PII S0016-7037(98)00121-5

Chemical composition of the continental crust as revealed by studies in East China

SHAN GAO,* TING-CHUAN LUO, BEN-REN ZHANG, HONG-FEI ZHANG, YIN-WEN HAN, ZI-DAN ZHAO, and YI-KEN HU

Department of Geochemistry, China University of Geosciences, Wuhan 430074, People’s Republic of China

(Received May21, 1997;accepted in revised form April3, 1998)

Abstract—We report abundances of sixty-three major, trace, and rare earth elements in the upper crust in fivetectonic units (the interior and southern margin of the North China craton, the North and South Qinlingorogenic belts, and the Yangtze craton) of central East China and the study area as a whole. The estimates arebased on sampling of 11,451 individual rock samples over an area of 950,000 km2, from which 905 largecomposite samples were prepared and analyzed by thirteen methods. Some of the trace elements (i.e., Ag, As,Ge, Mo, Pd, Pt, Sb, Se, Sn, W) have never been subjected to systematic analysis in previous regional crustalcomposition studies. The middle, lower, and total crust compositions of the tectonic units are also estimatedfrom studies of exposed crustal cross-sections and granulite xenoliths and by correlation of seismic data fromeleven regional seismic refraction profiles with lithologies.

The proposed granodioritic total crust composition has the following ratios of element pairs exhibitingsimilar compatibility, that are identical or close to the primitive mantle values: Zr/Hf5 37, Nb/Ta5 17.5,Ba/Th5 87, K/Pb5 0.123 104, Rb/Cs5 25, Ba/Rb5 8.94, Sn/Sm5 0.31, Se/Cd5 1.64, La/As5 10.3,Ce/Sb5 271, Pb/Bi5 57, Rb/Tl5 177, Er/Ag5 52, Cu/Au5 3.23104, Sm/Mo5 7.5, Nd/W5 40, Cl/Li 510.8, F/Nd5 21.9, and La/B5 1.8. Them (238U/204Pb) value is calculated at;5.

The upper crust composition is less evolved and higher in TiO2, total FeO, Co, Cr, Ni, Sc and V, and lowerin Na2O, K2O and Nb, Ta, Rb, Th, U, and Zr, than previous estimates based on shield samplings. Becauseusually the uppermost layer of the crust, where mafic volcanics tend to concentrate, has been removed fromPrecambrian shields, and since our study involves Phanerozoic orogenic belts, the results are suggested to bebetter representative of the upper crust in a general sense. Trace elements associated with mineralization (e.g.,B, Cl, Se, As, Bi, Pd, W, Th, Cs, Ta, Tl, Hg, Au, and Pb) show considerable inter-unit variations by a factorof 2–5 in the upper crust. In addition, the North Qinling paleoactive margin is characterized by anomalousenrichment in Th, U, and Pb in particular and has a marked lowerm value (3.3) compared to the two cratonsand the South Qinling paleopassive margin (m 5 4.5–6.2).

Each tectonic unit has a relatively homogenous middle crust composition which is broadly similar to thecomposition of the total crust. The lower crust in East China can be divided into two layers both seismicallyand chemically. The upper lower crust is characterized by Vp5 6.7 km s21 and an intermediate compositionand the lowermost crust by Vp5 7.1 km s21 and a mafic composition. The bulk lower crust is stillintermediate in composition with 58% SiO2 due to the dominance of the upper lower crust.

P-wave velocities of both the lower and total crusts in East China are slower by 0.2–0.4 km s21 comparedto various global estimates. Correspondingly, the total crust shows an more evolved composition and ischaracterized by a significant negative Eu anomaly (Eu/Eu*5 0.80), low Sr/Nd (10.4), and high La/Nb (3.0)ratios. The obtained SiO2 is 64% on a volatile-free basis. The near arc magma La/Nb ratio implies thatintraplate crustal growth contributes,10% of the continental crust. The relative deficits in Eu, Sr, andtransition metals (Cr, Ni, Co, V, and Ti) in the derived crustal compositions of East China, along with slowercrustal velocity and thin crustal thickness for the Paleozoic to Mesozoic Qinling-Dabie-Sulu orogenic belt,strongly suggest that lower crust delamination of eclogites, as represented by those from the Dabie-Sulu belt,had played an important role in modification of the East China crust during the Phanerozoic era.Copyright© 1998 Elsevier Science Ltd

1. INTRODUCTION

The chemical composition of the continental crust and itslateral and vertical variability have critical importance in ourunderstanding of crust formation and evolution and for quan-tifying other geodynamic processes. Major uncertainties lie inthe lower crust, as well exemplified by the mafic/intermediatecomposition paradox of the lower crust revealed, respectively,by granulite xenoliths exhumed mainly by basaltic volcanicsand by crustal cross sections exposed by means of tectonicprocesses (Fountain and Salisbury, 1981; Rudnick and Presper,

1990; Gao et al., 1992; Rudnick, 1992; Rudnick and Fountain,1995; and references therein). Although granulite xenoliths andexposed granulite terrains provide direct windows into thepresent and/or fossil lower crust, seismic refractions are still themost important approach to probing the generally inaccessibledeep crust over large areas. Refraction studies give P- andS-wave velocities which are determined by the rock composi-tion in addition to pressure, temperature, anisotropy, and theinfluence of fluids (e.g., Holbrook et al., 1992; Kern, 1993;Christensen and Mooney, 1995; Rudnick and Fountain, 1995).The nonuniqueness in relating seismic velocity and rock lithol-ogy is one of the major problems in the interpretation ofgeophysical data, which can be reduced by measurement ofseismic properties of regional deep crustal rocks at in situ

*Author to whom correspondence should be addressed (gaoshan@cug.edu.cn).

Pergamon

Geochimica et Cosmochimica Acta, Vol. 62, No. 11, pp. 1959–1975, 1998Copyright © 1998 Elsevier Science LtdPrinted in the USA. All rights reserved

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conditions and with combined use of compressional and shearwave velocities and heat flow data (Holbrook et al., 1992;Rudnick and Fountain, 1995).

The lower crust is complicated further by the so calledseismic vs. petrological Moho, in which crustal rocks likeeclogite may exhibit mantle-like P-wave velocities (Mengeland Kern, 1992; Kern et al., 1998). Moreover, underplating ofmantle-derived magma may lead to a transitional crust-mantleboundary (Drummond and Collins, 1986; Griffin and O’Reilly,1987; Rudnick, 1992).

