From the Indoor Lab to the Outdoor Lab: Using XRF Datasubmitted by
Higher Order Thinking Skills:
Role of Activity in a Course:
Data, Tools and Logistics
Extensive mapping and laboratory studies by M.E. Bickford and colleagues have revealed the presence of (at least) two caldera complexes in SFM, the largest of which is highlighted in the above map. This so-called "Butler Hill" caldera is characterized by central plutons of fine- to coarse-grained high-silica granite, by medium-grained ring plutons of intermediate silica content, and by voluminous occurrences of rhyolitic ignimbrites. The ring plutons are colored on the above map, and one of these - the Silvermines Pluton - was the main focus of this project.
At Hughes Mountain, a rhyolitic ignimbrite with a well-developed subhorizotal fabric consisting of flattened pumice fragments is characterized by beautiful columnar jointing.
The Knoblick Granite, another of the ring plutons, is a shallow-level intrusion that was emplaced into its own volcanic cover.
The Silvermines Granite is locally cut by basaltic dikes, one of which is shown here.
At "Tiemanns Shut-ins," a locality near the center of the Silvermines Granite, typical medium-grained, pale pinkish-gray granite is intruded by a finer-grained diorite, and by later fine-grained aplites. The diorite has commingled with the granite, as manifested by cuspate contacts and widespread globules of diorite within granite.
Although granites and rhyolites were analyzed by the class from several localities within the SFM, we focused on the striking features found at Tiemanns Shut-ins. In particular, we wanted to test the hypothesis that the diorites represent the product of mixing between granite and basalt, as proposed for similar-looking mafic enclaves in other settings. So, samples of Silvermines Granites, from Tiemanns and elsewhere, samples of diorite and an aplite from Tiemanns, and basaltic dikes were analyzed for major and trace elements by XRF methods. The resultant data are illustrated in this panel.
In the Qtz-Or-An+Ab, the analyzed granites and diorites form a linear array, approximately radial to the plagioclase corner. The "granites" lie in the granodiorite field, whereas the "diorites" lie in the quartz-monzodiorite field. Of particular note is the fact that the "Tiemanns granite" are close in composition to "Silvermines granite" from other localities in the pluton.
In the An-Ab-Or ternary, all of the "granites" and the "aplite" lie in the granite field, whereas the "diorites" analyze as granodiorite and trondhjemite.
On the Harker variation diagrams for major elements, the diorites lie between the fields for the granites and the basalts, which permits a "mixed" origin for the diorites. The composition of the aplite is consistent with it being a late differentiate of the granites.
The distribution of data points on the Ba-Sr plot is consistent with a "mixed" origin for the diorites. However, on the Zr-Nb plot, the diorite analyses overlap with those of the basalts, which is problematic for the mixing hypothesis.
The granites and rhyolites of the SFM are widely regarded by some investigators as examples of A-type or anorogenic granites. Hence, we might expect their compositions to fall within the proscribed fields on various discriminant plots. However, on the Nb—Y and Nb+Y—Rb plots of Pearce et al., the SFM granites (and diorites) lie in the field of "volcanic arc granites" (VAG) rather than the field of "within plate granites" (WPG).
On the Ti—V discriminant plot of Shervais (1982), the basaltic dikes fall in the field of alkalic basalts and continental flood basalts, which is consistent with their setting and overall chemical compositions.