CRITERIA FOR THE EXISTENCE OF PRE-ISLAND ARC GRANITOID ROCKS IN THE BASEMENT OF KUNASHIR ISLAND

Abstract

The development of the Kurile island arc is though to have begun in the Late Cretaceous. In the Neogene deposits on northern and southern islands of this ridge, including Kunashir, there are well-rounded fragments of granite and granite-gneiss, compositionally atypical of island-arc granitoid rocks in the same region. Chemical analysis of the granitoid rocks exposed on Kunashir Island revealed much compositional similarity between the rocks found at the foot of the Tyatya volcano and on the Lovtsov Peninsula. The Rb-Sr age of the granitoids from the Lovtsov Peninsula, as calculated for biotite with an initial Sr87/Sr86 ratio of 0.704, coincides with the earlier determined Rb-Sr age of the granite boulder at the foot of the Tyatya volcano. The data indicate that in the Kunashir Island basement there are pre-island arc granitoid rocks of Late Triassic-Early Jurassic age.