Abstract
The outcrop of the Cape Sukoton intrusion, a constituent of Miocene doleritic sill swarm in the northern part of Rebun Island, north Hokkaido, is 1, 600 × 600m wide and more than 240m thick, and dips gently eastward. It can be lithologically divided into three zones; the upper Columnar Joint Zone, middle Banded Structure Zone, and lower Massive Zone. The appearance of columnar joints is remarkable in doleritic rocks of the Columnar Joint Zone. Banded structures and columnar joints are also well developed in the doleritic rocks of the Banded Structure Zone, whereas both banded structures and columnar joints are not observed in porphyrites of the Massive Zone. Based on structural analysis of columnar joints, lithological descriptions of rocks in each zone, and the presence of interlayered sillsediment structure at the boundary between the intrusion and sedimentary rocks, the Cape Sukoton intrusion is infered to have formed in the following main three stage magmatic events. 1) first stage (formation of the Columnar Joint Zone); intrusion of a basaltic magma into wet and poorly consolidated sediments, and melanocratic to leucocratic dolerites and leucocratic andesitic veins were generated during subsequent cooling contraction and crystallization of magma, and columnar joints were formed throughout the body, 2) second stage (formation of the Banded Structure Zone); intrusion of a new basaltic magma into the Columnar Joint Zone and wet poorly consolidated sediments. During cooling of the magma, banded structures composed ostratified vesicular and non-vesicular layers have been formed in melanocratic dolerites, and 3) third stage (formation of the Massive Zone); intrusion of another magma into the Banded Structure Zone, prior to the complete cooling of the Banded Structure Zone. During the third stage, melanocratic dolerites of the Banded Structure Zone might have been brought into the intruding magma, which resulted in the appearance of melanocratic enclaves contained in the Massive Zone.
