Abstract: Rb-Sr isochronal ages and chemical data are reported for the Alxa alkaline-rich intrusive rocks, western Inner Mongolia, China. These alkaline-rich intrusive rocks have elevated contents of total alkali w(K₂O+Na2O) with an average of 12.70%. They are consisting mainly of K-feldspar, quartz, plagioclase, biotite and/or amphibole. Rb-Sr whole rock-minerals ages determination indicates that the isochronal ages are(250±18) Ma and (213.0±9.8) Ma for Suhaitu and Wenduerhao intrusion, respectively. The dated isotopic ages demonstrate that the Alxa alkaline-rich intrusive rocks belong to Indosinian alkaline magma activity. The similarities and differences of the alkaline-rich intrusive rocks distributed in the Alxa Fault Block and those distributed along the northern margin of the North China Block and the Tarim Block are compared and discussed. The authors deduce that during late Palaeozoic-Indosinian period, the Alxa Fault Block and the North China Block may be located in the same geodynamic system, but the Tarim Block may be located in a different geodynamic system.

Key words: Rb-Sr isochronal age, the Alxa Fault Block, alkaline-rich intrusion, Suhaitu intrusion, Wenduerhao intrusion

摘要： 采用Rb-Sr法对阿拉善断块有代表性的苏亥图、温都尔浩富碱侵入岩体的年龄进行了测定，分别获得(250±18)Ma和(213.0±9.8)Ma的年龄值，提供了属于印支期的年代学证据，认为阿拉善断块北缘富碱侵入岩和华北断块北缘燕辽—阴山印支期富碱性侵入岩带应该划归同一印支期富碱性侵入岩带。推断阿拉善断块和华北断块在晚古生代—印支期可能处于同一地球动力学体系内，而塔里木断块可能处于另一地球动力学体系。

关键词: Rb-Sr年龄, 阿拉善断块, 富碱侵入岩, 苏亥图岩体, 温都尔浩岩体