DID THE JOHNSON GRANITE PORPHYRY ERUPT?

The Late Cretaceous Johnson Granite Porphyry (JGP) is the youngest unit in the Tuolumne Intrusive Suite (TIS) 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. This model can be tested geochemically because REE patterns and Sr-Y systematics of high-silica rhyolites differ from aplites (Glazner et al., 2008). In particular, rhyolites with Y/Sr < 0.1 are rare in the volcanic record. 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. The mapped JGP consists of 2 phases, one an aplite-leucogranite mix (classic JGP) and the other a relatively fine-grained granite with ~5-8% biotite. Texturally diverse felsic dikes cut both units. Field relations are not clear enough to reveal the relative age of the two main units; they are commonly interfingered and have some gradational contacts. Classic JGP hosts ellipsoidal xenoliths of slightly older Cathedral Peak Granodiorite, with medium-grained groundmass mantling K-feldspar megacrysts. The ellipsoidal nature of the Cathedral Peak xenoliths suggests that they were not entrained in the JGP through volcanic brecciation.

Analysis of 11 samples of JGP indicates that the unit is compositionally similar to aplites rather than rhyolites. In particular, Y/Sr is low (0.03-0.07), consistent with separation from titanite-bearing residue. This characteristic is common in high-silica intrusive rocks but rare in rhyolites; coupled with the megacryst-bearing xenoliths (a feature essentially unknown in rhyolites), this suggests that the JGP is not the unerupted remains of a volcanic unit.