内蒙古赤峰敖汉旗地区义县组地层划分与对比

内蒙古赤峰敖汉旗地区属于宁城-捣格朗营子火山喷发带北段。通过实测剖面和大比例尺地质调查,确定敖汉旗地区中生界以火山岩为主的地层为义县组,并将其进一步划分为4个岩性段:一段由复成分砾岩和含砾粗砂组成,二段由酸性流纹质火山岩组成,三段由一套安山质的火山岩及沉积相的凝灰质砂砾岩等组成,四段由中性偏碱性的粗面英安质火山岩组成。本区各段火山岩LA-ICP-MS锆石U-Pb测年结果表明,敖汉旗地区义县组火山活动时限为127.7~121.6Ma。该结果与北部阜新-义县盆地义县组标准剖面年龄可对比。依据岩石组合特征、同位素测年结果,敖汉地区义县组可以与标准剖面的中部对比,缺失可与标准剖面上部对比的层位。

Stratigraphic division and correlation of the Yixian Formation in Aohan area, Chi-feng, Inner Mongolia

The Aohan area of Chifeng belongs to "northern volcanic eruption belt of Ningcheng-Daogelangyingzi". Regional sec-tion measuring and large-scale geological survey has shown that the strata which are composed mainly of volcanic rocks in the Aohan area should be grouped into Yixian Formation, and can be further divided into four lithologic members in the following ascending or-der:The first member consists of polymictic conglomerate and coarse sandstone gravel; the second member consists of acidic rhyolitic volcanics; the third member consists of a series of andesitic volcanic rocks and sedimentary facies tuff gritstone; and the fourth mem-ber consists of trachydacite volcanic rocks with medium to high alkaline features. The LA-ICP-MS zircon U-Pb data of volcanic rocks from each lithologic member in the area indicate that the volcanic activity occurred at 127.7~121.6Ma, and this result can com-pare with things of the typical section of Yixian Fomation in northern Fuxin-Yixian basin. Rock assemblages and isotope dating re-sults show that the strata in Aohan area have similar lower and medium members, but the upper strata are absent, as shown by a corre-lation with strata of the typical section of Yixian Formation.