CRETACEOUS AND EOCENE MAGMATISM IN THE SOUTHERN PRIEST RIVER CORE COMPLEX (PRC) OF NORTHEASTERN WASHINGTON AND NORTHERN IDAHO

In the Mt. Spokane quadrangle, the Newman Lake Gneiss (NLG) and an unnamed biotitic orthogneiss (BOG) are structurally above the Hauser Lake Gneiss (HLG). HLG is a sillimanite-bearing paragneiss (1.47 Ga Prichard Fm. equivalent) that dominates the southern PRC. In the central portion of the quadrangle is the Mt. Spokane pluton (MSP), a two-mica granite. In the southeast, the weakly foliated and lineated Eocene Rathdrum granite (RG) intrudes both orthogneisses and the HLG.

The NLG is sheet-like and occurs between the HLG (east) and the MSP (west). The NLG is well foliated, biotitic (with relict Hbl), and contains conspicuous zoned Kfs megacrysts (< 5 cm) and trace Ms. The BOG, considered Precambrian by previous workers, is tonalitic (with visible Mt), well-foliated, with 10 to 20-cm thick biotite-rich and biotite-poor bands. The coarse grained MSP is non-foliated, with subhedral Bt and Ms. The RG is medium-grained and biotitic with rare Ms.

Geochemically, the NLG is a magnesian, calc-alkalic granodiorite that is weakly peraluminous (1.1 ASI). The BOG is calcic and also weakly peraluminous. Trace element plots are enriched and have moderate LILE:HFSE slopes with negative spikes in Nb, Ta, P, and Ti. REE plots have moderate LREE:HREE slopes with defined negative Eu anomalies. In general, the NLG and MSP overlap, but are distinct from the BOG. The RG is calc-alkalic and weakly peraluminous (1.1 ASI), and exhibits similar trace element and REE patterns. However, the RG is higher in K₂O, Ba, La, Rb, Zr, and FeO_t/MgO, and lower in Na₂O and Sr.

We report a new U/Pb (Zrc) date for the NLG of 65.4 ± 0.9 Ma from its type locality, and a new U/Pb (Zrc) date of 98.9 ± 1.2 Ma for the BOG, confirming that it is not basement but part of the extensive regional Cretaceous magmatism. The NLG coincides with peak regional metamorphism (68-64 Ma) of the HLG. Recent U/Pb dates report magmatic ages of 76.5 ± 0.1 Ma and 46.4 ± 1.8 Ma for the MSP and RG respectively. A few inherited zircons from BOG and the NLG range from 1584 to 1899 Ma. A probable source for the Paleoproterozoic inherited zircons is 1.85 Ga crystalline basement, such as that exposed south near Coeur d’Alene. The 1584 Ma inherited zircons may indicate that, besides the exposed bodies near Priest River, ID., additional basement of this unique age is present beneath the PRC.