CRYSTAL RECORDS OF EVOLVING SILICIC MAGMA: INTERMEDIATE VOLCANICS OF THE HIGHLAND RANGE, NV

The Highland Range volcanic sequence, located in southern Nevada, is comprised of a series of Miocene lava flows and tuffs that vary in composition. Intermediate volcanics are found at the base of the sequence followed by silicic volcanics, which are capped by another layer of intermediates. In this study, we sought to characterize populations of feldspars and pyroxenes using crystal textures and major element compositions in intermediate volcanics from the base of the sequence. The goal of this study was to identify the magmatic processes that occurred prior to eruption and how eruptible silicic magmas evolve over time. Crystal textures and compositions were evaluated using backscattered electron (BSE) imaging on a scanning electron microscope (SEM) and wavelength dispersive spectroscopy (WDS) on an electron microprobe (EPMA); respectively. Crystal populations were distinguished based on textural differences corresponding with variations in major element composition. Our results reveal six feldspar populations with compositions ranging from An_2-61, Ab_29-75, and Or_{2- 66}. The populations include euhedral crystals with oscillatory zoning, crystals with resorbed/patchy cores, anti-rapakivi feldspars, “sandwich” crystals with uneven rims, patchy/cellular-zoned crystals, and crystals that interlock to form clots. Orthoclase compositions are Ba-rich, reaching up to 2 wt. % BaO. In comparison to the feldspars, there are two dominant pyroxene populations: clinopyroxenes, and rare orthopyroxenes with clinopyroxene rims. The clinopyroxenes range from Wo_43-46, En_39-44, and Fe_11-14 while orthopyroxenes ranged from Wo_2-3, En_65-67, and Fe_30-32. This diversity of crystal populations suggests complex magmatic processes occurred early in the history of this volcanic sequence, prior to eruption of the intermediate volcanics; future work to elucidate magmatic conditions will involve investigating mineral trace elements to identify evidence of open-system processes.