Xianglin Tu

Petrogenesis of the Neoproterozoic bimodal volcanic rocks along the western margin of the Yangtze Block: New constraints from Hf isotopes and Fe/Mn ratios

High-precision major element and Hf isotope data are reported for the Neoproterozoic Suxiong volcanic rocks along the western margin of the Yangtze Block. These volcanic rocks have variable εHf(T) values and Fe/Mn ratios. The relatively primitive basalts have high Fe/Mn ratios and high Hf-Nd isotopic compositions, indicating that they were generated by partial melting of garnet clinopyroxene in mantle plume at high pressure. Thus, the Suxiong basalts are genetically related to the proposed Neoproterozoic superplume. On the contrary, a few differentiated basalts have low Fe/Mn ratios and low Hf-Nd isotopic compositions. They are likely to experience assimilation-fractional crystallization process. The Suxiong rhyolites have consistent Hf and Nd model ages of 1.3–1.4 Ga. They are likely generated by shallow dehydration melting of pre-existing young arc igneous rocks associated with the basaltic underplating/intrusion in a continental rift.

The Bikou basalts in the northwestern Yangtze block, South China: Remnants of 820-810 Ma continental flood basalts?

National Natural Science foundation of China[40721063]; Chinese Academy of Sciences[KZCX3-SW-141]; Australian Research Council[DP0770228]

High Oxygen Fugacity and Slab Melting Linked to Cu Mineralization: Evidence from Dexing Porphyry Copper Deposits, Southeastern China

The Dexing porphyry Cu deposit is the largest Cu deposit in eastern China, with total reserves of 8.4 Mt Cu. The Dexing porphyries have geochemical characteristics typical of adakites: they are similar to examples in the Circum-Pacific Belt and in the Lower Yangtze River Belt but different from adakites from the Dabie Mountains and the Tibetan Plateau. Ce4+/Ce3+ and values calculated from zircon trace-element compositions vary from 495 to 1922 and from 0.51 to 0.82, respectively, and reflect high oxygen fugacity similar to that measured in or inferred for porphyry Cu-Au deposits in the South America. The high oxygen fugacity is consistent with abundant anhydrite and magnetite-hematite intergrowths in the porphyry, which indicate that the highest oxygen fugacity of Dexing porphyry reached the hematite-magnetite buffer. Based on the geochemical characteristics and the drifting history of the Pacific Plate, we propose that the Dexing adakitic porphyries formed through slab melting, most likely during subduction of an aseismic ridge in the Pacific Plate in the Mid-Jurassic.

Geochemical and zircon U–Pb study of the Huangmeijian A-type granite: implications for geological evolution of the Lower Yangtze River belt

The Early Cretaceous Huangmeijian Pluton is an A-type granite located on the northern bank of the Lower Yangtze River in Anhui Province, east-central China. It intruded the SE edge of the Early Cretaceous Luzong volcanic basin. The moderate- to coarse-grained granite is mainly composed of alkali feldspar, plagioclase, and quartz and has a typical A-type geochemical signature. LA-ICP-MS zircon dating yielded a weighted mean 206Pb/238U age of 127.1 ± 1.4 Ma, similar to other A-type granites in the Lower Yangtze River belt, indicating an Early Cretaceous extensional environment. Temperatures calculated using the Ti-in-zircon thermometer suggest that the magma formed under high-temperature conditions (720–880°C). The low calculated Ce(IV)/Ce(III) ratio based on zircon rare earth element patterns indicates low oxygen fugacity for this A-type magma. Previous studies suggested that eastern China was an active plate margin related to the Early Cretaceous subduction of the Pacific and Izanagi plates. The ridge between these two plates probably passed under the Lower Yangtze River belt, forming A-type granites and adakites. The Huangmeijian Pluton is roughly the same age within error but is marginally older than the Baijuhuajian A-type granite in the eastern part of the Lower Yangtze River belt. A-type granite genesis in the Lower Yangtze River belt only lasted for 2–3 million years and slightly predates the transition from regional extension to compression. All these can be plausibly interpreted by the ridge subduction model, that is, A-type granites formed because of mantle upwelling through the slab window during subduction of the ridge separating the Pacific and Izanagi plates.

Geochronology and geochemistry of single-grain zircons: Simultaneous in-situ analysis of U-Pb age and trace elements by LAM-ICP-MS

Simultaneous in-situ analysis of U-Pb age and 26 trace elements for single-grain zircon using the LAMICPMS technique was successfully achieved in this study. The precision of analysis is 1 to 3 % for 207 Pb/ 206 Pb isotopic ratios, 2 and 8 % for 206 Pb/ 238 U ratios and 5 to 15 % for most of the other trace elements. The concordia U-Pb and the mean 206 Pb/ 238 U ages for zircon grain of 1800 Ma to 150 Ma by LAM-ICP-MS spot analysis are in good agreement within analytical errors with the TIMS and SHRIMP results. All zircon grains are uniquely LREE-depleted in chondrite-normalized REE patterns. Zircon formed in mantle-derived magmas is significantly different from that of granitoids with regard to contents of many trace elements ( i.e. , U, Th, ΣREE, Y, Nb and P) and REE patterns ( i.e. , Nd/Yb ratio and Ce and Eu anomalies). This combined geochronological and geochemical investigation provides valuable information on timing and genesis of zircon.

