An AFC (assimilation and fractional crystallization) process as the petrogenesis of andesites from the Pliocene Myojin-iwa Formation, the back-arc side of the Northeast Japan. Combined major-and trace-element and Sr-Nd isotope constraints.

Abstract

Andesitic rocks, of the Pliocene Myojin-iwa volcanic field, in the northern part of Niigata Prefecture, Northeast Japan, show transitional chemical characteristics between typical calc-alkaline and tholeiitic series. The most primitive basaltic andesite among these andesitic rocks (the Myojin-iwa Formation) has geochemical characteristics similar to those of other Pliocene-Quaternary basaltic rocks from the back-arc side of the NE Japan, and has initial Sr isotope ratio slightly higher than those of the latter basaltic rocks.Andesites from the Myojin-iwa area show SiO2 contents ranging from 53.3% to 62.8, and initial Sr and Nd isotope ratios vary from 0.70324 to 0.70378 and 0.512791 to 0.512926, respectively. The initial Sr isotope ratios display positive correlation with SiO2, and their initial Nd isotope ratios gradually decrease with increasing SiO2. This suggests that the genesis of andesites from the area can not be attributed to simple fractional crystallization of the primary basaltic magma, but to an assimilation and fractional crystallization (AFC) process. Based on petrographical evidence, major and trace element compositions and Sr-Nd isotope systematics, the Paleogene granitoid rocks constituting the upper crust beneath the Myojin-iwa and surrounding areas are the possible candidates for assimilants. An AFC model using the granitoid rocks as the assimilant can successfully reproduce the chemical variations of the andesites from the Myojin-iwa Formation by assuming the r values (ratio of the rate of assimilation to the rate of fractional crystallization) of less than 0.2.