Analysis of Plutonic Texture by GIS Analysis of Thin Sections (2008)

Undergraduates: Christopher Estes, none none Allen Glazner

Faculty Advisor: Allen Glazner
Department: Geology

---

It is difficult to quantify the textural properties of rocks beyond simple modal analysis using standard thin-section examination. In order to better quantify plutonic textures we made a GIS-based geologic map of a thin section of equigranular Granodiorite from Yosemite National Park, CA. The resulting geodatabase can be queried for many textural properties._x000D_
Grain boundaries were digitized by using both polarized and unpolarized scans checked against microscopic views of the original thin section. The resulting map gives a much clearer picture of textural relations than the microscopic view because extraneous information (e.g., twinning and brightness variation) is suppressed. Plagioclase (40% by area) and quartz (25%) compose the majority of the slide and occur in large, sinuous clusters of 20 or more grains. All other minerals occur as discrete crystals with little or no clustering. Linear grain clusters of quartz and plagioclase evident in the map are clearly not randomly distributed. K-feldspar is nearly as abundant as quartz but is unclustered, occurring as relatively large, isolated grains. Rogers and Bogy (Science, 1958) found similar results from a variety of granites and speculated K-feldspar crystals suppress nucleation of other K-feldspars nearby. During crystal aging, small K-feldspar crystals dissolved and were accreted onto larger ones, producing K-feldspar megacrysts in the porphyritic Half Dome and Cathedral Peak Granodiorites (Johnson et al., Eos, 2006)._x000D_