RARE-EARTH ELEMENT DISTRIBUTION IN ROCKS OF DIORITE-GRANITOID PLUTIONS OF URUP ISLAND, KURILE ISLANDS, AS RELATED TO THE PROBLEM OF THEIR ORIGIN

Abstract

Diorite-granitoid plutons are among the principal structural components of continental orogenic belts but are considerably less widespread on present island arcs. On Urup Island, chiefly on its Pacific side, there are five diorite-granitoid plutons, ranging in size from 1 to 10 km2. In chemical composition, the diorite-granitoid plutons of Urup Island are assigned not to the calcalkalic rock series, like those of the Aleutians, but chiefly to the tholeiitic. Their rocks consist of plagioclase, potassic and potassisodic feldspars ortho- and clinopyroxenes, amphibole and quartz. The accessory minerals are spatite, sphene and zircon. The most silicic rock varieties commonly contain xenoliths of the earlier, more mafic rocks, mostly diorite and tonalite. The concentrations of rare-earth elements (REE) in three principal rock types of a pluton in the Galinka River basin were determined by the isotopic dilution method to within ±1 percent. The results are given. The authors attempt to estimate the possible course of processes that formed the intermediate and silicic, plutonic rocks of Urup Island on the basis of the assumption that they resulted from the fractional crystallization of principal rock-forming minerals from basalt melt equivalent in chemical composition to what Ewart describes as average island-arc tholeiitic basalt.