DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF CAMBRIAN SANDSTONES IN THE SOUTHERN OKLAHOMA AULACOGEN (MEERS AND PRATT HILL QUARTZITES)

The Southern Oklahoma aulacogen, which strikes NW into the interior of Laurentia, is a major rift that formed during early Cambrian opening of the southern Iapetus Ocean. Outcrop and subsurface data show the rift to be filled dominantly with bimodal igneous rocks. The upper parts of the rift fill consist mostly of rhyolite and related subvolcanic granite, which have yielded U-Pb zircon ages of ~540-530 Ma. In the subsurface, terrigenous detritus derived from cratonic sources has been penetrated by one well close to the northern rift margin, but the only exposed rocks that might record clastic influx from older Laurentian crust into the developing rift comprise the Meers Quartzite and Pratt Hill quartzite (informal term). Both units form small outcrops of contact-metamorphosed quartz-rich sandstone in the eastern Wichita Mountains, which expose a part of the rift farther in from the margins. The Meers Quartzite was deposited on an erosional surface carved into a tilted layered mafic complex that represents an early, pre-rhyolite phase of synrift magmatism. The Pratt Hill quartzite is intercalated within the rhyolitic volcanic pile. The protoliths of both quartzites contained up to 90% well rounded quartz grains and were contact metamorphosed during intrusion of granite.

Results of LA-MC-ICP-MS analyses of 1063 single detrital zircon grains show that three samples of the Meers Quartzite yield unimodal weighted mean U-Pb ages ranging from ~543 to 530 Ma. A sample of the Pratt Hill quartzite yields preliminary detrital zircon age data falling in the same range, except for two grains with dates of ~2000 Ma and ~2040 Ma. These data imply that almost all the clastic detritus in the exposed quartzite units was locally derived from lower Cambrian felsic igneous sources within the rift. It is likely that magma was supplied to the rift rapidly enough to keep pace with extension, and the Southern Oklahoma aulacogen may not have become a significant depocenter for craton-derived clastic sediment until later in the Cambrian, possibly during thermal subsidence after cessation of igneous activity.