OLIVINE PHENOCRYSTS AND SPINEL INCLUSIONS IN THE WOZHONG HIGH-MG AND K-RICH LAVAS FROM THE WESTERN YUNNAN, CHINA: PETROGENESIS AND GEODYNAMIC IMPLICATIONS

Abstract

Cenozoic high-Mg and K-rich lavas from Wozhong, western Yunnan province contain abundant olivine phenocrysts with Fo in mineral cores varying between 94 ~ 75%. As a whole, Fo contents decrease from macrophyric olivines (>1mm) to medium sized olivines (0.3 ~ 1mm) and to the small olivines (<0.3mm). All the olivine phenocrysts contain high CaO (>0.1wt% ), which is indicative of a magmatic origin. NiO contents decrease with decreasing Fo contents in the phenocrysts, which reflects the fractional differentiation trend in magmas. The origin of these olivine phenocrysts is complex. The disequilibrium between whole rock MgO and Fo contents of the macrophyric and medium olivine cores suggests that they are xenocrysts. The spinel inclusions in olivine phenocrysts have high Cr#(0.77 ~ 0.94), low Al2 O3 contents (<10%) and low Mg# values (0.17 ~ 0.65). TiO2 contents in most of the spinel inclusions are lower than 1.0wt%. This suggests that the majority of olivine phenocrysts were crystallized from arc-type magmas derived from partial melting of metasomatic harzburgite. However, a small number of spinel inclusions are high in TiO2 (1.36 ~ 3.96wt%), which could be related to the Emeishan flood volcanism. Both macrophyric and medium olivine phenocrysts are compositionally zoned with Fo and NiO contents decreasing, MnO and CaO content increasing from core via mantle to rim. The rims of these large phenocrysts are compositionally similar to small olivines. At least two stages of magmatic processes have been involved in the genesis of the Wozhong high-Mg and K-rich lavas. The compositional variation in the core of the macrophyric and medium phenocrysts may have resulted from fractional differentiation of the early highly magnesian magmas which may not be genetically related to the host magmas. In contrast, compositional trends in the rims and the small olivines were likely related to the differentiation of the more evolved host magmas.