内蒙古赤峰北部二叠纪基性—中性次火山岩系地球化学特征

Geochemical features of the Permian basic-intermediate subvolcanics in northern Chifeng, Inner Mongolia

赤峰北部早二叠世末至晚二叠世初基性—中性次火山岩锆石U Pb和谐线年龄为 (2 76 .3±4 .2 )Ma ,岩石组合为玄武岩、玄武安山岩及安山岩等。岩石化学上属亚碱性系列、钙碱性火山岩系、钠质类型 ;稀土总量中低 ,属轻稀土富集型 ,稀土分布模式为右倾平滑曲线或略有正、负铕异常 ;过渡族元素球粒陨石标准化曲线形态呈“W”型 ,锶初始值为 0 .70 5398,反映物质来源较深。该火山岩系形成于华力西中期强烈造山后均衡调整阶段的引张构造环境。

The basic intermediate subvolcanic rocks from northern Chifeng generated at the end of early Permian epoch to the beginning of late Permian epoch yield U Pb concordant ages of 275.3±4.2 Ma. These subvolcanics, in which basalt, basaltic andesite and andesite are dominant, are petrochemically of calc alkaline affinity of subalkaline series and sodic type, and tend to have moderate low total REE, being LREE enriched. The distribution patterns of REE are manifested as some right sloping flat curves with slight positive negative Eu anomalies. The chondrite normalized plots of transitional elements take “W”shape in general. The initial value of Sr is 0.705 398, implying a deep source. From the above it is suggested that the volcanic series was formed in a tensional state at the isostatic readjustment stage after the intense middle Vriscan orogeny.