Abstract

HYBRIDIZATION AND SHEARING IN BANDED GNEISSES NEAR PHILADELPHIA

BY ELIZABETH ARMSTRONG

The area under discussion extends for approximately 30 miles along the strike of the pre-Cambrian gneiss of the Piedmont Plateau from Trenton, New Jersey, to the Delaware County line near Bryn Mawr, Pennsylvania.

Injection of felsic material into a plagioclase amphibolite has produced a banded hybrid gneiss which has been subsequently sheared as evidenced by vertical, winding mylonite zones west of Wissahickon Creek and by schistose zones to the east. The character of the injection varies from magmatic intrusion of bodies showing well-defined contacts to hydrothermal penetration. The following reactions are observable in the mixed types: biotite replacing hornblende; plagioclase becoming more sodic with formation of epidote in enclosed laths; potash feldspar replacing plagioclase; quartz, potash feldspar, and plagioclase replacing hornblende and biotite.

Cataclastic deformation of the mixed rock has resulted in the trituration of all the minerals except, . . .

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