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Highly magnesian olivines and green-core clinopyroxenes in ultrapotassic lavas from western Yunnan, China

¹ Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, China
² Geology Department, Royal Holloway University of London, Egham, UK
³ Department of Earth Sciences, Nanjing University, 210093 Nanjing, China

^* Corresponding author: e-mail: yigangxu{at}gig.ac.cn

Oligocene ultrapotassic rocks from western Yunnan have abundant olivines of varying size and green-core clinopyroxenes. Olivine macrocrysts (1–5 mm) have very high forsterite contents (Fo_92–94), and high CaO (0.15–0.30 wt%) and Cr₂O₃ (up to 0.23 wt%) contents. They are believed to be magmatic rather than disintegrated crystals from the upper mantle. These magnesian olivines are not in equilibrium with melts corresponding to the whole-rock compositions of host lavas and, as such, may represent xenocrysts entrained by ultrapotassic magmas during their ascent to the surface. While komaiites, boninites and olivine lamproites are potential sources of these xenocrysts, a genetic link to the late Permian Emeishan large igneous province (LIP) is favoured, because the Emeishan flood basalt is the only known geologic event in the studied area that could have produced highly magnesian olivines. The olivine xenocrysts may have been entrained from the high velocity lower crust (V_p = 7.1–7.8 km/s), which is believed to have formed from high Mg basalts (picrites?) that underplated and intruded the crust during the Emeishan flood basalt episode. Given the presence of olivine xenocrysts and reverse-zoned clinopyroxenes, it is apparent that some magnesian ultrapotassic lavas from Yunnan have a complex hybrid origin. While the former point to ultramafic/primary melts the latter indicate interaction between evolved melts and relatively primary melts thus indicating complex magma mixing processes.

Key-words: magnesian olivine, green-core clinopyroxene, ultrapotassic lavas, Yunnan.

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