PETROLOGIC SIGNALS RELATED TO THE RIFTING PROCESS IN THE CAMBRIAN SOUTHERN OKLAHOMA AULACOGEN

The Wichita Mountains area of SW Oklahoma contains the largest outcrop area, and the strongest geophysical signature, of the Cambrian Southern Oklahoma Aulacogen. Rift rocks, in the age range 540-525 Ma, are a bimodal sequence of tholeitic gabbros (anorthositic Glen Mountains Layered Complex [GMLC] and Roosevelt Gabbro Group [RGG]), topped by A-type rhyolites (Carlton Rhyolite Group) and sheet granites (Wichita Granite Group). No substantial rift sediments are known, implying a topographically high, and therefore hot, rift. Rift width estimates vary between about 50 km and 100-150 km. All exposed intrusive bodies are relatively fine-grained indicating emplacements near the top of the rift. However, the gravity signal is positive and large, indicating a substantial mass of basaltic liquid remained in the middle to lower rift crust, and also that most of the silicic liquids (possibly as much as 40,000 km³) came up near and at the surface during the rifting process. There should not be much Cambrian granite at depth.

Emplacement and geochemical signals from the gabbros include: 1) fine- to medium-grained (average up to 1 cm); 2) distinct layers with strong planar plagioclase fabrics in the GMLC; 3) isotopic signatures (ε_Nd +3.6 to +5.4 GMLC; +3.3 to +5.1 RGG) indicative of derivation from subcontinental lithospheric mantle. Additionally, diabase dikes that cross-cut all other units have similar isotopic signatures (ε_Nd +3.1 to +5.1).

Equivalent signals from the rhyolites and granites are: 1) the dominant rhyolite lavas were hot and dry and erupted in subaerial environments; 2) sheet forms of the granites; 3) contrasting granite textures: fine-grained, granophyric to medium-grained, seriate, 4) isotopic signatures (ε_Nd +2.5 to +4.1) very similar to the mafic rocks.

Emplacement of sheet forms is compatible with extension. Magmas mostly were hot and dry but well-mixed chemically; contrasting textures indicate variable rhyolite cover thicknesses and therefore on-going erosion throughout rifting history. Geochemical signals show new rift crust with little contamination of older, bordering 1.4 Ga Arbuckle crust. All signals show shallow emplacement of new crust, layering partly due to seismic shaking during emplacement and crystallization, and a pronounced AMCG signature applied to the igneous assemblage as a whole.