Contrasted petrological relations between tholeiitic and calc-alkalic series from Funagata volcano,northeast Japan.

Abstract

Funagata volcano, on the volcanic front of northeast Japan arc, is situated at about 35km northwest of Sendai city, Miyagi prefecture. It is made up of three major volcanic bodies, consisting predominantly of lavas with subordinate pyroclastic rocks, of which about 95 vol. % is calc-alkalic andesite and about 5 vol. % is tholeiitic basalt and andesite. Chemical and mineralogical data are presented to show the contrasted petrological relations between tholeiitic and talc-alkalic series from Funagata volcano. The different compositional trends between both series in the 1/K2O-oxides diagram suggest that the calc-alkalic magma was not produced by continuous fractional crystallization of the tholefitic magma. Analysed mineral phases in the tholeiitic rocks display systematic compositional variations, obeying the process of simple fractional crystallization, On the other hand, mineralogical evidences of some calc-alkalic rocks, such as disequilibrium phenocryst assemblages and reverse zoning of phenocrysts, indicate that magma mixing may have played a key role in their formation. Calculation of magmatic temperatures by pyroxene geothermometer indicates that the tholeiitic magmas crystallized under about 1180-1000°C, whereas the calc-alkalic magmas did under almost constant 950°C. The liquidus temperatures of plagioclase at 1 atm were estimated to be nearly the same range (1070-1210°C) between tholeiitic and calc-alkalic series. The difference between the calculated 1 atm liquidus temperature and pyroxene equilibration temperature was supposed to be ascribed to the difference of H2O contents in magmas at the time of phenocrysts crystallization. Evaluated H20 contents on the basis of experimental data of Sekine et al. (1979) range from 0.5-1.6 wt. % in the tholeiitic magma and 2.0-3.8 wt. % in the calc-alkalic magma. The extremely low H2O contents of the tholeiitic magma imply that the primary magma of the tholeiitic series is unlikely to have originated by hydrous partial melting of mantle peridotite.