Compositional zoning and its implication in a toroidal circulation inside the Yakushima Pluton, SW Japan

Abstract

The epizonal Yakushima pluton at the north end of the Ryukyu arc displays an asymmetrical compositional zoning. The core and southeastern periphery of the pluton are per-aluminous cordierite granitoids, whereas the main constituent of the pluton is cordierite-free and rather primitive in Rb-Sr and Nd-Sm isotopes. This doughnut-like compositional zoning in the Yakushima pluton share the same symmetry as the patterns of magmatic flow fabrics defined by alignment of rigid orthoclase megacrysts, suggesting a single toroidal circulation cell about an axis inclined toward northwest. Lithological contours inside this zoned pluton are locally truncated by the contours of intensities of solid-state deformation fabrics that developed during emplacement. Thus, the compositional zoning is likely to be inherited from an earlier stages of magmatic layering and relates the ascent processes. This paper attributes the doughnut-like compositional zoning of the Yakushima pluton to the drag along the pluton\u27s contact that circulated granite interior and formed the magmatic flow fabrics during its ascent. Such doughnut-like compositional zoning is expected in a magmatic pluton that rose through a ductile media as a diapir