Variations of crustal composition with age and crustal typesand from region to region are another major uncertainty inestablishing global models. Few regions have been subjected tosystematic sampling and chemical analysis for the purpose ofregional composition studies. Among these are the CanadianShield (e.g., Shaw et al., 1967, 1976, 1986, 1994; Fahrig andEade, 1968; Truscott and Shaw, 1990), the Baltic and Ukrai-nian shields and the basement of the Russian Platform (Ronovand Yaroshevsky, 1969), the northwest Scotland Highland(Bowes, 1972), the Precambrian Llano Uplift, central Texas,USA (Johnson et al., 1976), and the Qinling orogenic belt andthe adjacent margins of the North China and Yangtze cratons(Gao et al., 1991, 1992). The Canadian Shield is best studied,and its surface composition has been used as a standard for theupper crust (Taylor and McLennan, 1985, 1995; Rudnick andFountain, 1995; Wedepohl, 1995). Most of these studies arerestricted to Precambrian shields and do not involve orogensand their associated continental margins, which have been themajor site of continental growth and crust-mantle interaction.As noted by Condie (1993), equating the average compositionof Precambrian shield to average upper crust should be donewith cautions, because 5–20 km or more of upper crust hasbeen eroded from shields and thus is not included in theaverage composition based on surface sampling. Also, becausemost shield areas comprise chiefly Precambrian rocks, thePhanerozoic contribution to average upper crust is largelyignored in this approach. It would seem that at best, shield areasgive us the average chemical composition of presently pre-served Precambrian upper crust, which may or may not besimilar in composition to the global upper continental crust.Therefore, even for the best known upper crust, problems stillexist regarding its lateral compositional variation, let alone thedeep crust.

In previous compilations of average compositions of rocktypes, upon which estimates of crustal composition were based(e.g., Shaw et al., 1986; Rudnick and Presper, 1990; Rudnickand Fountain, 1995; Wedepohl, 1995), chemical analyses fromliterature were used. This possibly leads to additional uncer-tainties due to adoption of different analytical methods and dataof varying precision and accuracy produced in individual stud-ies.

The above uncertainties and complexities emphasize theimportance of regional composition studies and show thatgeochemical, geophysical, and geological results must be com-bined to derive a reasonable compositional estimate of thecontinental crust.

In Gao et al. (1992), we reported abundances of thirteenmajor, sixteen trace, and fourteen rare earth elements in thecrust of the Qinling orogenic belt and the adjacent margins ofthe North China and Yangtze cratons based on regional sam-

pling and chemical analysis. During the last five years the studyhas been extended to cover a much larger area in central EastChina with a total sampling area of 950,000 km2. This com-pares to 153,200 km2 in the previous study (Gao et al., 1991,1992). Sixty-three elements or components were analyzed us-ing various methods for 905 large composite samples, whichwere made from 11,451 individual rock samples. Some of thetrace elements reported in this study (i.e., Ag, As, Ge, Mo, Pd,Pt, Sb, Se, Sn, W) have never been subjected to systematicanalysis in previous regional crustal composition studies. In acompanion paper (Gao et al., 1998), we summarize crustalstructures of major tectonic units in China from data of eigh-teen seismic refraction profiles (Fig. 1) and discuss their im-plications for composition and evolution of the lower crust. Inthe present paper, we will concentrate mainly on chemicalcompositions of various crustal layers and the total crust ofdifferent tectonic units in central East China.

2. GEOLOGIC SETTING

The study area includes the East Qinling orogenic belt andthe North China and the Yangtze cratons in central East Chinaand covers a total area of 950,000 km2 excluding unsampledQuaternary sediments (Figs. 1 and 2). Within this area rocktypes with variable metamorphic grades and ages are wellexposed. For example, most of the granulite occurrences inChina are distributed in the area. In the following discussion,we divide the study area into five tectonic units (Fig. 2). Theseare, from north to south, the interior of the North China craton(INC), the southern margin of the North China craton (SNC),the North Qinling belt (NQ), the South Qinling belt (SQ), andthe Yangtze craton (YC).

The Qinling mountains are an E-W-trending mountain rangethat separates, both geographically and geologically, North and

Fig. 1. Generalized tectonic map of China showing distributions ofseismic refraction profiles (heavy lines) and the area of geochemicalsampling (dash line box). NC5 North China craton; YC5 Yangtzecraton; SC5 South China orogen. The profiles are numbered where1–12 represent geoscience transects and 13–18 indicate seismic refrac-tion profiles.

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South China and extends westward to the Qilian, the Kunlun,the Pamir, and eastward to the Dabie Mountains (Fig. 1). Thepaleo-Qinling ocean is considered to have comprised the east-ernmost part of the Paleotethyan ocean and separated North andSouth China prior to the Paleozoic to Early Mesozoic subduc-tion and continental collision (Mattauer et al., 1985; Hsu et al.,1987; Gao et al., 1995; Zhang et al., 1997). It is commonlyagreed that the North Qinling belt was an active continentalmargin of the North China craton, whereas the South Qinlingbelt was a passive margin and constitutes the northernmostportion of the Yangtze craton (Zhang et al., 1996).

Simplified stratigraphic columns and descriptions of Ar-chean and post-Archean rocks for the five tectonic units exceptINC have been given in Gao et al. (1991, 1992, 1996). Sinian(800–570 Ma) and Phanerozoic tectonic developments andstratigraphic records in INC correlate with those in SNC.Hence, descriptions are necessary only for the Precambrianrocks in INC.

The North China craton is one of the world’s oldest Archeancratons and preserves crustal remnants as old as$3.8 Ga inboth the Qianxi and Anshan regions (Liu et al., 1992), whichare located within and west of the study area, respectively (Fig.2). In the amphibolite- to granulite-facies Archean Qianxi ter-rain, the Caozhuang Supracrustal Association contains calc-silicate rocks interlayered with amphibolitic layers probably oftuffaceous protoliths, fuchsitic quartzite, metapelite, andbanded iron formation (BIF). Detrital zircon ages of 3550–

3850 Ma were determined by SHRIMP for the fuchsitic quartz-ite (Liu et al., 1992). The Qianan and Qianxi Gneisses consistof charnockite and TTG (tonalitic-trondhjemitic-granodioritic)and granitic gneisses of probable mid and late Archean age,respectively (Sills et al., 1987; Wu et al., 1991).

