Xiao-Han Gong

Iron and magnesium isotopic constraints on the origin of chemical heterogeneity in podiform chromitite from the Luobusa ophiolite, Tibet

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 Fe-56 values ranging from 0 to +0.083 in olivine, from -0.034 to +0.081 in orthopyroxene and Mg-26 values ranging from -0.25 parts per thousand to -0.20 parts per thousand in olivine, from -0.29 parts per thousand to -0.26 parts per thousand in orthopyroxene, respectively. The olivines of two dunites from the Luobusa display small Fe and Mg isotopic variations, with Fe-56 values of +0.014 parts per thousand and +0.116 parts per thousand and Mg-26 values of -0.21 parts per thousand and -0.29 parts per thousand. All chromites in the Luobusa chromitites have lighter Fe isotopic compositions than the coexisting olivines, with Fe-56 values ranging from -0.247 parts per thousand to +0.043 parts per thousand in chromite and from -0.146 parts per thousand to +0.215 parts per thousand in olivine (Fe-56(Chr-Ol)=-0.294 to -0.101 parts per thousand). The chromite Mg-26 values span a significant range from -0.41 parts per thousand to +0.14 parts per thousand. Large disequilibrium Fe and Mg isotope fractionation between chromite and olivine, as well as positive correlation of chromite Fe-56 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.

Removal of deep lithosphere in ancient continental collisional orogens: A case study from central Tibet, China

Widespread but small-volume Late Cretaceous volcanic rocks in central Tibet contain important information on the Lhasa–Qiangtang collision process. In this contribution, we focus on Late Cretaceous volcanics in the southern Qiangtang subterrane, and present zircon LA–ICP–MS U–Pb ages, whole-rock major and trace element compositions, and Sr–Nd isotopic data. Zircon LA–ICP–MS U–Pb dating yielded a concordant age of 80 Ma, which postdates the Early Cretaceous collision of the Qiangtang and Lhasa terranes. The volcanic rocks are potassium-rich alkaline andesites with high contents of K2O (3.45–5.11 wt.%) and Th (13.39–25.02 ppm), as well as high K2O/Na2O ratios (0.6–0.9). They have higher REE and HFSE contents than coeval Mg-rich and adakite-like magmatic rocks that can be related to partial melting of a thickened lower crust. Moreover, they have higher values of Mg# and lower contents of SiO2 than lower continental crust-derived rocks in central Tibet and experimental data of mafic rocks. We argue that the andesites were generated after the removal of thickened lithospheric mantle and subsequent to the final Lhasa–Qiangtang amalgamation in a post-collisional setting. The high-K characteristics can be explained by producing the primitive andesite magmas from partial melting of the residual and shallow metasomatized lithospheric mantle (the K-rich layer) during heating by upwelling asthenosphere; subsequently, these primitive andesite magmas were subjected to fractional crystallization to generate the Amdo andesites. The way in which these andesites were formed provides evidence for the lithospheric thickening and uplifting of central Tibet during the Late Cretaceous prior to India–Asia collision. This article is protected by copyright. All rights reserved.