关键词: 早古生代, 火山岩, 白乃庙岛弧, 兴蒙造山带, 古亚洲洋

Abstract: 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.

Key words: early Paleozoic, volcanic rocks, Bainaimiao arc, Xing-Meng Orogenic Belt, Paleo-Asian Ocean