Like several other stops on the trip, this one featured an igneous intrusion—the Emigrant Pass pluton, emplaced 41 to 34 million years ago in three phases. Rock in the Devil's Playground area is part of the youngest phase (Egger et al. 2003). This pluton is especially interesting to geologists studying metamorphic core complexes (MCCs), for it is in the southern part of the Albion–Raft River–Grouse Creek MCC (ARG in map below).
|Black blobs are MCCs; arrow points to Emigrant Pass pluton. After Strickland et al. 2011.|
|Note multiple plutons in Albion, Raft River, Grouse Creek Mts. After Egger et al. 2003.|
|Based on Peterson & Buddington 2014, DeCourten & Biggar 2017.|
I like metamorphic core complexes very much, largely for their mystery. But they're difficult. It's challenging just to spot one, even with a guidebook. These are giant structures, visible only as parts exposed here and there. Plutons are much easier to understand, fairly common, and yet still worth contemplating. I became a fan when I realized that if I can see a pluton, something dramatic must have happened.
|It seems plutons are often sculpted into intriguing forms, like the Harrison Pass pluton in the Ruby Mountains.|
|Tors carved from one of our local plutons, at the crest of the southern Laramie Mountains.|
|Notch Peak intrusive at the base of the west face of the House Range; photo by Mike Nelson.|
|Emigrant Pass pluton—a tilted world. Did it tilt during uplift?|
Rocks in this part of the Emigrant Pass pluton have been called granitic and granitoid. These are handy terms because the range of granitic rock types is broad and hard to subdivide neatly. Egger et al. (2003) are more specific: "a virtually homogeneous coarse-grained biotite granite". The granite is criss-crossed with aplite and pegmatite dikes, which formed when still-molten magma—hydrous and therefore last to crystallize—was injected into fractures in the solidifying pluton.
|Aplite is more resistant to erosion so dikes stick out a bit from the granite.|
|Fine example of spheroidal or onion-skin weathering at the Devil's Playground. Photo courtesy scienceteacherexplorer (click link for more great shots).|
|Geocacher enjoying spheroidal weathering (source).|
|Are these young devils, recently emerged? Or elderly ones, to dust returning?|
|In the company of plants and rocks :)|
|Stenotus acaulis; those of us who have been around for awhile may know it as Haplopappus acaulis.|
|Such a lovely parasite!|