The Sierra Nevada Batholith: Close-up Shots

The next time you hike across a outcrop of intrusive rocks in one of our national parks, keep your eyes open for interesting features that reflect its formation. A granitic body often displays areas of mafic rock, and mafic bodies often display felsic areas. Some areas appear to have been liquid when they were injected into the larger intrusive magma: they show rounded edges and can be bent. Other areas entered the large intrusive body as solid rock: they show angular edges and lithologies that may match rocks surrounding the granite. These are termed xenoliths (foreign rocks). Geologists examine the lithology and especially the chemistry of both types of enclaves in order to understand the processs of intrusive rock formation and the geology of areas deep beneath the surface.

This photo shows a variety of mafic enclaves in the Sierra Nevada Batholith. The elongated dark ones near the bottom look like pods of magma injected into the still-molten granite. Others are more angular and may be xenoliths. You'd have to do more work to be sure!
Processes of magma injection, flow, and mineral crystallization helped to produce the mafic banding in this Sierra granite. Note the small felsic dike above and to the left of the red jack knife. Photo by CE Jones
Here is an example of felsic enclaves in a gabbro. The rounded shape and the thin connection between several pods at the bottom of the photo suggest the felsic material was liquid when it was incorporated into the gabbro. Photo by Kent Ratejeski
This block of gabbro in a granite body shows angular edges, suggesting that it is a xenolith. It was ripped off the sides of the passage used by the felsic magma as it migrated up through the crust. This photo was take in Donner Pass in the high Sierras. Photo by Norris W. Jones

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