Microstructure and geochronology of pseudotachylite from the Hamm Peak, East Antarctica, and its geological significances

The pseudotachylite in granulite facies granitic gneisses from the Hamm Peak, southwestern Prydz Bay, East Antarctica, occurs along the east-west-trending ductile-brittle shear zone. The characteristics of microstructure show that the pseudotachylite was formed by the frictional-melt during the rapid faulting along the paleoseismic zone. This inference is supported by the common presence of spherulites and different morphological microlites, such as skeletal, dendritic, acicular and globular in the matrix of pseudotachylite. There exist two kinds of microlite mineral assemblage. One consists mainly of hyperite and plagioclase, which developed in the northeastern part of the shear zone. The other consists of biotite, plagioclase, alkali feldspar and quartz, etc, which developed in the southwestern part of the shear zone. The occurrence of different kinds of microlite mineral assemblage indicates the differences of tectonic surrounding and stress distribution along different parts of the shear zone. Moreover, the presence of aluminous-rich hyperite may indicates the relatively high temperature and high pressure in the ambient physical condition during the pseudotachylite formation and crystallization afterwards, i.e., under the granulite facies conditions. The K-Ar age of bulk matrix of pseudotachylite is 878.1±16.8 Ma. Bulk 40Ar/39Ar step-heating release spectrum gave the varying ages mainly from 925 to 626 Ma. Combined with the regional comparison, we conclude that the pseudotachylite formed during the Grenvillian tectonic events.