High-precision analysis of Sr/Ca and Mg/Ca ratios in corals by laser ablation inductively coupled plasma optical emission spectrometry

A method has been developed to determine high-precision Sr/Ca and Mg/Ca ratios in corals by laser ablation inductively coupled plasma optical emission spectrometry (LA-ICP-OES) using aqueous solution standard calibration. Simultaneous determination of the signals of the entire analytical wavelengths by ICP-OES and the high performance of the new type of LA system (Resonetics 193 nm ArF excimer laser-ablation system, RESOlution M-50) improve the precision for elemental ratios. Repeated measurements on a coral base synthesized working standard, BH-7, provide precisions of about 0.4% and 0.8% for the Sr/Ca and Mg/Ca ratios, respectively, which are better than the formerly reported precision by the LA-ICP-MS method, about 1%. Such precision is comparable to those obtained by the solution nebulization-ICP-OES (SN-ICP-OES) method, and is adequate for paleoclimate reconstruction. In addition, the LA-ICP-OES can provide results with much higher spatial/time resolution. Comparisons between the LA and SN methods were handled by measuring along the same track of a coral. The Sr/Ca results by these two methods agree quite well with each other. The LA-ICP-OES method is very promising for the analysis of element/Ca ratios in coral and other carbonates used in paleoclimate studies such as stalagmite. Systematic discrepancy, however, was observed in the Mg/Ca ratios, likely due to the existing state of magnesium in the coral skeleton.

Separation of magnesium from meteorites and terrestrial silicate rocks for high-precision isotopic analysis using multiple collector-inductively coupled plasma-mass spectrometry

We report a modified procedure for separating Mg and Al from meteorites and terrestrial igneous rocks for high-precision analysis of Mg isotopes with multiple collector-inductively coupled plasma-mass spectrometry. The separating procedure was carried out in a single ion-exchange column filled with AG50W-X12 resin, and Mg was eluted with 1 M HNO3, followed by Al eluted with 4M HNO3. The modified procedure efficiently eliminates most matrix elements (except for Ni, Co, and Cu) with a recovery yield of Mg > 99%. Measurements of Ni-, Co-, and Cu-bearing simulation solutions revealed no detectable matrix effects. However, test runs demonstrated significant mass-dependent fractionation during the chromatographic process; consequently, a high recovery yield of Mg (>99%) is required to limit the deviation to less than 0.05‰. Furthermore, analysis of Mg standard solution with a wide range of concentrations demonstrated negligible deviation for samples with concentrations of 0.3–2.5 μg ml−1Mg relative to Mg standard solution concentrations of 1 μg ml−1Mg. The total procedure bank is less than 2 ng. The long-term reproducibility of instrumental measurements of Mg isotopes is ±0.05‰ for δ25Mg, ±0.10‰ for δ26Mg, and ±0.06‰ for δ26Mg* (2 SD, n = 211), based on analyzing five pure Mg standard solutions. Analysis of three chondrites and seven igneous rock standards showed ranges of δ26Mg(DSM3) from 0.02‰ to 0.30‰ for the former and from 0.03‰ to 0.30‰ for the latter. This method is simple and actionable.

Platinum-group elements for the mantle peridotites in the Dazhuka ophiolite, Tibet, China

The total PGE amounts of mantle peridotites in the Dazhuka ophiolite, Tibet, are 28.37–50.67 ng/g, slightly higher than those of mantle peridotites in the primitive mantle, and typical ophiolites in the world, and the Alps-type mantle peridotites. The PGE distribution patterns in the Dazhuka mantle peridotites are also different from those of the mantle peridotites of partial melting relict origin. The Dazhuka mantle peridotites have relatively high total PGE amounts and are enriched in Pt, Pd, and Ru. Their PGE distribution patterns belong to the positively inclinedor swallow-type patterns. The PGE distribution patterns in the mantle peridotites of partial melting relict origin belong to the negative-slope patterns or flat patterns. This reflects the unique features of the upper mantle in this region. Relative enrichment in Pt and Pd, as well as in the incompatible elements Cu, Au, Cs, Rb, Ba, Th, U and LREE, indicates that the partial melting-derived relict mantle peridotites in the Dazhuka ophiolite had experienced intensive permeating and mixing processes of the melt and fluid both containing abundant incompatible elements.