ORIGIN AND TECTONIC IMPLICATIONS OF THE EOCENE GOLDEN HORN BATHOLITH: A UNIQUE “A-TYPE” GRANITE IN THE NORTH CASCADES, WASHINGTON

The 310 km²Golden Horn batholith (GH) was emplaced during the enigmatic “Challis Event” and has A-type affinities not seen in other batholiths in the North Cascades orogen. Our study utilizes petrology and high precision geochronology to identify the processes and tectonic environment that led to generation of this unusual granitoid.

In order of decreasing age the GH consists of: (1) F-rich alkaline granite (AG), (2) hypersolvus biotite granite (HG), (3) rapakivi granite (RG), and (4) calc-alkaline granodiorite (CG). Contemporaneous with these felsic units are compositionally diverse mafic stocks and dikes that include calc-alkaline basalts (CAB), high silica adakites (HSA), low silica adakites (LSA) and rare basalts with OIB traits. All GH granitoids have arc signatures (e.g., HFSE depletions) and similar, mantle-like isotopic compositions (206/204 = 18.82-18.88; 207/204 = 15.58 – 15.59; 208/204 = 38.37 – 38.44; εNd = +4.3 to +4.8) that overlap those of the CAB and HSA (206/204 = 18.85 – 19.07; 207/204 = 15.57 - 15.60; 208/204 = 38.43 – 38.62; εNd = +2.7 to +4.9). The AG and HG are highly evolved (Mg# = 0.01 -0.09; Eu/Eu* = 0.01 – 0.19) and compositionally similar to melts experimentally produced from tonalite or basalt at low fH₂O (e.g., Turner and Rushmer, 2009). These data are consistent with derivation of the AG, HG, and CG by melting of underplated arc magmas, perhaps represented by the CAB. Textural and compositional data suggest the RG formed by mixing of AG/HG and CG.

High-precision U-Pb zircon CA-ID-TIMS geochronology indicates the AG, HG, and RG were emplaced in less than 300 ky at ~48 Ma. This assembly time is considerably shorter than those observed in arc-related intrusive complexes (built over >1 my) and suggests that the GHB is related to an unusually brief and focused melting event. We propose that extensive melting of basaltic lower crust (and overlying tonalite?) and the rapid assembly of the GH were a response to elevated lower crustal temperatures caused by subduction of the Kula-Farallon ridge or rupture/rollback of the Farallon slab. HSA and OIB dikes are consistent with slab window or slab breakoff/rollback magmatism during GH assembly. Regionally, GH magmatism appears to be part of a southward-propagating event and we are continuing to investigate its cause.