Strontium isotopic age and the origin of including basalt pebbles of the Sashida Limestone in south Izu region, Japan

Abstract

In the previous paper, the geological ages of the Late Cretaceous, Paleocene, late Middle to Late Eocene, Oligocene and early Early Miocene were confirmed at the first time, based on the occurrences of both the age diagnostic larger foraminifera and associated planktonic foraminifera, from the Sashida Limestone included basalt pebbles, Sashida, Kamo-Gun, south Izu Peninsula. Therefore, the Sashida Limestone was made up of clastic carbonate rocks of various ages in essence as well as in outward form. In this paper, the age determination by the microfossil of larger and planktonic foraminifera was re-investigated by the Sr isotopic age determination from the same microfossil sample materials of the Sashida Limestone in order to get the age security. As a result, the ages from 69.3 to 71.8 Ma（Late Cretaceous; Maastrichtian）and 28.8 to 33.3 Ma（Early Oligocene；Rupelian）were determined and supported the ages of foraminifera partial. While, analysis of chemical composition of the basalt pebbles was investigated in order to estimate the original generation tectonics. As a result, the basalt pebbles showed a unique composition of high magnesium and high alkali, similar to basalts of the lower Yugashima Group and were assigned to the alkali basalt series, which were thought to have emerged in the very early stage of the Shikoku Basin spreading. The antecedent of the Izu Peninsula was defined as the Izu landmass from the previous paper, which was inferred the lateral mobility toward east from the spreading, and the last, bordered on the north by the Honshu.