Barrovian metamorphism linked to crustal thickening in the southeast Coast Plutonic Complex of British Columbia and Washington is proposed in this paper to be the product of magmatic loading. Relevant observations and arguments are the following. (1) Isotopic ages coupled with structural and fabric relations document that throughout the region Barrovian metamorphism and plutonism are broadly coeval. (2) Baric patterns in country rock indicate steep-sided crustal loads originating within the high-grade parts of the orogen.(3) Patterns of subsidence and uplift are localized and diachronous, precluding a single-event regional thickening mechanism. (4) Country-rock fabrics are dominated by steep, orogen-parallel foliations and shallow, strike-parallel lineations, features not easily reconciled with tectonic thickening mechanisms. (5) Abroad zone of schists flanking the large (25 x 100 km) Scuzzy batholith in British Columbia bears metamorphic isograds and foliations that track the border of the pluton across regional structural trends, implicating pluton emplacement as the cause of metamorphism. (6) The crustal-loading event in the vicinity of the Scuzzy pluton is bracketed by mineral and textural features as being coeval with emplacement of the pluton, which occurred over a period of ∼7 m.y. (7) The regionally developed Chiwaukum Schist in Washington has petrologic and structural features and a metamorphic age that suggest it was formed by, and represents the floor of, an eroded extension of the Scuzzy pluton.
The proposed magma-loading model invokes the diapir and ballooning concepts of previous workers in which rising plutons displace country rock downward. This mechanism results in a type of convective crustal overturn, and at least partially solves the space problem related to batholithic emplacement. As a result of this process, relatively broad tracts of country rock acquire Barrovian mineralogy. Following plutonism, magmatically thickened crust is eroded and isostatically up-lifted to expose the plutonic roots and high-grade country-rock basement.