In this paper, a high sensitivity method for measurements of Nd isotopes as NdO+, on a TIMS using a single W filament with TaF5 as an ion emitter is presented. Although analyzing Nd isotopes as oxides (NdO+) is a well known technique, this is the first report to analyze Nd isotopic compositions as oxides using W filaments and the TaF5 emitter. When 0.5–1 ng loads of a Neodymium isotopic reference reagent, JNdi-1, were measured using this method, the ion yields were found to be mostly in excess of 15% and could be as high as 32%. Internal precision on 143Nd/144Nd could be better than 10 ppm (2SE) for 1 ng JNdi-1 loads and better than 15 ppm (2SE) for 0.5 ng JNdi-1 loads; thirteen replicates of 0.5–1 ng JNdi-1 loads yielded a 143Nd/144Nd value of 0.512112 ± 0.000028 (2SD). Compared with the previously reported NdO+ measurement method using the Re (or W) filaments + Silica-gel + H3PO4 loading techniques, this method has advantages including higher sensitivity, a more stable ion beam, and no need for oxygen gas to be bled into the ion source chamber. Sm isotopes were analyzed as Sm+ using the W filaments and the TaF5 emitter, and high sensitivity and good ion beam stability were also obtained. Several international rock reference materials, including an ultramafic rock reference material USGS PCC-1 that contains very low amounts of Sm and Nd, were analyzed with full column chemistry and the TaF5 method, and the results of Sm, Nd concentrations and Nd isotopic data are in good agreement with the reported values. Combined with a highly efficient and low-blank column chemistry to separate Nd from Sm, Ce, and Pr, this method holds potential to analyze Sm, Nd concentrations and Nd isotopic compositions of highly depleted peridotites; very small aliquots of minerals such as garnets; extra-terrestrial materials of limited sample size; and environmental samples that contain very low quantities of Sm and Nd.
The emplacement age of the Muntele Mare Variscan granite (Apuseni Mountains, Romania) Like the Alps and Western Carpathians, the Apuseni Mountains represent a fragment of the Variscan orogen involved in the Alpine crustal shortenings. Thus the more extensive Alpine tectonic unit in the Apuseni Mountains, the Bihor Autochthonous Unit is overlain by several nappe systems. During the Variscan orogeny, the Bihor Unit was a part of the Someş terrane involved as the upper plate in subduction, continental collision and finally in the orogen collapse and exhumation. The Variscan thermotectonic events were marked in the future Bihor Unit by the large Muntele Mare granitoid intrusion, an S-type anatectic body. Zircon U-Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) dating yielded a weighted mean age of 290.9 ± 3.0 Ma and a concordia age of 291.1 ± 1.1 Ma. U-Pb isotope dilution zircon analyses yielded a lower intercept crystallization age of 296.6 + 5.7/-6.2 Ma. These two ages coincide in the error limits. Thus, the Muntele Mare granitoid pluton is a sign of the last stage in the Variscan history of the Apuseni Mountains. Many zircon grains show inheritance and/or Pb loss, typical for anatectic granitoid, overprinted by later thermotectonic events.
The South China craton was formed by collision of the Yangtze and Cathaysia blocks during the Neoproterozoic Jiangnan orogeny (also termed the Jingnin or Sibao orogeny in Chinese literature). Basement rocks within the Yangtze block consist mainly of Proterozoic sediments of the Lengjiaxi and Banxi groups. U‐Pb ages of detrital zircons obtained by the laser ablation inductively coupled plasma mass spectrometry dating technique imply that the deposition of the Lengjiaxi Group continued until the Neoproterozoic. The youngest detrital zircons suggest a minimum deposition age of ∼830 Ma for the Lengjiaxi Group and an initial deposition age of $$785\pm 12$$ Ma for the Banxi Group, indicating a temporal hiatus of $$48\pm 13$$ Ma between these Neoproterozoic sedimentary rocks distributed in northwestern Hunan Province, South China craton. Detrital zircons from both the Lengjiaxi and Banxi groups have a wide range of εHf(t) values from −12 to 14.2 and a continuous Nd and Hf model age spectrum from ∼820 Ma to 2200 Ma. Model ages of many detrital zircon grains reach up to ca. 2.9–3.5 Ga, indicating that both juvenile mantle material and ancient crust provided sedimentary detritus. This is also consistent with the Nd isotopic signature of sedimentary rocks recorded in the Lengjiaxi Group, suggesting a back‐arc tectonic setting. The Banxi Group has slightly enriched Nd isotopic signatures relative to the Lengjiaxi Group, implying a higher percentage of old continental material in the sedimentary source. Combined with previously published data, new results help us to reconstruct the Neoproterozoic tectonic evolution of the South China craton.
AbstractThe South Yellow Sea basin in eastern China has experienced a multi-stage tectonic evolution history. The major structures were created when the basin was a foreland basin during the Mesozoic. However the geological evolution of the basin has not yet been corroborated by direct evidence from the underlying basement rocks. Qianliyan Island in the southern Yellow Sea provides an opportunity to study the formation and evolution of the basin by means of direct geochronological and geochemical evidence. On Qianliyan Island, basement rocks are exposed that consist of granitic gneiss, felsic gneiss and minor mylonite, and lenses of eclogite. Major and trace element characteristics of these four types of gneiss indicate that they originated from crustal material, varying in composition from pelite to greywacke. SHRIMP U-Pb zircon dating results of a felsic gneiss sample show that this rock crystallized between 659 and 796 Ma and underwent a metamorphic overprint at 229 ± 4 Ma. This age pattern resembles that of gneisses from the ultra-high-pressure terrain in the Dabie–Sulu belt. We conclude that the study area was part of the northern margin of the Yangtze Block during the Neoproterozoic. Neoproterozoic magmatic activity occurred along this margin and the basement sequence underwent Triassic metamorphic overprint during the northward subduction of the Yangtze Block beneath the North China Block. We further conclude that the deformation associated with this metamorphic event led to the formation of the southern Yellow Sea foreland basin.Keywords: Yangtze BlockYellow Sea basinSulu orogengneisszircon age AcknowledgementsE. Kolaray and an anonymous reviewer are sincerely acknowledged for constructive suggestions for the manuscript. We gratefully thank Lilin Du and Zhiqing Lai for technical support during SHRIMP analysis, and Anlong Li, Jinqing Liu, and Li Zhao for assistance during fieldwork.
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