Freshly exposed igneous rock at 3M quarry near Little Rock, Arkansas. About 100 million years ago, molten rock from the earth’s hot interior was forcibly injected into the bedrock in this area. The dark-colored rock you see at the center of the picture is what geologists call a xenolith, or accidental inclusion. It’s basically a hunk of the bedrock that broke off and got trapped in the magma before it cooled. The bedrock was originally shale, but the heat and fluids from the igneous rock cooked and altered the shale so it’s now a metamorphic rock called hornfels.
This week we took a break from our regular field area, and were treated to a three days of field trips relating to the sparse though important igneous rocks of Arkansas led by our own Mike Howard. Mike, who is regarded as the preeminent expert on the mineralogy of Arkansas, will be retiring in a few weeks after a 39 year career at our agency,
We stopped at numerous locations in Pope, Conway, Pulaski, Saline, Hot Spring, and Pike counties. The first day, we headed up to the northernmost igneous intrusion in the state which is north of Dover just off Highway 7 on Dare Knob. From there we stopped at several carbonatite sills (igneous intrusions that follow sedimentary bedding planes) in the Arkansas River Valley near Oppelo.
The next day we started out with a visit to the 3M quarry on Granite Mountain. Granite Mountain is actually a relatively small exposure of an igneous batholith which extends to an area of over 250 square miles in the subsurface. Also, it is not actually granite at all, but nepheline syenite. It is being processed mainly into roofing granules. The other large active quarry in the area, Granite Mountain Quarry, produces mostly aggregate materials.
From there we went to Bauxite where we saw a large olistolith in the syenite. A xenolith is a piece of the country rock that is incorporated into the melt of an igneous intrusion. In this case the country rock was probably Jackfork Sandstone as evidenced by tight folding in a quartzitic rock slightly metamorphosed by contact with the igneous body. From there we made several stops in the Magnet Cove area, where we saw broken phenocrysts (large crystals in a finer matrix) of pseudoleucite, carbonatite, garnet, rutile and pyrite.
We also saw the largest barite pit in the U.S. (now abandoned), and a nearby vein of smoky quartz that was easily accessible.
The next day we drove to the diamond mine at Murfreesboro, with which Mike has had an association since even before it was made a state park. I found a good barite crystal, but unfortunately, no diamonds.
These trips were attended by almost the entire staff, and we much appreciated getting to see these rare rocks and minerals with knowledgeable commentary by the state’s most expert resident. We will miss having him at the survey to answer our many questions, but hope that he enjoys many years of well-deserved leisure activities. Thanks, Mike!!
Until next week, I’ll see you on the outcrop!