Thermometry of Quartz Mylonites

We have investigated natural quartz mylonites from a well-studied area offering ‘laboratory conditions in nature’ to test the applicability of the Ti-in-quartz geothermometer [Wark and Watson, 2006] on rocks deformed in a temperature range from ~280-700 °C. Quartz mylonite samples were systematically collected across the Tonale fault zone (northern Italy) [Stipp et al., 2004]. The eastern part of the fault zone was contact metamorphosed by the synkinematic intrusion of the Adamello pluton [Stipp et al., 2004]. Across the ~800 m-wide ductile shear zone, this resulted in a synkinematic temperature gradient from ~280 °C at the frictional-viscous transition to ~700 °C at the pluton contact as derived from conventional thermobarometry [Stipp et al., 2002; Werling, 1992]. Deformation microstructures display the entire range of dynamic recrystallisation in quartz: bulging recrystallisation (BLG), subgrain rotation recrystallisation (SGR), and grain boundary migration recrystallisation (GBM) [Stipp et al., 2002]. We have analyzed 18 samples from the shear zone on the SHRIMP-RG (reverse geometry) ion microprobe at Stanford University. We demonstrate that the Ti-in-quartz geothermometer yields the near-peak temperature of deformation for the mylonites deformed at temperatures above ~500 °C. However, in the mylonites deformed under lower temperature conditions, the previously established Ti concentrations were not reset. We suggest that this is due to the sluggish Ti diffusion rates below 500 °C [Cherniak et al., 2007] and the small volume-% of recrystallised grains. Our results show that GBM promotes the equilibration of Ti in quartz, while quartz mylonites deformed by SGR or BLG will likely yield inherited temperatures. In sum, for low-temperature mylonites (< ~500 °C) the applicability of the thermometer strongly depends on how long the rocks stayed at these conditions and how intense were dynamic recrystallisation processes. Conversely, for high-temperature mylonites the question remains to which extent was the Ti concentration affected by retrogression and recrystallisation during exhumation. Cherniak, D. J., E. B. Watson, and D. A. Wark (2007), Ti diffusion in quartz, Chemical Geology, 236, 65-74. Stipp, M., H. Stünitz, R. Heilbronner, and S. M. Schmid (2002), The eastern Tonale fault zone: a 'natural laboratory' for crystal plastic deformation of quartz over a temperature range from 250 to 700 °C, Journal of Structural Geology, 24, 1861-1884. Stipp, M., B. Fügenschuh, L. P. Gromet, H. Stünitz, and S. M. Schmid (2004), Contemporaneous plutonism and strike-slip faulting: A case study from the Tonale fault zone north of the Adamello pluton (Italian Alps), Tectonics, 23, TC3004, doi:3010.1029/2003TC001515. Wark, D. A., and E. B. Watson (2006), TitaniQ: a titanium-in-quartz geothermometer, Contributions to Mineralogy and Petrology, 152, 743-754. Werling, E. (1992), Tonale-, Pejo- und Judicarien-Linie: Kinematik, Mikrostrukturen und Metamorphose von Tektoniten aus räumlich interferierenden aber verschiedenaltrigen Verwerfungszonen, ETH, Zürich.