Petrographic contrast between ilmenite- and magnetite-series gabbroids in the Ryoke and San-in belts, southwestern Japan Arc

Abstract

To determine the petrographic contrasts between ilmenite- and magnetite-series magmas, we assessed the rock descriptions, modal compositions, and whole-rock chemical compositions for ilmenite- and magnetite-series gabbroic masses in the Ryoke and San-in belts and focused on the primitive phases of the two plutonic belts. We obtained the following results: (1) The San-in gabbroids contain variable modal amounts of magnetite, up to 5.8%, whereas no magnetite was detected in the Ryoke gabbroids. The outcrop measurements of magnetic susceptibility exhibited good positive correlation with the magnetite contents of the outcrop samples. (2) As compared to the San-in gabbroids, the Ryoke gabbroids are generally less abundant in alteration minerals such as chlorite and fibrous actinolite and more abundant in olivine- and orthopyroxene-bearing phases. (3) The rock texture and whole-rock chemical compositions indicate that the Ryoke gabbroids have more cumulative characteristics than the San-in gabbroids. The compositional differences between them can be mainly explained by the differences between their degrees of crystal accumulation. (4) We were unable to detect any significant differences between the N-MORB normalized patterns of trace-elements and REEs in the Ryoke and San-in gabbroids. Features (2) and (3) may be responsible for the differences between the emplacement depths of the Ryoke and San-in gabbroic masses, although these features are independent of redox states. Feature (4) suggests that the two gabbroids have a common petrogenetic background. Based on these results, we conclude that the contamination of Ryoke gabbroic magma by sediments is insignificant, having little or no effect on redox states. It is more likely that the divergent redox states of the two gabbroic magmas are primarily attributable to the differences in the volatile components involved in the fO2 buffer reactions.