GRANITE INTRUSION IN ISLAND ARC SETTING: CONSTRAINTS FROM AMS AND ROCK FABRICS ON THE YAKUSHIMA PLUTON, SW JAPAN

The Yakushima granite pluton in the Outer Zone of Southwest Japan is located in between the central axis of the Ryukyu trench in the east, that marks boundary between the subducting Philippine Sea plate and overriding Eurasian plate, and an active volcanic arc fringing the eastern margin of a spreading back-arc basin, namely Okinawa trough. Zircon U-Pb and fission track ages indicate that the Yakushima pluton segregated, ascended and emplaced in a short period during 16-13 Ma into the Tertiary-Cretaceous accretionary prism sediments of the Shimanto group. Patterns of magmatic flow fabrics determined for each 1 km x 1 km grid inside the 400 square km large pluton had been suggested to imply internal circulation of the pluton due to drag along the contact surface during its rise (Anma, 1997). This paper further compares the rock fabrics and AMS fabrics to deduce the sense of shear in outcrop scales. About 70 % of the 441 studied specimens have a magnetic susceptibility less than 250 x 10E-6 (SI). These specimens usually display the Pj - Km characteristics of biotite-bearing paramagnetic granites, whereas the rest of specimens have features indicative of a ferromagnetic contribution. Directions of [010] face of euhedral orthoclase megacrysts were used to calculate the shapes and intensities of magmatic flow fabrics and compared with orientations of AMS fabrics for selected outcrops where the susceptibilities and their anisotropies indicate paramagnetic contributions. Magnetic planes are usually plotted to the same point maxima when the distribution of [010] faces on a stereonet is symmetrical implying uniaxial deformation. In contrast, magnetic planes become sub-parallel to the X-Y planes of orthoclase alignment when the distribution of [010] faces is asymmetrical and imply shear deformation. The shear directions deduced from the obliquity of two planar structures and asymmetrical distribution of [010] faces are concordant and provide a good shear sense indicator for the post-kinematic granite with no visible deformation structures. Our new data implies that the Yakushima pluton developed strong planar structures and then additional flow at the emplacement level folded and sheared them along the contact. Emplacement mechanism for the Yakushima pluton appears to be rather complex than previously suggested.