FABRIC EVOLUTION VIA SOLUTION-PRECIPITATION CREEP IN THE SLATY LISHAN FAULT ZONE, TAIWAN

Textural analysis of slates and quartzites in the Lishan fault zone of central Taiwan suggest that solution-creep was the dominate deformation mechanism active during shearing at shallow crustal levels (< 10 km). The Lishan fault zone defines the boundary between interbedded quartzites and slates of the Hsüehshan Range to the west and the mudstones and pencil slates of the Backbone Range to the east.

Most commonly seen in the slates and only conspicuous in cathodoluminescent (CL) images are dark red metamorphic quartz overgrowths around bright blue and red detrital quartz and albite grains. These overgrowths also occur around detrital muscovite and chlorite-muscovite stacks and are confined to strain shadows indicating that the quartz precipitated in the regions of low strain during deformation. Chlorite-muscovite stacks, detrital grains, and the quartz overgrowths in strain shadows are all truncated by anastomosing folia dominated by muscovite with minor amounts of chlorite. The sharp contacts of the folia against the older grains imply dissolution-precipitation as the mechanism of cleavage generation (e.g. 3Ab + K⁺ + 2H⁺ = Ms + 6Qtz + 3Na⁺). Other common replacement reactions visible via electron backscatter imagery include extensive but arrested replacement of dolomite and albite by ankerite, pyrite by Fe-rich chlorite folia, and calcite by apatite. In the quartzites the quartz grains show undulose extinction in polarized light suggesting intracrystalline deformation. However, the same grains viewed in CL show an early bright blue, followed by medium red, followed by dark mottled red and blue generations of digenetic and metamorphic quartz overgrowths surrounding detrital grains of mixed provenance. In contrast to the slates, these overgrowths in quartzites occur in all directions, albeit are thickest parallel to the maximum elongation direction. In summary, these petrographic observations demonstrate that dissolution-precipitation processes involving both single phase (e.g. quartz) and phase changes dominate fabric development under greenschist facies metamorphic conditions.