GLASSY PSEUDOTACHYLYTE VEINS FROM THE FUYUN FAULT ZONE, NORTHWEST CHINA

Glassy pseudotachylytes occur in granitic mylonites from the Fuyun fau lt zone, northwest China. Powder X-ray diffraction data indicate that the fine-grained matrices of the Fuyun pseudotachylytes vary from almo st crystalline to complete glass (a few to 90 wt% glass), Powder X-ray diffraction patterns also indicate that clasts in the glass type pseu dotachylyte veins mostly consist of quartz grains. Microlites displayi ng quenched or rapid-cooling textures, vesicles and amygdules, flow st ructures, and rounded fragments are also common in the Fuyun pseudotac hylytes. The average bulk chemical compositions of these pseudotachyly tes are similar to those of the host rocks. But the glass matrix has a low SiO2 component which is 5-15 wt% lower than that of the host rock s. The pseudotachylytes contain 2-3 wt% water (H2O). Using experimenta l data which relate the solubility of water to pressure, approximately 400 bars lithostatic pressure corresponding to approximately 1.5 km d epth, of pseudotachylyte formation is estimated. A minimum temperature estimate of 1450-degrees-C can be estimated due to the presence of th e pure SiO2 glass. The X-ray diffraction and chemical data indicate th at the Fuyun pseudotachylytes formed during seismic faulting at shallo w depths by selective melting under water-saturated conditions rather than by total melting or by crushing of the host rocks along fault sur faces.