25 June 2012

Leopard rock

Posted by Callan Bentley

Here’s a pretty rock to liven up your Monday:

This is a deeply porphyritic rock known locally and colloquially as “leopard rock.” It outcrops along Route 212 (the Beartooth Highway) between Cooke City and Red Lodge, Montana. This location is northeast of Yellowstone National Park.

The spots you see are feldspar mega-crysts: it takes time to grow crystals this big, so these must have formed over a long time, as the magma was only slowly losing heat, deep in Earth’s interior. The darker color is a finer-grained ~basaltic groundmass – the magma that chilled rapidly after emplacement of the dike into cold country rock. And so we would annotate the previous picture something like this:

These rocks were intruded into the Archean “Wyoming Terrane” basement as mafic dikes during the Proterozoic.

Some of the feldspar megacrysts are really big:

An outcrop of “leopard rock,” showing the effects of weathering on highlighting the two parts of the rock:


Lastly, consider this weird one: perhaps the contact zone where the leopard rock dike met country rock that was pliable and goopy during intrusion? The country rock foliation definitely appears to wrap around the feldspar megacrysts…

(Note: we wetted this last sample for better photo contrast.)