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

We report abundances of sixty-three major, trace, and rare earth elements in the upper crust in five tectonic units (the interior and southern margin of the North China craton, the North and South Qinling orogenic belts, and the Yangtze craton) of central East China and the study area as a whole. Th...

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Veröffentlicht in:Geochimica et cosmochimica acta 1998-06, Vol.62 (11), p.1959-1975
Hauptverfasser: Gao, Shan, Luo, Ting-Chuan, Zhang, Ben-Ren, Zhang, Hong-Fei, Han, Yin-wen, Zhao, Zi-Dan, Hu, Yi-Ken
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Sprache:eng
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Zusammenfassung:We report abundances of sixty-three major, trace, and rare earth elements in the upper crust in five tectonic units (the interior and southern margin of the North China craton, the North and South Qinling orogenic belts, and the Yangtze craton) of central East China and the study area as a whole. The estimates are based on sampling of 11,451 individual rock samples over an area of 950,000 km 2, from which 905 large composite 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 crustal composition studies. The middle, lower, and total crust compositions of the tectonic units are also estimated from studies of exposed crustal cross-sections and granulite xenoliths and by correlation of seismic data from eleven regional seismic refraction profiles with lithologies. The proposed granodioritic total crust composition has the following ratios of element pairs exhibiting similar compatibility, that are identical or close to the primitive mantle values: Zr/Hf = 37, Nb/Ta = 17.5, Ba/Th = 87, K/Pb = 0.12 × 10 4, Rb/Cs = 25, Ba/Rb = 8.94, Sn/Sm = 0.31, Se/Cd = 1.64, La/As = 10.3, Ce/Sb = 271, Pb/Bi = 57, Rb/Tl = 177, Er/Ag = 52, Cu/Au = 3.2×10 4, Sm/Mo = 7.5, Nd/W = 40, Cl/Li = 10.8, F/Nd = 21.9, and La/B = 1.8. The μ ( 238U/ 204Pb) value is calculated at ∼5. The upper crust composition is less evolved and higher in TiO 2, total FeO, Co, Cr, Ni, Sc and V, and lower in Na 2O, K 2O and Nb, Ta, Rb, Th, U, and Zr, than previous estimates based on shield samplings. Because usually the uppermost layer of the crust, where mafic volcanics tend to concentrate, has been removed from Precambrian shields, and since our study involves Phanerozoic orogenic belts, the results are suggested to be better 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 factor of 2–5 in the upper crust. In addition, the North Qinling paleoactive margin is characterized by anomalous enrichment in Th, U, and Pb in particular and has a marked lower μ value (3.3) compared to the two cratons and the South Qinling paleopassive margin (μ = 4.5–6.2). Each tectonic unit has a relatively homogenous middle crust composition which is broadly similar to the composition of the total crust. The lower crust in East
ISSN:0016-7037
1872-9533
DOI:10.1016/S0016-7037(98)00121-5