Lu Zeng

U–Pb and Lu–Hf isotope systematics of detrital zircons from the Songpan–Ganzi Triassic flysch, NE Tibetan Plateau: implications for provenance and crustal growth

We conducted in situ U–Pb and Lu–Hf isotope analyses of 401 detrital zircons collected from the Songpan–Ganzi Triassic turbidite complex in an attempt to understand the provenance variations of the siliciclastic rocks and the crustal growth history of central China. These detrital zircons exhibit a wide age spectrum with three major peaks at 1.7–2.0 Ga, 750–1050 Ma, and 210–500 Ma. They are dominated by negative ϵHf(t) values with a large range. Synthesis of the zircon U–Pb and Lu–Hf isotopic data indicate that the Triassic Songpan–Ganzi turbiditic succession could have been derived dominantly from the Tibetan terrains + the Kunlun and Qinling orogens. Our samples are characterized by a common, prominent group of Hf crust formation model ages at 0.8–4.1 Ga with a peak at 2.7–3.4 Ga. This fact indicates that (1) Phanerozoic magmatism in central China could have been predominantly products of crustal reworking with insignificant formation of juvenile crust and (2) the Neoarchaean was an important period of continental growth in central China. In addition, our data set also reveal that three widespread tectonothermal events could have occurred in the region during the late Mesoproterzoic, Palaeozoic, and early Mesozoic, respectively.

Nonlinear transport properties and Joule heating effect in charge ordered LuFe2O4

The nonlinear current-voltage (I-V) relation of charge ordered (CO) LuFe2O4 has been studied in the temperature range from 220 to 520 K. Strong nonlinear effects are identified in the CO state and disappear in high-temperature charge disordered state. Alternation of the CO state under applied electric field is directly revealed by in situ transmission electron microscopy observations. Careful investigations of time-dependent resistance under the applied electric field near the threshold value (Eth) often show three distinct steps for the nonlinear transitions. The CO collapse likely at a critical conductivity yields a sharp drop in resistance.

Late Permian–Triassic siliciclastic provenance, palaeogeography, and crustal growth of the Songpan terrane, eastern Tibetan Plateau: evidence from U–Pb ages, trace elements, and Hf isotopes of detrital zircons

In order to constrain the detrital provenance of the siliciclastic rocks, palaeogeographic variations, and crustal growth history of central China, we carried out simultaneously in situ U–Pb dating and trace element and Hf isotope analyses on 368 detrital zircons obtained from upper Permian–Triassic sandstones of the Songpan terrane, eastern Tibetan Plateau. Two groups of detrital zircons, i.e. magmatic and metamorphic in origin, have been identified based on cathodoluminescence images, zircon Ti-temperatures, and Th/U ratios. Our data suggest that the derivation of siliciclastic rocks in the Songpan terrane was mainly from the Qinling, Qilian, and Kunlun orogens, whereas the Yangtze and North China Cratons served as minor source areas during late Permian–Triassic times. The detrital zircons from Middle–Late Triassic siliciclastic rocks exhibit wide age spectra with two dominant populations of 230–600 Ma and >1600 Ma, peaking at ~1.8–1.9 Ga and ~2.4–2.5 Ga, suggestive of a derivation from the Qinling, Qilian, and Kunlun orogens and the Yangtze Craton being the minor source area. The proportions of detrital zircon populations from the northern Qinling, Qilian, and Kunlun orogens distinctly decreased during Middle–Late Triassic time, demonstrating that the initial uplift of the western Qinling occurred then and it could have blocked most of the detritus from the Qilian–northern Qinling orogens and North China Cratons into the main Songpan–Ganzi depositional basin. The relatively detrital zircon proportions of the Yangtze Craton source decreased during Early-Middle Late Triassic time, indicating that the Longmenshan orogen was probably being elevated, since the early Late Triassic and gradually formed a barrier between the Yangtze Craton and the Songpan terrane. In addition, our Lu–Hf isotopic results also reveal that the Phanerozoic magmatic rocks in central China had been the primary products of crustal reworking with insignificant formation of a juvenile crust.

