四川茂县渭门组凝灰岩锆石SHRIMPU-Pb测年及地质意义

对出露于松潘-甘孜褶皱带四川茂县渭门一带的渭门组的形成时代一直令人关注,但迄今为止未见其形成年龄的报道,在开展1∶25万绵阳幅区域地质调查(修测)时,在渭门组中上段发育近100 m厚的具地层意义的凝灰岩中采集高精度SHRIMP测年样品一件,获得SHRIMP U-Pb加权平均年龄为530.1±4.3 Ma(MSWD=1.5)。样品中的锆石Th/U介于0.36~1.09 Ma之间,并具清晰的振荡环带结构,均为岩浆成因的锆石,表明渭门组形成于早寒武世,为准确地界定渭门组的时代和层序提供了新的参考依据,对正确认识松潘-甘孜褶皱带的地层格架和构造濱化具有重要意义。

Zircon SHRIMP U-Pb Dating and Geological Significance of the Weimen Group Tufts From Mao County,Sichuan Province

The age of the Weimen Group that is exposed in Songpan-Ganzi (Sichuan) fold belt is of great interest. But so far there are no reports of its formation age. During a 1:250000 scale survey of Mianyang area, a nearly 100 m thick tuff bed in the Upper Weimen Group was identified and one sample was collected for high precision SHRIMP dating. A SHRIMP U-Pb weighted mean age of 530.1 ± 4.3 Ma (MSWD = 1.5) was obtained. The zircon Th / U ratios of the sample are from 0.36 to 1.09, and zircon grains show a clear oscillatory banding structure, indicating that zircons are of magmatic origin. The age results suggest that the Weimen Group was formed in the early Cambrian. The new age provides a chronologic basis for understanding the tectonic evolution of the Songpan-Ganzi fold belt.