Plutonic and metamorphic rocks of Massif-A in the Yanato Mountains, East Antarctica

Geological and petrographical observations of crystalline basement rocks occurring in Massif-A of the Yamato Mountains are briefly described as a report of the geological works of the 21st Japanese Antarctic Research Expedition in 1979-1981. The basement rocks are composed of various kinds of acid to intermediate plutonic rocks and subordinate amounts of acid, basic and calcareous metamorphic rocks. The metamorphic rocks such as banded two-pyroxene biotite gneiss, two-pyroxene metabasites and calc-silicate gneiss, and the plutonic rock such as massive quartz syenitic rock make up the oldest rocks, followed by intrusions of the quartz syenitic charnockites and the subsequent quartz monzonite. The latest plutonism and related metamorphism are represented by intrusion of the granitic migmatite, with biotite gneiss and amphibolite paleosomes, into the quartz monzonite, and by development of the migmatitic biotite gneiss with granitic neosomes, though their occurrence is local. The rocks composing Massif-A could be grouped into two stages of plutonic and metamorphic rock association mainly on the basis of their modes of occurrence; the older association includes the rocks from the oldest ones to the quartz monzonite, and the younger association all the migmatitic rocks. Mineral assemblages of the metamorphic rocks enclosed in the plutonic rocks of the both associations suggest that the older plutono-metamorphic activity would have taken place under the granulite facies condition, while the younger activity under the amphibolite facies condition.