PRECISE U-PB ZIRCON GEOCHRONOLOGICAL CONSTRAINTS ON VOLUMINOUS CAMBRIAN MAGMATISM IN THE SOUTHERN OKLAHOMA AULACOGEN

Final breakup of Rodinia near the end of the Precambrian was preceded and accompanied by widespread Neoproterozoic to Cambrian anorogenic magmatism along the margins of various cratons now dispersed worldwide. Such magmatism is well-documented along the western, northern and eastern margins of Laurentia, where it has an overall age range of ~780–540 Ma and provides insight into relations between mantle convection and supercontinent fragmentation. There is much less evidence for such magmatism along the largely buried southern Laurentian margin. An outstanding exception is the bimodal Wichita Igneous province (WIP), which was emplaced within the Southern Oklahoma Aulacogen during rifting preceding Cambrian opening of the southern Iapetus Ocean. Major units of the WIP are exposed in the Wichita Mountains in SW Oklahoma, with smaller outcrops occurring in the Arbuckle Mountains ~100 km to the east. Buried parts of the WIP extend for ~40,000 km² in the subsurface, and geophysical evidence shows that the total volume of magma emplaced within the province was probably > 250,000 km³.

We present the first high-precision CA-ID-TIMS U-Pb zircon geochronological constraints for the WIP. Our data reveal an age of 532.49 ± 0.15 Ma for anorthositic gabbro in the Glen Mountains Layered Complex (GMLC), which is the oldest main igneous unit exposed in the Wichitas. The GMLC was tilted during an early phase of extension, and its upper, more differentiated parts along with an unknown thickness of roof rock were eroded away prior to eruption of A-type rhyolites, which have an exposed thickness of 2 km. Lava flows from the base and top of this sequence yield ages of 530.98 ± 0.15 and 530.70 ± 0.12 Ma, respectively, and the extensive Mt. Scott sheet granite, which intruded along the unconformity between the layered complex and the rhyolites has an age of 530.45 ± 0.14 Ma. These data suggest that significant parts of the WIP were emplaced in a narrow time frame (~2 m.y.) within a tectonically active, rapidly evolving rift setting, possibly in association with a mantle plume that triggered opening of the southern portion of the Iapetus Ocean. Rhyolite exposed in the Arbuckles has an age of 539.20 ± 0.15 Ma, raising the possibility that magmatism was diachronous along strike in the aulacogen, although this needs to be tested by further isotopic dating.