Liquid immiscibility of boninite in Xiangcheng, southwestern China, and its implication to genetic relationship between boninite and komatiitic basalt

Boninitic rocks and associated high-magnesian basalt and high-iron tholeiit e in the Xiangcheng area constitute the basal horizon of the are volcanic s equence in the Triassic Yidun Island-Are, southwestern China. The boninite occurs as pillow, massive and ocellar lavas; the last one possesses well-de veloped globular structure and alternates with the former two. The boninite is characterized by the absence of phenocrysts of olivine and low-Ca pyrox enes and by low CaO/Al2O3 ratios (<0.67) and high Cr (> 1000 ppm) and Ni (> 250 ppm). The normalized abundance patterns (NAP) of trace elements to pri mitive mantle are similar to the NAP of low-Ca modern boninites and SHMB in the Archaean and Proterozoic. As a mechanism of ocellar texture, liquid immiscibility in boninite is supp orted by the following lines of evidence: (a) sharp contact between ocelli and matrix, (b) constant volumetric ratios of ocelli/matrix and common coal escence of ocelli in ocellar rocks, (c) identical micro-spinifex textures a nd mineral assemblages with different modal mineral contents in ocelli-matr ix pairs, (d) bubbles and acicular clinopyroxene crystal strand over the bo undary between ocelli and matrix, and (e) chemical compositions of ocelli a nd matrix corresponding to high-Mg andesite and komatiitic basalt, respecti vely. The close association and geochemical similarities between ocellar bo ninites and pillow boninite/massive boninite suggest that these are comagma tic rocks. The primary features of the ocellar boninite indicated by high M g/(Mg+Fe2+) ratio and high Cr and Ni abundance show that liquid immiscibili ty took place in the early evolution stage of the boninitic magma. The misc ibility gap in boninite which is smaller than that in tholeiite is likely t o be due to the low FeO*/MgO+FeO* ratio and high MgO content of the boninit ic magma. The association of komatiite-komatiitic basalt-boninite (or SHMB and the im miscibility phenomenon in high-Mg lavas in some Archaean greenstone belts a nd ophiolites is also reexamined. It is considered that liquid immiscibilit y in parental boninitic magma probably produced komatiitic basalt, which of ten occurs in close association with boninite (or high-Mg andesite).