THE UGLY GUTS OF BEAUTIFUL HORNBLENDE PHENOCRYSTS IN THE HALF DOME GRANODIORITE, YOSEMITE NATIONAL PARK, CALIFORNIA

The Late Cretaceous Half Dome Granodiorite is one of the classic plutonic rocks of the world. It makes up many remarkable landforms (e.g., Half Dome and Cloud’s Rest) and is exceptionally well exposed in Yosemite National Park. In outcrop it is a fresh, coherent, equigranular to porphyritic granodiorite with euhedral phenocrysts of hornblende (up to 2 cm in length), biotite (1 cm), titanite (1 cm), and K-feldspar (several cm). In thin section the only hints of alteration are mild chloritization of biotite and saussuritization of plagioclase, but the rocks hold a dark secret: the hornblende phenocrysts are little greenschist-facies reaction cells that contain a remarkably complex low-T mineral assemblage inside their beautiful, black, euhedral exteriors.

Hornblende crystals weather out whole in grus, and we mounted several in epoxy and cut them through their centers perpendicular to the c-axis. Microprobe analyses show that the crystals vary from magnesiohornblende with 8 wt% Al₂O₃ to actinolite with 1 wt% Al₂O₃ over distances as short as 10 μm; no systematic igneous zoning is present but healed fractures are apparent. The crystals average 45 vol% non-amphibole inclusions including every igneous mineral in the rock and a full set of greenschist-facies minerals including albite, chlorite, epidote, clinozoisite, magnetite, and muscovite. Whole-crystal analyses by XRF demonstrate that the alteration process was nearly isochemical, assuming that the starting composition was equivalent to the high-Al₂O₃hornblende; thus, the metamorphic mineral assemblage represents a complex, texture-preserving reaction of hornblende to actinolite.

Presence of a full subsolidus metamorphic mineral assemblage within the phenocrysts, which look for all the world like completely happy igneous minerals, is consistent with other indicators of extensive low-T reequilibration of the mineral assemblage. In particular, recrystallization to 3-feldspar assemblages of Or₈₅ + albite + oligoclase indicates equilibration at T < 400 °C, consistent with greenschist facies. It appears that the Half Dome and other granodiorites tried to adjust to dropping temperature as they cooled, but held on to what we have been trained to interpret as fully igneous textures.