EVALUATING THE ERUPTIVE NATURE OF THE JOHNSON PORPHYRY GRANITE; A GEOCHEMICAL ANALYSIS

The Johnson Granite Porphyry (JGP) is the youngest unit in the Tuolumne Intrusive Suite of Yosemite National Park, California. It has long been suggested that the JGP magma body vented, that the observed JGP is unerupted rhyolite, and that the diverse petrography of the unit reflects chaotic eruption dynamics. The JGP is broadly aplitic in character, and the REE patterns and Sr-Y systematics of high-silica rhyolites differ from aplites (Glazner et al., 2008). In particular, low Y and high Sr in aplites relative to rhyolites suggests fractionation of titanite but not of significant plagioclase. Analyses of 2 JGP samples by Gray et al. (2008) suggest aplitic (intrusive) chemistry rather than volcanic. We investigated the geochemistry of the JGP in order to better understand whether the unit might have erupted.

We collected twenty samples of the JGP and analyzed eleven for major and trace elements. Field observations suggest that the JGP consists of 2 phases, one being aplite and leucogranite (classic JGP) and the other a relatively fine-grained granite with ~20% biotite. Field relations are not clear in revealing the relative age of the two units, but the classic unit is lower in silica and higher in Zr than the fine-grained granite. XRF data are regrettably inconclusive about the intrusive/extrusive affinity of the JGP, as the analyzed samples plot in the region of overlap between aplites and rhyolites on a Sr-Y plot.