MAGMA EMPLACEMENT PROCESSES IN THE RAPIDLY CONSTRUCTED GOLDEN HORN BATHOLITH, WASHINGTON

The plutonic record provides an opportunity to study how magma reservoirs are emplaced and evolve at depth. One critical question about reservoir construction is whether space for the intrusion is accommodated through vertical or lateral translation of host rock. Here we describe the intrusive relationships surrounding the upper-crustal (ca. 6-7 km paleodepth) Eocene Golden Horn batholith in the North Cascades, Washington. This intrusion was emplaced during regional transtension and widespread strike-slip faulting. It has a well-characterized internal structure consisting of distinct, subhorizontal tabular granitoid bodies marked by mostly sharp contacts that were emplaced over ca. 739 ka. Magma emplacement rates calculated for the units within this pluton suggest incremental emplacement of small volumes of very felsic magma early in the history of the pluton, which is unusual for most Cordilleran batholiths. These early units have margins characterized by intrusive breccias that are up to a few hundred meters in thickness and extend for at least 23 km along strike. The angular xenoliths of the host metasedimentary and metavolcanic rock range from cm-scale to tens of meters in length. Following incremental emplacement of the oldest parts of the pluton, a large (>424 km³) unit of rapakivi granite was emplaced and crystallized over a few 10s of kyr. The external contacts of this unit are sharp and very steep with little to no evidence for brittle fracturing. We interpret this difference to represent the progressive heating and weakening of the wall rock throughout batholith construction. More broadly we see little evidence for significant uplift of the pluton’s roof nor lateral shortening along the pluton’s margin, as host rock bedding is not deflected nor is there any ductile deformation evident near the pluton. We suggest that the pluton grew through floor subsidence and discuss the significance of this process in evaluating the growth and development of silicic magma reservoirs through geophysical means.