ZONATION OF TITANITE FROM THE NOTCH PEAK (JURASSIC) AND LITTLE COTTONWOOD (OLIGOCENE) GRANITES, UTAH, PROVIDE CLUES TO MAGMATIC AND HYDROTHERMAL HISTORIES

Titanite is an accessory mineral in many granites. Due to its incorporation of a wide variety of trace elements and low elemental diffusion rates, titanite is useful in the study of petrogenetic processes.

Titanite grains in the Notch Peak granite have 3 types of zonation. Primary magmatic titanite contains prominent sector and oscillatory zones. In BSE images, the sector zones are bright, while oscillatory zones alternate between light and dark bands. BSE bright zones have relatively high LREEs (0.02-0.03 apfu) and low F (0.03-0.05 apfu). Some grains contain secondary, partially replaced titanite, as well as interstitial extensions. It grew on irregular dissolution surfaces and contains significantly lower LREEs (0.01-0.02 apfu) and higher F (0.06-0.08 apfu) consistent with precipitation from a fluid. Locally, titanite was partially to completely replaced by fine-grained quartz, ilmenite, magnetite, and rutile–where oxidizing fluids reacted with it. This is a third phase in the history of titanite in the pluton. Primary titanite crystalized ~166 Ma while secondary titanite crystalized ~165-157 Ma, based on U-Pb dating.

The Little Cottonwood Stock titanite typically has primary interiors with patchy zoning and abundant inclusions of apatite and rounded ilmenite (exsolved to rutile/anatase and magnetite) mantled by sector and oscillatory zoned titanite. From this, we infer that ilmenite precipitated first in a lower fO₂ magma which mixed with a higher fO₂ magma causing the dissolution of ilmenite, and stabilization of titanite that crystalized in irregular patches around it. Mantles, with sector and oscillatory zones, then precipitated on the patchy cores. These distinctive mantled titanite grains are found across most of the pluton, suggesting a large continuous magma reservoir existed. Later, secondary titanite formed on wavy resorption surfaces as thin rims and interstitial extensions. These zones have low LREE and high F suggesting they crystallized from a late to post-magmatic fluid. (Later oxidation and titanite destruction are less prominent in the Little Cottonwood stock than in the Notch Peak granite.) U-Pb dates indicate that magmatic titanite grew ~31.8 Ma and secondary titanite is as young as 29 Ma suggesting the occurrence of a post-magmatic hydrothermal event like that at Notch Peak.