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