鄂尔多斯盆地东南缘延长组凝灰岩的时空分布特征

Spatial and Temporal Distribution and Characteristics of Tuff Intervals from Yanchang Formation in the Southeastern Margin of Ordos Basin

以山西永和延长组2段凝灰岩夹层为重点研究对象,通过详细野外调查及年代学分析,明确了该区凝灰岩岩石学特征,精确限定该区凝灰岩的关键时限,同时与鄂尔多斯盆地南缘陕西延长组7段凝灰岩特征对比,探讨鄂尔多斯盆地延长组凝灰岩形成的大地构造背景。研究结果表明:山西永和凝灰岩夹层厚度较薄(10~30 cm)且蚀变强烈,具有典型凝灰岩结构及中酸性火山岩特点;分布具有明显的区域性,主要集中于鄂尔多斯盆地东南缘,北至宁武盆地,南到大宁地区,东到永和交口,具有由西南向东北方向厚度逐渐减薄的趋势;两个凝灰岩样品锆石加权平均年龄分别为(241±3)Ma(n=29)和(238±3)Ma(n=29),与陕西延长组7段多井凝灰岩年代学数据(227~241 Ma)对比指出,不同地区凝灰岩的喷发时限并不完全具有等时性,但总体处于早三叠世晚期至中三叠世早期。

This study focuses on the tuff intervals involving zircon U-Pb geochronology, which are located at the bottom of Yanchang Formation in Yonghe outcrop section, Shanxi Province. In conjunction with the field exploration and microscopic observation, they show obvious regional differences. They mainly occur in the Yonghe and Daning counties, the southeast margin of Ordos Basin, and in the Ningwu Basin, and show a trend of thinning from west to east. The weighted average ages of the dominant component in the two Yonghe tuff samples are(241±3)Ma(n=29) and(238±3)Ma(n=29), respectively. Combined with the different well data in Shaanxi Province(227~241 Ma), these results indicate that the eruption of tuff is dated at the late Early Triassic to early Middle Triassic.