2007 Volume 102 Issue 2 Pages 127-136
Orbicular granite occurs at Minederayama in the Tsukuba Mountains, Ibaraki Prefecture, Japan. The orbs consist of a migmatitic core, originally pelitic hornfels, and a mantle that consists mainly of radial cordierite containing apatite inclusions and subordinate feldspar and micas. The orientation and size distribution of the orbs indicates that isolated orbs had floated within the host granitic magma. According to the geometrical selection theory of crystal growth, we propose that radial cordierite grew from the orb shells to the orb cores. Melting experiments demonstrate that the mineral assemblage of the orbicular granite reflects the incongruent melting of pelitic hornfels. Important factors in the formation of the orbicular granite are the unmixing between the incongruent melt of cordierite hornfels and host granite melt, and the infiltration of the incongruent melt into the host granite melt as the cordierite crystallized. The temperature of the host granite magma, as estimated from melting experiments and mineral assemblage, was above 730 °C at 0.3 GPa, which may represent superheated conditions for the host granitic magma. Cooling of the host granite magma placed the orbs under supercooled conditions as the radial cordierite grew from the shells of the orbs to the cores.