Chrome spinel in normal MORB-type greenstones from the Paleozoic–Mesozoic Mino terrane, East Takayama area, central Japan: Crystallization course with a U-turn

摘要

Phenocrystic chrome spinel crystallized in normal MORB-type greenstones in the East Takayama area. Associated phenocryst minerals show a crystallization sequence that was olivine first, followed by plagioclase, and finally clinopyroxene. Chrome spinel ranges from 0.54 to 0.77 in Mg/(Mg+Fe~(2+)) and 0.21 to 0.53 in Cr/(Cr+Al); the Fe~(3+) content varies from 0.07 to 0.22 p.f.u. (O = 4). Significant compositional differences of spinel were observed among the phenocryst mineral assemblages. Chrome spinel in the olivine–spinel assemblage shows a wide range in Cr/(Cr+Al), and is depleted in Fe~(2+) and Fe~(3+). Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage is Fe~(2+)- and Fe~(3+)-rich at relatively high Cr/(Cr+Al) ratios. Basalt with the olivine–plagioclase–spinel assemblage contains both aluminous spinel and Fe~(2+)- and Fe~(3+)-rich spinel. The assumed olivine–spinel equilibrium suggests that chrome spinel in the olivine–spinel assemblage changed in composition from Cr- and Fe~(2+)-rich to Al- and Mg-rich with the progress of fractional crystallization. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage, on the other hand, exhibits the reversed variations in Mg/(Mg+Fe~(2+)) and in Cr/(Cr+Al) ratios that decrease and increase with the fractional crystallization, respectively. The entire crystallization course of chrome spinel, projected onto the Mg/(Mg+Fe~(2+))–Cr/(Cr+Al) diagram, exhibits a U-turn, and appears to be set on a double-lane route. The U-turn point lies in the compositional field of chrome spinel in the olivine–plagioclase–spinel assemblage, and may be explained by plagioclase fractionation that began during the formation of the olivine–plagioclase–spinel assemblage.