Microstructure and tetragonal-to-orthorhombic phase transition of AFe(2)As(2) (A=Sr,Ca) as seen via transmission electron microscopy

The structural properties of the SrFe2As2 and CaFe2As2 compounds have been extensively analyzed by transmission electron microscopy (TEM) from room temperature down to 20K. The experimental results demonstrate that the SrFe2As2 crystal, in consistence with previous x-ray data, has a tetragonal structure at room temperature and undergoes a tetragonal (T)-orthorhombic (O) phase transition at about 210K. Moreover, twinning lamella arising from T-O transition evidently appears in the orthorhombic phase. On the other hand, TEM observations of CaFe2As2 reveal the presence of a pseudo-periodic structural modulation with the periodicity of around 40nm at room temperature. This modulation is likely in connection with the local structural distortions within the Ca layer. In-situ cooling TEM observations of CaFe2As2 reveal the presence of complex domain structures in the low-temperature orthorhombic phase.

Late Palaeozoic and early Mesozoic tectonic and palaeogeographic evolution of central China: evidence from U–Pb and Lu–Hf isotope systematics of detrital zircons from the western Qinling region

The western Qinling region of central China is situated centrally in the Kunlun, Qilian, Qinling, Longmenshan, and Songpan–Ganzi orogens. Late Palaeozoic and Early Mesozoic sediments deposited here may provide keys to understanding the tectonic evolution of the Palaeo-Tethys and collision of the North China and Yangtze Cratons. We conducted in situ U–Pb and Lu–Hf isotope analyses of 568 detrital zircons collected from Upper Palaeozoic to Mesozoic sandstones in the central Qinling block, Taohe depression, and Bailongjiang block in western Qinling to constrain the sources of these sandstones. Our results reveal that the Bailongjiang block has affinities with the Yangtze Craton, from which it may have been rifted. Therefore, the Palaeo-Tethyan Animaqen suture between the two cratons lies north of the Bailongjiang block. We identified the North China Craton as the main source for Triassic flysch in central China. It is possible that the Bailongjiang block could have blocked detritus shed from the North China Craton into the main depositional basins in the Songpan–Ganzi area. The dominance of 300–200 Ma detrital zircons of metamorphic origin in Lower Jurassic sandstones indicates that the Dabie–Qinling orogen was elevated during Early Jurassic time. In addition, our Lu–Hf isotopic results also reveal that Phanerozoic igneous rocks in central China were mostly products of crustal reworking with insignificant formation of juvenile crust.

Effects of layered structural features on charge/orbital ordering in (La, Sr)n+1MnnO3n+1 (n = 1 and 2)

The charge/orbital ordering (COO) of the layered mixed-valence manganites (La,Sr)(n+1)Mn(n)O(3n+1) (n = 1 and 2) is examined by first-principles calculations and discussed in comparison with the La(0.5)Ca(0.5)MnO(3) perovskite phase ([Formula: see text]). The results demonstrated that the layered structural features could yield not only visibly weak coupling between Mn-O layers but also various features in the orbital ordering associated with different types of local structural distortions. In both La(0.5)Sr(1.5)MnO(4) (n = 1) and LaSr(2)Mn(2)O(7) (n = 2) phases, the orbital ordering can be chiefly assigned to the d(x(2)-y(2)) orbital, in contrast with the zigzag-type d(z(2)) orbital ordering in the [Formula: see text] perovskite phase. Our theoretical analysis shows that a variety of essential factors, including the local structural distortions of the MnO(6) octahedra, the on-site Coulomb interaction, and magnetic interaction, have to be properly considered in order to achieve acceptable COO ground states for the layered variants in (La,Sr)(n+1)Mn(n)O(3n+1).

