Shuang-Qing Li

Petrology and geochemistry of Early Cretaceous A-type granitoids and late Mesozoic mafic dikes and their relationship to adakitic intrusions in the lower Yangtze River belt, Southeast China

ABSTRACT The relationship among magmatism, large-scale metallogenesis of Southeast China, and subduction of the Pacific plate has long been debated. The lower Yangtze River belt (LYRB) in the northeastern edge of Southeast China is characterized by intense late Mesozoic magmatism and associated polymetallic mineralization such as copper, gold, iron, tungsten, molybdenum, etc. The copper-related adakitic rocks (148–130 Ma) in this belt are the oldest episode of magmatism and intruded as small intermediate-acid intrusive bodies. The Huayuangong granitoids (HYG), located in the southern part of this belt, however, are copper-barren. Three granitoid samples from this pluton give zircon U–Pb ages of 126.4 ± 1.6 Ma, 125.9 ± 1.9 Ma, and 126.2 ± 1.2 Ma, respectively. The HYG has A-type affinity with metaluminous to weakly peraluminous, high FeOT/(FeOT+MgO) ratios, and high Zr+Nb+Ce+Yb contents. Meanwhile, 10 late Mesozoic mafic samples from the LYRB exhibit similar trace element characteristics to those of ‘continental arc andesite’ (CAA) and suggest an enriched lithospheric mantle source with depletion in high field strength elements (e.g. Nb, Ta, Zr, Hf, and Ti) and enrichment of large ion lithophile elements (e.g. Rb, Th, U, and Pb). Although the HYG exhibits similar Sr–Nd isotope composition with the mafic dikes, distinct whole-rock Pb isotope ratios imply that the granitoids and mafic magmas originated from heterogeneous mantle sources. Compared with coeval Baijuhuajian A-type rocks that are exposed along the Jiang–Shao fault of Southeast China, the HYG shows enriched Hf isotope ratios of zircon with εHf(t) values ranging from −4.8 to −11.1. In the Yb/Ta versus Y/Nb diagram, being different from the major asthenospheric mantle-origin Baijuhuajian pluton, a large range of and high Y/Nb ratios as well as high Zr contents of the HYG pluton suggest a magmatic source of mixing between the asthenospheric and enriched crustal component in the LYRB. Compared with early-stage copper-related adakitic rocks (148–130 Ma) with subduction-related affinities and high oxygen fugacity, the copper-barren HYG has with-plate A-type affinities and lower oxygen fugacity. Summarizing, the production of early-stage (i.e. subduction related) adakitic rocks followed by late-stage A-type granitoids in the LYRB is ascribed to the rollback of the Palaeo-Pacific plate beneath Southeast China and associated with asthenospheric upwelling and lithospheric thinning during the late Mesozoic era.

X-ray photoemission studies of praseodymium thin films on SiO2/Si(100)

Pr overlayers, with thicknesses of about 1.8 and 0.6 nm, were deposited on 1.2 nm SiO2/Si(100) at room temperature and x-ray photoelectron spectroscopy (XPS) was used to investigate the reactions at the Pr/SiO2/Si interfaces as a function of annealing temperature. The results show that the Pr overlayers reduce SiO2 at room temperature, forming Pr2O3 and a Pr silicate (Pr–O–Si). For the 1.8 nm Pr/SiO2/Si system, the 1.2 nm SiO2 layer is mostly reduced at room temperature and a Pr silicide was also observed. The Pr–O–Si silicate increases in intensity while the Pr2O3 intensity decreases with annealing temperature. Angle-dependent XPS, taken after annealing, indicates that the silicate is located at the top surface. In the case of the 0.6 nm Pr/SiO2/Si system, the Pr overlayer is almost oxidized after annealing, and some SiO2 remains even after annealing at 1090 K, which can serve as a low-defect-density buffer layer between the high-κ film (Pr2O3 and Pr–O–Si) and the silicon.

Age Constraints on Late Mesozoic Lithospheric Extension and Origin of Felsic Volcanism in the Songliao Basin, NE China

Late Mesozoic lithospheric extension in NE Asia resulted in the development of a large extensional province and widespread formation of volcanosedimentary basins. Songliao basin is the biggest in that region, situated between the Siberian and Sino-Korean (North China) cratons. The Songliao basin contains voluminous volcanic rocks as a major part of the basin fill. Volcanic successions form a significant but unexposed part of the late Mesozoic magmatic province in NE China. Here we report zircon U-Pb ages and geochemical data for felsic volcanic rocks from the Songliao basin. Zircon populations of two types of rhyolites with distinct I- and A-type affinities, analyzed by laser ablation inductively coupled plasma mass spectrometry, yielded similar U-Pb ages of 114 ± 2 and 113 ± 2 Ma, respectively. Whole-rock Nd and zircon Hf isotope data of the felsic rocks indicate an origin from newly formed crustal protoliths. The I-type dacites and rhyolites show geochemical signatures with subduction-related affinity, indicating generation by partial melting of juvenile crust that originated from melting of subduction-modified mantle sources, whereas the A-type rhyolites have higher heavy rare earth element and high field strength element concentrations and lower Ba/Nd ratios that are interpreted as evidence for melting of I-type felsic lower crustal sources in an intraplate tectonic environment. Typical geochemical compositions of A-type rhyolites indicate anorthite-rich plagioclase as a residual magmatic phase and imply melting at shallow crustal levels. The A-type felsic rocks were generated during a period of maximum lithospheric extension below eastern China around 110 Ma, and this thinning process was probably related to the retreat of the Paleo-Pacific trench.

