Delineating Terrane Boundaries in Northeastern Washington Using Isotopic and Geochemical Analysis of Cretaceous Plutons.

Abstract:

Late Cretaceous granitoids are exposed across a ~300 km wide swath of NE Washington and Idaho, and can be grouped from west to east into four geographic regions: (1) North Cascades (2) North Central Washington (3) Northeast Washington and (4) Idaho Batholith. We focus on the petrology and U-Pb geochronology of seven intrusions in the North Central region (NC) with the goals of: (1) comparing chemical and Sr-Nd isotopic characteristics of these plutons with those of similar age plutons to the east and west, (2) delineating terrane boundaries using Nd model ages, and (3) identifying geographical trends in age or composition that could help explain the anomalous breadth of Cretaceous magmatism in the area.

NC plutons are primarily biotite granites; some also contain hornblende, garnet, and/or K-spar megacrysts. All are peraluminous (ASI = 1.10 – 1.41) and have SiO₂ ranging from 65-74 wt% and arc chemical signatures (HFSE depletions). The less silicic ones are high-Si adakites. Isotopic data (⁸⁷Sr/⁸⁶Sr_i  = 0.7034 to 0.7053; εNd (t) = +4.87 to -3.48) suggest the NC plutons are mixtures of material from depleted mantle and older crust, some of which was eclogitic. Depleted mantle Nd model ages cluster between 750-950 Ma; younger than those of Northeastern region plutons (1250-1500 Ma) but older than those of North Cascades plutons (250-450 Ma) (Loewen et al., AGU 2007). We interpret these discontinuities in model ages as terrane boundaries, with NC plutons demarking the southern extension of Quesnellia, a Paleozoic-Mesozoic island arc accreted to North America by the late Jurassic as part of the Intermontane Superterrane. Ongoing U-Pb dating will clarify whether there are differences in the timing of Cretaceous magmatism between the different regions.