Which pluton are you?

The Wichita Mountains of southwestern Oklahoma exposes the products Cambrian rift magmatism. The bulk of the felsic rocks represent a series of shallow granite intrusions. Outcrops in the east show immense continuity, but also reveal intrusion relationships among several magmatic systems as well as complications due to local thermal regimes, dynamics, and accompanying changes due to crystallization and exsolving volatiles. Moving west, the exposures are found on isolated hills separated by younger cover; individual intrusive bodies are expressed as an "archipelago" of outcrops.

The Cambrian granites are collectively mapped as the Wichita Granite Group (WGG). A subdivision (lithodeme) collects outcrops into individual units of similar characteristics. The implication is that each lithodeme is an expression of a magma input (a pluton). The current lithodemography was best refined by two papers: Powell et al. (1980) and Myers et al. (1981), and further amplified by Oklahoma Geo. Survey guidebooks by Gilbert (1982 and 1986). Subsequent work led to the mapping effort of Stanley and Miller, and the current definition of WGG lithodemes.

It is likely that the various lithodemes arise from a limited number of petrogenic batches with similar chemical composition. Myers et al. (1981) proposed that the compositions could be grouped into classes, an idea further explored in Gilbert (1982). The proposed classes were largely based on ranges in the whole-rock element oxide data, and constrained the Mount Scott, Reformatory, and Mountain Park Classes.

With accumulation of data on the WGG in the subsequent four decades permits the further evaluation of the classes. However, Harker diagrams of this expanded data set fails to hold to the original class ranges, or consistent ranges whatsoever. Although some of the variation can be related to documented genetic processes, there is no clear grouping even among parental representatives.

Principal component analysis is a statistical technique that can resolve multivariant influence on the system. Using the work of Ragland et al. (1997) as a template, I was able to run an analysis using the major-element oxides and four trace elements (Ba, Rb, Sr, and Zr). The results show greatest lithodemic-relevant clustering in PC1 vs. PC3, less definition in PC2. Sr, Na2O, Ba, and Al2O3 have the largest loadings. Genetic relationships are generally negative correlations that produce orthogonal trends in principal component space, but poorly correlated (high-angle) loading vectors produce some interesting discriminators, even if their causes are more obscure.

For example, Ba and Sr variations seem most useful. Ba/Sr ratios are indeed affected by commonly crystallizing phases plagioclase and hornblende; plotting the ratio can be useful for looking for fractional crystallization processes in relatable suites. But such a plot does little to resolve classes. However, plotting Ba versus Sr is a reasonably robust way to group the WGG lithodemes.

Despite the departure seen in data acquired since the definition of the three classes, the Ba vs. Sr classification manages to contain the lithodemes into groupings evocative of the original system; as a low Sr and Ba Mountain Park class, an intermediate Sr Reformatory class (with a high and low Ba subclass), and a high Ba and Sr Mount Scott Class. This is not free of outliers, most notably in the Lugert Granite. At this juncture, further investigation into current mapping is warranted in many of these cases (and underway in the case of the Cache Granite.. But the diagram has already proved useful in classifying problematic outcrops documented in Stevenson (2020), showing the close affinity of the mineralogically distinct Craterville and Pophyritic French lake units to the much larger adjacent Quanah Pluton.

Which pluton are you? Show me your large ion lithophiles, and I'll let you know.

1980, Geological Society of America Bulletin, v. 91, p. 509-514

1980, Geological Society of America Bulletin, v. 91, p. 509-514

1980, Geological Society of America Bulletin, v. 91, p. 509-514