Discussion on the age of the Xiong'er Group in the Southern margin of the North China Craton

Abstract

The volcanic sequence of Xiong'er Group, distributed in Henan, Shanxi and Shaanxi provinces, overlies unconformably the Archean and Palaeo-Proterozoic crystalline basement, and is covered by the Middle-Upper Proterozoic terrestrial clastic rocks and carbonate rocks of the Ruyang and Guandaokou Groups. The available ages of the Xiong'er Group ranging from 1.85 to 1.40 Ga, so it was generally thought that the Xiong'er Group was formed at the beginning of Meso-Proterozoic, corresponding to the Changcheng Group. We have obtained ages of the zircons from the rhyolite porphyry at the top of the Majiahe Formation as well as the pyroxene diorite intruded in the Jidanping Formation, using the conventional zircon U-Pb and Lasser Probe-Inductively Coupled Plasma Mass Sepectrometry (LP-ICPMS). It is proved that the Xiong'er Group was formed in 1.75 ∼ 1.95 Ga, earlier than the formation of Changcheng Group. The earliest eruption of the Xiong'er Group volcanics occurred in the Xiong'er and Xiaoshan Mountains, forming the Xushan and Jidanping Formations. At ca. 1.85 Ga, the volcanism developed along a trifurcate rift, that was the Majiahe Formation, covering all the aera. The latest magmatism, represented by some small intrusive pyroxene diorites, probably occurred at 1.75 Ga and was confined in the Xiong'er and Waifangshan Mountains. The formation time of the Yunmengshan Formation, the Ruyang Group, and the Gaoshanhe Formation, the Guandaokou Group overlying the Xiong'er Group, probably corresponds to that of the Changcheng Group.