GENERATION OF THE RUBY BATHOLITH, NORTH-CENTRAL ALASKA, BY AN EARLY CRETACEOUS SHORT-LIVED VOLUMINOUS INTRUSIVE EVENT

The Ruby terrane of west-central Alaska has a remarkably similar protolith and Jurassic history to the southern Brooks Range. Both contain continentally-derived blueschist facies metasediments and Devonian orthogneisses and are structurally overlain by oceanic assemblages of the Angayucham/Tozitna terrane. We have previously proposed that the Ruby terrane emerged from the Arctic Alaska orogen through tectonic escape via dextral slip, driven by collision of Arctic Alaska with the Yukon-Koyukuk arc and rotational opening of the Canada basin.

The Ruby terrane’s voluminous E. Cretaceous batholith is unique in northern Alaska. Previous age constraints for the biotite granite - granodiorite batholith were dominantly K-Ar hbd.-bio. pairs, which range from 103-110 Ma. Three new U-Pb zircon ages (SHRIMP and TIMS) considered representative of the batholith range from 111.4 – 113.6 ± ~1 Ma. Several of the plutons intrude both the Ruby terrane schists and the structurally overlying Angayucham basalts. Overall the batholith has little to no fabric.

In contrast, a slightly older suite of granitoids at the SW end of the batholith have a wide range of compositions (gabbro to leucogranite) and fabric development. Twelve zircon and monazite ages from the complete range of compositions range from 118 – 111 Ma. The more mafic phases (tonalitic orthogneisses with mafic enclaves) have ages of ~118 Ma (SHRIMP and TIMS) and the youngest ages are from more evolved compositions. All of the igneous rocks have relatively high initial Sr isotopes and are moderately potassic, including the most mafic rocks. An U-Pb monazite age of 116.4 ± 0.5 from granulite facies metamorphic country rocks indicate peak metamorphic temperatures are associated with the mafic intrusive phases.

At the time of mafic magmatism, the Arctic Alaska subcontinent was being subducted under the waning Koyukuk arc, producing unusually radiogenic and potassic melts. We propose that the mafic magma in the Ruby terrane originated from melting of enriched mantle lithosphere of the Arctic Alaska subcontinent. The magma subsequently underplated the Ruby terrane’s hydrated continental crust at moderate crustal depths. Thermal buoyancy could explain the observed decompression, which would enhance crustal melting and produce a positive feedback loop with buoyancy forces.