Low-temperature thermochronology

Description

The Huangling dome is one of the largest tectonic domes in the South China continent. According to previous geological mapping results, the dome was formed between the Late Jurassic and Early Cretaceous, when regional compression was eventually replaced by regional extension. This transition fuels a dispute about whether the dome was generated by compression or extension. This study carried out single-grain zircon (U–Th)/He (ZHe) dating for samples collected from the dome and adjacent synclines. All measured ZHe ages are younger than the depositional or crystallization ages of the sampled rocks, and the western and eastern dome sides have much older ZHe mean ages than those of the dome and synclines' cores. These reflect an complex interactions between uplift, denudation and heating processes during dome formation. In order to explore the exhumation history of the dome, we developed one-dimensional heat-conduction models that cover a wide range of heating and exhumation scenarios (assuming that uplift rate equals denudation rate) within an interval of 152.1–121.4 Ma. These models were solved and searched for the predictions of the measured ages. The best models have a fast decrease in uplift rate with time and a constant heating rate, indicative of an early-interval shaping of the dome in a compressional setting.

Steps to reproduce

Five samples were collected along an E–W transect that covers the dome core, the western and eastern dome sides, and the cores of the synclines on the east and west. These samples are ~2 kg in weight and fresh. Zircon grains were separated using conventional crushing, heavy liquid and magnetic techniques. Among them, four or more euhedral grains (>75 µm in width, transparent and free of clear inclusions) were handpicked under a binocular microscope. Zircon (U–Th)/He analyses were carried out at the (U–Th–Sm)/He facility of The University of Melbourne. Analytical methods follow an established laboratory routine for laser He extraction: 4He is extracted from a zircon grain using a semiconductor diode Coherent Quattro FAP laser with fibre-optic coupling to the sample chamber, and He concentration is measured using a Balzers quadrupole (QMS 200-Prisma) mass spectrometer. The outgassed grain is then dissolved in HNO3 and analyzed using a second-generation Varian quadrupole ICP–MS. Fish Canyon Tuff zircons are used as standards to check the analytical accuracy.

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