CHEMICAL-COMPOSITION OF THE CONTINENTAL-CRUST AS REVEALED BY STUDIES IN EAST CHINA

We report abundances of sixty-three major, trace, and rare earth eleme nts in the upper crust in five tectonic units (the interior and southe rn margin of the North China craton, the North and South Qinling oroge nic 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 indivi dual 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) h ave never been subjected to systematic analysis in previous regional c rustal composition studies. The middle, lower, and total crust composi tions 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 lit hologies. The proposed granodioritic total crust composition has the f ollowing ratios of element pairs exhibiting similar compatibility, tha t are identical or close to the primitive mantle values: Zr/Hf = 37, N b/Ta = 17.5, Ba/Th = 87, K/Pb = 0.12 x 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, R b/Ti = 177, Er/Ag = 52, Cu/Au = 3.2 x 10(4), Sm/Mo = 7.5, Nd/W = 40, C l/Li = 10.8, F/Nd = 21.9, acid La/B = 1.8. The mu (U-238/Pb-204) value is calculated at similar to 5. The upper crust composition is less ev olved and higher in TiO2, total FeO, Co, Cr, Ni, Sc and V, and lower i n Na2O, K2O and No, Ta, Rb, Th, U, and Zr, than previous estimates bas ed on shield samplings. Because usually the uppermost layer of the cru st, where mafic volcanics tend to concentrate, has been removed from P recambrian shields, and since our study involves Phanerozoic orogenic belts, the results are suggested to be better representative of the up per crust in a general sense. Trace elements associated with mineraliz ation (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 upp er crust. In addition, the North Qinling paleoactive margin is charact erized by anomalous enrichment in Th, U, and Pb in particular and has a marked lower CL value (3.3) compared to the two cratons and the Sout h Qinling paleopassive margin (mu = 4.5-6.2). Each tectonic unit has a relatively homogenous middle crust composition which is broadly simil ar to the composition of the total crust. The lower crust in East Chin a can be divided into two layers both seismically and chemically. The upper lower crust is characterized by Vp = 6.7 km s(-1) and an interme diate composition and the lowermost crust by Vp = 7.1 km s(-1) and a m afic composition. The bulk lower crust is still intermediate in compos ition with 58% SiO2 due to the dominance of the upper lower crust. P-w ave velocities of both the lower and total crusts in East China are sl ower by 0.2-0.4 km s(-1) compared to various global estimates. Corresp ondingly, the total crust shows an more evolved composition and is cha racterized by a significant negative Eu anomaly (Eu/Eu = 0.80), low S r/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 that int raplate crustal growth contributes <10% of the continental crust. The relative deficits in Eu, Sr, and transition metals (Cr, Ni, Co, V, and Ti) in the derived crustal compositions of East China, along with slo wer crustal velocity and thin crustal thickness for the Paleozoic to M esozoic Qinling-Dabie-Sulu orogenic belt, strongly suggest that lower crust delamination of eclogites, as represented by those from the Dabi e-Sulu belt, had played an important role in modification of the East China crust during the Phanerozoic era. Copyright (C) 1998 Elsevier Sc ience Ltd.