Mechanical twinning of plagioclase in a deformed meta-anorthosite ? the production of M-twinning

Abstract

Mechanical twins on both the Albite and Pericline laws are well developed on an optical scale in basic plagioclase from the Harris meta-anorthosite where the rock is affected by closely spaced shear fractures associated with pseudotachylite formation. In some cases the twinning is accompanied by kinking or intracrystalline fractures. The twins may be periodic and form a ladder structure and sometimes appear to intersect on an optical scale showing structures which very closely resemble those seen in microcline. Both Albite and Pericline twins are lenticular as observed by TEM, but isolated twin tips were rarely seen. No dislocations are associated with the twin tips. The twins in the optically observed ladder structure (when thicker than ∼2μm), have themselves a very fine secondary ladder structure, consisting of periodic (200–600 nm) lenticular twins which impinge on the perpendicular twin wall. Intersecting twins were also observed and where two sets of equally thick twins intersect, M-twinning develops as shown by selected-area diffraction. The twin formation is analyzed in terms of the plagioclase structure and a coherent model for twin nucleation proposed. True twins can develop in plagioclase only if the Al/Si distribution is unchanged or nearly so after deformation. Mechanical twinning is quite easy in low plagio-clases between An100 and about An30 which have P¯1 or I¯1 lattices or domains with I¯1-type structures. Only one kind of nucleus can develop coherently in a single crystal, but it may grow to give either an Albite or a Pericline twin. Periodic nucleation and growth give complex textures. The qualitative variation of the twin energy as a function of the obliquity is given for different twin shapes and degrees of Al/Si order. M-twinning arises mechanically by coherent growth and interference of the strain fields.