IMPLICATIONS FOR TECTONIC AND MAGMATIC PROCESSES FROM THE ~5 TO 40 KM NORTH CASCADES CRUSTAL SECTION, WASHINGTON

The crystalline core of the North Cascades comprises oceanic and arc terranes that were metamorphosed to amphibolite facies and intruded by 96-45 Ma plutons. The southern core preserves an ~5 to 40 km crustal section, facilitating an evaluation of magmatic and structural styles during arc-normal shortening. Magma bodies ranging from <10 cm-thick sheets to large (>1000 km3) plutons intruded after terrane juxtaposition. Metamorphic and geochronologic data indicate rapid burial of plutons and host rocks during mid-Cretaceous deformation, and the deepest rocks of the section are meta-supracrustal units of amphibolite rather than granulite facies metamorphic conditions. The thickness of the mid-Cretaceous crust was most likely >60 km and may have approached Andean Altiplano-Puna plateau thickness.

The dominantly tonalitic plutons are steep-sided, vertically extensive bodies that commonly are sheeted. Compositions of the plutons do not vary significantly with depth. The 96-84 Ma, most voluminous pulse of magmatism is recorded in both focused and unfocused magmatic flow at all crustal levels, which resulted in large plutons and abundant narrow sheets, respectively. Individual, large-volume intrusions were constructed over time periods of up to ~5.6 Myr during punctuated intervals of magma emplacement. Petrologic variations, internal magmatic contacts and local magma mingling indicate that these intrusions were constructed from multiple batches of magma. At deeper crustal levels, large plutons such as the Tenpeak pluton are constructed of magma sheets of distinct composition and texture with little evidence of homogenization between magma pulses. In contrast, the shallow level Mount Stuart batholith (MSB) shows gradational contacts between magma pulses of differing composition and contains only cryptic contacts between tonalitic pulses that are significantly different in age. The mid- to lower-crustal intrusions, such as the Tenpeak pluton, may represent conduits between lower crustal zones of magma generation and mixing, and upper crustal zones of large, relatively homogenous intrusions such as the MSB. The magmatic, metamorphic, and structural processes recorded in the North Cascades exemplify the dynamic evolution of arcs and the vertical and lateral displacements during arc construction.