Crust In China Research Articles

How mafic is the lower continental crust?

Crustal structures for nine broad tectonic units in China, except the Tarim craton, are derived from seismic data for 18 geophysical refraction profiles, which include twelve geoscience transects and have a total length of 16 216 km. All the tectonic units except the Qinling orogen show a four-layered crustal structure, consisting of upper, middle, upper lower, and lowermost crust. If corrections are made to room temperature and 600 MPa, the upper lower crust has P-wave velocities of 6.7–6.8 km s −1 and the lowermost crust 7.0–7.2 km s −1. Velocities of the bulk lower crust and total crust are 6.8–7.0 and 6.4–6.5 km s −1, respectively. They are slower by 0.2–0.4 km s −1, than the global averages. By correlation with experimental results on velocities of typical lower crustal rocks, the upper lower crust is suggested to be intermediate, while the lowermost crust is mafic in composition. Due to dominance of the former, the bulk lower crust is still intermediate for most tectonic units. However, inter-unit compositional variations are also evident, as indicated by large variations in the thickness ratio (0.3–3.0) of the upper lower to lowermost crust. The lower crust in East China as a whole is suggested to have ∼57% SiO 2. The results contrast with generally accepted models of mafic lower crust. Our estimates of the total crust composition in central East China show a more evolved character compared to models of Rudnick and Fountain, and Taylor and McLennan and are characterized by a prominent negative Eu anomaly (Eu/Eu * = 0.80), higher SiO 2 (61.8%), and lower Sr/Nd (∼10) as well as lower Sr/Nd, Cr/Nd, Ni/Nd, Co/Nd, V/Nd and Ti/Nd ratios. This, together with slower crustal velocities and remarkably thin crustal thicknesses (34 km) for the Paleozoic–Mesozoic Qinling–Dabie orogenic belt, leads to the suggestion that lower crustal delamination played an important role in modification of the East China crust. Mass balance modeling further suggests that eclogite from the Dabie–Sulu ultrahigh pressure metamorphic belt is the most likely candidate as the delaminated material, and that a cumulative 37–82 km thick eclogitic lower crust is required to have been delaminated in order to explain the relative Eu, Sr and transition metal deficits in the crust of central East China. Delamination of eclogites can also explain the significantly higher than eclogite Poisson's ratio in the present Dabie lower crust and upper mantle and lack of eclogite in Cenozoic xenolith populations of the lower crust and upper mantle in East China.

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. 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 China 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 intermediate composition and the lowermost crust by Vp = 7.1 km s −1 and a mafic composition. The bulk lower crust is still intermediate in composition with 58% SiO 2 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 s −1 compared to various global estimates. Correspondingly, the total crust shows an more evolved composition and is characterized by a significant negative Eu anomaly (Eu/Eu∗ = 0.80), low Sr/Nd (10.4), and high La/Nb (3.0) ratios. The obtained SiO 2 is 64% on a volatile-free basis. The near arc magma La/Nb ratio implies that intraplate 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 slower crustal 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.

The early Precambrian crustal evolution of China

The Archaean continental crust in China originated from dispersed nuclei under the influence of whirl tectonics 2.5 Ga ago. From the beginning of the early Proterozoic, the crustal evolution process was controlled by the plate tectonics. A tectonic regime of coexisting mobile belts of different characteristics and rigid blocks emerged. The diversity of tectonic setting and structures marked the beginning of a new epoch of crustal evolution. The north China protoplatform, the Tarim protoplatform and the Yangtze protoplatform are considered to have separated from each other by least at the end of Archaean, based on the early Proterozoic accretion zones developed between them. Cathaysia gradually moved closer to the Yangtze protoplatform in the early Proterozoic. The nearly north-south primary structural trend of the early Proterozoic formations in the northern margin of the modern north China platform indicates that the north China protoplatform had been connected with the Siberian craton during and before early Proterozoic times.

Types, Metallogenic Environments and Characteristics of Temporal and Spatial Distribution of Copper Deposits in China

Abstract Based upon a comprehensive study of 123 copper deposits in China, this paper reaches the following conclusions: 1. The crust in China was solidified pretty late; with the accretion of the plate, the copper mineralization moved towards its margin in space and became successively younger in age. 2. The major copper ore types ever found in the world have mostly been discovered in China, in which the porphyry type seems to be the most important and the massive sulfide type in the transitional bed between marine clastic and carbonate rocks is clearly defined. 3. Carbonate strata are widespread in China and their deposition lasted for a long geological period, so the proportion of copper deposits occurring in them is large as compared with other parts of the world. 4. Seven metallogenic epochs can be recognized, in which the Mesozoic one plays the leading role and the middle (late) Palaeozoic and Cenozoic ones are next in importance. Mineralization was comparatively simple in the early geological period, and became diversified later on. In the early epochs copper deposits related to basaltic magmatism and metamorphosed marine sediments predominated, whereas in the later epochs those. related to granitic magmatism and continental sedimentation were dominant. 5. There exist in China thirteen metallogenic provinces, of which the Lower Yangtze downwarping belt, Sanjiang fold system, Jiangnan axis and Xikang‐ Yunnan axis are of greater significance. 6. The crustal mobility in China was rather pronounced, the polycyclic evolution of the crust has resulted in such obvious phenomena as the inheritance of mineralization, the coexistence of various types and the superposition of different genetic types.

Preliminary study of REE iron formations in China

Geological and geochemical characteristics of REE iron formation (REEIF), a term proposed by Prof. Tu Kuang-chih to specify a special type of Precambrian iron formations rich in REE, are discussed in this paper with special reference to its REE contents, REE distribution patterns, the formation mechanism, the relationship between its development and the multi-stage evolution of the continental crust in China, and the implications of REE as an indicator of oxidation state for ancient atmosphere. Major conclusions are outlined as follows: (1) REEIFs are characterized by high REE concentrations against the very low REE levels in normal Precambrian iron formations. (2) REEIFs are formed by marine sedimentary-diagenetic processes in miogeosynclines or transition zones during Proterozoic times To some extent, volcanic activity may play an important role in the deposition of ore-forming materials. In a broader sense, REEIFs belong to Fe-bearing dolomite formations. Most REEIFs in China may be superimposed by late geological processes such as hydrothermal-metasomatism, migmatization and metamorphism. Generally, REEIFs have much in common with stratabound ore deposits in respect to their characteristic features. (3) Similar to Precambrian iron formations, REE are enriched in LREE. But, the degree of LREE enrichment is noticed to increase of total REE content. Most REEIFs are characterized by high ratios of σ Ce/σY, (Mg+Fe)/Ca, Na/K, Nb/Ta, Zr/Hf, Th/U, Ba/Sr, etc. (4) The extensive occurrence of REEIFs indicates higher REE abundance in the continental crust of China, thus lending further support to the multi-stage theory regarding the evolution of chemical elements and the differentiation in the continental crust of China. (5) Preliminary data seem to support the time-dependence of REE distribution patterns and relative Eu contents of REEIFs in China.