栃木県茂木町北方地域に産する第三紀高TiO₂ソレアイト

抄録

Tholeiitic basalts characterized by very high TiO₂ contents have been found in the northeast Japan arc. These basalts (the Motokozawa basalts) occur as lavas and minor dikes in the lower Miocene Motokozawa Formation which is distributed in the eastern part of Tochigi Prefecture, Northeast Japan.
The Motokozawa basalts are uniform, non-porphyritic and subophitic in texture. The essential constituent minerals are plagioclase, clinopyroxenes (augite, ferroaugite, subcalcic augite and pigeonite) and Fe-Ti oxides. Fe-Ti oxides occur sporadically as euhedral crystals (3.8-6.4 modal%) and also as very fine grained crystals in the mesostasis. The Motokozawa basalts are high alkali tholeiite or quartz tholeiite according to the classification of Kuno (1968b) and Yoder and Tilley (1962), respectively.
The quartz-normative tholeiites having high TiO₂, FeO^* (FeO+0.9 Fe₂O₃) and low K₂O contents are predominantly found in the east Pacific Ocean floor, though less abundantly in other ocean floors.
Discussions and considerations on the genesis of ocean floor tholeiites with high TiO₂ and FeO_* contents suggest that the Motokozawa basalts may not be produced by the fractional crystallization from the primary magma having low TiO₂ and K₂O contents. One of the reasonable explanations for the genesis of the Motokozawa basalts is the fractional crystallization from the primary TiO₂-rich, K₂O-poor magma generated in the upper mantle which had been previously enriched in Ti and depleted in K through a preceded partial melting under hydrous conditions.