Llanite

Abstract: Blue quartz phenocrysts from the Llano rhyolite (llanite), Llano County, Texas, derive their coloration from Rayleigh scattering by abundant submicrometer-sized (type l) ilmenite inclusions. Also included are larger, less abundant ribbon-shaped (type 2) ilmenite inclusions lying on the rhombohedral growth surfaces of the host q\artz. These type 2 inclusions produce chatoyance in certain orientations; however, they are, in general, individually too large (-0.1 by I by 20 pm) to contribute to the blue color by Rayleigh scattering. The total amount of ilmenite in the llanite blue quartz is calculated to be -0.02 volo/0. Llanite blue quartz and groundmass exhibit distinct trace-element crystal/liquid partition coefficients that deviate from the flat patterns characteristic of other quartzlrhyolite pairs. Partition coefrcients for Hf (0.335),Zr (0.38), Cr (0.10), and Lu (0.28) are significantly greater than those for Rb (0.01), Ba (0.013), Th (0.004), La (0.003), and Tb (0.008), suggesting that a majority of the ilmenite inclusions crystallized from the llanite melt. This conclusion assumes that quartz equally partitions all elements except Eu, which in the llanite blue quartz as well as some colorless quartz exhibits partition coefficients with a positive anomaly (Eu/Eu* > 2.6).The trace-element data are compatible with either of the following scenarios: (1) both type I and type 2 ilmenite inclusions originated by crystallization from the llanite magma, or (2) the volumetrically major portion of the ilmenite inclusions (probably type l) was derived by crystallization from the magma, whereas the remainder (probably only type 2) was formed by exsolution processes. If entrapment of early crystallizing ilmenite is a generally applicable model for the origin of blue quartz, two implications arise. First, calibrations of the temperature and oxygen fugacity of Fe-Ti oxide exsolution in blue quartz should not be made unless an exsolution origin for the inclusions is assured. Second, the dominant occurrence of blue quartz irr rocks of middle to late Proterozoic age may reflect preferential conditions that promoted early ilmenite saturation during this time. These conditions remain largely undetermined but could involve particular physical parameters of magma equilibrium such as low temperature, high pressure, or high oxygen fugacity and/or processes of magma production resulting in Ti-, Fe-, alkali-, and REE-rich high-silica compositions.