AN OVERVIEW OF RECENT WORK ON CAMBRIAN A-TYPE FELSIC VOLCANISM IN THE SOUTHERN OKLAHOMA AULACOGEN

Extensive Cambrian A-type magmatism in the Southern Oklahoma aulacogen resulted in the extrusion of the Carlton Rhyolite Group over an area of ~40,000 km² (now present mostly in the subsurface), together with intrusion of sheet granites into and beneath the rhyolitic volcanic cover. Mafic igneous activity overlapped in time with the felsic magmatism and undoubtedly played an important role in the petrogenesis of the felsic rocks. Numerous diabase intrusions cut the rhyolites and granites, and recent work has shown that abundant basaltic lavas are intercalated with the rhyolites in the subsurface. Trace-element data for the rhyolites (~100 samples) suggest that the felsic magmas were produced at least partly from OIB-type mafic precursors, with varying involvement of older crust.

Detailed mapping of rhyolite outcrops in the Wichita and Arbuckle Mountains has documented at least 37 different rhyolite flows as much as 600 m thick. Flows are stacked on top of each other or are separated by volcaniclastic interbeds deposited at least partly in lakes and ≤1 m to ~100 m thick. Most flows can only be traced a short distance laterally because of limited outcrop; the most extensive flow so far documented is ≥18.5 km long. Individual flows show a standard vertical zonal sequence. Flow-banded and locally flow-brecciated upper and lower flow margins originally contained abundant glass, based on the presence of relict perlitic texture. Intervals rich in lithophysae occur in the inner parts of the glassy zones and pass into homogeneous felsitic interiors, which in thin section contain abundant, randomly oriented tridymite needles (now inverted to quartz) that increase in length toward flow centers. We infer that the flows were extruded as thick sheets that underwent slow, uniform cooling after emplacement; they may represent remnants of laterally extensive “flood rhyolites” comparable to those documented from other A-type provinces. Assuming the tridymite in flow interiors formed within its equilibrium stability range, the flows must have come to rest at temperatures >870° C. Ongoing work on wells drilled into the volcanic succession indicates that similar thick flows with groundmass tridymite in felsitic interiors are also common in the subsurface, and this type of effusive unit may dominate the entire rhyolitic volcanic field.