Application of the revised Ti-in-zircon thermometer and SIMS zircon U-Pb dating of high-pressure pelitic granulites from the Qianlishan-Helanshan Complex of the Khondalite Belt, North China Craton

Abstract

The Ti-in-zircon thermometer is a newly established, single-mineral trace element thermometer, which has been successfully used to estimate the peak and post-peak temperatures of high-grade metamorphic rocks. Here the revised Ti-in-zircon thermometer and in situ SIMS U-Pb dating are applied to detrital and metamorphic zircons of pelitic granulites from the Qianlishan-Helanshan Complex. The SIMS U-Pb results show that detrital zircons yield 207Pb/206Pb ages scattering from 2021 to 2449Ma with a peak age of 2030Ma, possibly sourced from felsic to intermediate igneous rocks at a relatively low temperature magmatic environment. Most metamorphic zircons yield ages of 1953±15Ma, 1954±20Ma and 1948±18Ma with Ti contents of 3.9–82.6ppm, corresponding to metamorphic temperatures of 664–992°C. Minor metamorphic zircons give 207Pb/206Pb ages of 1923±9Ma and Ti contents of 5.8–16.3ppm with temperatures of 697–794°C. These define metamorphic ages and temperature conditions at peak and post-peak metamorphic stages of pelitic granulites, respectively. Combined with previous studies, our new data establish a linking between zircon U-Pb ages and metamorphic thermal evolution. This demonstrates a ∼1.95Ga peak temperature granulite-facies metamorphism and subsequent near-isothermal decompression process at ∼1.92Ga, which is considered to record a ∼1.95Ga continent-continent collisional event between the Yinshan and Ordos blocks, followed by post-orogenic exhumation at ∼1.92Ga.