Collections > UNC Chapel Hill Undergraduate Honors Theses Collection > Al Zoning and Mineral Inclusions in Hornblende Crystals from the Half Dome Granodiorite
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The Al-hornblende geobarometer is a widely applied tool for calculating the depth of a pluton’s emplacement. Though its widespread use has been questioned, it is still often used with little investigation of its applicability, as there are few geobarometers available in these systems (Anderson and Smith 1995, Ague 1997). To examine the applicability of the hornblende geobarometer within the Tuolumne Intrusive Suite within Yosemity National Park, California, samples of hornblende phenocrysts from the Half Dome Granodiorite were collected and analyzed. From them, maps of mineral inclusions and Al content were made using scanning electron microscopy and remote sensing techniques. Instead of the expected regular concentric zoning in Al content, zoning in Al is patchy and irregular. Furthermore, the crystals are riddled with inclusions of other minerals, some of them late crystallization products. These features suggest that Al zoning within these crystals is not controlled by pressure and temperature, and that these crystals would be unsuitable for geobarometry. Because the included and zoned nature of these crystals is not obvious in hand sample, it is necessary that hornblende crystals be carefully examined before Al geobarometry is attempted. Furthermore, the inclusion of late-crystallizing minerals within hornblende strongly suggests that they could not have formed in a single crystallization event. This indicates that their origin must be more complicated than traditional models of pluton formation account for.