北祁连山西段巴个峡-黑大坂一带几个花岗闪长岩体的侵入时代讨论--兼论古阿尔金断裂活动时间

北祁连山西段巴个峡-黑大坂一带分布一套花岗闪长岩,前人未做同位素年龄测定,根据岩体与围岩的接触关系确定为华力西晚期;笔者根据单颗粒锆石U-Pb年龄测定确定了这套岩石的侵入年龄为481.6±3.3 Ma,为加里东晚期.应属早中奥陶世,并对其进行了讨论;进而指出阿尔金断裂的形成时代可能大于481.6±3.3 Ma,且进行了论证.其地质意义有两点①阿尔金断裂带可能穿入敦煌地块-北山杂岩中,但为后期近Ew向推覆构造所"掩盖",侵入活动是一重要证据;②区域加里东期岩浆活动应为一期重要的岩浆-热事件.

A Discussion on Intrusion Epochs of Granodiorites along the Bagexia-Heidaban Zone in the Western Part of the North Qilian Mountains

No isotopic dating of granodiorites along the Bagexia-Heidaban zone in the western part of North Qilian Mountains was made by previous workers.They were commonly thought to be products of Late Variscan.Nevertheless,U-Pb single-zircon dating conducted by the authors show that these granodiorites are 481.6±3.3 Ma in age,thus belonging to Ordovician.The Altun fault belt might have been formed earlier than 481.6±3.3 Ma.Geological significance of these age data is also discussed in this paper.