Geochemistry, zircon U-Pb dating and tectonic implications of the Palaeoproterozoic Ji’an and Laoling groups, northeastern Jiao-Liao-Ji Belt, North China Craton

摘要

The Jiao-Liao-Ji Belt is located between the Longgang and Langrim blocks in the Eastern Block of the North China Craton (NCC). The belt comprises Palaeoproterozoic volcanic–sedimentary successions that are divided into the Ji’an, South Liaohe, and Jingshan groups in the south and the Laoling, North Liaohe and Fenzishan groups in the north. The tectonic setting and depositional environments of the Liao-Ji area are debated. This paper presents geochemical and detrital zircon U-Pb data for the boron-bearing, graphite-bearing, aluminum-rich Ji’an Group and the clastic sediments and carbonates of the Laoling Group in southern Jilin Province, in order to constrain their depositional ages, provenance and tectonic setting. Zircon geochronology for both the Ji’an and Laoling groups yields age peaks of 2.57–2.41, 2.23–2.09, and 2.04–2.02?Ga, indicating that the sediments were derived from the Longgang and/or Liaonan block(s), the 2.2–2.1?Ga Liaoji granitoids and ～2.0?Ga igneous rocks, respectively. On the basis of the youngest detrital zircons and the oldest metamorphic zircon peaks, depositional ages can be constrained to the period of 2.13–1.94, 2.10–1.94, and 1.95–1.90?Ga for the Mayihe, Huangchagou, and Dadongcha formations of the Ji’an Group, and 2.18–1.90, 2.08–1.91, 1.99–1.90, 1.96–1.90, and 1.94–1.90?Ga for the Dataishan, Zhenzhumen, Huashan, Linjiang, and Dalizi formations of the Laoling Group, respectively. The zircon age spectra of each formation indicate that deposition of the Ji’an and Laoling groups was related to the opening and closing of a back-arc during the period 2.2–1.9?Ga. The Dataishan Formation (Laoling Group) was deposited in the rift environment during opening of the back-arc. During the early stage, the Mayihe and Huangchagou formations (Ji’an Group) were deposited on the side of the magmatic arc, and the Zhenzhumen Formation (Laoling Group) was deposited on the passive-margin side. During the late stage, both Ji’an and Laoling