An unusual ‘crack-seal’ vein geometry

Inclusion bands in quartz that fills extensional veins are considered to be the result of a mechanism named ‘crack-seal’ by Ramsay. A microstructure is decribed here in which chlorite grains, occurring as inclusions arranged in inclusion bands, are oriented with their long axes oblique to both the quartz high-angle grain boundaries and inclusion-band traces. Syntaxial growth of chlorite on wall-rock phyllosilicates may have caused this microstructure. Consequences of this process in the determination of the strain history of extensional veins are discussed.