Rendeng Shi

Petrology, geochemistry and isotopic ages of eclogites from the Dulan UHPM Terrane, the North Qaidam, NW China

The Dulan eclogite–gneiss region is located in the eastern part of the North Qaidam eclogite belt, NW China. Widespread evidence demonstrates that this region is a typical ultrahigh-pressure (UHP) metamorphic terrane. Eclogites occur as lenses or layers in both granitic and pelitic gneisses. Two distinguished sub-belts can be recognized and differ in mineralogy, petrology and geochemistry. The North Dulan Belt (NDB) has tholeiitic protoliths with high TiO2 and lower Al2O3 and MgO contents. REE patterns and trace element contents resemble those of N-type and E-type MORB. In contrast, eclogites in the South Dulan Belt (SDB) are of island arc protoliths with low TiO2, high Al2O3 and show LREE-enriched and HFSE-depleted patterns. Sm–Nd isotope analyses give isochron ages of 458–497 Ma for eclogite-facies metamorphism for the two sub-belts. The ages are similar to those of Yuka and Altun eclogites in the western extension of the North Qaidam-Altun eclogite belt. The Dulan UHP metamorphic terrane, together with several other recently recognized eclogite-bearing terrenes within the North Qaidam-Altun HP-UHP belt, constitute the key to the understanding of the tectonic evolution of the northern Tibetan Plateau. The entire UHP belt extends for more than 1000 km from the Dulan UHP terrane in the southeast to the Altun eclogite–gneiss terrane in the west. This super-belt marks an early Paleozoic continental collision zone between the Qaidam Massif and the Qilian Massif.

Tibetan chromitites: Excavating the slab graveyard

Podiform chromitites enclosed in depleted harzburgites of the Luobusa massif (southeastern Tibet) contain diamond and a highly reduced trace-mineral association. Exsolution of diopside and coesite from chromite suggests inversion from the Ca-ferrite structure in the upper part of the mantle transition zone (>400 km). However, the trace-element signatures of the chromites are typical of ophiolitic chromitites, implying primary crystallization at shallow depths. Os-Ir nuggets in the chromitites have Re-Os model ages (T RD ) of 234 ± 3 Ma, while T RD ages of in situ Ru-Os-Ir sulfides range from 290 to 630 Ma, peaking at ca. 325 Ma. Euhedral zircons in the chromitites give U-Pb ages of 376 ± 7 Ma, e Hf = 9.7 ± 4.6, and d 18 O = 4.8‰–8.2‰. The sulfide and zircon ages may date formation of the chromitites from boninite-like melts in a supra-subduction-zone environment, while the model ages of Os-Ir nuggets may date local reduction in the transition zone following Devonian subduction. Thermo-mechanical modeling suggests a rapid (≲10 m.y.) rise of the buoyant harzburgites from >400 km depth during the early Tertiary and/or Late Cretaceous rollback of the Indian slab. This process may occur in other collision zones; mantle samples from the transition zone may be more widespread than currently recognized.

Discovery of coesite in the North Qaidam Early Palaeozoic ultrahigh pressure (UHP) metamorphic belt, NW China

Abstract Coesite and graphite were discovered as inclusions in zircon separates from pelitic gneiss associated with a large eclogite body in the North Qaidam UHP terrane. This finding suggests UHP metamorphism at pressures below the diamond stability field. This supports previous indirect UHP evidences, such as polycrystalline quartz inclusions in eclogitic garnet, quartz lamellae in omphacite and P–T estimates for both eclogite and garnet peridotite. The U/Pb and Sm/Nd isotopic ages from the North Qaidam eclogite indicated that continental subduction occurred in Early Palaeozoic, most probably in relation with the collision between the Sino-Korean and Yangtze plates.

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.

SHRIMP U-Pb zircon dating for Qiashikansayi granodiorite, the northern Altyn Tagh mountains and its geological implications

The Qiashikansayi granodiorite is foliated resulting in a granodioritic gneiss. Its geochemical features, such as alumina saturation index(A/CNK) of 0.81–0.99, Na2O/K2O values> 1, TiO2 contents < 1.0%, LREE enrichment with high fractionation factors, weakly negative or no Eu anomalies, and significant Ba and Ti negative anomalies, suggest that it is similar to a typical island arc pluton. The trace elements of the Qiashikansayi granodiorite are plotted in the island arc field in the tectonic setting discrimination diagrams as well. Cathodoluminescence images demonstrated that the zircons have clear rhythmic crystallized zoning, without any remnant core and new crystallized rim, suggesting the zircons be magmatic ones. Their U and Th contents vary in the range of 574–870 μg/g, and 279–556 μg/g respectively, with the Th/U ratio in the range of 0.52–0.68. SHRIMP zircon U-Pb dating yielded the 481.5±5.3 Ma age for the intrusion of the granodiorite, which is coeval with the island arc volcanic rocks in the northern Qilian Mountain, and confirms that there is an early Paleozoic island arc in the Hongliugou-Lapeiquan area.

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.