MICROLITE MORPHOLOGY AND CHEMISTRY IN PSEUDOTACHYLITE, FROM THE FUYUNFAULT ZONE, CHINA

Microlites in pseudotachylite veins from the Fuyun fault zone, northwe st China, can be divided into four morphological groups: simple, skele tal, dendritic, and spherulitic. From the margin to the center of micr ocrystalline and microlitic pseudotachylite veins, the size of single crystals or crystal fibers becomes progressively larger from a few mum to as much as 15 mum. The morphologies of microlites vary from simple shapes, such as acicular and globular forms, to more complex forms su ch as skeletal, dendritic, and coarse spherulitic forms, In glassy pse udotachylites, only simple and fine spherulitic microlites, generally smaller than 5-6 mum, 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, grossu lar garnet, biotite, hornblende and some magnetic minerals. The biotit e and hornblende microlites have a variable Ti content, generally high er than that of biotite and hornblende fragments from the country rock s. The wide variety of morphologies and textures of microlites within the pseudotachylite, the presence of sanidine, anorthoclase, An-high p lagioclase, pyroxene, and Ti-high biotite and hornblende indicate that these microlites are primary crystals formed from rapid cooling or qu enching of a melt.