T272 Recent Advances in Studies of Large-Volume Silicic Volcanism: Stratigraphy, Architecture, Evolution

Wednesday, 12 October 2011
1:30 PM - 5:30 PM
Room 101FG

VOLUMINOUS A-TYPE RHYOLITES WITHIN A MAJOR, LARGELY BURIED CAMBRIAN RIFT ZONE IN SOUTHERN OKLAHOMA

Paper number: T272-1
Program: Technical Sessions
Time: 3:40 PM – 4:00 PM

Authors

Abstract

Intraplate magmatism occurred in many places along the margins of the Laurentian craton during supercontinent breakup at the end of the Precambrian. One of the largest accumulations of felsic igneous rocks formed at this time is present within a largely buried Cambrian rift zone in southern Oklahoma. Rhyolites and related sheet granites within the rift record generation of voluminous A-type felsic magmas during bimodal igneous activity leading to opening of the Iapetus Ocean along the southern Laurentian margin. The rhyolites extend over 40,000 km2 in the subsurface and in places were clearly ponded against rift-bounding normal faults. Available U-Pb zircon ages of ~539-530 Ma for the felsic rocks indicate emplacement in a relatively narrow interval in the Early Cambrian. Basalt lavas underlie and are intercalated with the rhyolites in the subsurface, and the extent of the volcanic field is comparable to the Miocene-Recent Snake River Plain-Yellowstone system. In spite of this, the southern Oklahoma rift has received limited attention from volcanologists.

The rhyolites are only exposed in restricted areas in southern Oklahoma, and constraints on emplacement mechanisms must be gleaned primarily from these isolated outcrops, complemented by data from basement wells. Exposed rhyolite sequences are up to ~2 km thick and typically consist of a series of stacked, tabular flow units 80-400 m thick that lack pyroclastic textures and are separated in places by generally thin intervals of air-fall tuff or reworked volcaniclastic debris. Most flows can only be traced a few km laterally before trending beneath cover, but they typically show a standard vertical zonation, in which basal and upper chilled glassy margins (now altered) pass inward into lithophysae-rich and prominently flow-banded zones. Flow interiors consist of massive felsite with random tridymite needles that increase in size inward, recording uniform cooling after emplacement. We interpret these units to represent erosional remnants of relatively large-volume sheet-like lava flows comparable to those documented from other A-type felsic provinces. Outcrop studies and subsurface data suggest that eruption of these distinctive flows in rapid succession was an important control on volcanic architecture of the rift fill.