STOPING HAPPENS!

Since R.A. Daly elegantly suggested a mechanism to explain discordant intrusive contacts that were not faults in 1903, there have been relatively few published studies that deal directly with magmatic stoping. In fact, most magma emplacement studies dismiss stoping based on the argument that stoped blocks are not widely observed. We have mapped and examined a number of pluton-host rock systems with crystallization depths from 2-25 km. Some plutons contain stoped blocks of various lithologies and in sizes larger than a house (Sausfjellet, Norway; Andalshatten, Norway; Jack Ass Lakes, CA; Copper World, CA; Wooley Creek, CA), others do not (e.g., White Horse, NV; Crater Island, UT; Chita, Argentina). In most cases the host rocks contain evidence for ductile deformation (folding, dynamothermal metamorphism) that is emplacement related. However, all of these plutons have variably to strongly discordant contacts with host rock structure/stratigraphy at outcrop to map scales. Such observations are consistent with a rheological transition from early emplacement-related ductile deformation in the host rocks to late emplacement-related brittle deformation (i.e., diking and stoping). In some cases it is possible to retro-deform cross sections or maps of the host rocks across the intrusive contact (e.g., White Horse, Chita, Andalshatten) and evaluate whether or not the host rock stratigraphy can be restored. For these three plutons, the host rock stratigraphy cannot be restored indicating that material has been removed from the section/map along the discordant contact. Over 50% of the map space currently occupied by the White Horse pluton and up to 80% of the exposed volume of the Chita pluton may have been created by stoping. Our field, petrological and geochemical studies indicate that perhaps as much as 25% of the outer zone of the Sausfjellet pluton is the result of assimilated stoped blocks. Because variably discordant contacts around plutons are ubiquitous, we suspect that stoping may be a pervasive and ubiquitous magma emplacement mechanism throughout the crust. While existing analytical models suggest that stoping is thermally detrimental to the migration of magmas in crustal environments, we find the observation of discordant intrusive contacts to be compelling evidence for stoping.