To the west, the Jining granulite-facies terrain (Fig. 2) iscomposed of charnokite, TTG, granitic gneisses, and maficgranulite of probable intrusive origin, as well as supracrustalrocks. The supracrustal rocks show khondalite characteristicsand are dominated by metasandstone, quartzite and metapelitethat commonly contain sillimanite, garnet, and graphite (Shenet al., 1992; Kern et al., 1996). A preliminary U-Pb age from asingle zircon of the TTG gneiss yields an age of 3.3 Ga Ma(Kroner et al., 1987). This terrain may extend to the west andcorrelate with the Wulashan and Qianlishan granulite terrains,and they collectively constitute a;1300 km E-W trendinggranulite belt along the northern North China craton.

South of the Jining terrain, the comparable Fuping andHengshan amphibolite- to granulite-facies terrains (Fig. 2) aredominated by TTG and granitic gneisses with minor metapelite,carbonate, and quartzite. The felsic gneisses frequently containamphibolite and locally preserve mafic granulite inclusions(Kern et al., 1996). Sm-Nd and zircon U-Pb dating gives agesof 2.8 Ga and 2.5 Ga, for the amphibolite-mafic granulite andtonalitic gneiss, respectively (Shen et al., 1992; Li and Qian,1994). The adjacent Wutai granite-greenstone terrain (Fig. 2)consists of bimodal mafic-ultramafic and felsic metavolcanic

Fig. 2. Distribution of sampling localities for stratigraphic sections (solid circles) and granitoid and mafic-ultramaficintrusion traverses (open circles) in central East China. Shaded areas indicate Archean terrains with those sampled beingnumbered. INC5 interior of the North China craton; SNC5 southern margin of the North China craton; NQ5 NorthQinling belt; SQ5 South Qinling belt; YC5 Yangtze craton.

1961Chemical composition of East China crust

rocks, with thick sequences of metagraywackes in the upperunit. These volcanic and sedimentary rocks were intruded bylarge volumes of contemporaneous TTG and granitic gneisses.Single zircon dating by SHRIMP and evaporation methodsconstrain the age of the volcanic rocks and felsic gneiss to bewithin 2.5–2.8 Ga (Bai et al., 1992).

The Paleoproterozoic, unconformably overlying the Wutaiterrain, is a 11 km thick sequence of dominantly carbonate,slate, phyllite, quartzite, and sandstone with thin layers ofbasalt in the lower unit. The basalt has a single zircon U-Pb ageof 23586 96 Ma (Bai et al., 1992). The Paleoproterozoic in theJining and Qianxi areas consists of biotite gneiss, mica schist,greenschist, phyllite, quartzite, and carbonate. The Meso- andNeoproterozoic consists of carbonate, shale, sandstone, andquartzite or their low-grade metamorphosed equivalents.

Post-Archean granitoids account for 15–30% of the totalsurface area in the present study area and range in age fromPaleoproterozoic to Mesozoic.

3. SAMPLING AND ANALYTICAL METHODS

11,451 individual rock samples were taken from type sections ofArchean terrains and post-Archean stratigraphies as well as intrusivebodies. They are representative of all the Early Archean to Neogenerock units and two-thirds of the total exposed area of post-Archeangranitoids as well as major mafic-ultramafic intrusions of the study area(Fig. 2). The sampling includes new samples from INC and the interiorof YC which were not covered in our previous study and additionalArchean samples from SNC and YC. The additional sampling allowsbetter characterization of the Archean crust, as it is generally believedthat Archean crustal growth accounts for the majority ($60%) of thepresent continental mass (Taylor and McLennan, 1985, 1995; Reymerand Schubert, 1987; McCulloch and Bennett, 1994; Gao and Wede-pohl, 1995). In most cases, the sections and traverses were selectedalong roadcuts, river banks, or mountains valleys where fresh samplesare usually available and remote from the influence of mineralization.Sample sizes were commonly 1–5 kg depending on lithologies.

After petrographic examination to insure freshness, rock sampleswere crushed in specially designed Al2O3 or steel jaw crushers and thenpowdered in an agate or Al2O3 mill to a grain size,200 mesh. Fromthe 11,451 individual rock samples, 905 large composite samples weremade according to tectonic unit, lithology, and age, and by remixingand regrinding equal amounts of aliquots of the powder samples or rockchips again in an agate mill. In contrast to our previous study in whichindividual samples were analyzed for forty-three elements, compositesamples were used in the present study for analysis of sixty-threemajor, trace, and rare earth elements or components. Thirteen differentanalytical methods were utilized for specific elements. These methodsfollow those for certifying values of the standard reference samples ofrocks GSR1–6 and of stream sediments GSD 1–12 (GeochemicalStandard Reference Research Group, 1986, 1987), which have beenused as international reference samples (Govindaraju, 1994). The an-alytical methods are listed in Table 1, along with detection limits andaverage relative differences between obtained and recommended val-ues (RE) of reference samples as well as differences of replicateanalyses (RD). Analytical precision, as indicated by both RD (Table 1)and relative standard deviations of analyses of standard referencesamples, is usually,10% for major elements except the volatilecomponents H2O

1, CO2, S, and organic carbon, and#15% for traceand rare earth elements except Ag, Au, B, Be, Bi, Cd, Cl, Ge, Hg, Mo,Pd, Pt, Sb, Se, Sn, Tl, and W, which range between 15 and 30%. Asmeasured by RE(Table 1), accuracy is,7.1% for all the elements exceptS.

4. CRUSTAL STRUCTURE AND COMPOSITIONAL MODEL

4.1. Characteristics of Crustal Structure

We derived crustal structures for nine broad tectonic units inChina using seismic data from eighteen geophysical refraction

profiles, which include twelve geoscience transects and have atotal length of 16,216 km (Fig. 1; Gao et al., 1998). Amongthem, nine geoscience transects and two refraction profilestraverse the North China Craton, the Qinling-Dabie-Sulu belt,and the Yangtze craton with a total length of 8,531 km (profiles1–9, 13, and 14; Fig. 1). From the geophysical data, thick-nesses, and P-wave velocities of crustal layers and heat flowsfor the nine tectonic units are acquired (Gao et al., 1998).Figure 3 illustrates the crustal structures for INC, SNC, YC,NQ, SQ, central East China (CEC) representing the samplingarea and East China as well as the global continental crust. Thestructure for the global continental crust is calculated fromstructures and areal proportions of various continental tectonicsettings compiled in Rudnick and Fountain (1995), where av-erages of areal data from two models are used (Rudnick andFountain, 1995). Below we summarize the major characteris-tics of the crustal structure relevant to the present study.

