青藏高原东北缘多福屯第三纪钠质基性火山岩及构造启示

位于青藏高原东北缘的多福屯第三纪火山岩,为钠质基性火山岩系。该火山岩(La/Yb)N值在6~11,ΣREE平均为117,样品普遍具有不相容元素富集、Pb亏损以及弱的Nb、Ta正异常,Nb/U和Ce/Pb平均值分别为30和17;样品的87Sr/86Sr值为0.7041~0.7069,Nd初始值为0.5129(εNd(t)=6),Δ207Pb/204Pb和Δ208Pb/204Pb分别在11~19和73~84之间,结合高的Sr初始值,表现了特征的Dupal(高放射成因铅)异常。地球化学特征表明,该火山岩属于似OIB性质的陆内火山岩,源区具有DM与EMⅡ混合特点。个别岩石表现出明显陆壳混染的迹象。火山岩源区特征反映了OIB类岩石的复杂性,可能由区域原、古特提斯地幔继承而来,因而是原地的和固有的,并非与青藏高原物质向东挤出有关。通过与邻区同时代火山岩的对比推断,整个高原东北缘新生代火山岩幔源区具有小尺度的不均一性。高原东北缘新生代火山岩的发育可能与较大规模的贺兰-川滇南北复合构造有关。

Tertiary Duofuton Na-rich mafic volcanic rocks, on northeastern margin of the Qinghai-Tibet Plateau and their tectonic implications

The Tertiary Duofuton volcanic rocks, located on the northeastern margin of the Qinghai-Tibet Plateau, belong to a Na-rich mafic volcanic system. The rocks are characterized by the(La/Yb)N ratios of 6-11, average ΣREE of 117, general enrichment in incompatible elements, Pb depletion and weak Nb and Ta positive anomalies. Their Nb/U and Ce/Pb ratios are 30 and 17 on average, respectively. The 87Sr/86Sr ratios are 0.704 1-0.706 9, 143Nd/144Nd=0.512 9(εNd(t)=6), and Δ207Pb/204Pb and Δ208Pb/204Pb =11-19 and 73-84, respectively. Coupled with high 87Sr/86Sr ratios, they show Dupal anomalies. Generally, these mafic rocks are OIB-like intracontinental volcanics and the source region exhibits a mixture between DM and EMⅡ, some of the rocks are obviously contaminated by crustal materials. The complexity of OIB for these mafic volcanic rocks may have been inherited from the proto-and paleo-Tethyan mantle in the region and hence it is in-situ and original in nature, and possibly not related to the eastward extrusion of materials from the Qinghai-Tibet Plateau. In comparison with coeval volcanics in adjacent areas, it can be inferred that the mantle region in the northeastern margin is heterogeneous on a small scale. The development of the Cenozoic volcanics could have been triggered by the N-S trending tectonics.