CHEMICAL AND TEXTURAL ANALYSIS OF ENCLAVE TITANITE IN THE OLIGOCENE LITTLE COTTONWOOD GRANITIC STOCK TO DETERMINE CRYSTALLIZATION HISTORY, UTAH

The origin of the mafic enclaves in the Oligocene Little Cottonwood (LC) Stock, Utah, was originally thought to be cumulate fragments ripped from the walls of the crystallizing pluton. In contrast, enclaves are interpreted to have formed by magma mixing in many other plutons. Here we use titanite (Ttn), an accessory mineral common in granites, to evaluate the role of magma mixing. Ttn incorporates a wide variety of trace elements and shows striking textural and zoning patterns due to temperature fluctuations, oxygen availability, and magma composition. In LC enclaves, grains range from being distinctly euhedral to anhedral, poikilitic grains precipitated around feldspar and rare quartz. Oscillatory, sector, and mottled zoning, with abundant ilmenite amoeboids (exsolved to rutile/anatase) common in mottled cores, were observed in all enclave samples. Magnetite is rare within Ttn but is common in the pluton itself. Euhedral grains with mottled cores and oscillatory and/or sector zones are also found in the LC granite, but poikilitic grains are nearly exclusive to enclaves. Ttn grains also tend to be larger in enclaves (150-3000 μm vs. 100-2000 μm) and more abundant.

Based on these observations, we propose that a hotter, reduced, TiO₂-rich—and thus ilmenite-rich—mafic magma intruded and mixed into an oxidized, magnetite-bearing felsic magma. This caused ilmenite to dissolve and stabilize Ttn, forming the mottled cores with abundant amoeboids. Ttn with oscillatory growth and sector zoning then mantled these cores. Simultaneously, the enclave magma quenched against the cooler felsic magma, causing elongate apatite and poikilitic Ttn grains to precipitate in the enclaves after feldspars; these likely did not interact much with the rest of the melt after quenching. Ttn could also have been forming in the felsic magma body prior to and after the mixing event; these would be the euhedral grains with no mottled cores.

Such a model is supported by chemical data as well. REE, Ti, Si, Ca, Al, and Fe concentrations and δ¹⁸O (~5.5‰) are similar for both types of Ttn. This indicates the two magmas had enough time to thoroughly mix. Zr, however, is higher in enclave Ttn. From this, temperatures of the mafic enclaves appear to be higher than the felsic stock (745°C vs. 724°C in the mantled cores, 741°C vs. 697°C in the oscillatory zones).