Johnson Granite Porphyry is found in Tuolumne Meadows, Yosemite National Park.
| Johnson Granite Porphyry | |
|---|---|
| Stratigraphic range: 85.4 Ma [1] | |
| Type | Geological formation |
| Area | 1,100 square kilometres (420 sq mi) |
| Location | |
| Location | Yosemite National Park |
| Country | United States of America |
| Type section | |
| Named for | Johnson Peak |
| Location | Yosemite National Park |
Johnson Granite Porphyry was named for Johnson Peak, the highest mountain in Tuolumne Meadows, Yosemite National Park.[1]
Its area is estimated at 1,100 square kilometres (420 sq mi), the exposed area covering roughly 40 square kilometres (15 sq mi).[2] Johnson Granite Porphyry is part of the Tuolumne Batholith.[3] Its boundaries are difficult to trace, and are irregular.[1] It is both the youngest, and the most siliceous rock of Tuolumne Intrusive Suite.[4][5]
Geology
The innermost, most felsic part of the Tuolumne Batholith is the Johnson Granite Porphyry.[6] It is a highly irregular, elongated body. Completely surrounding it is the much larger body of Cathedral Peak granite.[1]
Field observations and magnetic fabrics suggest an extrusive event, that is, Johnson Granite Porphyry may record possible volcanism. This may relate to deformation along active shear zones.[7][5][8]
Johnson Granite Porphyry is composed of
- quartz
- K-feldspar
- zoned plagioclase, and
- locally contains coarse-grained feldspars
- Biotite, the only mafic mineral, with accessory phases of;