Abstract
On the basis of the volcanic successions and the petrological data, the activity of Kitamatsuura basalts (KMB) is divided into five stages. Basalts of the first to third stages belong to the alkaline rock series. On the other hand, the fourth and fifth stages rocks are sub-alkaline (tholeiite and calc-alkaline rock series, respectively). In the variation diagrams of the major and trace element chemistry, alkaline and tholeiite rock series follow different paths of trends. This suggests that these two series were derived from different primary basalt magmas. The MORB-normalized incompatible element patterns for KMB are similar to those of within-plate basalts. The source of KMB was relatively homogeneous in trace element and Nd-Sr isotopic compositions. By reference to melting experiments at high pressures, it is suggested that the primary magma of the alkaline rock series was derived by a small degree of partial melting of a source mantle at 10-15 kbar and that that of the tholeiite rock series was produced by a large degree of partial melting of the same source at 5-10 kbar. On the other hand, the petrographical and petrological data indicate that the calc-alkaline rock series was a product of magma mixing.
