Zhenhui Hou

High-precision barium isotope measurements by MC-ICP-MS

We present a high precision method to measure Ba isotopes by multiple-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). Barium is separated from matrices by using a cation exchange resin (AG50W-X12, 200–400 mesh). Instrumental mass bias of Ba isotopes was corrected by a sample-standard bracketing method using SRM3104a as the bracketing standard. Potential effects of different matrices from resin and samples, and acid molarity and concentration mismatch were rigorously evaluated in this study. The precision and accuracy of this method were tested by the measurement of a synthetic solution made by mixing SRM3104a Ba with other matrix elements. The average δ137/134Ba of the synthetic solution is −0.005 ± 0.047‰ (2SD, n = 36) relative to SRM3104a. The robustness of this method was further assessed by replicated analyses of 8 reference materials, including igneous rocks with mafic to felsic compositions. The δ137/134Ba of basalt standards BCR-2, BHVO-2, and JB-2 is 0.050 ± 0.039‰ (2SD, n = 13), 0.047 ± 0.028‰ (2SD, n = 22), and 0.085 ± 0.035‰ (2SD, n = 19), respectively; diabase standard W-2 is 0.035 ± 0.022‰ (2SD, n = 11); andesite standard AGV-1 is 0.047 ± 0.040‰ (2SD, n = 11) and JA-2 is 0.038 ± 0.048‰ (2SD, n = 17); rhyolite standard RGM-1 is 0.142 ± 0.030‰ (2SD, n = 15); and granodiorite standard GSP-2 is 0.013 ± 0.046‰ (2SD, n = 15). Two late Mesozoic basalts from China have a δ137/134Ba of −0.132 ± 0.020‰ (2SD, n = 7) and 0.001 ± 0.034‰ (2SD, n = 7), respectively. Based on repeated analyses of the synthetic standard and a carbonate standard IAEA-CO-9, the long-term external precision of our method is better than ±0.05‰, much smaller than the variation of δ137/134Ba in these reference standards and samples (up to 0.27‰). Therefore, the Ba isotopic composition can be used as a novel tracer to study geochemical processes.

A combined study of SHRIMP U-Pb dating, trace element and mineral inclusions on high-pressure metamorphic overgrowth zircon in eclogite from Qinglongshan in the Sulu terrane

Methods recently advanced for discrimination on the genesis of metamorphic zircon, such as analysis of mineral inclusions and trace elements, provide us powerful means to distinguish zircon overgrowth during high-pressure metamorphism. Zircons in ultrahigh-pressure eclogite from Qinglongshan in the Sulu terrane were studied by the SHRIMP U-Pb method in combining with trace element and mineral inclusion analyses. No inherited core was identified in the analyzed zircons by means of cathodoluminescence images. The occurrence of high-pressure metamorphic mineral inclusions in zircon, such as garnet, omphacite, rutile, and the flat HREE pattern in zircon indicate that the zircon formed at high-pressure metamorphic conditions. Therefore, a weighted average U-Pb age of 227.4 ± 3.5 Ma obtained from such a kind of zircon is interpreted to represent the timing of peak metamorphism for the Qinglongshan eclogite.

Paleo-Pacific Subduction in the Interior of Eastern China: Evidence from Adakitic Rocks in the Edong-Jiurui District

The Edong-Jiurui district is located more than 1000 km from the current Pacific subduction zone. It is part of the well-known middle and lower Yangtze River Cu-Fe-Au belt in central eastern China. Cu mineralization in this area is spatially and temporally associated with Late Mesozoic magmatic rocks. These rocks exhibit geochemical features of adakites, but their origin is not yet fully understood. To explore the relationship between Cu mineralization and Mesozoic magmatism, we report geochemical, Sr-Nd-Pb isotopic, and zircon U-Pb age data from adakitic rocks in the Edong-Jiurui area. Zircon U-Pb ages point to a protracted period of magmatic activity from 151 to 139 Ma. This time span coincides with the Cu mineralization (146–137 Ma) in the middle and lower Yangtze River belt. Adakitic features of the rocks are displayed by high contents of SiO2, Al2O3, Na2O, and Sr; enrichment of light rare earth elements (REEs) and large-ion lithophile elements; depletion of heavy REEs; positive Sr and negative Nb, Ta, and Ti anomalies; and high Sr/Y and La/Yb ratios. We favor a model of melt segregation from a plagioclase-free and garnet-bearing residue. Compared to non-Cu-bearing Mesozoic adakitic rocks in the Dabie terrane, adakitic rocks in the Edong-Jiurui area have higher initial ϵNd values (−3.4 to −6.3), Pb isotopic ratios, and Th contents and lower Pb/Ce values. Altogether, these features indicate that the melts were probably derived from subducted ocean mixed with marine sediment.

