15 April 2017

Basement xenoliths in Catoctin Formation, Compton Pass

Posted by Callan Bentley

My son and I hiked Compton Peak in Shenandoah National Park this morning, and saw these two lovely examples of xenoliths.

The example above is small, but it shows clearly the difference between the coarse, felsic basement rock (Mesoproterozoic granitoid, comprising the xenolith) and the surrounding fine-grained dark green metabasalt of the Catoctin Formation (Neoproterozoic). Here’s another, bigger example:

These two Blue Ridge examples both illustrate the principle of relative dating by inclusions – you cannot break off a piece of granitic basement to mix it into basaltic lava unless the granite already exists.

Hope you’re having as similarly insightful weekend!