PRECISE U-PB ZIRCON GEOCHRONOLOGY FOR THE WICHITA IGNEOUS PROVINCE IN SOUTHERN OKLAHOMA: GEOLOGICAL CONTEXT AND TECTONIC IMPLICATIONS

New work by Wall et al. (2018) provides precise U-Pb zircon geochronological constraints on extensive Cambrian magmatism recorded by the Wichita Igneous Province (WIP) in the Southern Oklahoma aulacogen (SOA). Here we expand on some of the implications of these results. The new data show that a major layered mafic intrusive complex as well as extensive felsic rocks exposed in the Wichta Mtns. in SW Oklahoma were emplaced in a narrow time frame at ~532-530 Ma. In smaller outcrops in the Arbuckle Mtns. a rhyolite lava has yielded an age of ~539 Ma, which may suggest that magmatism varied in time in different parts of the SOA. However, rhyolite lavas in the Arbuckles are cut by a complex series of mafic and felsic hypabyssal intrusions and volcanic vents emplaced over some length of time, and further dating is needed to constrain the age range of the igneous rocks there. Geophysical data and evidence from basement wells indicate that the SOA contains ~210,000 km³ of mafic rock oriented NW-SE along the axis of the SOA, along with ~40,000 km³ of granite and rhyolite present mostly at higher crustal levels. If the dates from the Wichitas are representative of the igneous province as a whole, the magma input rate was ~0.125 km³/yr. If magmatism occurred uniformly at 539-530 Ma, the input rate would be ~0.028 km³/yr. Both rates are within range of values for typical LIPs likely to be related to mantle plumes. Zircon trace-element patterns and whole-rock geochemistry for the mafic and felsic rocks are also consistent with a mantle-plume origin.

Protracted intraplate magmatism at ~765-540 Ma led to early Cambrian seafloor spreading and opening of the Iapetus Ocean along the eastern Laurentian margin, which contrasts with the SOA to the SW, where magmatism occurred over a much shorter time interval, and the rift-drift transition occurred later. In a popular model, the pronounced Ouachita embayment in that region was originally occupied by the Precordillera terrane, which began to detach from southern Laurentia due to a jump in the spreading center. The cause of this change in plate kinematics, however, has been unclear. We suggest that uprise of a mantle plume near the area now occupied by the SOA caused thermal weakening of the lithosphere, which triggered or facilitated separation of the Precordillera terrane from the Ouachita embayment.