Shuqiang Gao

Chemical composition of the continental crust in the Qinling Orogenic Belt and its adjacent North China and Yangtze cratons

The North China and Yangtze (South China) cratons are the most important tectonic units of China. The two cratons finally coalesced through collision during the Late Paleozoic to Middle Triassic with the formation of the E-W-trending Qinling-Dabie Mountains. We report abundances of thirteen major, sixteen trace, and fourteen rare earth elements of the sedimentary cover and upper crust of the Qinling Orogenic Belt (including the North and South Qinling) and the adjacent southern margin of the North China Craton and northern margin of the Yangtze Craton as well as the study area as a whole. The estimates are based on systematic sampling and analyses of >4500 individual rock samples taken over an area of 153200 km2. An attempt is also made to estimate deep and total crustal compositions of the tectonic units from an exposed crust cross section and exposed amphibolite and granulite facies rocks. The proposed granodioritic to quartz-dioritic total crustal composition for the study area is consistent with the regional mean crustal P-wave velocity (6.06 km s−1) and high surface heat flow estimates (72–109 mW m−2), and yields the key element ratios: ThK = 3.5 − 3.8 × 10 −4; SmNd = 0.20–0.21; and GaAl = 0.21 − 0.23 × 10−3, that are identical to those generally accepted. The dioritic to granodioritic deep crustal compositions agree with the characteristic low Vp(5.7–6.6 km s−1) of the regional middle-lower crust and with the step-like, sharp contrast in velocity (>1.0 km s−1) between the lower crust and upper mantle of the Qinling region which points out that mafic underplates/restites are probably not an important constitute of the present-day Qinling lower crust except for the North Qinling Belt. There are clear cratonic-orogenic distinctions in crustal, particularly upper crustal, trace element compositions but not in major and rare earth element compositions. The results imply a poorly differentiated crust, in terms of chemical composition, and contrasting patterns of heat-production distribution with depth for the Qinling region. The upper and deeper crusts exhibit broadly similar compositions except for carbonate components (CaO and CO2). Average compositions of granites are characterized by being low in SiO2 (69.41–70.29%), K2O/Na2O (0.90–1.05), and initial 87Sr86Sr (mostly 0.704–0.714) and less prominant negative Eu anomalies (EuEu∗ = 0.53–0.64) compared to granites in typical collision orogenic belts, the average worldwide granite and average S-type granite.