Partial melting of subducted Southern Qiangtang crust in northern Tibet: evidence from the geochemistry and geochronology of the Riwanchaka granodiorite porphyry in Central Qiangtang

<p>This article presents new zircon U–Pb geochronology, Hf isotopic, and whole-rock major- and trace-element geochemical data that provide insights into the petrogenesis and tectonic history of the Riwanchaka granodiorite porphyries of Central Qiangtang, Tibet. Zircon U–Pb ages of 236–230 Ma indicate an early Late Triassic age of emplacement of the porphyries, and zircon Hf isotopic data yield εHf(t) values of – 7.0 to – 1.5 and ancient zircon Hf crustal model ages (T_DM^C) of 1524–1220 Ma. The granodiorite porphyries are characterized by low K₂O contents, high Mg# values, and relatively high Cr and Ni contents. They are classified as I-type calc-alkaline granite and are considered to have formed through the anatexis of ancient mafic crustal rocks with contributions from mantle-derived components. The geochemistry and isotopic compositions of all samples are similar to those of magmatic rocks that originated in the South Qiangtang crust. However, field observations indicate that the pluton intrudes the North Qiangtang crust, and we propose that the granodiorite porphyries were derived by partial melting of subducted continental crust of the South Qiangtang terrane. These new data have been integrated with data from previous studies to construct a new model of slab rollback during northward subduction of the Southern Qiangtang continental crust at <i>ca</i>. 245–226 Ma, thereby improving our understanding of magmatic processes involved in continental subduction in collision settings.</p

High-Efficiency Broadband Meta-Hologram with Polarization-Controlled Dual Images

Holograms, the optical devices to reconstruct predesigned images, show many applications in our daily life. However, applications of hologram are still limited by the constituent materials and therefore their working range is trapped at a particular electromagnetic region. In recent years, the metasurfaces, an array of subwavelength antenna with varying sizes, show the abilities to manipulate the phase of incident electromagnetic wave from visible to microwave frequencies. Here, we present a reflective-type and high-efficiency meta-hologram fabricated by metasurface for visible wavelength. Using gold cross nanoantennas as building blocks to construct our meta-hologram devices with thickness ∼ λ/4, the reconstructed images of meta-hologram show polarization-controlled dual images with high contrast, functioning for both coherent and incoherent light sources within a broad spectral range and under a wide range of incidence angles. The flexibility demonstrated here for our meta-hologram paves the road to a wide range of applications related to holographic images at arbitrary electromagnetic wave region

High-Efficiency Broadband Meta-Hologram with Polarization-Controlled Dual Images

Holograms, the optical devices to reconstruct predesigned images, show many applications in our daily life. However, applications of hologram are still limited by the constituent materials and therefore their working range is trapped at a particular electromagnetic region. In recent years, the metasurfaces, an array of subwavelength antenna with varying sizes, show the abilities to manipulate the phase of incident electromagnetic wave from visible to microwave frequencies. Here, we present a reflective-type and high-efficiency meta-hologram fabricated by metasurface for visible wavelength. Using gold cross nanoantennas as building blocks to construct our meta-hologram devices with thickness ∼ λ/4, the reconstructed images of meta-hologram show polarization-controlled dual images with high contrast, functioning for both coherent and incoherent light sources within a broad spectral range and under a wide range of incidence angles. The flexibility demonstrated here for our meta-hologram paves the road to a wide range of applications related to holographic images at arbitrary electromagnetic wave region

High-Efficiency Broadband Anomalous Reflection by Gradient Meta-Surfaces

We combine theory and experiment to demonstrate that a carefully designed gradient meta-surface supports high-efficiency anomalous reflections for near-infrared light following the generalized Snell’s law, and the reflected wave becomes a bounded surface wave as the incident angle exceeds a critical value. Compared to previously fabricated gradient meta-surfaces in infrared regime, our samples work in a shorter wavelength regime with a broad bandwidth (750–900 nm), exhibit a much higher conversion efficiency (∼80%) to the anomalous reflection mode at normal incidence, and keep light polarization unchanged after the anomalous reflection. Finite-difference-time-domain (FDTD) simulations are in excellent agreement with experiments. Our findings may lead to many interesting applications, such as antireflection coating, polarization and spectral beam splitters, high-efficiency light absorbers, and surface plasmon couplers

Media 1: Fabrication of three dimensional split ring resonators by stress-driven assembly method

Originally published in Optics Express on 23 April 2012 (oe-20-9-9415