Feldspar Megacrysts as a Window into the Crystallization of Silicic Magmas: A Study in The Tuolumne Intrusive Complex, Yosemite National Park, CA

Abstract

The volcanic-plutonic connection remains one of the most significant conundrums facing igneous petrologists. We have developed a methodology and initial basis of observation to identify volcanic events using information recorded in alkali feldspar megacrysts contained in associated plutonic bodies. We collected samples, completed 3D imaging using x-ray computed tomography, gathered 2D images using back scatter electrons and energy dispersive spectroscopy in a scanning electron microscope, and produced 1D profiles using laser ablation inductively coupled plasma mass spectrometry. During 3D imaging, we found that beam hardening limited our ability to make meaningful analyses of inclusion distributions for each phase. To rectify this, we filtered incident x-rays through foils to dramatically reduce beam hardening. Aluminum, titanium, and copper foils were tested at a variety of thicknesses Ultimately, aluminum was found to be best for qualitative work, whereas copper was best for quantitative applications. Megacryst cores and zonation were observed to gain insight into the variety of crystal histories that could be identified in future work. We found four distinct core varieties and observed concentric zonation patterns in most megacrysts. These zones come in a variety of patterns (sawtooth, reverse sawtooth, peak, valley, and plain) that likely each correspond to a different crystallization process or trigger. Alkali feldspar megacrysts contain within them a wealth of information, and are capable of revealing the histories of silicic plutonic rocks generally hidden by time and the complexities of massive plutonic bodies.