Abstract
The Murotomisaki Gabbroic Intrusion is a sill-like layered gabbro emplaced in sedimentary strata of Tertiary age in Shikoku, Japan. We studied the zoning (including resorption structures) and the compositional variations of plagioclase from throughout the intrusion and found out that the zoning pattern may be classified into four types, A, B, B' and C, which may well correlated with the hosting rock types, the mode of occurrences and their stratigraphic positions in the intrusion. We successfully decoded the plagioclase zoning and deduced a sequences of events that took place during the magmatic differentiation and further interpreted them in the context of a stratified basal boundary layer slowly ascending in a solidifying magma body. It was revealed that various layered structures - modal layering, podiform gabbroic pegmatites and anorthositic layers - observed in the Murotomisaki Gabbro were formed within the moving basal boundary layer with the aid of flushing of water-enriched silicate melts from below but within the basal boundary layer. It was proposed that hydrous immiscible melt droplets that were generated near the base of the basal boundary layer had played an important role in displacing water upward, creating the water-enriched horizon within the basal boundary layer. Anorthositic plumes also generated in this horizon and ascended through the basal boundary layer and probably reached the main magma body lying above.
