- UW Department of Geology and Geophysics
The Priest pluton is part of the suite of 1.4-Ga granitic plutons in North America. The pluton is a corundum normative, peraluminous, K-feldspar megacrystic granodiorite to quartz monzonite. It is characterized by: (1) high Mg/Mg + Fe; (2) high abundances of A12O3, CaO, and large ion lithophile elements (e.g., Sr, Ba); and (3) low abundances of high field strength elements (e.g., Zr, Y). Major and trace element modeling suggests that variations in composition were produced by accumulation of K-feldspar and plagioclase, localized accumulation of plagioclase, biotite, and accessory minerals, and late-stage crystal fractionation of the residual melt.
Plutons in the suite of 1.4 Ga, so-called “anorogenic” granitic plutons have been classified as A-type granites. Major and trace element abundances suggest that the Priest pluton is not A-type, but has more in common with S-type granites. In addition, typical tectonic classification techniques suggest that the pluton is either a volcanic-arc/syn-collisional or late orogenic granite. The pluton's intrusion was accompanied by highly partitioned, compressional deformation that is not typically associated with anorogenic, A-type plutons. The Priest pluton also is distinct from the nearby Sandia pluton, although the two plutons are nearly the same age. The Priest pluton has higher Mg number, alumina saturation index, and large ion lithophile element abundances as well as lower abundances of high field strength elements. The Priest pluton contains only biotite as its mafic phase, whereas the Sandia pluton contains biotite and hornblende. Differences in geochemistry between the Priest and Sandia plutons and the rest of the suite of 1.4 Ga granitic plutons suggest that the lower crust is a heterogeneous source region. Therefore, geochemical signatures that commonly are associated with particular tectonic settings may instead reflect heterogeneities in the source.