关键词: 中国东北, 新生代玄武岩, 橄榄石, 辉石岩, 再循环地壳

Abstract: Cenozoic volcanic rocks are widespread in Northeast China, mainly surrounding the Songliao Basin. In the eastern part of the basin, volcanic rocks are distributed along Fushun-Mishan and Yilan-Yitong faults, especially near the Changbai Mountain area, and these volcanic rocks are dominated by sodium basalts. The genesis of these basalts is complex based on their significantly chemical heterogeneity found in the previous studies. Therefore, Cenozoic basalts from Jixi area in the north of Fushun-Mishan fault were selected to further identify the lithology in their mantle source. These basalts show a wide range of SiO₂ (44.2~49.6 wt%) and MgO (6.7~10.6 wt%) contents, and can be subdivided into basanite, trachybasalt and basalt. In the primitive-mantle normalized incompatible element diagram, they show positive anomalies of Nb and Ta and negative anomalies of Pb and Ti, which is similar to oceanic island basalts. The calculated the primary magmas compositions of these samples are rich in MgO but poor in SiO₂ and CaO; moreover, they fall in the region of silica-poor pyroxenite on the projections from or towards Diopside into the plane Olivine-Quartz-Calcium Tschermak’s (CATS). The high FC₃MS (FeO^T/CaO-3×MgO/SiO2) values of the basalts again indicate the presence of pyroxenite in their source. Furthermore, the olivine phenocrysts from these basalts are also analyzed, and the high-Fo olivines show lower Ca and Mn contents, and higher Ni contents and Fe/Mn ratios than the low- Fo olivines. The compositions of high-Fo olivines resemble to those olivines crystallized from pyroxenite-derived melt, strongly supporting the presence of pyroxenite in the mantle source. In summary, we highlight that pyroxenitic component, produced during the crustal recycling process, plays an important role in the formation of these mantle-derived magmas in Jixi area.

Key words: northeastern China, Cenozoic basalts, olivine, pyroxenite, crustal recycling