Abstract
Volcanic rocks of middle-upper Miocene age are widely distributed in the Tomari district, eastern part of the Shimokita Peninsula, Northeast Japan. High magnesian tholeiitic basalts in the Tomari volcanics are found as pillow lava and dikes These basalts are characterized by high Ni (47-276ppm), Cr (67-1044ppm) contents and low FeO*/MgO ratios (0.78-1.87). The bronzite bearing augite olivine basalt has very high MgO, Ni and Cr contents (11.70%, 276ppm and 1044ppm respectively), and its magnesian olivine phenocrysts have a narrow compositional range(Fo85-91) and high NiO content(0.21-0.32%). The basalt also contains chromite as inclusions in the olivine phenocrysts and rarely as isolated grains in the groundmass. Fe-Mg, Ni-Mg partitionings of the basalt suggest that the phenocrysts of magnesian olivine (Fo89.91 respectively) are in equiliblium with bulk rock composition.
These petrological features strongly indicate that the bronzite bearing augite olivine basalt has primitive nature and may represent the primary tholeiitic magma generated in the upper mantle.
