RAPID EMPLACEMENT OF THE GOLDEN HORN BATHOLITH: NORTH CASCADES, WA

The Golden Horn batholith (GHB) is a shallow granitoid intrusive complex that was emplaced along the Ross Lake fault zone in the North Cascades, WA during the middle Eocene. Its emplacement was approximately synchronous with exhumation of mid-crustal rocks in the North Cascades crystalline core, development of terrestrial basins, and regional dextral strike-slip faulting. The GHB is compositionally distinct from other 96-46 Ma intrusive complexes within the North Cascades as parts of the batholith have the geochemical characteristics of “A-Type” granite. Previous studies have defined four main phases that are exposed over 310 km²and ~1200m of topographic relief: 1) sodic amphibole bearing alkaline granite, 2) hypersolvus biotite granite, 3) two-feldspar biotite granite with rapakivi texture, and 4) smaller, predominantly tonalitic to granodioritic intrusions. Zircons from intrusive phases within the GHB define single age populations and contrast with the complex zircon systematics typical of calc-alkaline intrusions. High precision U-Pb zircon ID-TIMS dates from the GHB suggest that much of the batholith was intruded in 124 ± 26 kyrs, ca 48 Ma. This apparent emplacement rate is distinctly higher than the rates observed in other intrusive complexes within the orogen, which were built over periods > 1 my.

“A-Type” granites are commonly attributed to a) high-temperature crustal melting, b) extreme differentiation of basalt, or c) both processes. In either case, large amounts of basaltic melt are required to achieve crustal temperatures in excess of 900° C or provide the parental magma for differentiation. Evidence for contemporaneous mafic magmatism includes enclaves in GHB tonalite and granodiorite, mafic dikes, and the poorly dated Skymo layered mafic intrusion to the NW. Coeval felsic intrusive complexes to the S and SW of the GHB do not contain “A-Type” geochemistry, were emplaced over longer timescales, and likely represent hydrous melts. These observations suggest that the magmatic event that generated the GHB was limited in both space and time. The ca 48 Ma age of the batholith overlaps with dates from the Challis-Kamloops Volcanic Field to the east and the proposed timing of ridge subduction along the Cascadia margin. We continue to explore the relationship between the GHB and these regional events.