柴北缘赛什腾山滩间山群晚奥陶世富铌玄武岩成因及其地质意义

Genesis and Geological Significance of Late Ordovician Nb-rich Basalts from Tanjianshan Group in Saishitengshan Mountain,Northern Margin of Qaidam Tectonic belt

产于柴北缘构造带西段赛什腾山地区滩间山群中的变玄武岩的结晶年龄为(444±4)Ma,具有富Na_(2)O、贫K_(2)O、高TiO_(2)、Nb及低LILE/HFSE和HREE/HFSE值等特征,球粒陨石标准化稀土元素配分曲线整体表现为轻稀土相对富集、重稀土平坦的略向右缓倾型配分模式,且在原始地幔标准化微量元素蛛网图中显示Nb、Ta弱正异常,与富铌玄武岩地球化学特征一致。综合分析表明,赛什腾山富铌玄武岩岩浆源区为尖晶石相二辉橄榄岩,是俯冲大洋板片陡角度回转引起的上涌软流圈地幔在弧后盆地边缘(靠近岛弧侧)与亏损地幔楔混合的产物,指示晚奥陶世柴北缘西段仍处于弧后伸展阶段,陆陆碰撞尚未开始。结合区域已有资料,认为柴北缘滩间山群是晚寒武世—早中志留世洋陆转换过程中不同时期、不同构造背景下(包括洋岛、岛弧、弧后等)的火山-沉积产物,其经历了自大洋俯冲至陆陆碰撞前的整个俯冲消减过程,各类岩石因构造混杂最终保存于柴北缘狭长构造带内。

The crystallization age of meta-basalts from Tanjianshan Group in Saishitengshan mountain,in the western part of the northern margin of Qaidam tectonic belt,was 444±4 Ma,which has the characteristics of rich Na_(2)O,poor K_(2)O,high TiO_(2),Nb,and low LILE/HFSE and HREE/HFSE ratios.The chondrite-normalized REE distribution curve shows a slightly right-leaning distribution pattern with relatively enriched LREE and flat HREE.On the primitive mantle-normalized trace element diagrams,Nb and Ta show weak positive anomalies,which is consistent with the geochemical characteristics of Nb-rich basalts.The comprehensive analysis shows that the magma sources of Nb-rich basalt in Saishiteng Mountain maybe the spinel-phase lherzolite,which is the product of the mixture of upwelling asthenosphere mantle and depleted mantle wedge at the edge of the back-arc basin near the island arc side caused by the steep angleroll-back of the subduction oceanic slab.It indi-cates that the western part of the northern margin of Qaidam basin was under the stage of back-arc extension during the late Ordovician,and the continental collision had not begun.Combined with the existing regional da-ta,it is considered that the Tanjianshan Group in the northern margin of Qaidam Basin were the volcanic-sedi-mentary products of different periods and different tectonic settings in the process of ocean-continent transition from Late Cambrian to Early-middle Silurian,and had experienced the whole subduction process from oceanic subduction to continental collision.Due to tectonic mélange,various types of rocks were occurred in the north-ern margin of Qaidam basin.