PETROLOGY OF SERPENTINE SANDSTONE AS A KEY TO TECTONIC DEVELOPMENT OF SERPENTINE BELTS

The petrology of serpentine sandstone, which is ubiquitous in the serpentine belt, could provide some constraints on petrological characteristics for lost or eroded serpentinite masses. The chemistry of detrital chrome spinel and chrome diopside in particular is a useful indicator of the equilibrium temperature and lithology of the source peridotite. Combined with the petrology of the associated serpentinite masses this could provide the difference in the petrological characteristics, if any, between the lost serpentine and that now exposed in a serpentine belt, and consequently some constraints could be placed on the tectonic history of the serpentine belt itself. In the Mineoka belt, a narrow serpentine belt in central Japan, the serpentinite mass exposed during the Miocene has experienced a lower pyroxene temperature and wider lithology than the serpentinite masses now exposed in the same area. This fact possibly indicates that a vertical slice of the upper mantle peridotite has been uplifted and intermittently protruded since the Miocene. The Mineoka belt is probably placed on part of a fossil transcurrent fault, which had worked in the opening of the Shikoku Basin, of the northern end of the Philippine Sea plate. Along the transcurrent fault the mantle peridotite slices with various petrologic characteristics could have been continuously emplaced in the same way as in the present-day oceanic fracture zone. On the other hand, in the Kamuikotan belt of Hokkaido, northern Japan, the source of a Miocene serpentine sandstone at Furenai has the same petrologic characteristics as the now associated Sarugawa peridotite mass. The Sarugawa mass is a dismembered part of an ophiolite suite uplifted during the collision between the Eurasia plate and the Okhotsk (North America) plate in Miocene times. The Furenai serpentine sandstone was produced in a small sedimentary basin within the Sarugawa peridotite mass during the middle Miocene. Contrary to the Mineoka belt, large vertical uplift movements of the peridotite mass did not occur in the Kamuikotan zone after the Miocene.