Zong Keqing

Reappraisement and refinement of zircon U-Pb isotope and trace element analyses by LA-ICP-MS

A protocol was established for simultaneous measurements of zircon U-Pb ages and trace elements by LA-ICP-MS at spot sizes of 16–32 μm. This was accomplished by introducing N2 into ICP to increase the sensitivity. The obtained U-Pb ages for zircon standards GJ-1, TEMORA and SK10-2 are consistent with the preferred values within about 1% uncertainty (2 σ) by simple external calibration against zircon standard 91500. Different data reduction softwares could yield different uncertainties for calculation of U-Pb ages. The commercially available program GLITTER4.4 could apply an improper uncertainty calculation strategy, but it may yield artificial high precisions for single analyses. Our trace element analyses indicate that Si is not an ideal internal standard for zircon when calibrated against the NIST glasses. Calibration against the NIST glasses using Si as an internal standard, a systematic deviation of 10%–30% was found for most trace elements including Zr. However, the trace element compositions of zircon can be accurately measured by calibration against multiple reference materials with natural compositions (e.g., BCR-2G, BHVO-2G and BIR-1G), or calibration against NIST SRM 610 and using Zr as an internal standard. Analyses of two pieces of GJ-1 demonstrate that it is relatively homogenous for most trace elements (except for Ti).

Trace elemental records of short-lived heating during exhumation of the CCSD eclogites

Trace element compositions of garnet, omphacite and apatite in ultrahigh-pressure eclogites from the main hole of the Chinese Continental Scientific Drilling (CCSD) project were in situ analyzed by the LA-ICP-MS method. Although both garnet and omphacite have homogeneous major element compositions, their trace elements show zonations from core to rim in rare earth elements. In particular, middle rare earth elements in the garnet, heavy rare earth elements in the apatite and all rare earth elements in the omphacite increase from core to rim, respectively. Based on dependence of partition coefficients on temperature and pressure in these minerals, we suggest that the trace elemental zonations in these minerals may record a short-lived heating event during exhumation.

Provenance of Central Canyon in Qiongdongnan Basin as evidenced by detrital zircon U-Pb study of Upper Miocene sandstones

Deep-water canyon systems can provide important sandstone reservoirs for deep-water oil and gas exploration in the South China Sea; however, the sedimentary provenance of the Central Canyon in the Qiongdongnan Basin remains controversial. In this work, detrital zircon grains from three drilling sandstones in the Upper Miocene Huangliu Formation in the western part of the Central Canyon were analysed by LA-ICP-MS for U-Pb ages, in order to constrain their provenance. One hundred and ninety-one zircon grains yield concordant U-Pb ages ranging from 28.6 to 3285 Ma. Most of them show oscillatory or linear zoning in CL-images and high Th/U ratios (>0.1), suggesting that they are magmatic zircons. Three major age clusters at about 30 Ma (N=6), 220–270 Ma (N=29), and 420–440 Ma (N=13), and five minor age clusters at 70–110 Ma (N=7), 150–170 Ma (N=4), 800–850 Ma (N=11), 1800–2000 Ma (N=16), and 2400–2600 Ma (N=7), can be identified in the age spectrum, which are very similar to those of the Upper Miocene sandstones and modern river sands in the Red River area, but different from those of other nearby regions (e.g., Hainan Island, the Pearl River area, and the Mekong River area) in Southeast Asia. The major age peak at about 30 Ma in our samples is consistent with the timing of tectonothermal events in the Red River Fault Zone. Therefore, we suggest that the provenance of the western part of the Central Canyon, in the Qiongdongnan Basin, was fed dominantly by the Paleo-Red River system during the Late Miocene.