Diagenetic xenotime dating to constrain the initial depositional time of the Yan-Liao Rift

Abstract

In the Mesoproterozoic and Neoproterozoic, the North China Craton (NCC) underwent long-lived sedimentation with minor volcanism in the Yan-Liao Rift forming the Mesoproterozoic Changcheng, Jixian and Qingbaikou Groups in sequence. The basal boundary age of the Lower Changcheng Group indicates the initial time of the NCC as a stable cratonic block and can be used to make correlations among several representative basins worldwide. Recent studies have debated the initial depositional time of the Changcheng System, which is the key to understanding the Proterozoic tectonic evolution of the NCC. This study documented the xenotime and monazite from the lowermost Changzhougou and Chuanlinggou Formations of the Changcheng Group. The monazite grains from the Chuanlinggou Formation yielded U–Pb ages between 1810Ma and 2500Ma, which are comparable to the detrital zircon U–Pb ages in the same formation, indicating their detrital origin. Xenotime mostly occurs as overgrowths on the detrital zircon grains, supporting a post-depositional origin. The low U content of the Changzhougou Formation's xenotimes indicates a hydrothermal origin. Its peak age of 1570±15Ma may indicate a hydrothermal event. The xenotime from the Chuanlinggou Formation is characterized by a high U content and a REE pattern with a nonexistent or weak negative Eu anomaly and a higher Gd/Yb ratio, implying a diagenetic origin. These xenotime overgrowths yielded a wide age variation, even on a single grain, due to prolonged xenotime precipitation. The oldest population of 1716±3Ma is interpreted as the minimum depositional age of the Chuanlinggou Formation. The lowest boundary of the Changchengian System must be older than 1716Ma. This work demonstrates that diagenetic xenotime could better constrain the depositional time; however, extensive systematic research is required, especially with higher spatial resolution and more diagenetic xenotime grains.