Fukun Chen

Contrasting Late Carboniferous and Late Permian–Middle Triassic intrusive suites from the northern margin of the North China craton: Geochronology, petrogenesis, and tectonic implications

Two contrasting intrusive suites have been identified from the northern margin of the North China craton: a Late Carboniferous dioritegranodiorite suite mainly made up of quartz diorite, diorite, granodiorite, tonalite, and hornblende gabbro, and a Late Permian–Middle Triassic suite of granitoid intrusions consisting of monzogranite, syenogranite, and quartz monzonite. Plutons from the Late Carboniferous suite exhibit variable SiO2 contents and calc-alkaline or high-K calc-alkaline, metaluminous geochemical features. Most have low negative whole-rock ϵNd(T) values (where T is the crystallization age) of −17.1 to −11.5 and zircon ϵHf(T) values of −38.3 to −11.2, indicating that they were derived mainly from anatectic melting of the ancient lower crust with some involvement of mantle materials. However, an older pluton in the suite exhibits higher ϵNd(T) values of −11.5 to −9.9, Nd model ages of 1.82–1.64 Ga, lower initial 87Sr/86Sr ratios of 0.7046–0.7048, and it contains some zircon grains that are characterized by high negative to positive zircon ϵHf(T) values of −8.7 to 1.2, indicating strong involvement of juvenile materials derived from the lithospheric mantle. The Late Carboniferous plutons are interpreted as subduction-related and to have been emplaced in an Andean-style continental-margin arc during the southward subduction of the paleo–Asian oceanic plate beneath the North China craton. Rocks from the Late Permian–Middle Triassic intrusive suite display geochemical signatures ranging from highly fractionated I-type to A-type. They exhibit higher zircon ϵHf(T) values of −14.9 to −6.7, whole-rock ϵNd(T) values of −10.6 to −8.8, and younger Hf and Nd model ages than most of the Late Carboniferous plutons, indicating that they could have been produced by extreme fractional crystallization of hybrid magmas resulted from mixing of coeval mantle- and crust-derived melts. They are interpreted as postcollisional/postorogenic granitoids linked to lithospheric extension and asthenosphere upwelling due to slab break-off and subsequent sinking after final collision and suturing of the Mongolian arc terranes with the North China craton. These two contrasting intrusive suites suggest that the final closure of the paleo–Asian Ocean and collision between the Mongolian arc terranes and the North China craton occurred during the Late Permian, and these events were followed by postcollisional/postorogenic extension, large-volume magmatism, and significant continental growth. No significant syncollisional crustal thickening, high-pressure metamorphism, or S-type granitoid magmatism occurred during the collision process.

Fast tectonometamorphism and exhumation in the type area of the Barrovian and Buchan zones

The beginning of the Grampian episode of tectonometamorphism in Scotland is dated by collision and obduction of the Ballantrae Ophiolite Complex at 478 ± 8 Ma. For the first time we have used zircon and garnet to radiometrically date the peak of the Caledonian Buchan and Barrovian tectonometamorphism in their type areas as contemporaneous at 467 ± 2.5 Ma. Detrital garnet from the neighboring Southern Uplands flysch terrane has the same radiometric age and occurs in 465 ± 2.5 Ma molasse in the Midland Valley. We show that the entire Grampian episode lasted 15 ± 2.5 m.y. and that it took fewer than 7.6 m.y. to expose high-grade rocks following the peak of metamorphism. Those who study Precambrian orogeny should note the brevity of this Scottish tectonometamorphic episode and the speed of exhumation.

Precise determination of Sm, Nd concentrations and Nd isotopic compositions at the nanogram level in geological samples by thermal ionization mass spectrometry

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.

Nd–Sr–Pb isotopes of Tengchong Cenozoic volcanic rocks from western Yunnan, China: evidence for an enriched-mantle source

Abstract Tengchong Cenozoic volcanic activity in the southeastern Himalayan belt of western Yunnan is considered to be rift-related after the Indo-Eurasia collision. In this study, Nd–Sr–Pb isotopic compositions were analyzed in order to understand the genesis of the Cenozoic volcanic rocks. Five basalts and andesitic basalts are characterized by high 87Sr/86Sr ratios (0.7057–0.7081), low ϵNd values (−1.1 to −5.7), and particularly high 208Pb∗/206Pb∗ ratios (1.08–1.12). Twenty samples of rocks, which include granite, granodiorite, and amphibolite from the same region, were analyzed to evaluate the role of contamination and assimilation in the volcanic rocks. Granites have high 87Sr/86Sr ratios and low ϵNd values, indicating a crustal origin. Granodiorites have 87Sr/86Sr ratios of 0.7069–0.7100 and ϵNd-values of −2.9 to −7.6 that indicate contribution of a mantle component. Amphibolites that are characterized by high ϵNd-values of 9.4 to −1.7 may represent remnants of obducted oceanic crust of the Neo-Tethyan basin. The volcanic rocks are distinguished from the granitoids and amphibolites in terms of Pb isotopic compositions. This observation probably implies that crustal contamination played a minor role in the origin of the volcanic rocks. Isotopic evidence for the volcanic rocks indicates that they probably originated from melting of an enriched-mantle source, that is, ascribed to assimilation of subducted oceanic crust and sediments.

Provenance of the Beihuaiyang lower-grade metamorphic zone of the Dabie ultrahigh-pressure collisional orogen, China: evidence from zircon ages

The Beihuaiyang zone of the Dabie ultrahigh-pressure collisional belt, which was formed by the Early Mesozoic collision of the North and South China (Yangtze) Blocks, is diversely interpreted as an active or a passive sedimentary sequence. It comprises the Luzhenguan and Foziling complexes. Six granitoid rocks of the Luzhenguan complex are dated at 770 Ma to 720 Ma and one schist sample also contains detrital zircons of 760 Ma to 720 Ma. These data indicate that this complex was of the South China affinity and probably originated from the northern Yangtze Block. Two quartzite samples of the Foziling complex contain abundant detrital zircons of Archean to Paleozoic ages, clustering around 2.5 Ga, 1.9 ‐ 1.8 Ga, 1.0‐ 0.7 Ga, and 0.5 ‐ 0.4 Ga. This age spectrum demonstrates a mixed source of these two blocks. The youngest zircons suggest a Paleozoic deposition along an active continental margin south of the North China Block, while these Late Proterozoic zircons are characterized for the Yangtze Block. Subsequently, it is suggested that a terrain drifted from the northern Yangtze Block in Early Paleozoic must have been situated to the southern margin of the North China Block to provide material source prior to the final collision in Early Mesozoic. q 2003 Elsevier Ltd. All rights reserved.

Detrital Zircon Ages and Hf‐Nd Isotopic Composition of Neoproterozoic Sedimentary Rocks in the Yangtze Block: Constraints on the Deposition Age and Provenance

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 \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Zircon U-Pb and Pb-isotope fractionation during stepwise HF acid leaching and geochronological implications

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Ultra-low procedural blank and the single-grain mica Rb-Sr isochron dating

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