Like the other tectonic units in China (Gao et al., 1998), theNorth China and Yangtze cratons shows a four-layered struc-ture, consisting of the upper, middle, upper lower, and lower-most crust. The Qinling orogenic belt is an exception, whichlacks the high velocity (Vp$ 7.0 km s21) lowermost crustallayer. In addition, NQ exhibits a significant low velocity (5.9km s21) in the middle crust, which may be caused by partialmelting (Gao et al., 1992). CEC and East China have anaverage crustal thickness of 37–39 km, which is similar to theweighted global value (37–40 km) calculated from data com-plied by Rudnick and Fountain (1995; Fig. 3), but slightlythinner than the estimate of 41 km given by Christensen andMooney (1995).

If correction is made to room temperature and 600 MPa (allvelocity data are referenced to the same P-T conditions in thefollowing discussion, unless otherwise indicated), velocities ofthe middle, lower, and total crusts of each tectonic unit andCEC and East China are slower by 0.2–0.4 km s21, than theglobal averages (Rudnick and Fountain, 1995; Fig. 3). Theaverage observed total crust velocities are also slower by;0.2km s21 compared to the global average (6.45 km s21) ofChristensen and Mooney (1995). The lower velocity of theupper crust is because much of the tectonic units in China arecovered by thick post-Archean sedimentary rocks that are, ingeneral, unmetamorphosed and have slower velocity relative tocrystalline rocks. This is in contrast to the average upper crustalvelocity of 6.36 0.2 km s21 for shields and platforms (Rud-nick and Fountain, 1995).

CEC and East China has an identical average surface heatflow of 60 mW m22 (Wang, 1996; Gao et al., 1998), which issimilar to the value of the global continental crust (65 mWm22; Pollack et al., 1993).

4.2. Compositional Model

To calculate crustal composition we use a method similar toour previous study (Gao et al., 1992). Namely, the compositionof a given rock unit in each tectonic unit was calculated byaveraging relevant rock types according to their thickness andexposed area measured from geological maps. The upper crustwas taken to consist of sedimentary cover and the underlyingmetamorphosed crystalline basement. The depth of the sedi-mentary cover-crystalline basement boundary was estimated

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from the seismic PiP1 phase and aeromagnetic data, and therelative proportions of sedimentary cover to crystalline base-ment in the upper crust was determined from their respectiveexposure areas, assuming that the cover is underlain by thecrystalline basement, which extends down to the base of theupper crust.

Geological, geochemical, and geophysical studies (includinglinking measured rock velocity properties to seismic data)suggest that the Wutai-Jining and Dengfeng-Lushan zones rep-resent sections through the crust of INC and SNC, respectively.In the Wutai-Jining zone, the upper crust is represented by theWutai granite-greenstone terrain and overlying post-Archeansedimentary rocks, the middle crust by the comparable Heng-shan and Fuping amphibolite- to granulite-facies terrains, andthe upper lower crust by the granulite-facies Jining terrain. Thelowermost crust is not exposed in the section (Kern et al.,1996). In the Dengfeng-Lushan zone, the crust is representedby the Paleoproterozoic volcanic-sedimentary suites of green-

schist-facies and the Mesoproterozoic to Neogene unmetamor-phosed sedimentary rocks (upper crust), the Late ArcheanDengfeng granite-greenstone terrain of amphibolite facies(middle crust), and the Late Archean Taihu high-grade terrainof granulite-facies (lower crust) (Gao et al., 1992; Liu and Gao,1992; Liu et al., 1994).

In both the YC and SQ, unmetamorphosed to slightly meta-morphosed sedimentary sequences of the Neoproterozoic toJurassic are well preserved and cover much of the surface of thetwo areas. Mid- and Neoproterozoic amphibolite and LateArchean granulite facies rocks are exposed locally. This indi-cates small-scale uplift and erosion for the bulk YC and SQ,which agree with their relatively stable platform-like, passivemargin environment since the Neoproterozoic. Accordingly,the exposed high-grade rocks may be used as analogues to theircontemporary middle and upper lower crust. Owing to thedominance by felsic lithologies, they are however unlikely tocomprise the high velocity (7.1 km s21) lowermost crust of YC

1963Chemical composition of East China crust

(Fig. 3). In marked contrast, high-grade rocks dominate thesurface of the NQ, and geothermobarometric studies suggestthat a;26 km thickness of crust has been removed from thistectonic unit. Therefore, its present-day deep crust cannot beexpected to be composed of the exposed high-grade rocks butmay be similar to the underthrust SQ middle crust (Gao et al.,1992). This deduction is supported by the dramatic lead isoto-pic change at;360 Ma of the source of NQ granitoids from theNQ to SQ basements (Zhang et al., 1997).

5. COMPOSITION OF CONTINENTAL CRUST

5.1. The Upper Crust

5.1.1. General characteristics

Table 2 presents chemical compositions of upper crust forthe five tectonic units and CEC as a whole. For comparison, theupper crust compositional estimates from various authors arealso shown in Table 2. Abundances of major elements, REE,and the majority of trace elements given by Wedepohl (1995)were taken from the Canadian Shield (Shaw et al., 1967, 1976,1986). Taylor and McLennan (1985, 1995) also used major (ona volatile-free basis) and some trace element composition of theCanadian Shield in their upper crust compositional model.Their REE composition was derived from post-Archean shales.Compared to these earlier estimates, the upper crust of CEC ishigh in CaO, MgO, and volatile components (H2O

1 and CO2),and ferromagnesian elements TiO2, total FeO, Co, Cr, Ni, Sc,and V, and low in SiO2, Al2O3, Na2O, K2O, and Nb, Rb, Ta,Th, U, and Zr. If the composition is recalculated on a sedimen-tary carbonate rock-free basis, the results show general resem-blance to the earlier estimates (Table 2), but the above featuresof low Al2O3, Na2O, K2O, and Nb, Rb, Ta, Th, U, and Zr and

high TiO2, total FeO, Co, Cr, Ni, Sc, and V still hold or areeven more pronounced. The low Al2O3 content is due to thepresence of significant amounts of arenaceous rocks in theupper crust of INC, SNC, SQ, and YC, which have low Al2O3/SiO2 ratios (0.07–0.21). In comparison, the upper crust com-position of the gneiss dominated NQ shows a Al2O3 contentsimilar to those of earlier estimates (Table 2). The abovecharacteristics indicate more carbonate and mafic componentsin our upper crust composition, in accordance with our previousstudies (Gao et al., 1992).

