Glassy pseudotachylyte veins from the Fuyun fault zone, northwest China
Abstract
Glassy pseudotachylytes occur in granitic mylonites from the Fuyun fault zone, northwest China. Powder X-ray diffraction data indicate that the fine-grained matrices of the Fuyun pseudotachylytes vary from almost crystalline to complete glass (a few to 90 wt% glass). Powder X-ray diffraction patterns also indicate that clasts in the glass type pseudotachylyte veins mostly consist of quartz grains. Microlites displaying quenched or rapid-cooling textures, vesicles and amygdules, flow structures, and rounded fragments are also common in the Fuyun pseudotachylytes. The average bulk chemical compositions of these pseudotachylytes 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 rocks. The pseudotachylytes contain 2-3 wt% water (H2O). Using experimental data which relate the solubility of water to pressure, approximately 400 bars lithostatic pressure corresponding to approximately 1.5 km depth, of pseudotachylyte formation is estimated. A minimum temperature estimate of 1450°C can be estimated due to the presence of the pure SiO2 glass. The X-ray diffraction and chemical data indicate that the Fuyun pseudotachylytes formed during seismic faulting at shallow depths by selective melting under water-saturated conditions rather than by total melting or by crushing of the host rocks along fault surfaces.
- Publication:
-
Journal of Structural Geology
- Pub Date:
- January 1994
- DOI:
- 10.1016/0191-8141(94)90019-1
- Bibcode:
- 1994JSG....16...71L