Using a Ti-in-Zircon Thermometer to Determine Crystallization in a Granite and Eruption in a Coeval Silicic Tuff

In the extensional Chocolate Mountains area, the silicic Black Hills Tuff (BHt) and granite of Mount Barrow (MBg) yield zircon U/Pb ages of 23.4 ± 0.2 Ma and 24.2 ± 0.2 Ma, respectively, with an inherent uncertainty of 1 Ma. Both units have similar modal mineralogy, nearly identical lower-end HREE patterns, and are within 20 miles of each other. BHt is part of the regional silicic tuff sequence (STS), which contains several tuffs yielding different K-Ar ages. Within error, all tuffs yield zircon ages of ~23.5 ± 4 Ma. BHt is tentatively subdivided into four units; a basal, weakly to moderately welded ash-fall tuff varying in color from white to pink and containing around 5% lithic clasts; a second unit comprised of strongly welded, poorly and possibly only locally exposed red tuff with pumice fragments and lithic clasts; a third unit locally called “30 foot tuff” by Tarshis (1989), although regional thickness is believed to be greater; and a fourth unit called the “70 foot tuff,” although regional thickness is unknown.

Ti temperatures recorded in zircons were 687 - 707°C for BHt rims, and 764 - 842°C for cores. MBg rim temperatures were determined to be 668 - 788°C and cores were 725 - 749°C. Activities for silica (0.8) and titanium (0.6) were estimated based on modal mineralogy of samples, and temperatures were calibrated with mineral saturation temperatures and wet granite solidus in mind. Uncertainties associated with the technique are between ±12° at 600°C and ±11° at 750°C. Other tuffs in the STS yield average temperatures roughly between 750°C and 850°C for either rim or core, and none drop as low as BHt. Unlike the higher temperature silicic tuffs, BHt was at or near its wet solidus temperature when a destabilization, either decompression or heat injection, occurred, causing its eruption.