Origin of LateTriassic high-Mg adakitic granitoid rocks from the Dongjiangkou area,Qinling orogen,central China: Implications for subduction of continental crust

The origin of high-Mg adakitic granitoids in collisional orogens can provide important information about the nature of the lower crust and upper mantle during the orogenic process. Late-Triassic high-Mg adakitic granite and its mafic enclaves from the Dongjiangkou area, the Qinling orogenic belt, central China, were derived by partial melting of subducted continental crust and underwent interaction with the overlying mantle wedge peridotite. Adakitic affinity of the different facies of the Dongjiangkou granite body are: high Sr, Ba, high La/Yb and Sr/Y, low Y,Yb, Yb/Lu and Dy/Yb, and no significant Eu anomalies, suggesting amphibole + garnet and plagioclase-free restite in their source region. Evolved Sr-Nd-Pb isotopic compositions [(⁸⁷Sr/⁸⁶Sr)_i = 0.7050 to 0.7055,ε_Nd(t) = –6.6 to –3.3; (²⁰⁶Pb/²⁰⁴Pb)_i = 17.599 to 17.799, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.507 to 15.526, (²⁰⁸Pb/²⁰⁴Pb)_i = 37.775 to 37.795] and high K₂O, Rb, together with a large variation in zircon Hf isotopic composition (ε_Hf(t) = ?9.8 to + 5.0), suggest that the granite was derived from reworking of the ancient lower continental crust. CaO, P₂O₅, K₂O/Na₂O, Cr, Ni, Nb/Ta, Rb/Sr and Y increase, and SiO₂, Sr/Y and Eu/Eu* decrease with increasing MgO, consistent with interaction of primitive adakitic melt and overlying mantle peridotite. Zircons separated from the host granites have U-Pb concordia ages of 214 ± 2 Ma to 222 ± 2 Ma, compatible with exhumation ages of Triassic UHP metamorphic rocks in the Dabie orogenic belt. Mafic microgranular enclaves and mafic dykes associated with the granite have identical zircon U-Pb ages of 220 Ma, and are characterized by lower SiO₂, high TiO₂, Mg# and similar evolved Sr-Nd-Pb isotopic composition. Zircons from mafic microgranular enclaves (MMEs) and mafic dykes also show a large variation in Hf isotopic composition with ε_Hf(t) between ?11.3 and + 11.3. It is inferred that they were formed by partial melting of enriched mantle lithosphere and contaminated by the host adakitic granite magma.In combination with the regional geology, high-Mg# adakitic granitoid rocks in the Dongjiangkou area are considered to have resulted from interaction between subducted Yangtze continental crust and the overlying mantle wedge. Triassic continental collision caused detachment of the Yangtze continental lithosphere subducted beneath the North China Craton, at ca. 220 Ma causing asthenosphere upwelling and exhumation of the continental crust. Triassic clockwise rotation of the Yangtze Craton caused extension in the Dabie area which led to rapid exhumation of the subducted continental lithosphere, while compression in the Qinling area and high-P partial melting (amphibole ± garnet stability field) of the subducted continental crust produced adakitic granitic magma that reacted with peridotite to form Mg-rich hybrid magma.     

基于阻抗匹配条件的树枝状超材料吸收器

基于树枝状金属结构单元的电磁谐振特性,设计了一种双面大小树枝状结构的超材料吸收器,该结构分别是正面二级树枝及正对的背面三级树枝.通过仿真模拟设计了不同的阻抗匹配方式,调节相应的结构参数,找到了阻抗匹配对吸收特性的影响.在最佳的阻抗匹配条件下,得到最大吸收率.实验测量表明,阻抗匹配条件下双面大小树枝模型可以实现90.01%吸收率.增加样品层数可以有效的增加吸收率,3层样品就可以达到99%以上的吸收率,实现工程意义的完美吸收.     

The CLEAN method of spectral analysis and its application to the oscillations of the cataclysmic variable TT Arietis

We studied the one-dimensional spectral analysis CLEAN applied to time series with unequal intervals, tested it with simulated signal, and showed it to be also applicable to noisy series, only the quality of the spectrum is closely correlated with the noise intensity. Applying this method to photometric data of the cataclysmic variable TT Ari quasi-periodic oscillations with periods around 20 min are recovered.     

Magma mixing origin for the post-collisional adakitic monzogranite of the Triassic Yangba pluton, Northwestern margin of the South China block: geochemistry, Sr–Nd isotopic, zircon U–Pb dating and Hf isotopic evidences

Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major and trace element, whole-rock Sr–Nd isotope, zircon U–Pb and Hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental setting. The host monzogranite and MMEs from the Yangba pluton have zircon U–Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively. The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element and evolved Sr–Nd isotopic compositions [(⁸⁷Sr/⁸⁶Sr)_i = 0.707069–0.707138, and ε_Nd(t) = −6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive ε_Hf(t) values of 2.3–8.2 with single-stage Hf model ages of 531–764 Ma. Thus, the MMEs would be derived from partial melts of the Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5–14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81–0.90), suggesting that garnet was stable in their source during partial melting. Its evolved Sr–Nd isotopic compositions [(⁸⁷Sr/⁸⁶Sr)_i = 0.7041–0.7061, and ε_Nd(t) = −3.1 to −4.3] and high contents of K₂O (3.22–3.84%) and Th (13.7–19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51–55), Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the adakitic monzogranite show negative ε_Hf(t) values of −9.4 to −0.1 with two-stage Hf model ages of 1,043–1,517 Ma; some zircon grains display positive ε_Hf(t) of 0.1–3.9 with single-stage Hf ages of 704–856 Ma. These indicate that the source region of adakitic monzogranite contains the Neoproterozoic juvenile crust that has the positive ε_Hf(t) values in the Triassic. Thus, the high-Mg adakitic granites in the intracontinental setting would form by mixing between the crustal-derived adakitic magma and the SCLM-derived mafic magma. The mafic and adakitic magmas were generated coevally at Late Triassic, temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the South China Block. This bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the South China and North China Blocks.     

Zircon U-Pb Ages of Bottom and Top Parts of the Zhangjiakou Formation in Lingyuan Area （West Liaoning Province） and Luanping Area （North Hebei Province） and Their Significance

Thecharacteristicsofthetectonics,stratigraphy, paleontologyandvolcanicpetrologyoftheMesozoicin NorthHebeiandWestLiaoningaresimilarinmanyas pects,butthetworegionsalsohavesomeobvious differences.Thetworegionshavebeen“hotspots”inthegeologicalworldforafewy…