中文摘要：

文章以野外观察为基础确定了致矿侵入体,以岩相学特征确立了测年样品的代表性。在此基础上,选取大庙铁矿大乌素沟矿区浸染状铁矿石中的黑云母进行了40 Ar/39 Ar年龄测定,坪年龄为（395.8±3.7）Ma,反等时线年龄为（395.6±4.0）Ma（2σ;MSWD=0.9;n=8）。因此,大庙铁矿及其致矿苏长岩的形成年龄约为396Ma,相当于中泥盆世,而不是前人所认为的元古宙。新的测年结果与区域上近年来陆续识别出来的泥盆纪侵入岩类形成年龄一致,不仅表明华北克拉通的改造过程至少从泥盆纪就已经开始,而且暗示华北克拉通北缘仍有寻找其他＂大庙式铁矿＂的潜力。同时,文章提出,用成岩年龄作为成矿年龄时,需要有可靠的地质学和岩相学证据。

英文摘要：

The ore-bearing intrusion has been identified by field relationships in this paper and the dating sample is representative through petrographic characteristics.Based on the above,biotites were separated from the disseminated iron ore for 40Ar/39Ar dating.Perfect age plateau and inverse isochron-line were obtained,with the result of（395.8±3.7） Ma and（395.6±4.0） Ma（MSWD=0.9）,respectively.Thus this 40Ar/39Ar age（396 Ma） should represent the metallogenic epoch of Damiao ore deposits and the emplacement age of norite,which indicates that the former is the Middle Devonian rather than the Proterozoic as assumed previously.The new geochronologic result is consistent with that of the Devonian intrusions which were progressively recognized at the northern margin of North China Craton（NCC） recently,implicating that modification of the NCC was initiated in the Devonian at least and there are still exploration potential for＂Damiao-type＂ore deposits at the northern NCC.In addition,the authors point out that the reliable geological and petrographic evidences are required to justify the use of rock-forming age as the ore-forming age.