Retrograde Reactions of an Ultrahigh-Pressure Metamorphic Spinel Pyroxenite Lens, Northeast Sulu UHP Terrane, Eastern China

A ~5 cm thick garnet-rich layer, with relict spinel grains in its center, is intercalated within garnet pyroxenite at Rongcheng. Both the garnetite layer and the host garnet pyroxenite underwent the same P-T evolution, but exhibit quite different mineral assemblages, microtextures, and associated mineral reactions. Microtextures and mineral compositions confirm our previous suggestion that the Rongcheng garnet pyroxenite recrystallized from a spinel pyroxenite cumulate derived from a gabbroic magma. The present study concludes that: (1) The garnet-rich layer resulted from a net transfer reaction between primary spinel and clinopyroxene, Ca1.5(FeMg)0.5 Si2O6 + (FeMg)Al2O4 + TiO2 = Ca1.5(FeMg)1.5Al2(Si2Ti)O12, and mass-balance reactions between clinopyroxene end-members, 2[Ca0.75(FeMg)0.25][(FeMg)0.5Al0.5] (Si1.5Al0.5)O6 = Ca1.5(FeMg)1.5Al2Si3O12, under UHP conditions that produced garnet-I. (2) Garnet-I decomposed during exhumation of the UHP terrane, leading to the assemblage of Grt-II + Ilm + Cpx-II + Amp; ilmenites were partially released to the relict spinel along micro-fractures to form intergrowths of Spl-I + Ilm. A corona assemblage of Cpx-II + Spl-II + minor Amp developed along contacts between garnet and relict Spl-I at ~890°C and ~9 kbar. And (3), during a later stage of retrogression, aqueous fluid infiltrated heterogeneously through the garnet-rich layer, leading to the formation of hydrous minerals. Parageneses and mineral compositional maps suggest that Ti may have substituted for Si and entered the T site in the garnet structure, represented by a garnet component such as M3Al2[Si(3-x)Tix]O12, in addition to the commonly suggested M2+Ti4+ → 2Al3+ substitution (forming the garnet component M3-0.5x[Al2-xTix]Si3O12). Heterogeneous microtextures and mineral compositions indicate that the domain equilibrium was controlled by mass transfer at least at the mineral-grain scale due to various degrees of fluid infiltration.

Garnet amphibolites from the Ganzi–Litang fault zone, eastern Tibetan Plateau: mineralogy, geochemistry, and implications for evolution of the eastern Palaeo-Tethys Realm

ABSTRACT The Ganzi–Litang fault zone, an outstanding tectonic element in the eastern Tibetan Plateau has been intensively debated as an in-situ suture zone marking relict of a subducted Palaeo-Tethyan oceanic crust or a failed intracontinental rift. This paper reports the garnet amphibolites discovered along the Ganzi–Litang fault zone, eastern Tibetan Plateau. These garnet amphibolites are characterized by the garnet–hornblende–rutile–sphene–plagioclase–quartz assemblage. Conventional geothermobarometry figures out the metamorphic temperature and pressure conditions at 582–626°C and 1.61–1.82 GPa, respectively. Geochemical analysis (no Nb–Ta deletions and left-inclined to flat patterns of rare-earth elements) indicates that the garnet amphibolites could represent metamorphic product of the mid-ocean-ridge (MORB)-type mafic rocks that were contaminated by a mantle plume. The protolith of the garnet amphibolites was dated at 236 Ma using in-situ U–Pb zircon method, and the retrograde metamorphism was dated at 218 Ma using in-situ U–Pb sphene method. A comprehensive analysis combined with the development of the Palaeo-Tethys Ocean and the Yidun arc through geologic time indicates a Triassic to Early Jurassic age (236–195 Ma) for the metamorphism of the garnet amphibolites. The low geothermal gradient of 9.8ºC/km and the N-MORB nature of the garnet amphibolites suggest a subduction-zone environment for the high-pressure metamorphism. Therefore, the Ganzi–Litang fault zone is a Palaeo-Tethyan suture separating the Yidun arc and the Songpan–Ganzi terrane, representing the relics of a branch of the Palaeo-Tethys Ocean that was contaminated by a mantle plume.