Application of titanium-in-quartz thermobarometry to greenschist facies veins and recrystallized quartzites in the Hsüehshan range, Taiwan

摘要

The accuracy, reliability and best practises of Ti-in-quartz thermobarometry(TitaniQ) in greenschist facies rocks have not been established. Toaddress these issues, we measured Ti concentrations in rutile-bearing samplesof moderately deformed, partially recrystallized quartzite and vein quartzfrom the Hsüehshan range, Taiwan. The spread of Ti concentrations ofrecrystallized grains in quartzite correlates with recrystallized grainsize. Recrystallized quartz (grain size ~100–200 μm) thatformed during early deformation within the biotite stability field shows amarked increase in intermediate Ti-concentration grains (~1–10 ppm) relative to detrital porphyroclasts (Ti ~0.1–200 ppm).Fine recrystallized quartz (~5% of the samples byarea, grain size ~10–20 μm) has a further restrictedTi concentration peaking at 0.8–2 ppm. This trend suggests equilibration ofTi in recrystallized quartz with a matrix phase during deformation andcooling. Unlike previously documented examples, Ti concentration in thequartzite is inversely correlated with blue cathodoluminescence. Deformationwas associated with a minimum grain boundary diffusivity of Ti on the orderof 10−22m2 s?1. Vein emplacement and quartzite recrystallizationare independently shown to have occurred at 250–350 °C and300–410 °C, respectively, with lithostatic pressure of 3–4 kbar(assuming a geothermal gradient of 25° km?1), and with hydrostaticfluid pressure. Estimates of the accuracy of TitaniQ at these conditionsdepend on whether lithostatic or fluid pressure is used in the TitaniQcalibration. Using lithostatic pressure and these temperatures, the Thomaset al. (2010) calibration yields Ti concentrations within error ofconcentrations measured by SIMS. If fluid pressure is instead used,predicted temperatures are ~30–40 °C too low.TitaniQ has potential to yield accurate PT information for vein emplacementand dynamic recrystallization of quartz at temperatures as low as~250 °C, however clarification of the relevantpressure term and further tests in rutile-present rocks are warranted.