EVIDENCE FOR THE FORMATION OF PSEUDOTACHYLYTE IN LEWISIAN GNEISSES, MOINE THRUST HANGING WALL, SCOTLAND

Optical and electron petrography have been used to identify evidence for at least four stages of deformation in the Lewisian gneiss, located 71 miles northwest of Inverness, Scotland. Best-preserved is the pervasive, gently east-dipping gneissosity that overprints the migmatite complex. These fabrics are defined by bands of upper amphibolite facies mineral assemblages, including aligned amphibole needles and K-feldspar. Hornblende porphyroblasts in the gneiss are locally replaced by biotite, believed to be an example of reaction softening. The biotite is (in turn) kinked, indicating strain persisted during retrograde metamorphic conditions. These structures are overprinted by a gently east-dipping fracture set locally connected by a steeper fracture set. The fractures occur both within and at the contact between sheets of amphibolites. The fractures are filled by 50 micron to cm thick layers of very fine-grained black material. SEM observations show this material to contain randomly oriented, angular clasts of K-feldspar, plagioclase, quartz and amphibole set in a matrix dominated by randomly oriented <1 micron diameter grains of chlorite, quartz, albite and euhedral magnetite. The variation in the concentration of magnetite gives the matrix a banded appearance, and suggests that this matrix was initially chemically zoned. Rings of chlorite, albite and quartz isolate the various clasts from the magnetite-rich matrix, suggesting a reaction relationship between the matrix and clasts. We interpret these features to reflect the former presence of a pseudotachylyte that filled open fractures with flow-banded glass. These structures are cut by later fractures filled with K-feldspar. Where the veins intersect plagioclase grains, they are filled locally by albite. This control of host minerals on the composition of the fracture-filling minerals shows a local control of mineralogy, and suggests closed system behavior even as the gneisses were exhumed to very shallow depths. The east-west shortening and reverse faulting indicated by these structures open the possibility that this deformation was associated with the far-field effects of the Moine thrust.