Benren Zhang

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[punctuation space]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% SiO2. 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 SiO2 (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 Poissons 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.

Geochemical and Sr-Nd-Pb isotopic compositions of Cretaceous granitoids: constraints on tectonic framework and crustal structure of the Dabieshan ultrahigh-pressure metamorphic belt, China

Abstract The Dabie orogenic belt can be divided into four tectonic and lithological units. They are, from north to south, the North Huaiyang (NHY) unit, the Northern Dabie complex (NDC) unit, the ultrahigh-pressure (UHP) metamorphic unit, and high-pressure (HP) metamorphic unit. Cretaceous granitoids from the four tectonic units, irrespective of tectonic unit and lithology, show surprisingly similar major and trace element and particularly Sr–Nd–Pb isotopic compositions, indicating a similar crustal source for their magma genesis. They have initial ( 87 Sr/ 86 Sr) t values of 0.7068–0.7091 and Nd-depleted mantle model ages of 1.6–2.4 Ga and e Nd ( t ) values at the time of their emplacement (120 Ma) ranging from −13 to −26. Their present-day Pb isotopic compositions are 206 Pb/ 204 Pb=15.62–17.06, 207 Pb/ 204 Pb=15.21–15.44, and 208 Pb/ 204 Pb=36.59–37.81. With comparison to the Sr–Nd–Pb isotopic compositions of the basement rocks in the Dabie orogenic belt, the isotopic compositions of the Cretaceous granitoids indicate a source that is similar to the Northern Dabie complex but distinct from the ultrahigh-pressure metamorphic unit. This strongly suggests that the exposed Northern Dabie complex extends in the deep crust southward beneath the UHP/HP metamorphic units and northward beneath the North Huaiyang unit. The Northern Dabie complex is best interpreted as the core of a dome within the Dabie orogenic belt. The UHP/HP metamorphic unit is only a thin-skinned slab confined in shallower crustal levels over the Northern Dabie complex. Present tectonic framework for the UHP/HP terrain was formed by extensional tectonics and post the UHP/HP metamorphism. The Northern Dabie complex has an affinity to the Yangtze craton. The similar source for the Cretaceous granitoids also suggests that the suture between the North China and the Yangtze cratons is marked by the Xiaotian–Mozitang Fault separating the North Huaiyang unit and the Northern Dabie complex rather than the previously thought Wuhe–Shuiko Fault, which separates the Northern Dabie complex and the UHP unit.

MORB-type rocks from the Paleo-Tethyan Mian-Lueyang northern ophiolite in the Qinling Mountains, central China: implications for the source of the low 206Pb/204Pb and high 143Nd/144Nd mantle component in the Indian Ocean

Abstract Samples from a basalt and gabbro section of the Paleo-Tethyan (∼350 Ma) Mian-Lue northern ophiolites (MLNO) in the Qinling Mountains of central China display sub-parallel and relatively smooth incompatible trace element-depleted patterns and have high ϵ Nd (350 Ma) (8.1–11.3) and low 206 Pb/ 204 Pb (350 Ma) (16.90–17.25). The MLNO basalts and gabbros are compositionally similar to normal mid-ocean ridge basalts (MORB), particularly to those from the Carlsberg Ridge and Indian Ocean Ridge Triple Junction. The basalts and gabbros also have high Δ7/4 and Δ8/4 isotopic values characteristic of the Dupal isotopic anomaly in the southern hemisphere. Although the MLNO is presently in the northern hemisphere, it was previously located within the southerly location of the Indian Ocean based on paleomagnetic data. Thus, assuming that the low 206 Pb/ 204 Pb ratio of the MLNO basalts and gabbros is not due to seawater alteration or continental contamination, the unique isotopic signature of both the Paleo-Tethyan oceanic igneous crust and the modern Indian MORB may have come from a very similar, if not identical mantle reservoir. This indicates that a portion of the modern Indian MORB mantle isotopic domain could have been in existence for at least ∼350 Ma. We propose that the low 206 Pb/ 204 Pb and high 143 Nd/ 144 Nd isotopic character of the MLNO basalts and gabbros as well as similar Indian MORB originated either from a low μ sub-domain of the depleted asthenospheric mantle in the southern hemisphere or due to contamination of the depleted asthenosphere by deep-rooted plumes carrying a low 206 Pb/ 204 Pb mantle component. In contrast, the origin of the more common Indian MORB with low 206 Pb/ 204 Pb and low 143 Nd/ 144 Nd is most probably associated with the delamination of the Gondwanan continental lithosphere during formation of the Indian Ocean.

The recognizing of ca. 1.95 Ga tectono-thermal eventin Kongling nucleus and its significance for the evolution of Yangtze Block, South China

Some 1.95 Ga metamorphism of high-grade Archean rocks in Kongling terrain, continental nucleus of Yangtze Block, South China, are recognized by Sm-Nd dating. This event is further confirmed by the ages of single zircon SHRIMP U-Pb and K-Ar analyses of TTG and (or) paragneisses, and intrusions of potassic-granitic batholith in the region. The coincidence in period between the high-grade metamorphism in the Kongling area and the creation of major basement rocks of its neighborhoods of the Yangtze Block around the Kongling Archean nucleus makes it a favorable explanation for their tectonic connections that, in the whole Yangtze Block there was some tectono-thermal event of ca. 2.0-1.8 Ga, which resulted in the formation of unitive Yangtze basement.