Zircon U-Pb age of the Shacun gabbro body, Yuexi, Dabie orogen and its geological implications

The single grain zircon U-Pb age of the Shacun mafic-ultramafic intrusion, Yuexi, North Dabie is (128.1±2.0) Ma. This date indicates that the intrusion was implaced at Yanshanian when most Mesozoic granitoids were intruded. It was not the syn-collisional intrusion related to the subduction of the Yangtze Block and ultrahigh-pressure metamorphism at Indosinian.

Melt–peridotite interaction in the shallow lithospheric mantle of the North China Craton: evidence from melt inclusions in the quartz-bearing orthopyroxene-rich websterite from Hannuoba

To better understand the origin, migration, and evolution of melts in the lithospheric mantle and their roles on the destruction of the North China Craton (NCC), we conducted a petrological and geochemical study on a quartz-bearing orthopyroxene-rich websterite xenolith from Hannuoba, the NCC, and its hosted melt and fluid inclusions. Both clinopyroxene and orthopyroxene in the xenolith contain lots of primary and secondary inclusions. High-temperature microthermometry of melt inclusions combined with Raman spectroscopy analyses of coexisting fluid inclusions shows that the entrapment temperature of the densest inclusions was ~1215°C and the pressure ~11.47 kbar, corresponding to a depth of ~38 km, i.e. within the stability of the spinel lherzolite. Intermediate pressure inclusions probably reflect progressive fluid entrapment over a range of depths during ascent, whereas the low-pressure inclusions (P < 2 kbar) may represent decrepitated primary inclusions. In situ laser-ablation ICP-MS analyses of major and trace elements on individual melt inclusions show that the compositions of these silicate melt inclusions in clinopyroxene and orthopyroxene are rich in SiO2, Al2O3, and alkalis but poor in TiO2 and strongly enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), with negative anomalies of high-field strength elements (HFSEs). These characteristics suggest that the silica-rich melts could be derived from the partial melting of subducted oceanic slab. Therefore, this kind of quartz-bearing orthopyroxene-rich websterite may be produced by interaction between the slab-derived melts with the mantle peridotite. This study provides direct evidence for the origin, migration, and evolution of melts in the lithospheric mantle, which may play an important role in the destruction of the NCC.

Geochronology and geochemistry of felsic xenoliths in lamprophyre dikes from the southeastern margin of the North China Craton: implications for the interleaving of the Dabie-Sulu orogenic crust

Lamprophyre dikes emplaced in a Jurassic granite at the southeastern margin of the North China Craton (NCC) carry different types of xenoliths. Here, we report a combined study of zircon U–Pb ages and whole-rock geochemistry of the xenoliths, as well as an Ar–Ar age of the lamprophyre, providing constraints on the sources of the magmatism and tectonic evolution in the southeastern margin of NCC. Phlogopite from the lamprophyre dike gave a 40Ar/39Ar plateau age of 116.15 ± 0.33 Ma. The felsic xenoliths can be classified into three groups: monzogranite, banded biotite granitic gneiss, and garnet-bearing gneiss. The internal structures of zircons from the banded biotite granitic gneiss xenolith show complex growth patterns. The mantles of these zircons display low luminance with a concordant SHRIMP U–Pb age of 227 ± 10 Ma. The rims with lighter luminance provide a SHRIMP U–Pb age of 213 ± 10 Ma, both ages are identical within error and significantly different from the ages of the basement rocks in the surrounding Bengbu uplift. However, the age is identical to those of the ultrahigh-pressure metamorphic rocks in the adjacent Dabie–Sulu orogen. In addition, zircon mantle domains of the banded biotite granitic gneiss xenolith have low Th/U ratios (>0.01) and flat HREE patterns ((Yb/Dy)n < 10), which suggest growth in an assemblage with garnet during HP metamorphism. The rim domains show very low Th/U ratios (<0.01) and steep HREE patterns ((Yb/Dy)n > 10), implying growth during exhumation in the absence of a garnet. Our studies show that the banded biotite gneiss represents vestiges of the subducted South China crust injected into or thrust below the North China Craton and provides constraints on the process of underthrusting in a continental collision zone, as well as the Mesozoic tectonic history of the southeastern margin of the NCC.