摘要
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，∑REE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87^Sr／^86Sr值为0．7041～O．7069，Nd初始值为0．5129（εNd（t）=6），△^207Pb／^240Pb和△^208Pb／^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。
The Tertiary Duofuton volcanic rocks, located on the northeastern margin ol 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 EREE 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/86 Sr ratios are 0. 704 1-0. 706 9, ^143Nd/^144Nd=0. 512 9（εNa（t）=6）, and △^207rpb/^204Pb and △^208Pb/^2044Pb =11-19 and 73-84, respectively. Coupled with high ^87Sr/^86Sr ratios, they show Dupal anomalies. Generally, these marie 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 marie 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.
出处
《地学前缘》
EI
CAS
CSCD
北大核心
2007年第3期73-83,共11页
Earth Science Frontiers
基金
国家自然科学基金资助项目(40572138
40234041)
