Crystal-rich, chemically evolved, coarse-grained igneous rocks provide opportunities to investigate the differentiation and solidification of magmas within Earth’s crust. Specifically, by investigating the micro-scale textural, chemical, and chronological record of these crystal populations insights into the processes which operated during magmatic crystallization and intrusion of large magma bodies within Earth’s crust can be evaluated. This study evaluates a suite of megacrystic (>2.5cm) alkali feldspars hosted in granite from the Sheephole Wilderness area in southern California. Within these alkali feldspar megacrysts are inclusions of biotite, plagioclase, oxides, rarer amphiboles, and quartz, with zircon, titanite, and apatite present as accessory minerals. Quantifying trace elemental abundances (e.g., Rb, Sr) across these alkali feldspars via LA-ICP-MS permits evaluation of the changing magmatic conditions during megacryst formation. Preliminary trace element data across three transects of a sampled megacryst show variations on the order of 1000s of ppm. This observed chemical variation in part could be accounted for by the ablation of different megacryst components, e.g., mineral inclusions, along each transect. However, these variations could also be attributed to variable melt chemistry during crystallization, perhaps indicative of open magmatic processes. Meanwhile, the presence of zircon allows for the application of U-Pb geochronology via LA-ICP-MS hence the timescales over which megacryst formation occurs can potentially be evaluated. Nine zircon grains were dated through application of U-Pb geochronology. Ages range from ~220Ma to ~290 Ma spanning a total of 70 million years. This is interpreted as being indicative of the role of zircon inheritance (i.e., the presence of antecrystic zircons) which would also be consistent with an open magmatic system. This preliminary chemical and chronological dataset contribute to a growing body of literature (e.g., Chambers et al., 2020, Geology, 48(4)) investigating the timescales and chemical processes over which magmas, and their megacrystic feldspars, crystallize in Earth’s crust.
Author: Jessica Patrick
Faculty Advisor: Claire McLeod, Geology and Environmental Earth Science
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