Intraplate magmatism related to opening of the southern Iapetus Ocean: Cambrian Wichita igneous province in the Southern Oklahoma rift zone

Abstract

Southern Oklahoma and adjacent parts of Texas contain an extensive igneous province emplaced during Early Cambrian rifting within the Southern Oklahoma rift zone. The rift zone was initiated in association with the opening of the southern Iapetus Ocean during Rodinia supercontinent breakup and later became the site of a series of linked uplifts and basins as a result of late Paleozoic inversion. Igneous rocks within the rift are referred to as the Wichita province and are present mostly in the subsurface, although critical exposures occur in the Wichita and Arbuckle Mountains in southwestern and southern Oklahoma. Wells drilled into basement in the region provide a wealth of information on the distribution and relations of the major igneous units in the upper crust, and geophysical data provide important constraints on deeper levels of the rift zone.The upper parts of the igneous rift fill comprise the Carlton Rhyolite Group, which has an estimated subsurface areal extent of ~40,000km2, and the related Wichita sheet granites, which intrude the lower parts of the rhyolite succession. These rocks have A-type characteristics and were emplaced after intrusion, tilting and uplift of a large tholeiitic layered mafic complex; smaller bodies of gabbro also intruded the layered complex after it was tilted. U−Pb zircon and 40Ar/39Ar geochronology indicates the felsic rocks and at least some of the mafic units were emplaced in a relatively narrow time frame at ~539−530Ma. Basalts and intermediate lavas are present only in the subsurface. Our new work shows these lavas to have tholeiitic to mildly alkaline compositions and to be more extensive than previously realized, forming thick successions that both underlie and are intercalated with the rhyolites. Diabase dikes were intruded into older crust during initial opening of the rift, and a suite of late diabase intrusions also cuts the rhyolites and granites, indicating that mafic magma was supplied to the rift throughout its igneous history. Geophysical data show that an enormous mass of mafic rock occupies deeper parts of the rift, extending to depths of at least 10km beneath the surface. The total volume of igneous rock emplaced within the rift is estimated to be in excess of 250,000km3.Detailed studies of Carlton Rhyolite exposures in the Wichita Mountains, complemented by work on drill cuttings from basement wells, indicate that lava flows up to 400m thick make up major parts of the rhyolitic succession. The generally limited exposures of these rhyolite lavas in the Wichita Mountains are interpreted to be remnants of laterally extensive flow units similar to those documented from other A-type felsic provinces. Trace element abundances define three distinct geochemical groups of rhyolite, suggesting derivation from three different sources or magma reservoirs. Flows belonging to the different geochemical groups are intercalated in the Wichita Mountains exposures, which may indicate that laterally extensive flows from separate magma chambers came to rest on top of one another during accumulation of the rhyolitic volcanic pile.