5.1.2. The role of erosion

We attribute the above discrepancies between our estimatesand the estimates based mainly on sampling of the CanadianShield primarily to erosion. Various lines of evidence suggestthat mafic volcanics decrease in abundance at the expense ofgranitoid rocks with depth in the upper and middle crust.

The great majority of samples for the study of the surfacecomposition of the Canadian Shield were from amphibolitefacies localities (Shaw et al., 1967; D. M. Shaw, pers. com-mun.). It is however known that in the Phanerozoic and someProterozoic provinces, unmetamorphosed and low-grade meta-morphic rocks are important constituents of the upper crust.This is particularly true in China, where much of the tectonicunits, including the Archean North China craton, is covered byvariable but significant thickness of sedimentary and volcanicrocks, leading to a relatively low Vp in the upper crust, asdescribed above. Condie (1993) estimated relative proportionsof major rock types in provinces of different ages based ongeological maps. The results show that the ratio of plutonic tosupracrustal rocks decreases from Archean (2.5–2.3) through

Fig. 3. Average crustal structure for five tectonic units in central East China. For comparison also shown are structuresof central East China representing the geochemical sampling area and East China (Gao et al., 1998) as well as the globalcontinental crust. Data for the global continental crust are recalculated from structures and areal proportions for differentcrustal types, where areal data from two models are averaged (Rudnick and Fountain, 1995). All velocities (km s21) arereported at 600 MPa and room temperature. The underlined number below each column indicates average Vp for the totalcrust.

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Proterozoic (1.1) to Phanerozoic (0.55). The lower ratio ofPhanerozoic crust is suggested to reflect a shallower overallerosion level (Condie, 1993). In an attempt to restore theamount of crust lost by erosion, Condie (1993) added a layer ofcrust 10 km thick for Precambrian areas and 5 km thick forPhanerozoic areas to the present upper crust layer. His resto-ration model for the upper continental crust composition showsa surprisingly good agreement with the CEC upper crust com-position in Nb, Rb, Th, Zr, Co, Sc, and V as well as K2O (Table2). Although the Cr and Ni abundances of the restoration modelare significantly greater than our estimates, the difference issmall compared to estimates based on the Canadian Shield.

The Kapuskasing uplift is considered to represent a crosssection through;25 km crust of the Canadian Shield (Fountainet al., 1990a,b). The greenschist to low amphibolite faciesMichipicoten belt consists of 43% basalt. The abundance ofmafic rocks decreases significantly to 8% in the amphibolitefacies Wawa gneiss terrain and 15% in the granulite faciesKapuskasing terrain (Fountain et al., 1990b). The Wutai-Heng-shan zone represents a cross section though the upper andmiddle crust of the Archean North China craton (Kern et al.,1996). In this section, mafic volcanics make up 23% in thegreenschist and only 8% in the amphibolite facies terrains.

Detailed source provenance studies of metapelites fromsouthern Africa infer that, from the deep Moodies Group to theshallower Pongola Supergroup, proportions of both tholeiiteand granite components increase in the erosion products(Wronkiewicz and Condie, 1987, 1989; Jahn and Condie,1995). Their fractions decrease considerably in the Parkton,Brixton, and Promise Formation of the Witwatersrand Super-group because these successions represent an even deeper levelwhere tonalite dominates.

In addition, SQ and particularly the NQ paleo-active conti-nental margin exhibit higher Cr, Ni, and V than the adjacentcratonic regions (Table 2). This implies that the orogenic belt isless evolved than the cratons. It is concluded from these ob-servations that our model represents a more complete section ofthe upper crust. Therefore, the composition is suggested to bemore representative of the present upper continental crust in ageneral sense than the estimates based on shield sampling.

5.1.3. Lateral variations

Lateral variations of crustal composition can be determinedunambiguously from the study of the upper crust due to itsaccessibility to direct sampling. As shown in Table 2, on asedimentary carbonate rock-free basis, major elements showlittle variation between the five tectonic units. Figure 4 illus-trates variations of trace elements and REE. It can be seen fromthis figure that the majority of trace elements and REE varybetween 0.5 and 1.5 in terms of the values normalized to theCEC upper crust composition. Exceptions are B, Cl, Se, As, Bi,Pd, W, Th, Cs, Ta, Tl, Hg, Au, and Pb. Most of these elementsare siderophile and associated with mineralization. It is alsonoted that the exceptions mostly occur in the orogenic belts.NQ exhibits distinctly high U, Th, and Pb and Rb, Li, Cs, Ta,Tl, and Au to a lesser extent relative to the other tectonic units.The enrichment of Th, U, and Pb is substantiated by abundantallanite (;1.4%) observed under the microscope in the meta-graywackes from the Qinling metamorphic core complex (Youet al., 1987) and by the ubiquitous radiogenic lead isotopiccomposition of the Proterozoic-Early Paleozoic basements andgranitoids in NQ (Zhang et al., 1997).

Fig. 4. Inter-unit variations of upper crustal trace and rare earth element abundances relative to the upper crust of centralEast China as a whole. Elements are arranged in order of increasing normalized value for the North Qinling belt, whichexplains most of the inter-unit variations.

1966 S. Gao et al.

5.2. The Middle Crust

The middle crust represented by amphibolite-facies rocks inCEC has a tonalitic composition (Table 3). Average velocity ofthis layer is 6.4 km s21 (Fig. 3). Our measurements of deepcrustal rocks from the Wutai-Jining crustal cross section in theNorth China craton establish a well-defined linear correlationbetween Vp and SiO2 content for silicate rocks (Kern et al.,1996). From the linear regression for 600 MPa and roomtemperature, the 6.4 km s21 velocity yields a SiO2 content of63.2%. Our middle crust composition has 62.7% SiO2 on acarbonate-free basis. Therefore, the derived middle crust com-position agrees with the seismic data. The major and traceelement composition shows a broad similarity to the estimatesof Rudnick and Fountain (1995; Table 3), but our results aremore evolved and characterized by higher SiO2, K2O, Ba, Li,Zr, and LREE and LaN/YbN, with a significant negative Euanomaly. If the compositions are recalculated to 100% on avolatile-free basis, our SiO2 is 2.0% higher. This is reasonableon account of higher Vp for the Rudnick and Fountain (1995)global middle crust (Fig. 3).

