Microlite Morphology and Chemistry in Pseudotachylite, from the Fuyun Fault Zone, China

Microlites in pseudotachylite veins from the Fuyun fault zone, northwest China, can be divided into four morphological groups: simple, skeletal, dendritic, and spherulitic. From the margin to the center of microcrystalline and microlitic pseudotachylite veins, the size of single crystals or crystal fibers becomes progressively larger from a few µm to as much as 15 µm. The morphologies of microlites vary from simple shapes, such as acicular and globular forms, to more complex forms such as skeletal, dendritic, and coarse spherulitic forms. In glassy pseudotachylites, only simple and fine spherulitic microlites, generally smaller than 5-6 µm, occur. These microlites have a varied mineralogy but are mainly made up of plagioclase (An20-70), and alkali feldspar, including sanidine and anorthoclase, as well as clinopyroxene, grossular garnet, biotite, hornblende and some magnetic minerals. The biotite and hornblende microlites have a variable Ti content, generally higher than that of biotite and hornblende fragments from the country rocks. The wide variety of morphologies and textures of microlites within the pseudotachylite, the presence of sanidine, anorthoclase, An-high plagioclase, pyroxene, and Ti-high biotite and hornblende indicate that these microlites are primary crystals formed from rapid cooling or quenching of a melt.