PLAGIOCLASE TWIN LAWS AND FABRICS IN 3 SPECIMENS OF ANORTHOSITE

Twinning patterns and petrofabrics of plagioclases are examined in three specimens of anorthosite from the Bushveld Complex, the Quebec Massif, and the Fiskenaesset Complex. Their plagioclases have petrographical characteristics exhibiting their different petrogeneses. In an anorthosite from the Bushveld Complex, plagioclase grains are twinned after the albite-Carlsbad, pericline, albite and Carlsbad laws. Frequency percentage of the albite-Carlsbad and Carlsbad laws reaches 43% Plagioclase grains in the adcumulate layers are developed with their composition plane (010) subparallel to the cumulate plane, whereas those in the heteradcumulate layers are developed with their composition plane (010) subperpendicular to the cumulate plane. In an equigranular anorthosite from the Quebec Massif, plagioclase grains are polysynthetically twinned after the albite and pericline laws with rare examples of the albite-Carlsbad and Carlsbad laws. Frequency percentage of the latter two laws is only 1% together. Some regularities are recognized in the petrofabrics of c-axis and (010) plane. In a calcic anorthosite from the Fiskenaesset Complex, plagioclase grains are polysynthetically twinned, exclusively according to the pericline law or a combination of pericline and albite laws. The pericline law is predominant and reaches 64% and this twinning pattern cleaarly differs from that of the former two anorthosites. © 1979.