Magmatic evolution of the calk-alkaline rock in northern Kita-Hakkoda volcanic group, northern Honshu, Japan

Abstract:

Magmatic evolution in the northern Kita-Hakkoda, northern Honshu, Japan, was investigated from temporal variations of whole-rock chemistry and petrographic characteristics. The temporal variations are based on stratigrahic relation observed by geological survey and geomorphology. The volcanic activity of northern Kita-Hakkoda occurred about 0.4-0.2Ma. Rocks from northern Kita-Hakkoda are classified into two series on the AFM and the Miyashiro diagrams: calc-alkaline and tholeiite series. Calc-alkaline series rocks draw liner trends on Harker’s diagrams of compatible elements (Ni, Cr, MgO), whereas the trends of tholeiitic series are convex downward. Some calc-alkaline rocks have disequilibrium mineral assemblages, e.g., coexistence of magnesian olivine and embayed quartz. The calc-alkaline rocks exhibit mineralogical features that indicate open system processes, e.g., coexistence of reversely zoned pyroxenes and normally zoned pyroxenes, dusty zoning in plagioclase. In contrast, no evidence for open system process is recorded in phenocrysts in the tholeiitic rocks. Previous studies (Sasaki et al.,1985; Ohba et al.,2009) accounted for the chemical variation of tholeiitic magma by crystallization differentiation. The calc-alkaline trend intersects the tholeiitic trend on Miyashiro diagram; therefore, the mafic endmember is tholeiitic basalt magma with a composition of the intersection. In this study, we conclude that the calc-alkaline series rocks were formed by magma mixing. Compared with the stratigraphic order, the calc-alkaline rocks changed in composition from SiO₂ 60wt% to SiO₂ 50wt% with a stratigraphic order, but the compositional variation accompanied a small fluctuation. Because the magma compositions of calc-alkaline magma were influenced by magma mixing, the chronological chemical variation was caused by temporal variation in mixing ratios of endmember magmas.