MAJOR-ELEMENT AND TRACE-ELEMENT CONSTRAINTS ON THE PETROGENESIS OF A FAULT-RELATED PSEUDOTACHYLYTE, WESTERN BLUE RIDGE PROVINCE, NORTH-CAROLINA

A newly identified 6 cm thick, zoned pseudotachylyte from the western Blue Ridge province of North Carolina is interpreted to be the result of episodic melting on a reactivated brittle fault, previously active in the ductile regime. Kinematic indicators in cataclasites associated with the pseudotachylyte indicate top-to-the-northeast shear, whereas kinematic indicators in mylonites indicates top-to-the-northwest shear. Higher concentrations in the pseudotachylyte of Al2O3, K2O, FeO and depletion in SiO2 are explained by the total fusion of micas and partial fusion of K-feldspar (almost-equal-to 60%), leaving quartz and refractory phases as residual. Enrichment of the pseudotachylytes in Ba2+, Rb+, Sr2+ and Eu2+ provide additional evidence for the preferential melting of K-feldspar. The trace element signature of the pseudotachylytes (similar REE patterns, Ba/K,K/Rb, Ba/Rb, Zr/Hf, Ti/Cr ratios) and entrained lithoclasts identify the source of the melt as cataclastically deformed mylinite of the hangingwall. The similarity of the REE patterns in the pseudotachylytes and the source rock is explained by the entrainment and/or dissolution of REE-bearing phases in the melt in proportion to its volume. These data indicate that coseismic flash melting is distinctly different from equilibrium partial melting and that trace elements can be used to identify the source rock and distinguish those phases that melt from those that remain residual.