CONTACT RELATIONS BETWEEN MOUNT SCOTT GRANITE AND QUANAH GRANITE

The two dominant granites in the eastern Wichita Mountains are the older Mount Scott Granite (534 +/- 1 ½ Ma from Hogan, Wright, and Gilbert unpublished) and the younger Quanah Granite (~525 Ma from Tilton & Others, 1962). Their mutual contact relationships contain a fascinating story of their geologic and intrusive setting. The Mount Scott is the most widespread of the outcropping Cambrian Southern Oklahoma Aulacogen (SOA) granites. It is a sheet granite with estimated dimensions of 55 x 17 x ½? km. The Quanah is somewhat less sheet-like at 23 x 5 x 1?km, as its base is not seen.

These granites appear to be sitting on a substrate of gabbro and were intruded at roughly the same “stratigraphic” level under a cover of rhyolite. Thus MS had rhyolite originally against its sides as well as its roof. When Q intruded at this level, it intruded against one side of MS sheet. Along part of their present exposure, their presumed contact has been eroded away. However, the Elk Mountain-Sunset Peak area still contains a large exposure of their contact relations. In both granites, there are textural changes nearing the contact zone indicating that Q intruded against the old margin of MS. There are blocks of MS and of rhyolite trapped and surrounded by Quanah in this zone. Just west of Sunset Peak, Q appears to have intruded under basal MS generating dikes cutting both MS and the adjacent Sandy Creek Gabbro (one of the Roosevelt Gabbros), such as the Hale Spring Pegmatites. It is thus likely that the Q floor dropped below that of MS during the intrusion process. Support for this interpretation lies in geophysics where, to the south, the high gravity anomaly drops off and magnetic character becomes more subdued. If present outcrop can be taken as indicative of original magma volume, then MS was formed from at least 450 cu km, while Q was around 100 cu km. Both of these magmas appear to have formed from fractional crystallization of primary rift-related basalts. MS magma was relatively dry and carried early phenocrysts generated at depth on the rise toward the surface. In contrast, Q magma was relatively more hydrous and seems more likely to have reached emplacement position dominantly as a liquid. Since magma driving pressure for MS would have been higher than that for Q, these 2 granites were generated at different depths in the rift, and from different batches of basalt