Tectonics and Petrograpby of the East Ongul Islana, Lutzow-Holm Bukt, Antarctica

• Main Authors: キザキ コウシロウ, Koshiro KIZAKI
• Format: Report
• Language: English
• Published: Department of Geology and Mineralogy, Hokkaido University 1964
• Subjects: East Antarctica; Ongul; Ongul Island; East Ongul Island; Bukt; Antarc*; Antarctica
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=49; http://id.nii.ac.jp/1291/00000049/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=49&item_no=1&attribute_id=18&file_no=1

The East Ongul Island on the coast of the Lutzow-Holm Bukt, East Antarctica, was investigated from the tectonical and petrographical points of view. The rocks exposed in the island are petrographically classified into the following seven types : 1) Metabasites, 2) Pyroxene gneisses, 3) Hornblende gneisses, 4) Garnet gneisses, 5) Granites, 6) Feldspathic rocks, and 7) Pegmatites. It is verified that the unique structural feature of the island is represented by the isoclinal fold recumbent to the west with the frontal zone in which many small anticlinal as well as synclinal folds occur. The inner core of the main anticline is generally composed of the garnet gneisses with the pyroxene gneisses forming the outer mantle. The hornblende gneisses, which are distributed in front of the recumbent anticline, occur in the same horizon as the pyroxene gneiss, so the former may have been converted from the latter. This is also confirmed by the geological and petrographical observations. Besides, the hornblende gneiss zone coincides with the zone of granite and subsequent microcline pegmatite emplacement. The lineation of the rocks, represented by the undulating foliation, is defined to be a b-lineation parallel to the fold axis, but another lineation with the same character but perpendicular to the fold axis is also found. The fracture systems indicated by the pegmatite and joint patterns were analyzed by tectonic dating. The pegmatite intruded at the earlier folding stage, whereas the joint pattern was formed at the later upwarping stage. The gneisses and granites are classified mainly by the characteristic mafic minerals, and their modes of occurrence as well as the optical mineralogy are described. The optical data of the constituent minerals revealed that there is a tendency that the Fe/Mg ratio of the mafic minerals such as pyroxene, hornblende, biotite and garnet, may increase with acidity of the rocks, from the matabasites to the granites. The anorthite content of plagioclase is restricted within a rather narrow ...