating the orogenic thickening of the India—Asia collision. Volcanism may have commenced slightly earlier (~16—20 Ma) in the southern portion of the plateau arid then ceased around 10 Ma. The dominant lavas are pyroxeneand plagioclase-phyric shoshonites with subordinate occurrences of dacites and rhyolites. Their mineralogy reflects crystallization from high-temperature (^1100°C) magmas which had elevated oxygen and water fugaciti.es. Geochemically, they are characterized by relatively low TiO2, AI2O3 and Fe^Os, and high .A&^Q coupled with variable abundances of compatible trace elements and very high contents of incompatible trace elements. Normalized incompatible element patterns have marked negative Nb, la and Ti anomalies whereas K^O appears to be buffered at ~4% over a wide range of SiO^. Isotope data show a relatively broad and enriched range of^Sr/^Sr (0-7076-0-7106) at more restricted (—5 2 to -8 1). Pb isotopes are characterized by a range of 204 ^Pb/^Pb (38-67-39 SO) /204 pb/20 pb (,5.51_15.72) 207pb/pb (

/pdf/post-collision-shoshonitic-volcanism-on-the-tibetan-plateau-46befoqush.pdf

Post-collision, shoshonitic volcanism on the Tibetan Plateau: implications for convective thinning of the lithosphere and the source of ocean island basalts

We have melted metapelitic rocks from the High Himalayan Crystalline Sequence that are likely sources of leucogranite magmas. Starting materials were a muscovite schist and a tourmaline-bearing muscovite–biotite schist. Both are kyanite-zone rocks from the hanging wall of the Main Central Thrust. Experiments were conducted at 6, 8 and 10 kbar and 700–900°C, both without added H
 2 O (dehydration–melting) and with 1–4 wt % added H
 2 O. Dehydration-melting begins at 750–800°C, and produces melts that are virtually identical in composition to the Himalayan leucogranites. Adding H
 2 O lowers the solidus by promoting plagioclase + quartz melting. Melts produced from these starting materials at T ≤ 750°C by H
 2 O-fluxing are trondhjemitic, and different in composition from most Himalayan leucogranites. Leucogranite magmas in the Himalaya formed by dehydration-melting of metapelites during adiabatic decompression, at 6–8 kbar and 750–770°C. The dehydration-melting solidus for muscovite schist has a smaller dP/dT slope than that for biotite schist. In consequence, muscovite schist undergoes decompression-melting more readily than does biotite schist. The two solidi probably cross over at ∼10 kbar, so that muscovite may be a more important deep crustal H
 2 O reservoir than biotite.

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Experimental Constraints on Himalayan Anatexis

The uplift of the Tibetan plateau, an area that is 2,000 km wide, to an altitude of about 5,000 m has been shown to modify global climate and to influence monsoon intensity. Mechanical and thermal models for homogeneous thickening of the lithosphere make specific predictions about uplift rates of the Tibetan plateau, but the precise history of the uplift of the plateau has yet to be confirmed by observations. Here we present well-preserved fossil leaf assemblages from the Namling basin, southern Tibet, dated to approximately 15 Myr ago, which allow us to reconstruct the temperatures within the basin at that time. Using a numerical general circulation model to estimate moist static energy at the location of the fossil leaves, we reconstruct the elevation of the Namling basin 15 Myr ago to be 4,689 +/- 895 m or 4,638 +/- 847 m, depending on the reference data used. This is comparable to the present-day altitude of 4,600 m. We conclude that the elevation of the southern Tibetan plateau probably has remained unchanged for the past 15 Myr.

http://home.ustc.edu.cn/~yangyuan/document/spicer_nature01356%20(1).pdf

Nigel Harris

Papers

Trace element discrimination diagrams for the tectonic interpretation of granitic rocks

Geochemical characteristics of collision-zone magmatism

Post-collision, shoshonitic volcanism on the Tibetan Plateau: implications for convective thinning of the lithosphere and the source of ocean island basalts

Experimental Constraints on Himalayan Anatexis

Constant elevation of southern Tibet over the past 15 million years