Poisson's ratio of eclogite: the role of retrogression

Available laboratory measurements of combined P- and S-wave velocities of eclogites from the Dabie-Sulu ultrahigh-pressure metamorphic belt and data from Manghnani et al. [J. Gcophys. Res. 79 (1974) 5427-5446] show clear positive correlations between Poissons ratio and the intrinsic water content H(2)O(+) for samples with SiO(2) < 56% with the correlation coefficient r of 0.63-0.72, There are no significant correlations (/r/less than or equal to0.43) between Poissons ratio and other major oxides. The positive correlations are interpreted to result from hydration of eclogites during their exhumation and exposure at the surface. Our best estimates of the average Poissons ratios for unaltered quartz-bearing eclogites with H(2)O(+) < 1.0% are 0.24-0.25 (+/- 0.01), which is identical to that calculated from single crystal elastic properties, but lower than the averages (0.27 +/- 0.02) of measurements compiled by Rudnick and Fountain [Rev. Geophys. 33 (1995) 267-309] and Christensen [J. Geophys. Res. 101 (1996) 3139-3156]. Accordingly, unaltered quartz-bearing eclogites have Poissons ratios distinct from mafic granulites (0.28 +/- 0.01) and marginally different from ultramafic rocks (0.26 +/- 0.02)

Structure and composition of the continental crust in East China

Crustal structures of nine broad tectonic units in China, except the Tarim craton, are derived from 18 seismic refraction profiles including 12 geoscience transects. Abundances of 63 major, trace and rare earth elements in the upper crust in East China are estimated. The estimates are based on sampling of 11 451 individual rock samples over an area of 950 000 km2, from which 905 large composite samples are prepared and analyzed by 13 methods. The middle, lower and total crust compositions of East China are also estimated from studies of exposed crustal cross sections and granulite xenoliths and by correlation of seismic data with lithologies. All the tectonic units except the Tarim craton and the Qinling orogen show a four-layered crustal structure, consisting of the upper, middle, upper lower, and lowermost crusts. P-wave velocities of the bulk lower crust and total crust are 6.8–7.0 and 6:4–6.5 km/s, respectively. They are slower by 0.2–0.4 km/s than the global averages. The bulk lower crust is suggested to be intermediate with 58% SiO2 in East China. The results contrast with generally accepted global models of mafic lower crusi. The proposed total crust composition in East China is also more evolved than previous estimates and characterized by SiO2=64%, a significant negative Eu anomaly (Eu/Eu* = 0.80), deficits in Sr and transition metals, a near-arc magma La/Nd ratio (3.0), and a calculatedμ(238U/204Pb) value of 5. In addition, it has the following ratios of element pairs exhibiting similar compatibility, which are identical or close to the primitive mantle values: Zr/Hf=37, Nb/Ta=17.5, Ba/Th=87, K/Pb=0.12x104, 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/TI=177, Er/Ag=52, Cu/Au=3.2×104, Sm/Mo=7.5, Nd/W=40, CI/Li=10.8, F/Nd=21.9, and La/B=1.8.

U-Pb zircon age of the foliated garnet-bearing granites in western Dabie Mountains, Central China

U-Pb zircon dating on two foliated garnet-bearing granite samples in the western Dabie ultra-high-pressure (UHP) metamorphic unit yields concordant ages of (234±4) Ma and (227±5) Ma, respectively. These ages, following the UHP peak metamorphism, represent the magma emplacement ages for the foliated garnet-bearing granites. This, for the first time, shows that there are the Triassic granites in the Dabie Mountains. The foliated garnet-bearing granites resemble A-type granite in geochemical characteristics, indicating that they were formed in extensional geodynamic setting. The magma formation reflects a reheating event in the Dabie orogenic belt and it enhances the transfer of tectonic regime from collision into extension and promotes the rapid exhumation into lower crust for the UHP metamorphic rocks.

Discovery of the highly depleted N-MORB-type volcanic rocks: new evidence for the Mianlue paleo-ocean

The highly depleted N-MORB-type volcanic rocks have been discovered in Mianlue mélange of Qinling orogenic belt, central China. These mafic rocks are associated with the meta-peridotites, showing LREE-depleted pattern, similar to N-MORB and typical ophiolites. It is indicated that an ancient ocean existed in the Mianlue area of Qinling orogenic belt during the late Paleozoic.

DISCOVERY OF HIGH RADIOGENIC PB ISOTOPIC COMPOSITION FROM PROTEROZOIC MAFIC ROCKS IN NORTH QINLING AREA AND ITS IMPLICATION

NORTH and South China are considered parts of Laurasia and Gondwana Land in geological history. They are separated by the Qinling Mountains. As a result, study of the Qinling orogenic belt may help to determine the boundary between North and South China and hence between two largest continental blocks in the past. The Qinling orogenic belt may be divided into North Qinling (NQ) and South Qinling (SQ), which were thought to be respectively the continental...