Zhang B.,The Key Laboratory of Orogenic Belts and Crustal Evolution |
Yin C.Y.,The Key Laboratory of Orogenic Belts and Crustal Evolution |
Zhang J.J.,The Key Laboratory of Orogenic Belts and Crustal Evolution |
Wang J.M.,CAS Institute of Geology and Geophysics |
And 5 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2017
The Cenozoic Xuelong Shan antiformal dome is located along the northern segment of the Ailao Shan-Red River shear zone in Yunnan, China. Subhorizontal foliation in the gneiss core is recognized, representing a broad top-to-NE shear initiated under amphibolite facies conditions and propagating into greenschist facies in the mantling schist and strike-slip shear zone. Microfabrics of crystallographic-preferred orientations (CPOs) in quartz suggest that the deformation temperatures increased with increasing structural depth from the upper crust (300-500°C) in the mantling schist to the midcrust (15 km or more, ≥650°C) in the gneissic core. This trend is mirrored by variations in the metamorphic grade of the syn-kinematic mineral assemblages and microstructures, which range from garnet+amphibole+biotite+sillimanite+rutile+feldspar in the core to garnet+staurolite+biotite+epidote+muscovite within the limb units. The dome experienced the following deformation history: (1) a broad top-to-NE shear in the subhorizontal foliation of the gneiss core during the first stage of deformation (D1); (2) opposing reverse-sense shear along the two schist limbs of the dome during contraction-related doming (D2-D3); (3) sinistral strike-slip shearing within the eastern limb (D4); and (4) extensional deformation (D5). The structural-thermal patterns suggest the antiformal dome formation was roughly coeval with top-to-NE ductile shearing in the midcrust of Tibet at 32 Ma or earlier. A major implication is that there was a phase of contractional ductile deformation in the region prior to the initiation of strike-slip deformation. © 2017. American Geophysical Union. All Rights Reserved.
Xiao Y.,State Key Laboratory of Lithospheric EvolutionInstitute of Geology and Geophysics |
Zhou M.-F.,Hong Kong University of Science and Technology |
Zhu B.,State Key Laboratory of Lithospheric EvolutionInstitute of Geology and Geophysics |
Shi R.-D.,Key Laboratory of Continental Collision and Plateau UpliftInstitute of Tibetan Plateau Research |
And 3 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2016
We present high-precision measurements of iron (Fe) and magnesium (Mg) isotopic compositions of olivine, orthopyroxene, and chromite separates from harzburgites, dunites, and chromitites in the mantle section of the Luobusa ophiolite, southern Tibet, to investigate the origins of podiform chromitite. Two harzburgites in the Zedong ophiolite, southern Tibet, are also reported for comparison. The olivine and orthopyroxene in the Luobusa and Zedong harzburgites have similar Fe and Mg isotopic compositions, with δ56Fe values ranging from 0‰ to +0.083‰ in olivine, from -0.034‰ to +0.081‰ in orthopyroxene and δ26Mg values ranging from -0.25‰ to -0.20‰ in olivine, from -0.29‰ to -0.26‰ in orthopyroxene, respectively. The olivines of two dunites from the Luobusa display small Fe and Mg isotopic variations, with δ56Fe values of +0.014‰ and +0.116‰ and δ26Mg values of -0.21‰ and -0.29‰. All chromites in the Luobusa chromitites have lighter Fe isotopic compositions than the coexisting olivines, with δ56Fe values ranging from -0.247‰ to +0.043‰ in chromite and from -0.146‰ to +0.215‰ in olivine (Δ56FeChr-Ol=-0.294 to -0.101‰). The chromite δ26Mg values span a significant range from -0.41‰ to +0.14‰. Large disequilibrium Fe and Mg isotope fractionation between chromite and olivine, as well as positive correlation of chromite δ56Fe values with their MgO contents, could be attributed to Fe-Mg exchange between chromite and olivine. In the disseminated chromitites, the higher modal abundances of olivine than chromite would result in a more extensive Fe-Mg exchange, whereas chromite in the massive chromitite where olivine is rare could not be affected by this process. © 2016. American Geophysical Union.
Zan J.,Key Laboratory of Continental Collision and Plateau UpliftInstitute of Tibetan Plateau Research |
Fang X.,Key Laboratory of Continental Collision and Plateau UpliftInstitute of Tibetan Plateau Research |
Yang S.,Key Laboratory of Western Chinas Environmental SystemsMinistry of Education of China |
Yan M.,Key Laboratory of Continental Collision and Plateau UpliftInstitute of Tibetan Plateau Research
Geochemistry, Geophysics, Geosystems | Year: 2015
Recent studies demonstrate that particle size separation based on gravitational settling and detailed rock magnetic measurements of the resulting fractionated samples constitutes an effective approach to evaluating the relative contributions of pedogenic and detrital components in the loess and paleosol sequences on the Chinese Loess Plateau. So far, however, similar work has not been undertaken on the loess deposits in Central Asia. In this paper, 17 loess and paleosol samples from three representative loess sections in Central Asia were separated into four grain size fractions, and then systematic rock magnetic measurements were made on the fractions. Our results demonstrate that the content of the <4 μm fraction in the Central Asian loess deposits is relatively low and that the samples generally have a unimodal particle distribution with a peak in the medium-coarse silt range. We find no significant difference between the particle size distributions obtained by the laser diffraction and the pipette and wet sieving methods. Rock magnetic studies further demonstrate that the medium-coarse silt fraction (e.g., the 20-75 μm fraction) provides the main control on the magnetic properties of the loess and paleosol samples in Central Asia. The contribution of pedogenically produced superparamagnetic (SP) and stable single-domain (SD) magnetic particles to the bulk magnetic properties is very limited. In addition, the coarsest fraction (>75 μm) exhibits the minimum values of χ, χARM, and SIRM, demonstrating that the concentrations of ferrimagnetic grains are not positively correlated with the bulk particle size in the Central Asian loess deposits. © 2015 American Geophysical Union.