Inter-unit compositional variations are also obvious. Traceelements showing variation by a factor of.2 include As, B, Bi,Cl, Hg, Pt, Se, and W.

5.3. The Lower Crust

5.3.1. The upper lower crust

Compositions of granulite terrains for different tectonic unitsand CEC are given in Table 3. Granulite terrains from INC,SNC, and NQ have a similar composition, with INC beingenriched in total FeO, Cr, Ni, Pd, Pt, and depleted in Cs, Li, Rb,Th, U, and LREE. In contrast, the granulite terrain of YC ismuch more evolved. It has been argued that the lowermost crustis generally unexposed in crustal cross sections and granuliteterrains represent only the upper part of the lower crust (Bohlenand Mezger, 1989; Fountain et al., 1990a,b). Correlation ofmeasured velocities of deep crustal rocks with the seismicstructure in the North China craton also indicates that the 5-kmthick lowermost crust with Vp. 6.8 km s21 is not sampled inthe Wutai-Jining section (Kern et al., 1996). Therefore, gran-ulite terrains cannot be used to represent the entire lower crustexcept rare cases like the Ivrea Zone, where the crust-mantleboundary is suggested to have been exposed (Voshage et al.,1990). However, studies of the Wutai-Jining and Dengfeng-Taihua sections show that granulite terrains there correlate wellwith geophysical properties of the present upper lower crust inthe North China craton (Liu and Gao, 1992; Liu et al., 1994;Kern et al., 1996). We also use the granulite terrains to modelthe upper lower crust in the Qinling belt and Yangtze craton.

The CEC upper lower crust has Vp of 6.7 km s21, whichagrees with intermediate granulites (6.76 0.2 km s21; Rudnickand Fountain, 1995). However, the CEC granulite terrains, onaverage, have a significantly higher SiO2 of 62.72% relative toworldwide and CEC intermediate granulites, which consis-tently show an average SiO2 of ;58% (Rudnick and Fountain,1995; Appendix). As seen from Table 3, the discrepancy arisesprincipally from the Archean Kongling high-grade terrain ofYC (65.69% SiO2). Clastic metasedimentary rocks make up;50% of this terrain. They contain variable amounts of

high-Vp garnet and/or sillimanite (8.6–8.9 and 9.4 km s21,respectively; Kern and Schenk, 1985) and have an averageSiO2 of 65.0 %. Their Vp was determined to be 6.9 km s21 atpressure and temperature conditions corresponding to 30 kmdepth under the present regional geothermal regime. Granuliteto amphibolite facies TTG gneisses of the Kongling terrainhave an average SiO2 of 67.6 %, whose Vp should be 6.5 kms21 from comparison with velocities of felsic gneisses of thesame metamorphic grades (Rudnick and Fountain, 1995). Con-sequently, inclusion of 50% clastic metasediments will increasethe velocity of the Kongling terrain to 6.7 km s21 withoutsignificant change in SiO2. The similar bulk composition of themiddle and upper lower crusts but the latter with higher Vp canbe interpreted similarly by its higher metamorphic grade andpresence of garnet and sillimanite bearing clastic metasedimen-tary rocks.

5.3.2. The lowermost crust

The lowermost crust in CEC varies from 7.0 to 7.1 km s21

in Vp (Fig. 3). The velocities agree with our measured veloc-ities of mafic granulites from the North China craton and theQinling and Dabie belts (6.9–7.4 km s21) (Liu and Gao, 1992;Kern et al., 1996, 1998; Gao et al., 1998). They are alsoconsistent with velocities of amphibolite and granulite-faciesmafic rocks (7.0–7.3 km s21) compiled by Rudnick and Foun-tain (1995). Accordingly, two models are used for the consti-tution of the lowermost crust, consisting of the worldwideaverage mafic granulite xenolith (Rudnick and Fountain, 1995)and the average mafic granulite from high-grade terrains inCEC (Appendix), respectively. These two models are referredto as the xenolith and terrain models in the following discus-sion. Lower crustal xenoliths were recently recovered fromthree localities in East China and dominated by two-pyroxeneand garnet two-pyroxene mafic granulites (Fan and Liu, 1996;Xu et al., 1996, 1997; Gao et al., unpubl. data). Felsic granulitexenoliths of charnokitic and TTG and granitic gneisses are alsocommon. However, metapelitic xenoliths have not been re-ported. Thus, although the lowermost crust has Vp consistentwith granulite facies metapelites, they are not considered as animportant constitution in the lowermost crust. Geothermobaro-metric studies indicate that the mafic granulite xenoliths equil-ibrated at 800–1050°C and 800–1200 MPa, corresponding to25–40 km, and they were all derived from within;5 km abovethe regional Moho or crustal-mantle transition zone (Fan andLiu, 1996; Xu et al., 1996, 1997). Therefore, the xenolithevidence also indicates that the mafic granulites are confined tothe lowermost crust. Unfortunately, granulite xenoliths inChina have not yet been subjected to systematic geochemicalanalysis.

5.3.3. The bulk lower crust

From the above compositions and thicknesses of the upperand lowermost crusts, the bulk lower crust composition iscalculated for the xenolith and terrain models. The results arepresented in Table 3 (CEC2 and CEC3). Both models give quitesimilar compositions with 58% SiO2. The compositions are inturn close to the estimates of Wedepohl (1995) based on theEuropean Geotraverse (EGT). Major exceptions are that our Cr

1967Chemical composition of East China crust

1968 S. Gao et al.

and Ni are significantly lower and Cs is remarkably higher.Both our and the Wedepohl (1995) estimates are significantlymore felsic than those of Taylor and McLennan (1985, 1995)and Rudnick and Fountain (1995). These workers base theirstudies on crustal growth model and compilation of seismicrefraction profiles. The latter authors give the bulk lower crustalvelocities ranging between 6.9 and 7.2 km s21 for differentcrustal types, with an average of 7.1 km s21. The bulk lowercrustal velocities in East China vary from 6.8 to 7.0 km s21 andaverage 6.8 and 6.9 km s21 for CEC and East China, respec-tively (Gao et al., 1998). These velocities are slower by 0.2–0.3km s21 than the global estimate. If velocity versus depthprofiles are compared to the global continental crust of Chris-tensen and Mooney (1995), a similar conclusion can be derived(Gao et al., 1998). The CEC bulk lower crustal velocity is onlyslightly higher than the above velocities of intermediate gran-ulites and remarkably lower than those of amphibolite andgranulite facies mafic rocks.

