MINERALOGICAL AND TEXTURAL ANALYSIS OF FINE-GRAINED MICROVESICULAR CLINOPYROXENE-HORNBLENDE DACITE FROM SPRING HILL NEAR MOUNT SHASTA, CALIFORNIA

Compositions of plagioclase, hornblende, and clinopyroxene from samples of microvesicular grey dacite from the eastern face of Spring Hill, a satellite cone of Mount Shasta in the southern end of the Cascade Volcanic Arc, are presented here in conjunction with textural observations of the same samples in order to gain a more complete understanding of the evolution of the Mount Shasta volcanic system. Texturally, the samples contain 10% vesicules that rarely exceed 0.5 mm in diameter and possess plentiful, but fine to very fine crystals, with few phenocrysts exceeding 1 mm with occasional glomerocrysts of hornblende measuring 2-3 mm. Of the constituent minerals, the most commonly observed is plagioclase, which occurs mostly as trachytic laths with pronounced directional orientation though occasional anhedral to subhedral phenocrysts measuring <0.5 mm were observed. Plagioclase phenocrysts are mostly bytownite (An 70-76), whereas microlites are andesine (An 42-46). The next most common minerals were clinopyroxenes, occurring most commonly as euhedral phenocrysts <1mm in size, though there are instances of subhedral and, very rarely, anhedral phenocrysts; the compositions of observed clinopyroxenes are predominantly diopside with extremely low ferrosilite percentages, though some augite is present with higher ferrosilite composition (<20% Fs). Occurring nearly as often in some samples as clinopyroxene is quartz, present as ≤1 mm anhedral phenocrysts that often exhibit embayed margins. Less frequent in quantity of phenocrysts but dominant in regards to consistency of size is hornblende, occurring as subhedral to euhedral phenocrysts ≤1 mm in diameter as well as glomerocrysts as large as 2-3 mm in size, both of which exhibit reaction rims that range in thickness from <14 µm to as thick as 27 µm, which are less than neighboring Black Butte, suggesting the possibility of a faster ascent rate. Finally, present in lesser quantities and smaller size are both ilmenite and magnetite, typically found as euhedral phenocrysts no larger than 0.5 mm. These observations of mineralogy, composition, and texture suggest disequilibria indicative of mixing or mingling of different magmas which offers more information on the pre-eruptive conditions of Spring Hill and its relation to the evolution of the Mount Shasta.