The Effect of Recrystallization on Titanium Concentrations in Quartz, an Example from New Zealand's Alpine Fault

Results from crystallization experiments show a clear relationship between temperature, pressure, and trace concentrations of Ti in quartz. In theory, conditions associated with quartz recrystallization in deformed rocks can also be obtained using this relationship, however, doing so is complicated by uncertainties associated with Ti activity and the degree to which Ti concentrations have reached equilibrium values during dynamic recrystallization. We present 1,000 Ti-in-quartz SIMS analyses from two transects across New Zealand's Alpine Fault mylonite zone. Median Ti concentrations in a central region of the orogen (Gaunt Creek) are generally 2-3 ppm, regardless of distance (up to 1400 m structural distance) to the fault. Small grains (10-30 microns) generated during deformation show some resetting or partial resetting to 0.5-2 ppm. Further south, near Haast, median Ti concentrations are 1.5 ppm near the fault and increase systematically to 4 ppm away from the fault. This pattern is consistent with a previously documented inverted metamorphic field gradient formed by high-strain juxtaposition of rocks of different metamorphic grade. Quartz grains in quartzofeldspathic schists have similar Ti concentrations as grains in meta-basic schists. Ti-in-quartz values in metacherts are more variable, with about half the metacherts showing higher, lower, or more scattered values relative to neighboring schists. Ti-in-quartz concentrations in exposed rocks on the whole appear little-changed by intense Alpine Fault related deformation. A key exception is recrystallized quartz from a micro-scale C' shear band in the central part of the orogen ( 800 m structural distance) that contains markedly low Ti values (0.5-0.8 ppm) indicating resetting during a late stage of localized low-temperature deformation. An absence of such low Ti concentrations closer to the fault suggests that the outer part of the fault zone continued to experience deformation involving dynamic recrystallization of quartz late in the exhumation history when more fault-proximal parts of the fault zone were deforming by brittle processes.