ASPECTS OF MAPPING THE QUANAH GRANITE

The Quanah Granite has been mapped at a scale of about 1:10,000 through an USGS EDMAP project, for presentation at 1:24,000. This was done to assist the Oklahoma Geological Survey in its preparation of the Lawton Sheet at the 1:100,000 series. The Oklahoma Geological Survey is working on the Statemap project which will end up mapping the entire State of Oklahoma. The map area is part of the Wichita Mountains of southwest Oklahoma and represents the exposed portion of the Cambrian Southern Oklahoma Aulacogen. Most of the outcrops are in the federal Wichita Mountains Wildlife Refuge and easily accessible.

Quanah Granite was one of the first granites recognized as a separate unit within what is now known as the Wichita Granite Group. This group was defined by Powell et al. (1980) and Myers et al. (1981), with modifications by Price (1998), and all the granites have a sheet form and are A-type. The Quanah is one of the youngest granites in the eastern Wichitas based on field relationships. It is distinctive in its relatively large grain size (~1cm), compared to other Wichita granites, and it commonly carries riebeckitic amphibole and highly annitic biotite. There are internal fine-grained facies but these have not been mapped. Quanah contacts the Permian Post Oak Conglomerate (nonconformity), the older Glen Mountains Layered Complex, and the older Mount Scott Granite. The Mount Scott is distinctive in its micro-rapakivi texture.

This mapping project had both easy and difficult aspects. Contacts with anorthositic gabbro of the Glen Mountains Layered Complex are distinct. The Quanah contact with the Post Oak is normally clear except where recent erosion has left a thin veneer of Post Oak granite clasts on the substrate Quanah. However, the contact between the Quanah and the Mount Scott has been challenging for several reasons: 1) The Mount Scott fines and frequently loses its distinguishing grey ovoid feldspars toward the Quanah 2) the Quanah fines against the Mount Scott, while also including blocks of Mount Scott within its matrix. A zonal boundary rather than the traditional “line” boundary was determined to be the best method of dealing with these issues relating to this portion of the contact.