The seismic velocity can be used to infer the bulk lower crustcomposition independently. A mixture of 70% intermediategranulite (6.7 km s21), 15% mafic granulite (7.2 km s21), and15% metapelite (7.1 km s21; Rudnick and Fountain, 1995) willyield the CEC bulk lower crustal velocity of 6.8 km s21. Usingcompositions of respective rock types from CEC (Appendix),this simple model gives a composition, which is not at greatvariance with the above results (compare CEC3 and CEC4;Table 3).

In summary, an intermediate bulk composition best fits thegeophysical, geochemical and geological data of the lowercrust in central East China.

5.4. The Total Crust

5.4.1. General characteristics

Corresponding to the xenolith and terrain models for thelowermost crust, two sets of compositional estimates are ob-tained for the total crust. It can be seen from Table 4 that thetwo estimates (CEC2 and CEC3) are quite similar. This is alsotrue if the lower crust is assumed to be entirely composed ofgranulite terrains (CEC1), indicating that the total crust com-position is less model dependent. This is because the lowermostcrust accounts for only 12% of the CEC total crust in thickness.Since CEC and East China show a close resemblance in veloc-ity structure (Fig. 3) and heat flow, the derived compositionsshould represent East China as well. Calculation by substitutingwith crustal layer thicknesses of East China supports the con-clusion (compare CEC and EC; Table 4). Again, our totalcrustal compositions are similar to the estimates of Wedephol(1995) and more felsic than those of Rudnick and Fountain(1995) and Taylor and McLennan (1985, 1995) in particular.However, our estimates of heat-producing elements K9, Th, andU, and thus the value of heat production are significantly lowerthan those of Wedepohl (Table 4). As stated above, both theobserved and corrected mean crustal velocities of East Chinaare 0.2–0.3 km s21 slower than the global continental crust. Asa result, a more felsic composition is in accordance with geo-physical data.

5.4.2. Rarely analyzed elements and element ratios

Figure 5 shows average compositions of the CEC total crustand normal-type mid-ocean ridge basalts (Hofmann, 1988). Allconcentrations are normalized to the primitive mantle (PM)values (McDonough and Sun, 1995). Elements in the figure arearranged in the order of decreasing concentration in the crustand thus increasing degree of compatibility in the mantle, to theright. As suggested by Hofmann (1988), such plots demonstratethat the continental and oceanic crusts are complementary incomposition. The resultant sequence is similar but not identicalto the alternative sequence given by him. MORB displays anoverall regular pattern. Irregularity occurs for Ba, Th, Nb, Ta,Sr, and Si. The results are also clear in the MORB pattern ofHofmann (1988). But the Cu anomaly is not observed in ourstudy. Neighboring elements in Fig. 5 exhibit similar geo-chemical behavior and their ratios in the continental crustshould not deviate much from their PM values.

Our total crustal compositions give ratios of Zr/Hf, Nb/Ta,Ba/Th, K/Pb, Rb/Cs, Ba/Rb, Si/Ge, Sn/Sm, Se/Cd, La/As,Ce/Sb, Pb/Bi, Rb/Tl, Er/Ag, Cu/Au, Sm/Mo, Nd/W, Cl/Li,F/Nd, and La/B, that are identical or close to the respective PMvalues (Table 5). Constant Sn/Sm ratio has been noted in themantle and crustal rocks and was used to obtain a crustal Snabundance of 1.5 ppm by Jochum et al. (1993), which is inexcellent agreement with our study. Gold is a highly sidero-phile element which tends to concentrate in the upper mantlerelative to the continental crust. Pattou et al. (1996) and Hart-mann (1998) carried out precise analysis of Au by ICP-MS inupper mantle lherzolites from orogenic massifs and peridotitexenoliths. Their results show that Au is relatively homoge-neously distributed in fertile lherzolites and lherzolites of prim-itive mantle character and in xenoliths and has an averageconcentration of 1.0 (60.3) ppb. The value should be used asthe upper limit for the continental crust. Therefore, our lowerAu abundance seems to be more reasonable.

However, our Th/U and K/U ratios are higher and the U/Pbare lower, than PM and previous estimates of the continentalcrust (Table 5). This leads to a lower calculatedm (238U/204Pb)ratio. A similar lowm value (5.7) of the continental crust wasalso suggested by Zindler and Hart (1986). The North Qinlingpaleoactive continental margin has marked lowerm values(3.3–3.7) in the upper and total crust compared to the twocratons and the South Qinling paleopassive margin (m 5 4.2–6.2). Lead isotopic studies of island arcs and suspended loadsof rivers drainaging young arcs also show lowm (Tatsumoto,1978; Asmerom and Jacobsen, 1993). Loss of U during weath-ering may be another cause for the lowm values (Asmerom andJacobsen, 1993). Feldspars of Neoproterozoic to Mesozoicgranitoids from the Qinling belt show age-independent leadisotopic compositions, which can correlate either with the Pre-cambrian basements or with their underthrusting (Zhang et al.,1997). This implies lowm for the source of the granitoids andin turn suggests a deep origin for the lowm.

