白乃庙岛弧东段早古生代火山岩年代学、地球化学特征及地质意义

白乃庙群岛弧火山岩分布于兴蒙造山带南缘，是古亚洲洋与华北板块之间早古生代俯冲作用的岩浆记录，但以往工 作多集中在西段而较少关注其东延部分。本文对白乃庙岛弧带东段新识别出的早古生代火山岩进行了系统研究，以完善对 古亚洲洋早古生代演化的认识。白乃庙岛弧东段火山岩包括中-基性及酸性两种岩石类型，火山岩均富Cs，Rb，Th，U， 亏损Nb，Ta 等高场强元素，轻稀土富集，具有变化的Eu 异常。地球化学数据显示中-基性火山岩来源于俯冲板片熔体交 代的地幔熔融，并可能有大洋沉积物的加入，而酸性岩浆则来源于流体交代的下地壳。锆石LA-ICP-MS U-Pb 定年表明， 酸性及中-基性火山岩形成年代分别为430.5 Ma 和417.6 Ma，前者产出于俯冲岛弧环境，后者则可能为造山后环境下形成 的滞后型弧岩浆岩。年龄数据对比显示东段火山岩的形成年代晚于西段的白乃庙群地层，同时构造变质程度也更低，两者 的这种差异反映了东西两侧地质演化历史的不同，即早古生代古亚洲洋自西向东逐渐关闭。

Geochronology and Geochemistry of the Early Paleozoic Volcanic Rocks in Eastern Segment of the Bainaimiao Arc,and Their Geological Significances

The Bainaimiao arc occurs along the southern margin of Xing-Meng Orogenic Belt and was considered to have been formed by the subduction between paleo-Asian Ocean and North China Craton. It is the key area for reconstructing the tectonic history of the Central Asian Orogenic Belt(CAOB), however, most of previous studies mainly focused on the western part of the Bainaimiao arc and show less concern on the eastward extension. This study carries out systematic investigations on the Paleozoic volcanic rocks newly found in the eastern segment of Bainaimiao arc, aiming to constrain the tectonic evolution of Xing-Meng Orogenic Belt. The volcanic succession can be divided into an intermediate-basic sequence and a felsic sequence. All the volcanic rocks are enriched in Cs, Rb, Th, U and LREE, depleted in Nb, Ta, and with variable Eu anomalies. Geochemical results suggest that the intermediate-basic rocks were derived from a lithospheric mantle modified by the slab-derived melt, and sedimentary materials were also likely involved in the source. While the felsic rocks were formed by partial melting of lower crust which had been metasomatized by subduction slab fluids. Zircon LA-ICP-MS U-Pb dating shows that the ages of felsic and the intermediate-basic rocks are 430.5 Ma and 417.6 Ma, respectively. Felsic volcanic rocks were formed under the Paleo-Asian Ocean subduction setting, and the intermediate-basic rocks were likely generated in a post-collisional setting with signatures of lagged arc-type volcanic rocks. Zircon U-Pb ages indicate that the eastward extension of Bainaimiao arc was formed later than the western segment, and with a low degree of metamorphism and deformation, reflecting that the two flanks of the Bainaimiao arc might have underwent different evolution history and an eastward closure trend of the Paleo-Asian Ocean is thus inferred.