高知県足摺岬環状複合岩体のジルコンにおける特異な内部組織と微量元素組成

抄録

  Various internal textures of zircon from Cape Ashizuri Ring Complex, Kochi, SW Japan, were newly found by scanning electron microscope (SEM) and cathodoluminescence (CL) observations, and the chemical features including HfO₂, Y₂O₃, UO₂ and ThO₂ were described. Unusual internal textures of the zircon are classified into three types: Resorption Disturbance texture (RD), Local Disturbance texture (LD) and Hafnon-like Disturbance texture (HD). RD is cutting a part of original oscillatory zoning with rounded boundary, just like resorption-reprecipitation texture which commonly occurs in metamorphic zircon. LD is divided into two subtypes, LD1 and LD2: LD1 is developed at core and showing dendritic shape; on another front LD2 is mainly developed at rim and closely related to melt inclusions. Both subtypes have low concentrations of Y₂O₃ and ThO₂ and slightly high HfO₂ concentration, indicating that LD corresponds to recrystallized texture. HD is developing from zircon surface toward core deeply and associated with LD2. It has very unique chemical features: quite low Y₂O₃ and ThO₂ concentrations, twice HfO₂ concentration than that of igneous growth bands, and also nearly the same concentration of UO₂ as that of the igneous texture. The detailed phenomenon relating to HD formation is not clear. Through the observation, six growth stages of the zircon internal textures are suggested as follows: (1) igneous or hydrothermal core, (2) LD1, (3) RD, (4) igneous rim with enclosing melt, (5) LD2 around the melt inclusions and (6) outermost igneous rim. RD indicates ZrSiO₄-undersaturated environment, however all the other minerals from the Ring Complex except zircon have no record of such compositional change of the melt. In addition, RD and LD2 are observed in many zircons that have different compositions and included in different lithofacies. We think that these textural features may represent early magmatic events of Cape Ashizuri Ring Complex.