Ultramafic rock in the hornblende porphyrite dike of the Tamba Belt, Southwest Japan

Abstract

Ultramafic rock is found associated with the hornblende porphyrite dike (intermediate rockwith SiO2 53.39% and MgO 9.09%) which has intruded shale of late Jurassic age in Tamba Belt.It is mainly composed of euhedral to subhedral Mg-rich olivine and anhedral to subhedral brown Ca- amphibole with subordinate amounts of clinopyroxene, phlogopite, chromite, and etc.According to the classification after IUGS (1973), the modal composition indicates hornblendeperidotite class. Olivine is mostly replaced by serpentine, brucite, opaque iron oxide. Brown Ca- amphiboles have pargasitic compositions. Clinopyroxenes are diopsidic and are rich in Cr. According to the textural features, this rock can be called olivine-clinopyroxene orthocumulatepossessing well-developed interstitial amphiboles. Field and petrol ogical studies suggest that theultramafic rock has cognate relation to the hornblende porphyrite and was formed under thewater-rich condition.