Petrographic analysis of undeformed and deformed pseudotachylite from the Seaforth Head Region of the Outer Hebrides Thrust zone, Isle of Lewis, Scotland

Abstract

Undeformed and deformed pseudotachylite of the Seaforth Head region in the Outer Hebrides Thrust zone was analyzed by both light and SEM microscopy, and by x-ray analysis. An examination of the pseudotachylite and its microstructures was performed. Undeformed pseudotachylite which appears to be optically isotropic, consists of a fine, equant-grained matrix and porphyroclasts of plagioclase, quartz, and amphibole when examined using SEM microscopy. The deformed pseudotachylite's compositionis similar to that of the undeformed but contains a fabric defined by compositional layering and other deformational evidence. The deformed pseudotachylite has two distinct types of pseudotachylite bands-- Area A and Area B. Area A contains more porphyroclasts than Area B and Area B contains more K-feldspar. Compositional analysis was performed on the pseudotachylite indicated that the plagioclase ranged in composition from An40 to An65 with clustering of plagioclase composition for the deformed at An26 and An36 whereas the undeformed was found at An34 and An64. Calcic amphibole was found in both the deformed and undeformed pseudotachylite. The evidence found within the pseudotachylite supports the friction melt theory of formation caused by cyclic seismic activity.