Origin of Zircon Megacrysts from Cenozoic Basalts in Northeastern Cambodia: Evidence from U-Pb Age, Hf-O Isotopes, and Inclusions

Zircon megacrysts found in basalts may provide a window into metasomatic processes and element transport in the upper mantle. However, little is known about the formation conditions of these zircons. Here we report U-Pb ages, Hf-O isotope compositions, trace element data, and mineral inclusion analyses for six large fragments of zircon megacrysts from the Gem village in northeastern Cambodia. The zircons have low contents of U, Th, Y, P, and rare earth elements (REEs; 130–288 ppm). The normalized REE pattern is characterized by a steeply rising slope from light REEs to heavy REEs and positive Ce anomalies. Secondary ion mass spectrometer isotopic analyses on the zircon megacrysts yield a mean U-Pb age of 0.98 ±0.04 Ma (2σ) and homogeneous oxygen isotopic composition with a mean δ18O value of 5.0‰ ± 0.18‰ (2σ), identical to the δ18O mantle value. The 176Hf/177Hf values range from 0.282970 ± 0.000024 (2σ) to 0.283040 ± 0.000016 (2σ), with εHf (0.98 Ma) values of 6.6–9.0, obtained by the laser ablation–multicollector–inductively coupled plasma mass spectrometry technique, further points to the origin of the grains in the depleted mantle. The zircon megacrysts contain apatite inclusions and a reaction rim composed of baddeleyite and Al-Fe-Zr-silicate glass. This leads us to conclude that mantle zircon crystallized during a metasomatic event from phosphate-rich fluids and/or silicate melts enriched in zirconium.

Sedimentary Environment of Ediacaran Sequences of South China: Trace Element and Sr-Nd Isotope Constraints

Post-Marinoan sedimentary rocks from the Yangtze block in South China were investigated to seek constraints on their provenance, genetic relationship with paleoclimate, depositional environment, and changes. Sedimentary rocks from the Doushantuo Formation in the Jiulongwan section of the Yangtze block were collected for whole-rock trace elements and Sr-Nd isotopic analyses. Flat-to-slightly middle rare earth element (REE)–enriched REE-plus-yttrium patterns, negative-to-positive Ce anomalies (Ce/Ce* of 0.86–1.40), positive La anomalies (La/La* of 1.02–1.42), and suprachondritic Y/Ho ratios (36.8–49.5) obtained for the Doushantuo cap carbonates indicate their deposition in a fluctuating inner-shelf setting where freshwater and seawater mixed together and the influence of hydrothermal fluid from the seafloor is absent. All Doushantuo carbonates and shales have variable Sr isotopic compositions, with initial Sr isotopic ratios (0.704030–0.712720) that support significant river-water contribution during the deposition of the Doushantuo Formation. Samples of the Doushantuo Formation collected across sedimentary profiles show distinct variations in Nd isotopic compositions and diagnostic trace elemental ratios (Th/Sc). The cap carbonates received significant input from a broad continental area undergoing extensive weathering immediately after the cease of the glaciation era. In contrast, the upper Doushantuo Formation sediments received more mature detritus from the nearby Archean Kongling complex, reflecting receding sea levels. In contrast to the negative Ce anomalies observed in lower Doushantuo rocks, negligible Ce anomalies in upper Doushantuo rocks suggest an anoxic setting, in agreement with the growth of phosphatic concretions and the presence of framboids of pyrite and a higher concentration of organic matter.

Low temperature specific heat of Yb2Ti2O7 single crystals

We study the specific heat of Yb2Ti2O7 single crystals at low temperatures down to 0.4 K and in magnetic fields along the [110] crystalline direction. There is a small peak near 0.4 K at low magnetic fields, signaturing a first-order transition, and its field dependence is consistent with that for H || [100] but different from that for H || [111]. The experimental data also show a broad peak at about 2 K, which has a nonmonotonic change with increasing field. This road peak is originated from the spin correlations and its field dependence can be understood from the recent numerical linked-cluster calculations [Hayre et al., Phys. Rev. B 87, 184423 (2013)].

Early Cretaceous rift-related volcanism in the Songliao Basin, NE China – A geochemical study

ABSTRACT Following the amalgamation of the Siberian and North China Cratons, NE China, as part of the Central Asian Orogenic Belt (CAOB), underwent Late Mesozoic lithospheric extension that was associated with volcanic activity. The Songliao Basin is the most important rift structure formed during these processes and contains voluminous volcanic rocks interlayered with sedimentary infill. Mafic-to-intermediate lavas are associated with felsic ones. This study focusses on the geochemical compositions of the less-widespread Early Cretaceous mafic-to-intermediate lavas in the Songliao Basin and compares them with the more abundant felsic rocks. Two mafic-to-intermediate magma series, one with alkaline and the other with sub-alkaline affinity, were identified. High MgO and Cr contents, low Th/Nb and La/Nb ratios, and variable but depleted Nd isotope compositions indicate that both magma suites were most likely formed by the melting of enriched upper mantle sources. Sub-alkaline mafic-to-intermediate rocks and I-type rhyolites define a co-genetic magma series. This rock suite was produced by the melting of subduction-modified lithospheric mantle and subsequent magma evolution as well as crustal melting during lithospheric extension. Alkaline mafic-to-intermediate rocks and A-type rhyolites form another co-genetic magma suite that was produced under within-plate conditions from an OIB-type mantle source, supposed to be the heterogeneous shallow asthenosphere and/or the lower lithosphere. Decompression partial melting of this mantle source requires a relatively thin lithosphere. The development of alkaline mafic rocks and A-type rhyolites as typical bimodal volcanic assemblage reflects that lithospheric thinning below the Songliao Basin reached its maximum, whereas basin rifting terminated afterwards.