MOYIE-PURCELL MAGMATISM AS SEEN FROM AMPHIBOLITES WITHIN THE SOUTHERN PRIEST RIVER COMPLEX

The amphibolite body examined occurs within the Hauser Lake gneiss geographically located in the eastern Spokane Valley and structurally located in the southern portion of the Priest River complex. The Hauser Lake gneiss (upper amphibolite facies) has been correlated to the Prichard Formation of the Mesoproterozoic, Belt-Purcell Supergroup. The amphibolite studied is a tabular concordant body dipping at 48 degrees (west) with a total thickness of 17.7 m. Its western contact contains a 3 m thick garnet-rich border phase with 1-2 cm almandine garnets. The main portion of the amphibolite is coarse-grained, granoblastic with a mineral assemblage of amp+plag+pyx+qtz and minor amounts of bt+ap+zrc. The amphibolite also contains sporadic veinlets of plag+qtz. Structurally below the amphibolite is a 44.8 m thick body of white quartzite separated by 21.3 m of Hauser Lake gneiss. The amphibolite was analyzed for major and trace elements, and plots as a tholeiitic basaltic andesite with E-MORB affinity. Using a variety of bivariate elemental, normalized trace, and REE plots, the amphibolite closely correlates with mafic sills of the Paradise and Plains groups within the Prichard Formation of northwestern Montana. A third candidate may be mafic dikes of the Goat Flat Dike Group of western Montana. Based on its structural location and geochemistry, the amphibolite appears to correlate well with Moyie-Purcell aged (1.46-1.47 Ga) mafic magmatism within the Prichard Formation of the Belt-Purcell Supergroup. The geochemical data and conclusions presented here are consistent with three other amphibolite occurrences within the southern Priest River complex.