甘肃北山大山头南花岗闪长岩SHRIMP锆石U-Pb定年及其地质意义

SHRIMP Zircon U-Pb Age of the Dashantounan Granodioritein from Beishan Mountains in Gansu Province and its Signification

采用SHRIMP锆石U-Pb定年方法,首次获得大山头南出露的片麻状花岗闪长岩的^(206)Pb/^(208)Pb加权平均年龄为(386±6)Ma(MSWD=1.06),代表该岩体的成岩年龄,即大山头南花岗闪长岩岩体形成于中泥盆世。于此同时,首次获得了花岗闪长岩中部分继承锆石的年龄为(885±20)Ma(MSWD=1.18),该年龄是新元古代构造热事件的反映。该岩体成岩年龄介于其北侧的大山头超镁铁岩体(397±4)Ma和大山头南超镁铁岩体(374±3)Ma之间,结合研究区地质构造背景及前人研究成果,认为中晚泥盆世期间不仅有中酸性岩浆侵位,同时沿大断裂带也有部分镁铁-超镁铁岩侵位,中酸性岩体与镁铁-超镁铁岩体可能具有类似的地质背景,即它们均形成于后造山或同造山晚期阶段。

Using the SHRIMP zircon UPb dating methods, the diagenesisage of the Dashantounangranodiorite has been obtained for the first time. The obtained 206Pb/208Pb weighted average age is （386±6）Ma （MSWD=1 06）, standing for the Dashantounan granodiorite was formed in middle Devonian. At the same time, the inherited zircon age has been obtained for the first time, and the yielded age is （885±20）Ma （MSWD=1 18）, whichserves as a reflection of the Neoproterozoic tectonic thermal event. The diagenetic age between the age of Dashantourock mass （397±4）Ma and the ones of Dashantounan rock mass （374±3）Ma. After analyzing the previous research achievements, its believed that themagmatic intrusionin the Middle lateDevoniannot only includeintermediate acidic magmas,but also include basic ultrabasicmagma in the studying area. Theintermediate acidic and basic ultrabasicmagma might be formed in the same geological background, that is, theymight beformed in a post orogenic extensional setting or a late stage of syn orogenic setting.