6. IMPLICATIONS FOR STRUCTURE AND EVOLUTIONOF CONTINENTAL CRUST

6.1. Lower Crustal Structure

Recent summaries of global seismic data all give an one-layered lower crustal structure of mafic composition (Holbrook

1969Chemical composition of East China crust

1970 S. Gao et al.

et al., 1992; Christensen and Mooney, 1995; Rudnick andFountain, 1995; Zandt and Ammon, 1995; Christensen, 1996).In these studies, data from China were rarely considered. Re-sults presented above show a contrasting double layered lowercrustal structure, consisting of a felsic upper part and a maficlower part. The bulk lower crust is intermediate in composition.It may be argued that only the lower part corresponds to thelower crust in the global sense and the upper part belongs to themiddle crust. The lowermost crust is only 3–7 km thick in EastChina, which is thin compared to the;15 km thick globalmafic lower crust (Christensen and Mooney, 1995; Rudnickand Fountain, 1995). The upper and lower parts together havea total thickness of;14 km and make up about one-third of thecrust, which is consistent with the generally accepted definitionof the lower crust and comparable to the global lower crust indepth range (Holbrook et al., 1992; Christensen and Mooney,1995; Rudnick and Fountain, 1995). Such a double layeredlower crust structure can explain the systematically higherpressure and temperature conditions for the formation of over-whelmingly mafic granulite xenoliths relative to granulitesfrom high-grade terrains which are dominated by felsic lithol-ogies (Bohlen and Mezger, 1989; Rudnick and Presper, 1990;Rudnick and Fountain, 1995).

A similar double-layered lower crustal structure also char-

acterizes the;3000 km long European Geotraverse (EGT). Inthis case, the upper felsic and lower mafic parts have velocitiesof 6.5–6.9 and 6.9–7.5 km s21, corresponding to thickness of11.9 and 7.2 km, respectively (Wedepohl, 1995). The middlecrust was not distinguished in his model. It may correspond tothe lower part of the upper crust and the uppermost part of thelower crust for a 40 km thick crust compared to the global andEast China models. A double-layered lower crustal structure isnevertheless evident for EGT (Ansorge et al., 1992; Wedepohl,1995).

6.2. Formation and Destruction of Continental Crust

It has been disputed where and how the continental crustformed and their possible change with time. Major controver-sies lie around the island arc versus plume-related oceanicplateau accretion as the major mechanism responsible forcrustal growth (Abbott and Mooney, 1995; Rudnick, 1995;Taylor and McLennan, 1995; Stein and Goldstein, 1996). Inany case, primary mantle additions into the crust are mainlybasaltic, and yet the present-day continental crust has an inter-mediate bulk composition. These observations point to delami-nation of mafic/ultramafic lower crust as an important recyclingprocess for refining the primary crust. Dense mafic lower crust

Fig. 5. Average compositions of continental crust in central East China and normal-type mid-ocean ridge basalts(Hoffmann, 1988) showing increasing compatibility to the right. All concentrations are normalized to the primitive mantle(McDonough and Sun, 1995). The sequence of elements is determined by decreasing concentrations in the continental crust.

1971Chemical composition of East China crust

may form as eclogite during crustal thickening or throughunderplating and fractionation of basaltic magma at the base ofthe crust, or as granulite residues left after intracrustal meltingproducing granitic magma (Arndt and Goldstein, 1989; Kayand Kay, 1991; Gao et al., 1992; Rudnick, 1995; Wedepohl,1995).

The europium anomaly (Eu/Eu*) and Sr/Nd and La/Nb ratiosare important chemical indicators of the processes of crustalformation, differentiation and crust-mantle interaction (Taylorand McLennan, 1985, 1995; Gao et al., 1992; Gao and Wede-pohl, 1995; Rudnick, 1995). Rudnick (1995) uses the La/Nbratio of the continental crust to determine relative importanceof arc vs. oceanic plateau mechanisms in crustal growth andconcludes that between 10 and 35% of an intraplate componentis required to explain the low La/Nb ratio (1.5; Table 5) of thecontinental crust compositions obtained by her and previousstudies. Unlike Sr/Nd which is fractionated by intracrustalprocesses and shows an increase from the upper to the lowercrust, La/Nb is nearly constant in various crustal layers and isabout 3.0 for CEC (Table 5). The value is identical to themedian of arc magmas and slightly lower than Archean TTG(tonalite-trondhjemite-granodiorite) (4.2; Rudnick, 1995). Us-ing the mixing relationships between convergent marginmagma and intraplate magma of Rudnick (1995), the intraplatecrustal component is implied to be,10%.

As discussed above, the total crust composition in CEC is

more evolved than previous models and characterized by aprominent negative Eu anomaly (Eu/Eu*5 0.80), lower Sr/Ndratio (;10), and higher SiO2 (64.0% on a volatile-free basis;Tables 4 and 5), as well as relative deficits in transition metals(Cr, Ni, Co, V, Ti; Gao et al., 1998). These features, togetherwith slower crustal velocities and remarkably thin crustal thick-ness for the Paleozoic-Mesozoic Qinling-Dabie orogenic belt(29–37 km), leads to the suggestion that lower crustal delami-nation played an important role in modification of the CECcrust (Gao et al., 1998). Mass balance modeling further sug-gests that eclogite from the Dabie-Sulu ultrahigh pressuremetamorphic belt is the most likely candidate as the delami-nated material, and that a cumulative 37–82 km thick eclogiticlower crust is required to have been delaminated in order toexplain the relative Eu, Sr, and transition metal deficits in theCEC crust (Gao et al., 1998). Delamination of the thick (;70km) eclogitic lower crustal root has also been proposed toexplain the very thin, mainly granitic crust of 30–40 kmcurrently in the southern Sierra Nevada (Wernicke et al., 1996;Ducea and Saleeby, 1997). Therefore, delamination of eclogiticlower crust is a common and important process for evolution ofcontinental crust and crust-mantle interaction. An intermediatelower crust has lower melting temperatures relative to a maficlower crust. This may partially explain the emplacement ofvoluminous, highly fractionated granites into the upper andmiddle crustal levels during the Mesozoic in East China, which

1972 S. Gao et al.

in turn leads to lower velocities in the middle crust of EastChina compared to the global continental crust.

Acknowledgments—This study is co-supported by the National NatureScience Foundation of China, the State Commission on Education, theMinistry of Geology and Mineral Resources of China, the Open Lab-oratory of Constitution, Interaction and Dynamics of the Crust-MantleSystem, as well as the Alexander-von-Humboldt Foundation of Ger-many. We thank M. C. Yan for help in analysis of the compositesamples and C.-L. Gao, Z.-J. Li, J.-P. Ouyang, D.-P. Wang, and Q.-L.Xie for sampling some of the individual samples. We also thank S. Hartand D. M. Shaw for discussion. Constructive review comments by R. L.Rudnick and two anonymous reviewers are greatly appreciated.

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