PRELIMINARY PCA EVALUATION OF FELSIC MAGMATISM ALONG THE LAURENTIAN MARGIN, WICHITA MOUNTAINS, SOUTHERN OKLAHOMA

The felsic rocks of the Wichita Mountains, Oklahoma, are part of the exposed expression of voluminous bimodal magmatism along the Eocambrian Laurentian margin. These exhibit typical within-plate (A-type) characteristics. The bulk of felsic rocks populate two groups: the rhyolite lava flows, hypabyssal intrusives, and meta-rhyolites of the Carlton Rhyolite Group (CRG), and the penecontemporaneous shallow-crustal sheet plutons of the Wichita Granite Group (WGG). Both groups are differentiated by location, microstructure, and mineral content into formation-level or lithodemic units, correlating to individual magmatic events. Compositional similarity hampers geochemical descrimination using traditional bi- and trivarient parameters.

To better assess geochemical attributes, this study enganged principal component analysis (PCA) using the Z-scores for WGG and published CRG analyses (n=164) on major-element oxides, selected LILE, and Zr. The eigenvalues for PC1, PC2, and PC3 are each greater than 1, and represent 35.2%, 24.2%, and 9.8% of the variance. Differentiation indices SiO₂ and Rb produce negative PC1 and positive PC2 loadings; most other major element oxides, Sr, and Ba have antithetical vectors. Trace elements Nb, Y, and Zr produce strongly positive PC1 and PC2 vectors. Most other components, including Rb are driven positively into PC3 space, although SiO₂, Ba, and Zr are negative.

The bulk of the CRG plots in positive PC1 and negative PC2 space, whereas the WGG are largely opposite, but extend into positive PC1, overlapping the Fort Sill rhyolite (southern CRG flow). CRG in the northern Wichitas and meta-CRG are strongly positive in PC1 space. WGG largely are in three clusters in PC1: high (2 to 4), almost entirely Mount Scott Granite, intermediate (-2 to 0), dominated by the Reformatory Granite, and low (-4 to -5), dominated by the Headquarters Granite. Geographical clustering is best seen in PC3: northern CRG are positive, southern are negative; the WGG lithodemes tend to cluster together, but some exhibit significant scatter and outliers. In terms of PCA, the northern CRG bear closest geochemical resemblance to the voluminous Mount Scott Granite; the Fort Sill rhyolite is distinct, but holds component values closest to Cache Granite. Overall, the distributions are consistent with the presence of two or three closely-related felsic sources.