Extremely High Magma Emplacement Rates Recorded in the Golden Horn Batholith, WA

Abstract:

High SiO₂ rhyolites emplaced during ‘super-eruptions’ demonstrate that large volumes of eruptible magma can exist in the upper crust. However, the timescale over which the magma reservoirs that source these eruptions are built remains controversial. Thermal models suggest that magma emplacement rates need to be > 0.005-0.01 km³/yr in order to accumulate enough eruptible magma to source a ‘super-eruption’. Yet, these rates are higher than the time-averaged rates (< 0.001 km³/yr) for nearly all well-studied granitoid plutonic complexes. This disparity contradicts geologic evidence suggesting that the high SiO₂ rhyolites emplaced during ‘super-eruptions’ are extracted from crystal rich magma chambers that should be preserved in the geologic record as granodioritic and granitic plutons.

We quantify time-averaged magma emplacement rates for the upper crustal Golden Horn batholith, WA based on new geologic mapping and U-Pb zircon CA-IDTIMS geochronology. The batholith is exposed over 310 km³ and can be separated in the field into five intrusive units. High topography allows the 3D geometry of each phase to be constrained and their volumes range from < 100 km³ to > 400 km³. U-Pb zircon geochronology reveals that four of the five phases were assembled incrementally and distinct zircon populations from samples within these phases suggest that individual magmatic pulses had fully crystallized before the next arrived. However, six nearly identical U-Pb zircon dates from a > 400 km³ rapakivi granite show that this phase was built in ca. 50 kyr and that large portions may have been emplaced nearly simultaneously. The implied emplacement rate for this phase (≥ 0.008 km³/yr) is in agreement with those predicted for assembly of the upper crustal magma chambers that source ‘super-eruptions’, and it may provide a rare and unprecedented opportunity to study the processes that occur in such chambers.