Magma mixing model of the Tertiary granitic rocks of the southern Kyushu

The Miocene granitic rocks in the southern Kyushu were analyzed by the INAA and gammaray counting for 17 trace elements, and by wet chemistry for major elements. Based on unusual differentiation trends of Na2O and Th/U, the Th/U-K2O/Na2O diagram is proposed as a clue to study of the mixing process between two differently originated magmas. Present results show that all the differentiation trends direct towards a fixed point on that diagram, the Th/U and K2O/Na2O ratios of which are 6.5 and 1.7 respectively. This unfamiliar converging differentiation trend is concluded to indicate the mixing of two magmas, one of which has the composition of the fixed point. The point is explained to represent the minimum melt by partial melting in the Qtz-Or-Ab-An-H2O system, based on comparison of its K2O/Na2O ratio with that of the Sierra Nevada batholith and melting experiments. The similarity of Th/U ratio between the fixed point and shales or sandstones of the Shimanto belt suggests that they have been source materials to be partly melted. The differentiation trends of the other elements are also explained by the mixingmodel.